Symposium SB11 : Multiphase Fluids for Materials Science—Droplets, Bubbles and Emulsions

Ideally, the outcome of materials design experiments would be determined automatically based on a limited quantity of readily obtainable data. This task is often possible for an experienced practitioner. Here we automate this classification step for the case of soft materials design. We want to know whether a particular attempt to create a bijel has been successful or not based on a single confocal micrograph. Our most effective classification approach is based on shape descriptors of the co-continuous fluid domains in the image. Here, I will show how adaptive design can be used to optimize the pre-processing of the confocal image to improve our classification performance.

Dynamic multiphase complex emulsions formed from two or more immiscible solvents offer a unique platform for the generation of new triggerable materials. In designing our methods, we make use of solvent combinations that are immiscible at room temperature but exhibit a lower (LCST) or upper critical solution temperature (UCST). Emulsification of the mixture below LCST or above UCST enables a simple one-step fabrication of complex multicomponent emulsions as well as structured soft-matter particles with highly uniform morphology via temperature-induced phase separation. The morphology of these dynamic liquid colloids is exclusively controlled by interfacial tensions and the droplet geometries can be controllably altered after emulsification. Dynamic liquid colloids can selectively invert morphology in response to external stimuli such as the presence of specific analytes, small pH changes, light or high energy irradiation, and the presence of an electric or magnetic field and thus provide a new active element for novel and existing applications of emulsions including chemotaxis, the fabrication of optical metamaterials, and chemical/biological sensing.
We explore the potential of our dynamic micro-colloids to manipulate the pathway of light in response to chemically triggered morphology changes. Dynamic morphological reconfiguration of microscale refractive components in combination with the potential to incorporate a variety of active optical media as well as stimuli-responsive elements enables the application of these purely liquid-based or solidified micro-colloids as new transduction materials for chemo- and bio-sensing. Here, we will demonstrate a series of optical transduction methods that are based on having control over the total internal reflection of light from outside and inside multicomponent colloids. An associated understanding of the unique chemical-morphological-optical coupling inside chemically functionalized active soft matter colloids creates a solid foundation for the development of sensing paradigms targeting a series of chemical and biological analytes, including a rapid and sensitive method for the detection of common foodborne pathogens Escherichia Coli and Salmonella enterica bacteria.

Using large scale molecular dynamics we study in detail the impact of nanometer droplets of low viscosity on at substrates and the effect of the wettability between the liquid and the plate. We show the maximal contact diameter during the nanodroplet impact (Dmax) as well as the time required to reach it (tmax) are in agreement with experimental data at the macroscale showing similarities between droplet impacts at the nano and the macro scales. The comparison between the MD simulations and di erent models reveals that most of these models do not take into account all the effects we observe at the nanoscale. Moreover, most of their predictions for the impact at the nanoscale do not correspond to the simulation results. Because of this, we have developed a simple model for Dmax which is in agreement not only with the simulation data but also the experimental observations and it also takes into account the effects of the liquid-solid wettability. We also propose a new scaling for tmax with respect to the impact velocity which is also in agreement with the experimental observations. With the new model for Dmax plus the scaling found for tmax, we present a new way to collapse in a master curve the evolution of the micro to nanometer drop contact diameter during impact for di erent wettabilities and di erent impact velocities. We believe our results may help to design better nanoprinters since they provide an estimation of the maximum impact velocities required to obtain a smooth and homogenous coverage of the surfaces without dry spots.

In ternary mixtures of two immiscible liquids and one particulate species, capillary forces are a strong driving force for particle aggregation. In such mixtures, a rich variety of microstructures appears from a coupling between interfacial tension, particle wetting phenomena, and viscous forces of mixing. For the simplest case of spherical particles, it is possible to construct a non-equilibrium state diagram based on rational considerations of the various phenomena. This talk will discuss the situation with a far more complex type of particle: fumed silica.

We examine melt-blended mixtures of two molten plastics, polyisobutylene (PIB) and polyethylene oxide (PEO), and fumed silica particles which have strong affinity for the PEO. The unusual structure of fumed silica – which comprises 15-30 nm primary particles permanently bonded together into highly porous, fractal-like aggregates – strongly influences the morphology at all compositions. At relatively low PEO loadings, the PEO is absorbed into the pores within the fumed silica aggregates. The morphology consists of a network of interlocked particle aggregates bonded together by PEO. At higher PEO loadings, the PEO and the particles form a combined phase that is solid-like even at low fumed silica loadings. The morphology somewhat resembles a conventional co-continuous morphology where one phase is PIB and the other is a fumed silica-filled PEO. What is most interesting and potentially useful is that morphologies with two percolating phases appear across almost the entire composition range examined. We construct a composition-microstructure map for mixtures containing fumed silica and contrast against the simpler case of spherical particles.

Epoxy-phenolic resins, valued for their ease of processing into various shapes and coating thicknesses, have been widely used as base materials for a wide variety of applications, such as integrated circuit packages, laminating resins, adhesives, binders, surface coatings, and impregnants. Here we report a simple method to make bicontinuous epoxy-based porous nanocomposite monoliths. We begin with a bijel comprised of immiscible mixtures of two fluids: epoxy and vegetable oil, stabilized by epoxidized soybean oil (ESO) as a surfactant and a means to improve the interfacial bonding between the epoxy resin and vegetable oil. By adding nanofillers and mixing at high shear, interfacial jamming occurs, which arrests the fluids in a bicontinuous configuration. During the evolution of the emulsion, the configuration maintains, creating an interconnected porous structure, which length scale is determined by the selection of particle. Our rheology results reveal the role of ESO in this process, with low quantities not participating in the epoxy curing due to interfacial segregation and high loadings modifying the curing kinetics and mechanical properties as a plasticizer. Through this fabrication method, we can produce porous monoliths at a wide range of feature length scales that shrink less than 2% after curing and removal of the oil porogen, and due to the non-volatile nature of the oil, have uniform porosity through the bulk of the structures. In addition, the surface of the porous monolith can be functionalized by selectively altering the nanofillers.

Capillary suspensions of particles in immiscible fluid mixtures are stabilised by pendular drops of the minor liquid phases forming bridges between the solid particulate phase, a structure distinctly different from that of a Pickering emulsion.[1] This behaviour requires two immiscible fluids one of which strongly wets the solid phase (the secondary liquid which is usually a relatively small volume fraction of the suspension), while the other shows a relatively high contact angle. Capillary suspensions have a distinct rheology with a transition from a rigid gel to a fluid emulsion occurring above a yield stress and the gel reforming if the fluid strain rate drops below a critical value. This rheological behaviour is similar to that found with particle suspensions in tri-block copolymer solutions (e.g. Pluronics) but can be achieved with immiscible small molecules, e.g. water and octanol. The rheology of capillary suspensions is ideal for 3D printing by direct write extrusion and has the advantage over current methods by not requiring large quantities of polymers that may need removal by secondary post-processing.
Here we demonstrate a printable graphene ink produced by adding a small amount (1-2 vol%) of the secondary liquid, in this case octanol, to an aqueous graphene suspension (GS). Octanol is immiscible with the primary liquid, water, and strongly wets the graphene flakes in the suspension, forming the graphene capillary suspension (GCS). Rheological studies show that the storage modulus of GCS is three orders of magnitude larger than that of the original GS with a yield stress of 95 Pa. Beyond the yield stress the GCS shows shear thinning behaviour with the viscosity decreasing as the shear rate increases. The GCS materials were used as inks for Robot-assisted direct writing to fabricated simple graphene structures. These show a relatively high porosity of 81% and a corresponding bulk density of 426 kgm^-3. The printed structures show a crushing strength of 1.4 MPa and high electrical conductivity of 2370 Sm^-1 after heat treatment. Computed X-Ray tomography and SEM are used to explore the internal structure of the extruded filaments. Comparing the X-Ray tomography images of GCS and GS filaments, it shows a shear aligned graphene plate structure with the GS ink and a more disordered flake distribution with the GCS ink. This is consistent with the flake alignment in the GCS occurring after extrusion has arrested and the capillary suspension gels. Similar behaviour is found using BN as the 2D particulate material but not with MoS₂, despite both materials showing similar surface energetics when dispersed in water and octanol. Reasons for this discrepancy are explored.


Reference
1. Koos, E. and N. Willenbacher, Capillary forces in suspension rheology. Science, 2011. 331(6019): p. 897-900.

Pyroelectric materials are emerging as an alternate to thermoelectrics for waste heat harvesting due to their potential for higher efficiency. The desire to maximize scenarios compatible with pyroelectric waste heat harvesting introduces the challenge of inducing temperature oscillations in a pyroelectric material based on near-constant temperature, or constant flux, heat sources. Prior efforts in this area have utilized electrowetting on dielectric (EWOD) and microelectronic mechanical systems (MEMSs) to fabricate compact waste-heat harvesting devices for testing. However, for large-scale applications, a compromise must be made between device efficiency, cost, and device miniaturization. In this presentation, we demonstrate a method using electrohydrodynamic (EHD) force to actuate a capillary bridge from a liquid droplet between a heat source and heat sink which serves as a proxy for a pyroelectric material. Enhancement of heat transfer is derived from the capillary bridge. Through EHD thermal experiments with different thermal fluids, we found the enhancement of heat transfer is achieved by different dominant mechanisms depending on the viscosity of the formulation, with high viscosity formulations relaying on thermal conductivity and lower viscosity relying more on the convection induced by the EHD and Marangoni flow. Through periodically forming and breaking the capillary bridge, temperature oscillations were successfully established due to the resulting differences in the heat transfer rate. Employing this approach, a laminate device is built to turn a steady thermal field to an oscillating thermal field, showing a scalable method compatible with the roll-to-roll fabrication of sheet devices.

Multiphase fluid systems are strongly influenced by gravitational phenomena that affect everything from transport dynamics to the self-assembly of ordered systems. Buoyancy-driven convection drives heat and mass transport, providing a source of mixing. Density-driven segregation leads to inhomogeneities and phase separation including sedimentation. One byproduct of the sedimentation is the occurrence of stratification phenomena in self-assembled multiphase fluid systems such as foams and emulsions. The removal of gravity as a force provides opportunities for the investigation of discrete phenomena such as diffusion or viscosity in isolation. For example, in a persistent microgravity environment, the interfacial energies in self-assembled structures can be studied in isolation from sedimentation and stratification effects.
The International Space Station (ISS) U.S. National Laboratory offers a unique environment in persistent microgravity that enables the decoupling of physical phenomena such as buoyancy-driven convection from diffusion. Areas of investigation onboard the ISS have included droplet formation, colloid interactions (emulsions, nanoparticles, biomolecules, etc.), foam formation and stability, gel formation (aerogels, hydrogels, etc.), and self-assembled structures. For example, microgravity provides the opportunity to study “wet” foams in the absence of drainage—a result that is difficult to replicate terrestrially.
We will introduce the underlying physical phenomena of multiphase liquids in microgravity. We will present case studies of multiphase liquid system investigations conducted onboard the ISS, including studies of droplet formation, gel and foam stability, and the self-assembly of hydrogels and aerogels, and compare results with those from terrestrial experiments. We will also discuss translational lessons learned from microgravity experiments that inform and direct terrestrial research and manufacturing. Finally, we will present opportunities for future microgravity experiments and access to ISS facilities through the ISS U.S. National Laboratory.

Topological defects, or vortices, play a crucial role in spatiotemporal organization of equilibrium and non-equilibrium systems. For example, immobilization, or pinning, of Abrikosov vortices in type-II superconductors is a formidable challenge both for fundamental science and technology. The problem is even more difficult in the context of the out-of-equilibrium system, such as a living nematic, a suspension of swimming bacteria in a lyotropic liquid crystal. An interplay between hydrodynamic flows, elastic forces, and swimming activity generates a variety of complex phenomena, such as the onset of spatiotemporal chaos and textures of half-integer topological defects (disclinations). Here we study possibility control of the emerged order of topological defects by the arrays of 3D printed microscopic obstacles (pillars). Our studies show that while -1/2 defects may be easily immobilized by the pillars, +1/2 defects remain motile and reside in the bulk. Due to an attraction between opposite defects, positive defects remain in the vicinity of pinned negative defects, significantly diminishing their diffusivity. Experimental findings are rationalized by computational modeling of a living nematic. Our results provide valuable insight into the control and manipulation of active systems via targeted immobilization of topological defects.

Many natural materials have optomized structures on different length-scales and locally varying compositions. These features impart unique mechanical properties to them. Inspired by nature, we are developing tools that enable the production of structured hydrogels whose conformation changes over 10s of micrometer length scales. This is achieved using drops with well-defined sizes and compositions that are produced with microfluidics. I will demonstrate microfluidic devices that offer a tight control over the arrangement of the drops. These assembled drops are subsequently converted into macroscopic structured hydrogels possessing compositions with well-defined, locally varying compositions. I will show how the local composition of hydrogels influences their macroscopic mechanical properties.

Crystal fouling due to saline spray leads to corrosion and degradation of many materials, including metals, stones, and organics. This degradation is visible in many coastal cities, where buildings and other structures degrade faster than their non-coastal counterparts due to salty ocean spray. Implementation of superhydrophobic surfaces is one strategy that may help prevent crystal fouling, but is only successful when no crystals are able to form at the surface, as any amount of nucleation will seed additional crystal formation at that surface and degrade the hydrophobicity. In this work, we demonstrate an unusual phenomenon in which sodium chloride crystals grown from a drop placed on a heated superhydrophobic surface self-eject from that surface. This phenomenon is related to the plastron (a thin film of air) layer within the texture of the superhydrophobic surface, which creates a channel for water vapor to escape from the evaporating drop. A large temperature gradient across the drop leads to enhanced vaporization at the surface, and escaping vapor creates crystalline micro-tubules which continue to grow as salt crystallizes at the tips. These tubules grow into "legs," causing the entire salt structure to lift off from the surface. We find that the self-removal of the crystals keeps the surface clean, and observe no degradation of the anti-fouling performance over extended use. This phenomenon could be useful in cooling towers using water spray heat exhange, where water typically must be very pure to avoid salt corrosion. By implementing substrates that cause self-ejection of crystals, it may be possible to use salt water rather than ultra pure water, and thus preserve fresh water resources while also cutting down on costs associated with water treatment.

2D materials are being widely explored for use in the fabrication of large area flexible, transparent electronic devices such as: field-effect transistors (FETs), sensors, and light emission devices.¹ The assembly of large area films from a 2D material dispersion is a key step in the manufacture of electronic devices. Close packing and edge to edge contact is believed to be crucial in order to ensure high carrier mobilities through the 2D material film. However, widely used film assembly methods such as spin coating and spray coating result in low transfer efficiency, flake restacking and loose random packing of flakes in the film. Here, we present a thin film assembly method which overcomes these issues using a molecularly flat assembly plane; the interface of two immiscible liquids.
In this work, dispersions of exfoliated 2D material are deposited at the liquid/liquid interface and are compressed laterally by an interfacial tension gradient away from the point of injection. The resulting thin film is a 2D jammed monolayer of flakes which has minimal overlap and predominantly edge to edge contact between flakes. The packing density of the resulting film is high (87 %) and the transfer efficiency is over 100 m² per gram of exfoliated 2D material. The method described has the additional benefit that the bulk liquid phases help to wash contaminants from the thin film. We demonstrate how the resulting large area films, which can coat the entire surface of a 4 inch wafer, are highly pristine and simple to produce.
FETs were manufactured using the aforementioned film deposition method with a dispersion of electrochemically exfoliated MoS₂.² The mobility and on/off current ratio of the back gate and bottom contact FETs were 7 cm².V^-1.s^-1 and 10⁵ respectively; comparable to chemical vapour deposition grown MoS₂ FETs. This work demonstrates the use of liquid/liquid interfaces as a tool for self-assembly of high performance monolayer thin film devices made from dispersions of ultrathin 2D material.
1. Petrone, N., Hone, J. & Akinwande, D. Two-dimensional flexible nanoelectronics. Nat. Commun. 5, 1–12 (2015).
2. Lin, Z. et al. Solution-processable 2D semiconductors for high-performance large-area electronics. Nature 562, 254–258 (2018).

Disasters are sometimes associated with the transportation of oil across big distances using oil tankers. When an accident occurs large volumes of oil are spilled into the ocean and have to be collected from the water. However, the traditional oil-removing methods fail to collect the micro-sized droplets of oil originated by the mechanical action of the waves.[1] Oil/water separation is an important field, not only for scientific research but also for practical applications aiming to resolve industrial oily wastewater and oil-spill pollution, as well as environmental protection[2,3].

This work focusses on the recovery of oil microdroplets suspended in water, using a manufactured cellulose acetate (AC) non-woven electrospun membrane coated with different patterns of cellulose nanocrystals (CNCs). Efficiency tests show that these membranes can remove up to 80% of the oil microdroplets present in a water emulsion.

The imprint the different designs of the CNCs layer was performed using screen-printing and the adhesion of the CNCs with the fibers of AC was promoted by a thermal treatment. The removal of the oily micro droplets is achieved when a water/oil emulsion flows through the membranes, due to the hydrophilic character of the AC fibers, and the CNC-coated regions collect micro droplets of oil, due to its oleophilic character.

This work demonstrates that it is possible to produce efficient all-cellulosic composite membranes for the capture of micro-oil droplets dispersed in water, with easy tailoring of the ratio AC membrane and annealed NCC allowing to maximize the micro-droplet oil collection and at the same time the water flow. More, the combination of annealed NCC with non-woven electrospun membranes opens the door to a low-cost environment-friendly method of treating polluted ocean and waste-oily waters.

References:
[1] Z. Xue, Y. Cao, N. Liu, L. Feng, L. Jiang, J. Mater. Chem. A 2014, 2, 2445.
[2] L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang, D. Zhu, Angew. Chemie - Int. Ed. 2004, 43, 2012.
[3] W. Zhang, Y. Zhu, X. Liu, D. Wang, J. Li, L. Jiang, J. Jin, Angew. Chemie - Int. Ed. 2014, 856.

The liquid-liquid interface is a high-energy interface that can be stabilized using interfacially active molecules. These surfactants self-assemble at the oil-water interface, exposing their polar head group to water and apolar tail to the oil phase, allowing for the liquid-liquid interface to be used as a template for two-dimensional assembly. Our laboratory had previously used carbon-rich amphiphiles to fabricate carbon nanosheets at the air-water interface.^[1] In the work presented here, we report the spontaneous, in-situ formation of 2D carbon nanosheets at the liquid-liquid interface at room temperature. To this end, we employ highly reactive, carbon-rich surfactants based on hexayne phosphonic acids, that readily self-assemble at the oil-water interface and then spontaneously undergo two-dimensional interfacial polymerization. The resulting membrane can be further crosslinked by UV irradiation, forming an ultrathin carbon nanosheet at the interface. The interface thus undergoes a transition from molecular surfactant to colloidal stabilization by Janus-type carbon nanosheets and shows distinct properties of a Pickering emulsion, such as buckling upon a reduction in the interfacial area, and also healing upon rupture. We investigate the formation process of the nanosheet at the interface, the change of chemical composition upon UV-irradiation, as well as the mechanical properties of the nanosheets.

Reference:
[1] Schrettl, S.; Frauenrath, H. et al, Nature Chem. 2014, 6, 468-476.

The advent of microfluidics has led to remarkable advances in the synthesis of functional particles and microcapsules for a variety of applications. The ability to precisely manipulate the flow of multiphasic fluids in microchannels enable production of highly uniform liquid droplets and gas bubbles with complex morphology. Despite these exciting developments, there remains some key challenges that must be addressed to enable successful commercialization of these technologies. In this talk, I will discuss our recent contributions in producing “designer” microparticles for drug delivery, diagnostics and regenerative medicine applications. The importance of understanding and harnessing the fundamental interfacial phenomena to engineer the structure and functionality of these particles will be described. I will also discuss our recent efforts to scale-up the production of particles via parallelization in solvent-resistant microfluidic devices.

If micro-wells were the size of biological vesicles, there would be room for 10^17 compartments in one cubic centimeter. In contrast, the state of the art in high-throughput screening uses 1536 well plates and about 3 ml of solution per plate. Motivated by a desire to understand biological compartmentalization and to use it for biotechnology, this presentation will describe the fabrication and applications of arrays of micrometer and nanometer scale lipid droplet microarrays. Arrays are fabricated by nanointaglio, which involves the transfer of fluid inks from the recesses of a microstructured stamp onto a substrate.^[1] Thousands of different materials can be integrated into the droplets using pin spotting technology. Lipophilic small molecules can be encapsulated into the oil droplet, and cell cultured over the arrays for phenotypic screening with pharmaceutical applications.^[2] Furthermore, exposure of these droplet arrays to lipid binding analytes such as proteins while observing scattered light from the arrays allows label free detection of remodeling events in the lipid droplet nanostructure.^[3] Droplet size, shape, and composition are central to these applications. Recent progress in the use of these multiphase fluids to investigate and control biological systems will be presented.

^[1] Lowry et. al., Advanced Materials Interfaces 2014, 1, 1300127.
^[2] A. E. Kusi-Appiah, Lab on a Chip 2015, 15, 3397.
^[3] T. W. Lowry, Small 2016, 12, 506.

A simplified theoretical model treats neurons as non-linear oscillators that when coupled together through excitatory and inhibitory connections give rise to complex spatio-temporal patterns. When organized, these patterns are capable of processing and storing sensory information, and actuating musculature. Extrapolating from this general definition of a neuronal network, we posit these dynamics can be captured on an abiologic reaction-diffusion platform. We reported advances in soft lithography that allow the engineering of synthetic reaction-diffusion networks capable of producing the same spatiotemporal dynamics of the eel’s CPG¹. The network is natural in the sense that the oscillators and couplings are physical-chemical processes that, once initiated, proceed without any external intervention. Our experimental model system uses the oscillatory Belousov-Zhabotinsky (BZ) reaction with which we created diffusively coupled networks over which we designed (i) the topology of the network, the (ii) boundary and (iii) initial conditions, (iv) the volume of each reactor, (v) the coupling strength, and (vi) whether the coupling is of an inhibitory or excitatory nature. In particular, Central Pattern Generators have been modeled theoretically as being networks of identical oscillators^2-4. With this BZ experimental system we will probe a number of fundamental questions addressing the suitability of chemical oscillators networked into CPGs for control of soft robotics. It is important to note that the engineering principles we identified are general and can be applied to other oscillatory reaction-diffusion systems besides BZ.

Network symmetry imposes constraints on dynamics in both expected and surprising ways^2-4. First, the impact of network symmetry is completely independent on the underlying chemical nature of the oscillator. When wiring together identical oscillators, symmetry imposes significant constraints on the dynamics. These constraints create invariant manifolds, privileged subspaces in phase space that are impenetrable to the dynamical flow of the system. Invariably, real systems are not strictly symmetric. Oscillators and their connections have heterogeneities. How much variability does a system need in order to break the rules arising from symmetry?

A related question arises in regards to control of a network. Optimal control theory asks what is the minimum effort required to move as system from one stable attractor to another. Because dynamics is constrained to move on invariant manifolds, symmetries influence control protocols. We can test the theoretical role of symmetry and the protocols of optimal control theory in an experimental system. As specific examples of the role symmetry plays in network dynamics, we consider the simple cases of a ring of 3 and 4 identical oscillators and study how the dynamics changes as we break symmetry by changing the natural frequency of oscillation. The ring of 3 oscillators has two stable attractors. Moving to a ring of 4 oscillators leads to richer dynamics corresponding to the gaits of quadrupeds^2-4. The general questions we ask are: How to optimally switch from one gait to another? Can one control the entire network by manipulating a subset? How robust is the stability of the attractors to noise?

References
1. Litschel, T., Norton, M. M., Tserunyan, V. and Fraden, S., "Engineering reaction-diffusion networks with properties of neural tissue," Lab on a Chip 18, 714-722 (2018).

2. Golubitsky, M., Stewart, I., Buono, P. L. and Collins, J. J., "A modular network for legged locomotion," Physica D 115, 56-72 (1998).

3. Golubitsky, M., Stewart, I., Buono, P. L. and Collins, J. J., "Symmetry in locomotor central pattern generators and animal gaits," Nature 401, 693-695 (1999).

4. Golubitsky, M. and Stewart, I., "Recent advances in symmetric and network dynamics," Chaos 25, 097612 (2015).

Porous liquid infused surfaces, commonly referred to as SLIPS (slippery liquid infused surfaces), can be fabricated from any nano/microstructured porous solid material with a lubricating liquid film used to create surfaces that exhibit liquid repellency, self-healing, optical transparency, pressure stability, and self-cleaning. If designed properly these surfaces can repel many fouling challenges including bacteria, ice, water, oil, dust, barnacles, or other contaminants, and have been proposed as coatings on industrial and medical surfaces. These surfaces are robust even under high temperature and pressure conditions which also positions them as a viable treatment for the walls of microelectronic cooling channels or on fuel lines to reduce coking in high-temperature fuel delivery. To establish this use case, we present the development of a microfluidic device incorporating a porous liquid infused surface, and we measure the pressure drop and heat transfer across this surface while tailoring the chemistry, porosity and infusing liquid on the surface.

Liquid metals are useful soft and fluidic conductors for electronics, composites, and microfluidics. In the presence of oxygen, these metals form a thin (~3 nm) surface oxide that acts as a deformable solid shell and adheres to many surfaces. The first portion of the talk will address the unusual wetting behavior of oxide-coated liquid metals (in comparison to water) on smooth, non-reactive substrates using conventional tools for measurement contact angles. These experiments demonstrate the unusual contact angle hysteresis behavior due to pinning of the oxide. Next, this talk will discuss the effect of surface roughness and surface chemistry on oxide adhesion, including strategies to prevent oxide adhesion. Overall, we find that the contact angle can be manipulated mechanically to be any value from 0° to >140° depending on the hysteresis. Thus, for oxide-coated liquid metals, conventional wetting measurements may be relatively uninformative and subject to different interpretations. Accordingly, the results provide fundamental insights on the adhesion of the oxide to substrates, which is important for additive manufacturing of metals, soft and biocompatible electrodes and interconnects, and reconfigurable electronics.

1. Joshipura I.D., Ayers H.R., Castillo G, Ladd C, Tabor C., Adams J.J., Dickey, M.D. Patterning and Reversible Actuation of Liquid Gallium Alloys by Preventing Adhesion on Rough Surfaces ACS Applied Materials & Interfaces 10, 51 44686-44695 (2018).
2. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM Release #: LLNL-ABS-777669

Liquid materials have emerged as a promising material in neuromorphic devices and energy storage devices because of the high ionic mobility of the liquid. We report liquid-based resistive-switching memory (LRSM) devices, exploiting the formation and rupture of silver filaments in the solution [1]. The devices are designed to have a metal-solution-metal structure, consisting of an Ag electrode and an inert electrode that together sandwich a solution. The switching behaviors of the LRSM devices occur at a very low operating voltage, similar to the action potential in biological synapses because the movement of silver ions does not need much energy to migrate under the electric field in the AgNO₃ solution. Furthermore, we also evaluate the potential use of LRSM devices in neuromorphic devices. The LRSM devices have neuromorphic characteristics including potentiation, depression, excitatory postsynaptic current (EPSC), and paired-pulse facilitation (PPF). Silver filaments that are formed in solution lead to changes in the conductivity of the device and provide neuromorphic characteristics. Also, LRSM devices have flexibility due to using a liquid as the main component. This study will offer the potential for the development of flexible and highly energy-efficient brain-mimicking devices. In this presentation, liquid-based resistive-switching memory characteristics will be presented in detail.

[1] D. Kim and J.-S. Lee, Nanoscale, 11, 9726 (2019)

Appropriate external forces or physicochemical constraints can reshape liquid into desired morphologies. Historically, early strategic techniques for shaping liquid (e.g., brushing and printing presses) have significantly affected civilization. Recently, numerous micro- and nanofluidics has substantially enriched applicable micro- and nanodevices made of functional liquid-processible materials (i.e., liquid-mediated materials) by enabling spatiotemporal manipulation of fluids at the micro- and nanoscale, simultaneously offering cost-effectiveness, simplicity, flexibility in choice of materials and substrates, and scalability. In the micro- and nanofluidic liquid-mediated patterning (MNLP), nanowires particularly have been studied to meet increasing demands for transparent and wearable electronics. However, the state-of-the-art methods have required expensive preparation steps for nanostructured templates, otherwise they have shown low-resolution spatial control of liquid. Though nanoscale liquid-air interface control by microstructured templates have been a compromise between the cost and resolution issues in the nanopatterning, it is limited to unidirectional wire patterns. To this end, liquid foam pervasive in nature can be useful in comparison to the reported approaches for patterning networked high-resolution nanowires, though it involves certain challenges such as their unmanageable drying, complex topological changes, and dynamic fluidity.

Herein, we report a MNLP technique to engineer 2D liquid foam, motivated by its potentials in micro- and nanoscale material patterning. The main idea is the design of micropost and microhole arrays in a hexahedral liquid space. The evaporative microholes allow the fast generation of discretized liquid–air interfaces and directed receding of the interfaces toward micropost. Then the microposts pin the generated interfaces in predefined locations to make them form networked-line pattern of liquid. The generated liquid pattern could be used as molds for evaporatively structuring of the target materials in liquid, enabling material patterns of several micrometers or hundreds of nanometer width, showing results that cannot be easily achieved by using conventional patterning technologies. First, multiple heterogeneous materials could be self-aligned respectively into multiple different patterns precisely without cumbersome high-resolution alignment. Second, conventional photolithography technique could be combined with our MNLP method. From the demonstration of microscale UV writing on the bottom-up nanoscale prepatterned uv-curable poly(ethylene glycol) diacrylate (PEGDA), we can do mask-less nanopatterning in a low-cost and high-through manner. Third, our suspended nanowire was made in a single step without any conventional fabrication process, otherwise the 3D structure is made commonly using relatively laborious multi-step top-down fabrication techniques. Furthermore, it is no doubt that other useful materials like organic-inorganinc hybrid perovskite, functional nanocrystals, graphene will be patternable. Therefore, our foam-based MNLP technique shows high potential in field of micro-/nanopatterning for various applications such as micro-/nanofluidic, optoelectronics, and multi-functional electronic deivces.

Though the dynamics of phase transition in many multi-phase liquid mixtures have been thoroughly investigated, there is still more to explore with mixtures containing common liquid crystalline (LC) compounds and low molar mass solvents. Especially now that much LC research has been devoted to the development of soft devices and composites [1-3], where the preparation of LC-solvent emulsions may be necessary for processing using microfluidics [4-5] or ink-jet printing [6], for instance, it is essential that experimentalists consider the phase equilibria of LCs in solvents. While numerous studies already exist on 4-cyano-4′-pentylbiphenyl (5CB), a low molar mass room temperature nematic, we now experimentally find that binary mixtures between this LC and anhydrous ethanol show a broad miscibility gap that leads to phase separation between two distinct isotropic phases. More surprisingly, contaminating the ethanol-5CB mixtures with water (3 vol.% suffices) dramatically raises the temperature range of the miscibility gap and the critical temperature at which phase separation by spinodal decomposition occurs to higher than 50°C. The phase diagrams presented here describing these phenomena, experimentally analyzed using temperature-controlled polarized optical microscopy and differential scanning calorimetry (DSC), corroborated by theory and numerical simulations, emphasize the often overlooked practical consequences that mixing LCs with common laboratory solvents can have in the formation and stabilization of future LC-composites.

References:
[1]. M. Urbanski, C. G. Reyes, J. Noh, A. Sharma, Y. Geng, V. S. R. Jampani, and J. P. Lagerwall, J. Phys.: Condens. Matter, 2017, 29, 133003
[2]. C. G. Reyes, A. Sharma, and J. Lagerwall, Liq. Cryst., 2016, 43, 1986–2001
[3]. J. Wang, A. Jákli and J. West, ChemPhysChem, 2015, 16, 1839–1841
[4]. J. Kang, S. Kim, A. Fernandez-Nieves and E. Reichmanis, J. Am. Chem. Soc., 2017, 139, 5708–5711
[5]. Y. Kim, X. Wang, P. Mondkar, E. Bukusoglu, and N. Abbott, Nature, 2018, 557, 539–544
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Slow protein crystal nucleation is a major barrier to using crystallization as a separation and purification strategy in protein drug manufacturing. This work demonstrates the use of functionalized nanoparticles in solution to act as in situ templates for initiating crystal nucleation. We used lysozyme, a protein with well-characterized crystallization conditions, to evaluate the nucleation rates of crystals grown on the functionalized particles. On a microfluidic chip, a supersaturated solution of lysozyme is mixed with a stream containing precipitants and functionalized nanoparticles. The nucleation rates are measured using an emulsion based technique. Nanoparticles functionalized with groups that covalently bind with proteins demonstrate up to four times faster crystal nucleation than controls, and faster nucleation than bare nanoparticles or nanoparticles with other types of functionalizations. Surface adsorption measurements on larger flat surfaces with the same functional groups are used to examine the mechanism by which the functional groups enhance crystal nucleation.

Hydrocolloidal microcrystalline cellulose (MCC) from plant sources, is already widely used in industry to regulate the stability, texture, rheology and organoleptic properties of many food and cosmetic formulations. Bacterial cellulose (BC) is produced biotechnologically by different microorganisms, but most efficiently by acetic acid bacteria from the genera Komagataeibacter. This biomaterial is a prominent alternative to the already marketed celluloses, being more pure, crystalline, and having nanoscale fibres with high aspect ratio which account for excellent mechanical properties. BC has already been used in its hydrated form for the stabilization of oil-in-water (o/w) Pickering emulsions (particle-stabilized systems, as an alternative for the conventional surfactant-stabilized). For the sake of storage, economy and practicality, additives for industries are preferentially provided in a dry or powder form. Co-drying cellulose fibres or crystals with water soluble polysaccharides helps maintaining the rheologic and structuring properties after rehydration.
The main objective of this study was to assess the stabilizing properties of BC in Pickering o/w emulsions. For this, an equimassic formulation of BC and 90 kDa carboxymethyl cellulose (BC:CMC) was prepared and spray dried. Isohexadecane-in-water emulsions (10:90) were prepared in the presence of 0.10%, 0.25% and 0.50% of the BC:CMC formulation. Visual and microscopic aspect of the emulsions was registered over time. Samples were also visualized in Cryo-SEM. Rheological tests were performed to assess the emulsion’s viscosity profile, storage and loss moduli. Interfacial tension between the immiscible phases was measured with the Pendant Drop and Du Noüy Ring methods. For benchmarking purposes, the same emulsion preparation and analysis protocol was made with several different commercial cellulosic products and xanthan gum.
Microscopic analyses showed large oil droplets, but stable over time. Despite a visible creaming in the emulsions with lower BC:CMC concentrations, there was no evident separation of the oil phase; with 0.50% BC:CMC, the emulsions were effectively stabilized against agglomeration, coalescence and creaming for up to 90 days without the need of any other emulsifying agents. The same did not happen for the commercial celluloses at the same concentration. Cryo-SEM images showed the entangled and disordered three-dimensional networks formed by BC fibres structuring the water phase and surrounding the dispersed oil droplets. The BC:CMC emulsions showed a characteristic viscoelastic and shear-thinning behaviour. An increase in BC:CMC concentration results in higher emulsion viscosity, higher than the emulsions prepared with other cellulosic products. IFT measurements showed that the presence of the BC:CMC formulation actually diminishes the interfacial energy between the immiscible phases, more than the other cellulose products. The results of our dry BC:CMC formulation were only comparable to the ones with xanthan gum.
In short, BC:CMC showed formation of a three-dimensional network and viscosity increasing (thickening) properties, crucial characteristics for emulsion stabilizing formulations. BC has technically superior properties that will allow it to compete with, or even replace, plant celluloses in industry.
The authors would like to acknowledge the Portuguese Foundation for Science and Technology (FCT) for supporting this study under the scope of the strategic funding of UID/BIO/04469 unit, COMPETE 2020 (POCI-01-0145-FEDER-006684) and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by the European Regional Development Fund under the scope of Norte2020 - Programa Operacional Regional do Norte. Daniela Martins also acknowledges FCT for the PhD scholarship SFRH/BD/115917/2016.

Due to the serious reflection/scattering of sound waves, the acoustic bubbles on the surface that are generated during the sonication are posing a critically detrimental effect on the signal resolution of sonochemical devices such as various sensors for water-based liquid detection and sonobuoys for underwater acoustic study and anti-submarine warfare applications (Adv. Mater. 2011, 1922). In parallel, a large variety of high-density polyethylene (HDPE) polymers are extensively used in different devices particularly with sensors due to the high chemical and mechanical stability(Polym. Degrad. Stab. 2007, 1219). This outstanding stability also poses a challenge on modifying the surface with designed functional groups. The original HDPE surface shows a contact angle large than 60^o, which generates plenty of bubbles when it suffers the ultrasound agitation. Even though there are a few reports about the hydrophilic coating created using versatile nanoparticles and polymers, forming a controllable hydrophilic coating on HDPE surface is not studied yet.
The ultrasonic bubbles on the solid surface of various sonochemical devices largely affect the signal resolution due to the serious reflection/scattering of sound waves. The Laplace pressure of the cavitation bubble can be tuned by constructing an ultra-thin hydrophilic layer, which leads to the solvation or pinching off of the bubbles from the surface.In this talk, we will introduce a polydopamine polymer layer coated on the HDPE surface (Ye et al.ACS Appl. Mater. Interfaces, 2019, 16944). The formed hydrophilic layer with contact angle less than 45 degree almost completely eliminates the bubbles in both water and 32.5 vol% diesel exhaust fluid (DEF) solutions upon sonication, which results in the operation of the piezoelectric sensor over 500 hours while the sensor with pure HDPE only ranless than 2 hours.Further, the coated sensors showed a high stability under the temperatures of 60-80 ^oC. An improved mechanical property was confirmed via abrasion test, enabling a long-term stability in hash environments including acidic urine and ultrasonic agitation.The acoustic bubble suppression via the hydrophilic polymer coating on HDPE surface displays broad applications particularly with acoustic sensors, sonobuoys and non-destructive surface detection in sonochemistry.

Recently, phase separations in multicomponent solutions have attracted attention due in part to their potential usefulness in understanding biological systems. Living cells possess membrane-less organelles, formed by liquid-liquid phase separations with the cell. Multicomponent phase separations of polymers in solution may occur under certain thermodynamic conditions of temperature and concentration. Here we present studies of the phase separation of two immiscible polymers during film formation. By taking real-time optical microscopy video of the blade coating process, we are able to observe the multicomponent polymer system as it transforms from solution to thin film. Additionally, we use Raman mapping to confirm localized regions of polymer components. Finally, we present results showing the effect of additional components on the system.

The development of next-generation biosensing technologies has picked up momentum in the past decade. Particularly, among a variety of biosensing principles, magnetic biosensing technologies based on magnetic particles and magnetic field sensors have attracted growing attention due to the unprecedented advantages brought by this unique sensing format.
Our contribution to this exciting field of research and technology includes the development of a compact droplet-based magnetofluidic platform encompassing integrated novel functionalities, e.g. analytics in a flow cytometry format [1-3], magnetic barcoding [4] and sorting of magnetically encoded emulsion droplets [5,6]. We put forth a novel high-capacity indexing scheme based on multiphase microfluidic networks for large-scale screening applications [5,6] and realized flexible microfluidic platform with integrated magnetoresistive sensorics [4]. The technology on how to integrate high-performance magnetic field sensors into multi-functional self-assembled tubular architectures [7-9] for lab-in-a-tube concept [10] will be discussed. These features are crucial to address the needs of modern medical research, e.g. drug discovery [11].


[1] G. Lin et al., “Magnetoresistive emulsion analyzer”. Sci. Rep. 3, 2548 (2013).
[2] G. Lin et al., “Magnetofluidic platform for multidimensional magnetic and optical barcoding of droplets”. Lab Chip 15, 216 (2015).
[3] D. Karnaushenko et al., “Monitoring microbial metabolites using an inductively coupled resonance circuit”. Sci. Rep. 5, 12878 (2015).
[4] G. Lin et al., “A highly flexible and compact magnetoresistive analytic device”. Lab Chip 14, 4050 (2014).
[5] G. Lin et al., “Magnetic suspension array technology: Controlled synthesis and screening in microfluidic networks”. Small 12, 4553 (2016).
[6] W. Song et al., “Encoding micro-reactors with droplet chains in microfluidics”. ACS Sensors 2, 1839 (2017).
[7] I. Mönch et al., “Rolled-up magnetic sensor: Nanomembrane architecture for in-flow detection of magnetic objects”. ACS Nano 5, 7436 (2011).
[8] D. Karnaushenko et al., “Self-assembled on-chip integrated giant magneto-impedance sensorics”. Adv. Mater. 27, 6582 (2015).
[9] T. Ueltzhöffer et al., “Magnetically patterned rolled-up exchange bias tubes: A paternoster for superparamagnetic beads”. ACS Nano 10, 8491 (2016).
[10] E. J. Smith et al., “Lab-in-a-tube: ultracompact components for on-chip capture and detection of individual micro-/nanoorganisms”. Lab Chip (Tutorial Review) 12, 1917 (2012).
[11] G. Lin et al., “Magnetic sensing platform technologies for biomedical applications”. Lab Chip (Critical Review) 17, 1884 (2017).

In recent years, numerous innovative methods have been developed to meet the increasing demand for high throughput production of monodisperse droplets.[1] These methods can be either passive, such as dripping[2], or active, such as acoustophoretic printing[3]. However, both often require elaborate fine-tuning or cost-heavy equipment, which inhibits the field’s growth as it poses an entrance barrier for new researchers.

As a cheap and simple alternative to these methods, we have explored the use of commercial inkjet printers for the production of picoliter droplets; thereby, benefitting from decades of optimization in their speed, precision, and miniaturization. Nowadays, a 300$ office printer can produce >10k droplets per second from nozzles spaced 80 µm apart and a size down to 1.5 picoliter. In addition, many modern inkjet printers allow for variation of the droplet size, which makes their versatility even more pronounced. Despite the clear advantages, challenges related to backlash induced printing repeatability and printer-roller introduced sample contamination have limited the exploration of the many possibilities.

Through simple hardware adjustments to the office printer, we have made it possible to print multiple times without smearing and improved the between-print positioning repeatability from millimeter range to below 50 µm. This improvement enables a manifold of new utilities for the inkjet system. The low-cost inkjet printer has six ink lines; thereby, immediately allowing for droplet production with six different liquids. We demonstrate simultaneous printing with both polar and non-polar liquids containing fluorescent dyes, drugs, bacteria, and colloids. In particular, we have focused on using the inkjet printer to load drugs into microcontainers for targeted delivery and personalized treatment. This new loading method is superior to its predecessor due to its possibility of easily customizable drug content.

References:
[1] P. Zhu and L. Wang, “Passive and active droplet generation with microfluidics: a review,” Lab Chip, vol. 17, no. 1, pp. 34–75, 2017.
[2] A. S. Utada, A. Fernandez-Nieves, H. A. Stone, and D. A. Weitz, “Dripping to jetting transitions in coflowing liquid streams,” Phys. Rev. Lett., vol. 99, no. 9, pp. 1–4, 2007.
[3] D. Foresti et al., “Acoustophoretic printing,” Sci. Adv., vol. 4, no. 8, pp. 1–10, 2018.

With the emergence of additive manufacturing technologies, such as "in-air microfluidics", compound drops are attracting an increasing attention. One of the critical challenges 3D printing applications is to control the deposition process of the impacting drop and therefore its spreading, potential rebound and splashing.
By studying the dynamics of compound drops consisting of immiscible liquids, we identified the mechanism of self-lubrication of water-in-oil compound drops impacting on a solid surface. The presence of an oil shell encapsulating a core water drop acts as a lubricating layer promoting water rebound even on a hydrophilic substrate, on which water deposition is typically expected.
We define the mechanisms and the conditions that lead to deposition or rebound of the inner water drop, as such providing design guidelines for the printing of compound drops to be used in additive manufacturing.

Interfacial characteristics of a material play a key role in its interaction with foreign species. Low adhesion of the deposited materials provides the advantage of their easy removal from the surface. Different approaches have been developed to reduce solid-bio/species adhesion by modifying chemical and physical atomic forces, tailoring polymer chains, micro/nano structuring of a surface and so on. Here, we developed the underlying physics of the stress-localization concept to minimize adhesion of a solid on a surface. The fundamental underpinnings of this concept are elucidated and a simple formulation that could be used to characterize solid adhesion on various surfaces is derived. Using this concept, we developed a new bio-polymer material system with superior characteristics while exhibiting long-term mechanical, chemical and environmental durability. This polymeric material contains dispersed organogels in a high shear modulus matrix. In this study, we focused on the interaction of bio-species including both soft and hard organisms with this biopolymer material. Interfacial cavitation induced at the interface of bio-species and organogels particles lead to stress-localization and detachment of bio-species from these surfaces with minimal shear stress. In a comprehensive study, the performance of these surfaces is assessed for both soft and hard-biofouling including Ulva, bacteria, diatoms, barnacles, and mussels and is compared with state-of-the-art surfaces. These surfaces show Ulva accumulation of less than 1%, minimal bacteria biofilm growth, diatom attachment of 2%, barnacle adhesion of 0.02 MPa and mussel adhesion of 7.5 N. These surfaces promise a new physics-based route to address the biofouling problem and avoid adverse effect of biofouling on environment and relevant technologies.

The behaviors of fluid have shown great importance in crystal growth and material assembly on various substrates. Herein, we studied the temperature dependence of the fluidic behavior of several organic solutions on both solid and liquid substrates. On solid substrates, we focused on the liquid wedge near the air-liquid-solid contact line. The temperature gradient of the liquid wedge was manipulated by using a top-heating-bottom-cooling (THBC) setup. A steady Marangoni flow with a single vortex was created in the thin liquid film which benefits the material patterning. We also studied the behavior of volatile organic droplets on the water surface. The liquid-liquid-air contact lines are tunable by varying the temperature. A spontaneous self-division process of the droplets was observed. By thus tuning the fluid behaviors, the crystal growth and material assembly processes were manipulated and CH₃NH₃PbI₃ crystal arrays and dumbbell-shape C₆₀ aggregates were prepared.
References
Giri, G. et al. Nature 480, 504-508 (2011).
Diao, Y. et al. Nature Mater. 12, 665-671 (2013).
Hu, H. & Larson, R. G. J. Phys. Chem. B 110, 7090-7094 (2006).
Grzybowski et al. Angew. Chem. Int. Ed. 49, 6756-6759, (2010).

Living beings have evolved to use coupled neurons to perform autonomous functions, from breathing to walking. Local, often small, clusters of neurons known as central pattern generators (CPGs) perform these essential functions in the absence of constant guiding input from the brain. This modular method of control of whole organisms may present advantages over centralized, computational robotic control. An experimental platform, composed of diffusively-coupled PDMS microreactors each containing the oscillatory, light sensitive Belousov-Zhabotinsky (BZ) chemical reaction forming CPG-like patterns has been studied. Understanding experimental observations of quadruped gaits, and an explanation of a systematic method making synthetic CPGs with desired steady state gaits will be the focus of the presentation.

We have investigated synthetic clay mineral nanosheets of different lateral sizes. The clay nanosheets have mechanical properties similar to
graphene oxide or graphene sheets, which already have been proven to be capable of wrapping at different lateral length-scales. It is lasp known that clays (e.g. mica) can also wrap magnetic nanoparticles, which might lead to improved control of magnetic particle transportation in porous media, e.g. for medical, or for oil-reservoir diagnostic purposes.

To achieve wrapping of liquid droplets, very dilute oil-in-water emulsions (0.01~1%) and exfoliated clay colloidal suspensions have been
prepared separately and subsequently mixed. The oil drops have diameter 10-25 µm, and the exfoliated clay (synthetic sodium fluorohectorite) have thickness 1 nm, and lateral dimensions 10-100 µm. Our results demonstrate wrapping of droplets by the clay nanosheets.

Furthermore, we have probed clay sheet deformation characteristics by depositing exfoliated clay particles on solid substrates with various
curvatures. This forms the base for ongoing work of establishing protocols for nanosheet clay wrapping of magnetic nanoparticles.

Acknowledgements: Research Council of Norway, Petromaks2 project number 268252: “Nanofluids for IOR and Tracer Technology”.

Many plants and insects have developed intriguing wetting properties that enable them to thrive in their natural habitats [1, 2]. Lotus leaf, which is one of the most well-known biomimetic examples, has served as the blueprint for designing superhydrophobic surfaces for over two decades. Specifically, the water repellency of lotus leaves mainly stems from the hydrophobic epicuticular wax coating and low fraction of solid surface (Φs) that is in direct contact with water [2]. As shown by the classical Cassie-Baxter equation (1944), superhydrophobicity can be achieved by a surface with solid fraction (Φs) less than 0.05 [3]. However, some insect surfaces exhibit exceptional water repellent characteristics at solid fractions (Φs) much greater than 0.05. For example, superhydrophobic mosquito eyes, springtails, and cicada wings possess solid fractions (Φs) as high as 0.25 – 0.64 [4 – 6]. In addition, the texture size on these insect surfaces is typically on the order of 100 – 300 nm. To understand why both high solid fraction and nanoscale textures are important on these superhydrophobic insect surfaces, we systematically designed and fabricated a series of synthetic textured surfaces with feature size ranging from 100 nm to 30 µm with solid fractions (Φs) of 0.25 and 0.44, and investigated their static and dynamic wetting behaviors. We discover that nanoscopic textures (i.e., ~100 nm) at high solid fraction (i.e., Φs ~ 0.44) enable reduced droplet contact time (the duration that an impacting droplet is in contact with the solid) by as much as ~2.6 ms, which is ~14% of contact time reduction as compared to those of microscopic counterparts. The amount of contact time reduction is significant, as it is comparable to the timescale for a mosquito to escape from a lethal raindrop collision [7]. Detailed analysis of the results will be presented in the meeting. Our discovery may provide a new physical insight to the design of new superhydrophobic materials for highly dynamic environments.

Keywords: nanostructures | insects | superhydrophobic surfaces | drop impact | contact time

[1] Wagner, T., Neinhuis, C., & Barthlott, W. (1996). Wettability and contaminability of insect wings as a function of their surface sculptures. Acta Zoologica, 77(3), 213-225.
[2] Barthlott, W., & Neinhuis, C. (1997). Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta, 202(1), 1-8.
[3] Cassie, A. B. D., & Baxter, S. (1944). Wettability of porous surfaces. Transactions of the Faraday Society, 40, 546-551.
[4] Gao, X., Yan, X., Yao, X., Xu, L., Zhang, K., Zhang, J., ... & Jiang, L. (2007). The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography. Advanced Materials, 19(17), 2213-2217.
[5] Gundersen, H., Leinaas, H. P., & Thaulow, C. (2014). Surface structure and wetting characteristics of Collembola cuticles. PLoS One, 9(2), e86783.
[6] Oh, J., Dana, C. E., Hong, S., Romaan, J. K., Jo, K. D., Hong, J. W., ... & Miljkovic, N. (2017). Exploring the role of habitat on the wettability of cicada wings. ACS Applied Materials & Interfaces, 9(32), 27173-27184.
[7] Dickerson, A. K., Shankles, P. G., Madhavan, N. M., & Hu, D. L. (2012). Mosquitoes survive raindrop collisions by virtue of their low mass. Proceedings of the National Academy of Sciences, 109(25), 9822-9827.

The sorting and separation techniques of microscopic droplets have been progressed with the aid of microfluidic device. At present, various methods had been reported for the manipulation of droplets based on electric, acoustic, magnetic, hydrodynamic control[1]. In this presentation, we introduce a method utilizing optical radiation pressure. The magnitude of the optical radiation pressure depends upon the optical property (refractive index, scattering cross section and absorbance) of the materials. Therefore, the method will be able to distinguish the droplets by not only size-dependence but also color-dependence. We developed a micro fluidic device for screening droplets by optical radiation pressure. The device we developed is simple construction only irradiating laser beam to micro fluidic channel. Micro fluidic channel we designed contains inlet, main and outlet channels. The inlet section consists of three 50µm-width and 20µm-height channels crossing by 90 degree. The main channel has its width of 150µm and height of 20µm along 10mm. The main channel is branched out into two 75µm-width and 20µm-height outlet channels. The channel was made of poly(dimethylsiloxane) and fabricated by soft lithography. In our experiment, oil in water emulsion was infused to a center inlet channel. Aqueous solution without droplets was infused to the two side branched channels as sheath flow. The structure of three crossing channels caused hydrodynamic focusing, hence a line of flowing droplets was created at the main channel. A line shaped focused laser beam with 532 nm wavelength was irradiated to the main channel orthogonally. Flowing droplets in the main channel were irradiated by the laser beam. Depending on their optical property, only the droplets experienced strong optical radiation pressure were changed their flowing position. Eventually, they were introduced to one of outlet channels, however, the rest flew into the others. In the presentation, we will show experimental results showing screening droplets by their size and color.

[1] H. D. Xi, et al.,Lab Chip, 2017, 17, 751

Wetting-controlled surfaces are typically hydrophobic/hydrophilic patterned surfaces that can be wet rapidly to create a matrix of precisely aligned and shaped sessile liquid droplets. 2D wetting controlled surfaces have attracted much interest for both fundamental issues and applications that include the creation of small droplet arrays of proteins and cells for analysis and screening purposes. Wetting controlled surfaces are also applicable to generate effective solution separating walls for precise ink deposition or conductive layer deposition in producing electronic devices. A potentially beneficial attribute of this process is that the two-dimensional patterned surfaces create, along with the liquid, three-dimensional shaped features. These features can be used in molding processes that are versatile because of the flexibility of the 2D pattern, the liquid identity and the liquid volume (contact angle).
We present a novel 3D structure fabrication technique using 2D hydrophilic / hydrophobic patterned wetting surfaces. The preparation of controlled wetting surfaces and the subsequent liquid coating processes create reproducible sessile droplet arrays. The patterned surfaces show sufficient liquid trapping capability with a wide range of liquids with surface tension values ranging from 72.8 (water) to 18.8 mN/m (octamethylcyclotetrasiloxane). These droplet arrays can be covered with controlled thickness silicone prepolymers which are cured catalytically. The array serves as a mold, and the liquid is retained in the more hydrophilic regions due to a hydrophilic / hydrophobic confinement effect. The velocity of liquid application using a flow coating system affects the volume of coated liquid yielding molds with varying feature height while maintaining the width of droplets. Several types of liquid, including ionic liquids, glycerol, and water were studied to confirm the versatility of this molding process. Immiscibility between the silicone and certain ionic liquids permitted precise molding. A molded silicone resin showed light scattering behavior that was predicted by the 2D droplet array structure, indicating the successful replication of the array to the silicone resin. The shape of resulting molded silicones when water was used was a function of relative humidity, because of the significant water vapor permeability of silicones. Due to the finite vapor pressure of glycerol, we could control the height (and shape) of features using controlled condensation/evaporation after flow coating. In this presentation, we will present our work on the preparation of wetting controlled surfaces, the creation and effect of the liquid droplet as a mold, and subsequent molding processes.

Compatibility of ink and substrate is crucial for an optimal printed image. Currently in inkjet printing, the ink must be adjusted to each printer head and then precisely formulated for the printing process. Consequently the only degree of freedom for desired print image lies in the properties of the substrate’s surface. By choosing a proper substrate or modifying it, according to its surface free energy, the required wettability and adhesion properties can be obtained; sometimes even without any primer or additional sealing process, which reduces production costs. In addition to wetting and adhesion, the absorption process on absorbent substrates has an influence on the final image quality. Using a goniometer capable of delivering ink droplets to a volume as low as 30 picoliters, the wettability of different raw and modified substrates was determined by contact angle measurements and the surface free energy of the substrates was calculated. With this knowledge it becomes possible to estimate wetting and adhesion behavior of inks.. Using picodrop-level dosing with the goniometer, the ink-substrate interaction directly under inkjet drop dispensing conditions is observed. Contact angle changes due to absorption and wetting can be determined with a speed of up to 1 ms. This picodrop delivery technique be used for early-stage printing tests of newly developed ink formulations with conditions similar to the inkjet process without the risk of printer head blockage. These measurements enable fast and reliable determination of the surface properties of substrates and inks which simplifies the choice of substrate-ink combination for ideal image printing. Selected examples will be presented.

The measurement of contact angles and surface free energy is a well-known method to study and understand adhesion properties of liquids to solid surfaces. Work of adhesion between liquid and substrate can be estimated by determination of surface energy of a substrate, measurement of surface tension of a liquid and calculation of adhesion work between them using Young-Dupre equation. This methodology is therefore commonly used in the development of coatings, paints and varnishes, for example, as well as to characterize surfaces and the effects of surface treatments. In this lecture a new, more direct approach to measurement of adhesion using a tensiometer will be presented. This new method uses small platinum ring to hold a droplet of the test liquid, software-controlled movement of the substrate to attach the droplet to the test substrate and measure the force between the liquid and the substrate while the droplet is being pulled off from the substrate. An integrated camera records the process of the drop detachment and allows calculation of the contact area as well as contact angle between the drop and the substrate. With this novel technique it is now possible to directly observe and measure the liquid's behavior when in contact with a solid surface. Several examples will be presented including biosurfaces and membranes to test the adhesion of water droplets to them. The droplets are brought in contact with, pressed onto and then pulled off the biosurfaces and various membranes. During this process force distance curves were recorded and later compared to each other. An analysis of the maximum adhesion force shows good correlation to contact angles which can also be analyzed combining the tensiometer with this video system.
Furthermore liquid transfer to or into the material can be analyzed and different compression rates can be used to simulate different drop behavior.

Structured liquids are of interest as possible electrolytes for energy storage and other electrochemical applications, where one focus is the electrochemical behavior of oil-in-water microemulsions. Stable microemulsion systems consisting of water (plus salt), toluene (oil phase) and a Polysorbate 20/butanol surfactant/cosurfactant system with ferrocene, a model redox active agent, loaded into the toluene phase is the focus of our studies. Electrochemical results indicate remarkably reversible (fast) electrochemical kinetics in the region in which oil-in-water droplets or bicontinuous microemulsions are present. To understand this process further, we have examined the structure of the microemulsion in the bulk with x-ray and neutron scattering and near a surface with neutron reflectivity. The near surface studies will provide insight into the interaction and ordering of the microemulsion at an electrode surface. This talk will focus on the structure of this microemulsion near surfaces with well controlled hydrophobicity using neutron scattering, where initial results indicate a layering of the components near these surfaces.

Gallium based liquid metal (LM), Eutectic gallium indium (EGaIn) exists in liquid form at room temperature. EGaIn has high electrical conductivity and thermal conductivity in liquid phase. Many kinds of way to make stretchable electrode pattern with LM and study of its properties have been published in research papers. These researches show several applications such as microfluidic systems, soft electronics and wearable devices.
In the recently, micro-nano sized LM droplet or powders have received significant focus in fabrication methods, applications and its natural material properties. LM powders are a type of biphasic materials which have an inner core in liquid state and an outer shell of Ga oxide, is reported to be a 30-50A thickness. LM powder has been of interest as this is able to show high interesting characteristics coming from the difference material properties (electrical, mechanical) between core and shell. The shell is electric insulation material, and the core is conductive.
In this study, we propose a fabrication process of micro-nano size liquid metal (LM) powder. For showing application example, we implemented a stretchable EMI shielding sheet using LM powder and silicone resin. This sheet has electric insulation that reason why the shell of LM powder is the insulating oxide layer, and it has enough EMI shield property because the core of LM powder in the sheet is conductive metal.
At first step, we make ethanolic micro-nano scale LM colloids using sonication, ultrasonic homogenizer make high oscillating shear forces in vial with LM and to form LM particles. The LM is a eutectic gallium–indium (Ga 74.5% and In 25.5%, Melting point 16°C). During the sonication process, LM particle is exposed to oxygen. Gallium oxide continuously forms on LM particles. We can control size of the LM particle by sonication time. The longer time of sonication, the smaller size of LM particle is. When sonication time is 15min, we can produce LM particle size 100nm~150nm. After sonication, Precipitation makes LM particle separation in vial by its size. We extract the LM particles from vial using a syringe. Finally evaporating the ethanol, we can get dried LM powder.
The sheet of EMI shield is made by silicone resin (Dragon skin™ 10NV) and fabricated LM powder. We mix the LM powder with the silicone resin, and add the hardening agent. After curing 2hours, we fill off the EMI shield sheet applied LM powder from a substrate (Si wafer)
Generally, EMI shielding materials use conductive materials and there is a risk of electric short circuits. We have confirmed the possibility of liquid metal powder as EMI shielding materials with non-conductive properties. This study used LM powder as EMI shielding material while preventing electric short circuits on the surface. Silicone resin is easy to form and have a stretch, which is advantageous for applying a composite sheet to various fields.
We measured the electromagnetic shielding using a network analyzer and measured the insulation performance using an insulation resistance tester withstand voltage insulation resistance tester. The EMI shielding performance of LM powder, insulation was evaluated via experiment results of developed the LM powder shield sheet. In the future work, we will study the theoretical relationship between the size of LM powder and the EMI shielding performance. We expect the performance of the sheet depending on the volume ratio of silicone resin and LM powder.

There has been intense and longstanding interest in the use of biocompatible microparticles for a variety of applications such as bio-imaging, drug delivery, medical diagnostics, photonics, optical data storage, and display pigment. Batch synthesis has been the method of choice for preparing polymer particles due to practical advantages in productivity and accessibility, despite disadvantages in controllability and efficiency. In batch synthesis, multiple separated steps of synthesis, purification, loading and washing are required to prepare the particles. During this process, the amount and size of loadable materials is limited. Furthermore, an excessive portion of loading materials is wasted, which is especially problematic when expensive loading materials are used (ex. Quantum dots, biomaterials, drugs and etc.).
Compared to the batch processes, the microfluidic drop breakup scheme allows predominant production of uniform emulsions with highly controlled size in continuous manner. Combined with photopolymerization scheme, those emulsions can be instantly converted into crosslinked particles containing various functional materials. Even infinitesimal amount of precious materials can be handled without any loss, since the emulsions are prepared from shear induced breakup of one phase into another immiscible fluid. In this scheme, there is no additional washing step or development step which causes material dissipation. Furthermore, this scheme allows easy loading of relatively large dispersion materials up to several micrometers. Most of the microfluidic particle synthesis is operated in a dripping mode regime which allows precise size distribution control. However, dripping mode operation has limited production rate since the breakup occurs one by one, with relatively low frequency. In addition, the controllable size range is limited because the emulsion sizes are more dependent on the nozzle diameter and interfacial properties rather than the flow conditions. To get smaller emulsion sizes, dimensions of the microfluidic channels around the breakup junction must be decreased, causing unstable flow and significant pressure build up in the system. Viscous monomers used for the particle preparation creates even larger pressure accumulation and can lead to device failure. By applying multiphase flow in the microfluidic system, structural emulsions, such as janus or eyeballs, snowman and hemispheres can be fabricated.
In this study, we presented production of highly uniform emulsions with various structures using harmonic breakup of co-flowing jet with high Weber number system. In jet mode breakup, axisymmetric laminar jet of inner fluid is broken into a train of uniform tiny emulsions. Ideal harmonic fluctuation formed by a well-defined microchannel leads to series of disconnection with regular interval from the periodic modal points. Since the jet diameter is relatively small and widely variable for given nozzle diameter, the jet mode allows low pressure drop, high production rate and wide range of controllability, simultaneously. Here, biocompatible microspheres with various shapes were synthesized throughout a controllable size range of 10 ~ 100 μm and 3% of uniform size distribution using jet breakup. The production speed can be ranged from 400 Hz to 100,000 Hz, according to the microfluidic chip material.

Particle stabilized bicontinuous emulsions (bijels) have been introduced over 10 years ago.^[1] Soon after their discovery, their future use as continuously operated crossflow reactors for chemical reactions between immiscible reactants was proposed. This potential yet remains to be demonstrated. Here, we take a significant step towards realizing this vision by introducing advective flow in bijels via electroosmosis. Recently, we have introduced Solvent Transfer Induced Phase Separation (STrIPS), a straightforward technique for generating bijel fibers with asymmetric oil/water channels of micrometer dimensions.^[2] Our current work has advanced STrIPS to generate bijel fibers with submicron sized oil/water channels of high uniformity. Surface tension and contact angle measurements are employed to rationalize the structure formation mechanisms. The uniform channels are found to enhance the mechanical strength and elasticity of STrIPS bijels, as determined by microfluidic in-situ mechanical testing. ^[3] Last, we investigate electroosmotic flow by monitoring dye propagation within the bijel fibers. Electroosmotic flow with speeds of up to 1 cm per minute is observed in the bijels by increasing the voltage and the nanoparticle surface charge. We report our research as a major step towards employing bijels as media for multiphasic processes with potential applications in Pickering interfacial catalysis and as cross-flow microreactors.

References
[1] Herzig, E. M., et al., Nature Materials 6 12 2007, 966.
[2] Haase, M.F., et al., Advanced Materials 27 44 2015, 7065-7071.
[3] Haase, M.F., et al., ACS Nano 10.6 2016, 6338-6344.

Recently, centrifugation and chromatography are commonly used to sort micro/nano particles. These methods allow us to sort particles by mass difference or size difference. In this presentation, we propose another particle separation method, in which particles are sorted by the difference of optical properties. For this purpose, we use optical radiation pressure. When laser light is launched onto small particles, the particles scatter the light, the particles are received impulse exerted as reaction. This optical force is usually very small, in the order of a few pN, and this force is called optical radiation pressure. Since the cause of the pressure is scattering of light, the magnitude of optical radiation pressure depends on optical scattering efficiency of particles. To utilize this dependence to sort particles, we built a simple optical system, consisting of a glass microcapillary and laser. First, we filled the glass capillary with an aqueous suspension of particles. Then we focused laser light from the tip end. The laser light traveled into the tube by waveguide mode toward the opposite end. The particles floating in the capillary were pushed by optical radiation pressure. The particles moved in the capillary with different speed. In the experiment, we used Nd:YVO4 laser light emitting at 1064 nm. The inner and outer diameter of the capillary are 20 µm and 25 µm, respectively. To prevent temperature rising, we used heavy water for the dispersant. The motion of these materials was observed by a bright field optical microscope from the side of the capillary. We investigated 3 types of micro/nano particles, milk fat/water emulsions, polystyrene microspheres, and multi-walled carbon nanotubes. We observed the transport speed varied in material. We also show experimental results of sorting milk fat/water emulsions and polystyrene sphere.

The evolution of cellular compartments for spatially and temporally controlled assembly of biological processes was an essential step in developing life by evolution. Synthetic approaches to cellular-like compartments are still lacking well-controlled functionalities, as would be needed for more complex synthetic cells. With the ultimate aim to construct life-like materials such as a living cell, matter-to-life strives to reconstitute cellular phenomena in vitro – disentangled from the complex environment of a cell. In recent years, working towards this ambitious goal gave new insights into the mechanisms governing life. With the fast-growing library of functional modules assembled for synthetic cells, their classification and integration become increasingly important. We will discuss strategies to reverse-engineer and recombine functional parts for synthetic eukaryotes, mimicking the characteristics of nature’s own prototype. Particularly, we will focus on large outer compartments, complex endomembrane systems with organelles and versatile cytoskeletons as hallmarks of eukaryotic life. Moreover, we identify microfluidics and DNA nanotechnology as two highly promising technologies which can achieve the integration of these functional modules into sophisticated multifunctional synthetic cells.

Solid particles can adsorb at fluid-fluid interfaces, much like molecular surfactants. Unlike molecular surfactants, however, the adsorption of solid particles can be considered to be irreversible, due to a large capillary energy associated with adsorption. As a result, solid particles can be used to stabilise drops and bubbles. Remarkably, particle-stabilised bubbles (Pickering bubbles) are found to be indefinitely stable, because the particle monolayer can arrest bubble dissolution, which is otherwise driven by the Laplace pressure. I will present two phenomena, discovered in our laboratory, which occur when particle-stabilised bubbles are subjected to changes in temperature or pressure. We found that a decrease in temperature destabilises particle-coated microbubbles in water beyond dissolution arrest. A simple model describing the effect of the change in temperature on mass transfer suggests that the dominant mechanism of destabilization is the increased solubility of the gas in the liquid. We can also drive particle-stabilised bubbles into periodic compression-expansion at 10-100 kHz by ultrasound forcing. This causes ultrafast deformation and microstructural changes in the particle monolayer. For large-amplitude forcing, the compression of the interface leads to particle expulsion, and we have uncovered different expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particle release during non-spherical oscillations. For small-amplitude driving, we have observed the formation of a transient microstructure -- a network of strings -- which is consistent with anisotropic interparticle interactions. By comparing the experimental results with the predictions of particle-based simulations, we have found evidence of dynamic capillary interactions resulting from the deformation of the interface due to the non-negligible inertia of the colloidal particles at the extreme accelerations caused by ultrasonic forcing.

Gas vesicles (GVs) are remarkably stable nano-sized gas-filled protein shells proven effective in ultrasonic imaging, especially in traditionally difficult, small-scale vasculature and interstitial spaces around tumors. The many potential benefits of GVs arise from their strong gas equilibrium at a submicron size as produced by bacteria or algae from a known genome, producing significant contrast in ultrasound imaging. The actual vibration behavior of GVs, including buckling and collapse, is poorly understood since the GVs are too small for observation methods of sufficient speed to produce details of the GV deformation during exposure to ultrasound. Traditional optical or acoustic microscopy methods are, in any case, not useful, and ex-situ transmission electron microscopy produces useful images but without sufficient time resolution.

We propose to instead use laser Doppler vibrometry (LDV) to observe the vibration behavior of GVs. Employing interferometry, LDV offers a far better spatiotemporal resolution, in our case up to 2.4 GHz in frequency and as little as 200 fm in surface displacement over a spot size of 1 µm. While the typical GV is smaller than 1 µm, an agglomeration of GVs may be used with the LDV to produce a measurable displacement response from a controlled, acoustically-delivered pressure. In this talk, we report the fundamental and first harmonic resonance frequencies of GVs at 1.02 GHz and 1.70 GHz, interparticle resonances at ~300 MHz, and vibration to buckling and collapse at the clinically relevant frequency of 6.5 MHz. We also compare these results with predictions from classic theories of bubble and particle oscillations and finite difference-based computations.

The design of well-controlled three-dimensional (3D) structures from individual building blocks such as graphene nanosheet or carbon nanotube has technological and scientific importance since it can provide advanced physical properties compared to their bulk counterparts. In particular, for a number of practical applications including electrode, supercapacitor, sensor, fluid absorber, energy damping, thermal insulator and catalysis, it is essential to fabricate well-defined 3D graphene structures with high surface area. While progress toward creation 3D porous graphene materials, previous efforts have placed severe restrictions on their wide-spread utilization because it is hard to create regularized and ordered structures with controlled dimensions, shapes, and morphologies. To overcome these challenges, we investigated multiphasic fluid mixtures such as emulsions droplets as alternative templates for generating well-defined 3D structures. This study uses a hierarchical design approach starting from the functionalized graphene oxide nanoplatelets at the molecular- and nanoscale, leading to the microfluidic fabrication of solid bubbles at the microscale. Then, generated graphene microbubbles are assembled into centimeter-scale 3D structures. Importantly, these assembled structures are self-interconnected into completely space-filled and reinforced closed-cellular structure. The 3D graphene structure exhibits the Young’s modulus above 177 kPa with a light density of 4.67 mg cm⁻³ and structurally stable up to 87% of the compressive strain benefiting from systematic stress dissipation through the complete space-filled closed-cellular structure. The method opens a new horizon for designing lightweight, strong, and superelastic properties.

The transport and actuation of droplets in microfluidic -based devices strongly relies on the properties of the fluids. Nanofluids (fluid-containing nanomaterials) offer an appealing approach to modulate key fluid properties such as conductivity, viscosity and interfacial tension which directly impact the wettability on various surfaces. This approach also has the added advantage of utilizing the optical properties of the nanomaterial for microfluidic-based optoelectronic applications such as reflective displays as well as photodetectors. Nanofluids comprising IR-absorbing nanofluids have been less explored and also pose colloidal stability challenges. We will present our results on the engineered properties of nanofluids comprising Ag₂Se, PbS and SnTe semiconductor nanoparticles in different solvent systems. The study highlights the synthesis of nanoparticles with surface ligands capable of significantly altering the conductivity and interfacial tension of nanofluids. This work uses electrowetting (droplet actuation through voltage application) in a biphasic system to probe the wettability of the different nanofluids by evaluating the changes in droplet contact angle on a dielectric surface. We show unique spreading characteristics of nanofluids in non-aqueous (Dimethyl sulfoxide and Dimethylformamide) and aqueous dispersion media. The work presented will certainly shed more light on the use of nanofluids in microfluidic-based devices and the properties of nanoparticles in aqueous and non-aqueous media.

We describe the self-assembly process of emulsion polymers. By varying the concentration of surface DNA, we tune the number of droplet-droplet bonds to assemble dimers, polymers, or even cross-linked networks. We observe that 96% of droplets assemble into either linear or branched polymers under optimized assembly conditions. We also find that linear polymers are freely-jointed and that the end-to-end length fluctuations follow the 2D Flory model of polymer scaling, while the 2D self diffusion follows Zimm scaling with polymer length. These polymers are then collapsed via triggered secondary interactions into compact clusters, allowing us to monitor the distribution of folded structures for a given chain length N.

Colloidal systems that undergo gelation attract much attention in both fundamental studies and practical applications. Rational tuning of interparticle interactions allows researchers to precisely engineer colloidal material properties and microstructures. In conventional approaches, colloidal self-assembly and gelation are driven by manipulating attractions such as electrostatic attraction, depletion, polymer bridging and dipole-dipole interaction. On the other hand, modulating the repulsions between colloidal particles has been investigated much less than manipulating attractions as a means to drive self-assembly. Although some examples have shown that gelation can be induced by modulating repulsions — such as screening the electrostatic repulsion between like-charged colloids using electrolytes or controlling the protonation of molecules grafted on the colloids by adjusting pH, the processes require adding or removing another component, which can be challenging for material processing and manipulation. Therefore, a simple external stimulus like temperature that can modulate the interactions is highly desired.

Here, we report a stimulus-responsive gelling platform where interparticle interactions are modulated through a thermally-responsive electrostatic repulsion. By including nonionic thermally-responsive oligomer surfactants in colloidal suspensions that are charge-stabilized by ionic surfactants, the ionic surfactants on the colloids are displaced by the nonionic oligomers at elevated temperatures. Such thermally-triggered surfactant displacement results in the decrease in colloidal repulsion, and hence induces the colloidal self-assembly. Our system shows rich phase behaviors and mechanical properties. We demonstrate that this gelation platform is general and robust over a wide range of composition, colloid size and component chemistry. By carefully characterizing the colloids and obtaining a mechanistic understanding of their response, we construct the interparticle potential and explain trends in material behaviors. This stimulus-responsive gelation platform is general and offers new strategies to engineer complex viscoelastic soft materials.

Micron-sized nanosheets such as graphene or MoS₂ platelets can be used to make conducting inks or as fillers in composites to enhance their mechanical properties. At high concentrations beyond rigidity percolation, nanosheet suspensions become yield stress fluids with a finite storage modulus. In this regime the elastic response of nano-sheet suspensions appears to be universal. The storage modulus plateau of few-layer graphene in NMP solvent [1] and aqueous graphene oxide gels [2] exhibit a power law exponent close to 3 as a function of packing fraction.
We present a new analytical model that explains this behaviour and connects the bulk response to the elastic properties of single nanosheets and their size. We infer the bending stiffness of single nano-sheets from our rheological data which agrees well with previous AFM experiments on single sheets and simulations.
This model opens up the possibility to infer elastic properties of different nanosheets from rheological data of the suspension rather than performing AFM experiments on single nano-sheets. Furthermore, this model may explain the mechanical enhancement of nanosheet composites where a similar exponent can be observed [3].

1] S. Barwich, J.N. Coleman, M.E. Möbius, Soft Matter,11 2015, 3159-3164.
[2] S. Naficy, J. Rouhollah, S.H. Aboutalebi, R.A. Gorkin III, K. Konstantinov, P.C. Innis, G.M. Spinks, P. Poulinc and G.G. Wallace, Materials Horizons 1 2014, 326-331.
[3] C.S. Boland, U. Khan,G. Ryan, S. Barwich, R. Charifou, A. Harvey, C. Backes, Z. Li, M.S. Ferreira, M.E. Möbius, R.J. Young, J.N. Coleman, Science, 354 2016, 1257-1260.

The jamming of nanoparticle surfactant assemblies at liquid-liquid interfaces enables the generation of persistent liquid in liquid structures or Structured Liquids. In general, increasing the binding strength of the individual nanoparticles to the interface leads to an increase in the stiffness of the assemblies. However, we observe a region of anomalously low compressive stiffness as nanoparticle binding energy is increased. We use a combination of tensiometry and in-situ AFM to reveal that the mechanism of anomalous stiffness is surfactant molecules surrounding nanoparticles at the interface without phase transfer which facilitates nanoparticle-nanoparticle interactions normal to the plane of the interface leading to facile multilayer formation. This region of anomalous stiffness expands the parameter space for the creation of complex shapes of structured liquids.

Cells, the building blocks of life, are enclosed with semi-permeable membranes. The membranes protect important genetic materials from surroundings while selectively allowing the transmembrane transport of molecules. This molecule-specific permeation is of great importance in cell growth and cell-to-cell communications. Inspired from the cell membrane, microcapsules with molecular size- or charge-dependent permeability have been developed using the membranes with consistent pore sizes and surface charges. Beyond these simple regulations of transmembrane transport, here, we suggest a new microcapsule with molecular polarity-selective permeation whose rate is further adjustable with temperature. Highly monodisperse microcapsules are produced using a microfluidically-prepared template of water-in-oil-in-water (W/O/W) double-emulsion drops. As the middle oil phase of the double-emulsion drops, we use a ternary mixture of photocurable triacrylate monomer, phase change material (PCM), and molecular compatibilizer. Upon UV irradiation on the drops, the monomer forms a polymeric shell and the liquid PCM fills the voids of the polymeric framework. The molecular compatibilizer, composed of acrylate group and PCM-like moiety, prevents the macrophase separation between the polymer and PCM, maintaining a homogenous shell at sub-10 nm scale. As the PCM forms the continuous nanochannels across the shell, the molecules that are soluble in the liquid PCM can diffuse through the channels above the melting point of PCM. By contrast, the molecules insoluble in the PCM are rejected from the microcapsules. The rate of permeation through the shell depends on the partition coefficient of the molecules in the PCM relative to water. As a less polar molecule has a higher partition coefficient, it shows faster permeation. The rate of permeation can be further adjusted by temperature. When the temperature drops below the melting point of PCM, the solubility dramatically decreases, resulting in the significant retardation of permeation. Therefore, the microcapsules provide molecular polarity-selective and temperature-dependent permeability whose threshold values are adjustable by the selection of PCM. As the polymeric framework and PCM form a homogeneous shell in the presence of molecular compatibilizer, PCM is preserved in the shell during temperature swing below and above the melting point, enabling the reversible change of permeability. This advanced transmembrane regulation provides a new opportunity for microcapsules as drug carriers, microsensors, and microreactors. For example, the microcapsules can be used as an injectable and implantable drug carrier that releases relatively hydrophobic drugs in a sustained fashion when the body temperature rises above the melting point of PCM as we verified through in-vitro and in-vivo experiments.

Many materials consist of soft particles at or below the jamming transition. However, we incomplete understanding of the relationship between material properties–such as rheology– and microstructure. To aid in this study of soft materials, we have created an experimental tool that enables spatiotemoral control over jammed microstructure. The composite has a photo- triggered jamming transitions, so we can direct what micro-regions of the materials are jammed. We have characterized the properties of this material on multiple length scales. This tool assists in the quantitative assessment of individual microstructural elements to the bulk material rheology, and may aid in the design of new soft materials.

Structural color is created by the interaction of light with physical structures and is most commonly seen in hard materials with high refractive index contrast and nanoscale periodicity such as diffraction gratings or photonic crystals. Developing methods to harness structural color in soft materials at larger length scales, such as in droplets, would enable use of such coloration mechanism for more diverse applications such as sensors, camouflage, or displays. Recently, we have observed tunable structural coloration from microscale concave interfaces of oil-oil complex droplets suspended in an aqueous surfactant solution. We were able to model the observed color and found that this effect was from light propagating by TIR along the interface can have different numbers of reflections and thus different path lengths which leads to interference and iridescent color. The requirements to achieve this effect include a transition from a high to low refractive index and a microscale geometry to support multiple TIR, allowing generation of structural color in a multitude of different materials and geometries and by diverse fabrication methods. For example, we have generated this effect in complex liquid droplets, solid particles, water drops condensed on substrates with low wettability, and solid elastic surfaces. With a light responsive surfactant, we were able to pattern reflective colored images in complex emulsions by stabilizing droplets of varying shape. These simple geometric requirements provide new opportunities for fabricating, designing, and controlling structural color, enabling use of such structural coloration in materials where it previously would have not been possible.

Anisotropy, the ability of partially ordered materials to respond directionally to a stimulus, is key to the dynamics and functions in living systems. From transporting signalling molecules within cells to transmitting mechanical stresses within growing bacterial colonies, anisotropy underpins the physics of living matter. Outside biological systems, liquid crystals (LCs) – formed typically by locally oriented low-molecular-weight rod-like molecules – offer an experimentally tractable system within which the fundamentals of anisotropic interactions can be precisely tuned and studied. In my lab, we harness this setting to, on the one hand, understand the fundamentals of anisotropic cross-talks between bio-relevant fluids and cues, and on the other hand, apply this knowledge to uncover the dynamics of real living systems. During this talk, I will allude to this iterative learn-apply-n-learn strategy to showcase how topological defects emerge within biological systems, regulate their micro-environment, and thereby tune the local material, transport, and mechanical attributes. Analysing living matter as emergent systems where anisotropy mediate evolutionarily well-timed biological functions, introduces a novel, yet general, mechanistic framework applicable for disparate biological systems. I will conclude by discussing how our ability to harness anisotropy – in action across disparate material fields – could have meaningful implications, from designing novel materials to exploring open questions in biology and translational medicine.

Emulsions stabilized with surfactants above the critical micelle concentration undergo solubilization as oil from droplets is transferred into the micellar phase. Concentration gradients of solubilizate species can create interfacial tension differences across droplets leading to their movement via the Marangoni effect. We explore how long-ranged interactions between solubilizing oil droplets occur due to the overlap of their continually maintained solute profiles to generate active behavior. Mixing droplets of different oils under various surfactant conditions we observe unique nonreciprocal interactions which lead to chasing between droplets and drive their self-assembly into clusters with dynamic but predictable motions. We show how the underlying interactions are caused in part by oil exchange between droplets via oil-filled micelles. Control over droplet interactions and the directional exchange of chemical components can be utilized in the design of tunable active multiphase fluid systems with the ability to self-organize, adapt to environmental stimuli, or carry out self-regulated reactions.

Surfaces that prevent the adhesion of and remove droplets have received significant attention in the context of self-cleaning, anti-icing, and anti-fouling surfaces. Superhydrophobic surfaces offer a passive solution; however, they are susceptible to Cassie-to-Wenzel transition and depend on gravity to move the droplets. Active droplet manipulation has also been explored using electric, magnetic, and vibrational fields, but such technologies have difficulties with surface pinning and viscous liquids. In this context, systems in which droplets self-propel are promising avenues to remove contaminants from surfaces.

Here, we investigate the self-propulsion of small droplets of radii at and below the capillary length on heated thin oil films. At film temperatures above the boiling point of the droplet and low film viscosities, we observe that droplets can propel at velocities up to 16cm/s. The propulsion of the droplet originates from the asymmetric release of vapor from beneath the boiling droplet, propelling the droplet towards the edges of the substrate. Studies of film parameters, such as film oil, temperature, viscosity, and thickness, were conducted. Additionally, the effects of surface geometry and texture were explored. The droplet velocity was found to have the greatest dependence on film oil, temperature, and viscosity. We predict the droplet velocities using a model that balances the viscous dissipation in the oil film with the momentum of the vapor ejected from beneath the boiling droplet. Within our experimental range, the simple model accurately predicts droplet velocities at a variety of oil film thicknesses, oil viscosities, oil temperatures, and droplet radii.

The fertilization process in mammals as sperm traverse towards the fertilization site where the oocyte has been released. In the case of marine animals and plants, which release gametes into the sea, the motion of sperm occurs in a vast aquatic environment. In contrast, the fertilization process of mammals happens inside a complex environment known as the “female reproductive tract”. The intriguing, multifaceted question is, how do healthy sperm naturally navigate the correct path towards the fertilization site? And concurrently, how does the female reproductive tract select for the best sperm while they move towards the oocyte. Since performing in vivo studies to answer these questions is difficult and faces many technical and ethical issues, designing in vitro environments that mimic at least one facet of female reproductive tract is vital.
In the last two decades, “microfluidics”, with its high and unprecedented precision of preforming studies on microswimmers and active matters in microenvironments, has enabled us to study the navigation strategies of mammalian sperm. Although the journey of mammalian sperm is many-sided and includes complex biological and chemical processes, studying the motion of sperm in microfluidic geometries, and under fluid flows that mimic the biophysical aspects of the sperm swimming channel in vivo, is critical. These studies will reveal new insight about the physical and fluid-mechanical clues provided by the female reproductive tract to facilitate sperm navigation towards the fertilization site.
The fluid-mechanical clues that enable mammalian sperm to swim along a correct path towards the fertilization site include the upstream swimming of the sperm in a simple, shear flow known as “rheotaxis”. Furthermore, the hydrodynamic interactions of sperm with nearby rigid boundaries is another navigation mechanism that is referred to as “boundary-dependent navigation”. We have performed studies on the fluid-mechanical navigation of the mammalian sperm as well as the influence of specific, geometrical features on the sperm locomotion.
To study the sperm navigation in a geometry that mimics the shape of “uterotubal junction”, which is a narrow junction at the beginning of fallopian tube, we designed a microfluidic stricture and studied the sperm locomotion under a simple shear flow. We discovered that such junctions select for highly motile sperm that can pass through the stricture while the slower sperm accumulate before the stricture. Therefore, a microfluidic stricture functions as a fluid-mechanical gate. The accumulation before stricture occurs in a hierarchical manner so that the competition is fiercest among sperm with the highest motility. To study the role of flagellar beating pattern on the sperm navigation, we first studied the flagellar beating pattern of sperm discovering a zeroth harmonic in the spatially asymmetric beating of the flagellum. This asymmetric beating creates a net torque and, thus, rotational components in the motion of sperm. We discovered that this zeroth harmonic, and asymmetric beating, impairs the boundary-dependent navigation while rheotaxis is less dependent on flagellar beating pattern.
To use our basic understanding of the sperm locomotion for such medical applications, we also have designed a microfluidic, rheotaxis-based sperm separation method. The microfluidic sperm separation techniques are sought-after for assisted reproductive technologies and infertility treatment as they are passive and operate without exertion of external forces. These efficient methods, accordingly, provide sperm with less DNA fragmentation. Our “microfluidic corral system” is a passive technique that only isolates progressively motile sperm with motilities higher than a tunable cut-off value.

We will present a number of strategies for using multiphasic liquid-liquid-polymer systems to make a rich variety of colloidal structures and materials. The capillary forces originating at the liquid/liquid interfaces can serve for reconfigurable binding in soft matter systems, including Pickering emulsions, novel responsive capillary gels, and compositions for 3D printing. We will first present responsive structures made of filaments from lipid-coated magnetic nanoparticles suspended in water-oil systems. The nanocapillary binding results in ultra-high filament flexibility. As an example of the application of such structures, we will discuss the development of new 3D printing inks consisting of water, crosslinked PDMS microbeads and liquid PDMS phase. These Homocomposite Thixotropic Pastes (HTPs) can be directly extruded and shaped on a 3D printer. The curing of the PDMS bridges yields remarkably elastic, flexible and biocompatible structures. The HTP-3DP inks enable the 3D printing of “active” and magnetically reconfigurable structures. In alternative materials synthesis approach the liquid/liquid droplet interfaces can template the formation of a variety of polymer nanomaterials, including nanoparticles, nanofibers, nanoribbons, microrods, and microsheets. This allowed us to introduce a new class of soft dendritic polymer microparticles ("dendricolloids") with hierarchical morphology similar to molecular-scale polymer dendrimers, but two orders of magnitude larger in scale. The polymer particles with branched and fractal morphology are fabricated by a simple and scalable process of interfacial polymer precipitation in turbulently sheared liquid media. The dendricolloids combine the properties of two of the most fascinating and studied soft matter systems – the freely-suspended dendritic particles have very large excluded volume, while in contact their nanofiber corona possesses the highly adhesive abilities of the nanofiber-padded gecko legs. The fractal branching and contact splitting phenomena of the dendricolloids enable a range of highly unusual properties – gelation at very low volume fractions, strong adhesion to surfaces and to each other, and ability to bind strongly and form coatings, nonwoven sheets, and ultrasoft membranes.

Visible light driven nano/micro swimmers are promising candidates for potential biomedical and environmental applications. However, the previously reported mean squared displacement (MSD) values are low, typically in the range of up to 200 µm² (when measured over 10 s), even under the favourable UV light illumination.^[1,2] This is a severe drawback for the applications where the efficient transport of micromotors within a vessel is demanded.

Here, we demonstrate Ag/AgCl-based spherical Janus micromotors that reveal an efficient propulsion under visible blue light illumination.^[3] The proper design of an Ag/AgCl-based micromotor can boost the MSD to a remarkable value of 3000 µm² (over 10 s) in pure H₂O, even when activated with blue light (excitation λ = 450-490 nm). The revealed propulsion of micromotors owns a dependence of the intensity of visible light, which is contributed by the couple plasmonic light absorption of Ag/AgCl and the efficient photochemical decomposition of AgCl. With the motion comparisons of individual Janus particle, small cluster, and large cluster, the effect of suppressed rotational diffusion has been revealed experimentally and in numerical simulations. Furthermore, we show that Ag/AgCl-based Janus micromotors reveal efficient exclusion effect to their surrounding passive polystyrene (PS) beads in pure H₂O.^[4] The exclusion efficiency is controlled by the number of single Janus PS/Ag/AgCl particles that compose a cluster. Using numerical simulations of the Langevin equations, we gain a fundamental understanding not only the diffusion constants, but also the system-specific interaction parameter between Janus motors and passive beads.

1. Ibele, M., et al., Angew. Chem. Int. Ed. 2009, 48, 3308.
2. Simmchen, J., et al., ChemNanoMat 2017, 3, 65.
3. Wang, X., et al., Small 2018, 14, 1803613.
4. Wang, X., et al., Small 2018, 14, 1802537.

Biological active matter, such as populations of cells and animals, often change between different swarming states. One example is shoaling, milling and schooling fish. Synthetic active matter consist of self-propelled inanimate units and emulates biological active matter. We combine electric field induced attraction with electro-rolling propulsion [1] in a population of granular beads. A variety of swarming regimes is realized: living crystals and clusters, a stripe phase of clusters, and polar liquid swarms, reminiscent of transitions in active matter simulations [2,3,4]. Remarkably, the crystal to liquid transition occurs at a different velocity threshold than the local to global polar order transition. The stripe phase is reminiscent of those in quasi two-dimensional matter with competing interactions. The system links universal patterns in biological and synthetic active matter, and can open new routes for tuning self-assembly in soft matter technologies.

[1] A. Bricard, J.-B. Caussin, N. Desreumaux, O. Dauchot, D. Bartolo, Emergence of macroscopic directed motion in populations of motile colloids, Nature 503, 95 (2013)
[2] Living Clusters and Crystals from Low-Density Suspensions of Active Colloids, B. M. Mognetti, A. Šarić, S. Angioletti-Uberti, A. Cacciuto, C. Valeriani, and D. Frenkel, Phys. Rev. Lett. 111, 245702 –(2013)
[3] Hydrodynamic interactions in dense active suspensions: From polar order to dynamical clusters, N. Yoshinaga and T. B. Liverpool Phys. Rev. E 96, 020603(R) (2017)
[4] Spontaneous aggregation and global polar ordering in squirmer suspensions
F.Alarcón and I.Pagonabarraga, Journal of Molecular Liquids, 185, Pages 56-61 (2013)

This talk will describe the use of hydrogels to encapsulate single, or multiple cells in hydrogel particles to enable very high throughput screening of the cells to study their properties and their response to external stimuli, while also protecting the cells and providing a solid substrate on which they can grow. Single cells can provide a means to screen for drug efficacy, while multiples cells can probe cell-cell interactions. In addition, larger scale cell structures can provide an organ in a drop.

Vesicles are aqueous droplets stabilized by water-permeable membranes composed by amphiphilic molecules such as phospholipids or block copolymers. The amphiphilic nature of these membranes ensures retention of hydrophilic ingredients within the aqueous cores of the vesicles. If sufficiently concentrated, these hydrophilic ingredients may phase separate, yielding compartments within the cores of the vesicles, which resemble the crowded cytoplasm of cells. Simultaneously, membrane composition can be finely tuned to create membrane phase separations, which resemble rafts in cellular membranes. Moreover, controlling independently the composition of each membrane leaflet enables the fabrication of asymmetric vesicles, which mimic the compositionally asymmetric nature of biological membranes. Much like in cells, all these types of phase separations and asymmetries dictate vesicle properties and enable broadening their range of applications. However, these applications depend critically on the degree of control achieved over the vesicle properties during the fabrication process. Here, we address the adequacy of different types of emulsion droplets, fabricated with glass-capillary microfluidic devices and thus exhibiting well-controlled topologies, as vesicle templates. In particular, we show the utility of water-in-oil-in-water (W/O/W) double emulsion drops with ultrathin middle oil layers as templates to fabricate vesicles exhibiting either phase separations in the vesicle cores or domains in the vesicle membranes [1,2]. We also show the utility of W/O/O/W triple emulsion drops with two ultrathin middle oil layers as templates to fabricate vesicles with asymmetric membranes [3]. Both approaches have high encapsulation efficiency and yield vesicles with monodisperse sizes, owing to the size monodispersity of the emulsion drops used as templates. Furthermore, coupling microfluidic vesicle production techniques to confocal fluorescence microscopy observations enables the accurate determination of both the composition of phase-separated compartments in the vesicle cores and the degree of asymmetry in the vesicle membranes.

[1] L.R. Arriaga, S. Datta, S.-H. Kim, E. Amstad, T. Kodger, F. Monroy, D.A. Weitz. Ultra-thin shell double emulsion templated giant unilamellar lipid vesicles with controlled microdomain formation. Small 10, 950-956 (2014).
[2] J. Perrotton, R. Ahijado-Guzman, L.H. Moleiro, B. Tinao, A. Guerrero-Martinez, E. Amstad, F. Monroy, L.R. Arriaga. Microfluidic fabrication of vesicles with hybrid lipid/nanoparticle bilayer membranes. Soft Matter 15, 1388-1395 (2019).
[3] L.R. Arriaga, Y. Huang, S.-H. Kim, J.L. Aragones, R. Ziblat, S.A. Koehler, D.A. Weitz. Single-step assembly of asymmetric vesicles. Lab Chip 19, 749-756 (2019).

Complex coacervation is a well-studied liquid-liquid phase separation phenomenon between oppositely charged polyelectrolytes. Recently, it has been appreciated that complex coacervation likely plays a role in the formation and function of membraneless organelles in eukaryotic cells. For example, polyanionic RNA can condense with cationic intrinsically disordered regions of proteins to form membraneless organelles such as nuclear bodies. Here we investigate how liquid-liquid phase separation via complex coacervation can be engineered from the ground up. Using protein engineering the overall charge and charge distribution on model fluorescent proteins was systematically varied. In particular, this study explores the impact of GFPs with different supercharged tags on the formation of protein complex coacervates in vivo. GFP variants with the same net charge but different charge distributions were expressed and their intracellular localization was monitored by optical microscopy. We found that GFP variants with positively charged tags can form a condensed phase in E. coli, and this condensed phase has many of the properties of a complex coacervate. In summary, we have identified several supercharged amino acid tags that can promote liquid-liquid phase separation of proteins of interest intracellularly. This finding can be applied to engineer intracellular complex coacervation to form artificial membraneless organelles.

Type I collagen is a self-assembling protein that generates fibrous viscoelastic networks. In the body, cell-laden networks of collagen are remodeled to generate anisotropic fiber networks that can impact biological processes as diverse as cancer cell invasion and vascular network formation. Despite this ubiquity, fabricating cell-laden networks of aligned collagen fibers ex vivo remains challenging. Here, we use Marangoni flows in evaporating sessile droplets of monomeric collagen to fabricate cell-laden networks of aligned type I collagen fibers. Using time-lapse confocal reflection microscopy, we find that Marangoni flows give rise to radial flow in the evaporating droplet, which orients collagen fibers radially over mm-scale areas. Our data suggest that the resulting collagen fiber alignment depends on the relative humidity and the rate of collagen fiber self-assembly. Moreover, we found that the pattern of collagen fiber alignment can be tuned by changing the geometry of the droplet. By incorporating mammalian cells into evaporating droplets, we generate cell-laden collagen hydrogels that can be used for cell culture. We observe that cells collectively orient in the direction of collagen fiber alignment. Marangoni flow is a simple, rapid, and scalable approach that can be used to generate fibrillar cell-laden collagen hydrogels with broad applicability.

Exchange of molecular signals is essential for the coordination of cell behaviors in cellular consortia and tissues. This, in turn, enables the development and organization of large multicellular structures of higher complexity than typical single-celled organisms.
In recent years, a variety of attempts have therefore been made to emulate such communication processes in the context of artificial cells – e.g., to divide tasks between specialized synthetic organelles or for the creation of artificial tissue-like materials.
In this talk, we will present several examples, in which compartments containing chemical reaction systems and gene circuits communicate with each other via the exchange of molecular signals – e.g., on-chip gene expression, gel-based organelles that exchange RNA molecules, or emulsion droplets that communicate via small molecules.

Since the first discovery of supercooling in 1724, the study of supercooled matter has been mainly limited to varying temperature or pressure. Early this year we have demonstrated an electrochemical approach to generate supercooled sulfur and observe the dynamic process in situ (see Publication 1 below). Our methodology combines dark-field optical microscopy, a transparent electrochemical cell, and a fast camera to visualize the process at single microdroplet with millisecond time resolution. This platform may open up opportunities for studying supercooled liquids as the droplets approach either homogeneous nucleation to the crystalline state or enter into the glass transition. The sulfur droplets remain liquid at 155 °C below sulfur’s melting point (Tm = 115 °C), with fractional supercooling change (Tm − Tsc)/Tm larger than 0.40. In operando light microscopy captured the rapid merging and shape relaxation of sulfur droplets, indicating their liquid nature. Micropatterned electrode and electrochemical current allow precise control of the location and size of supercooled microdroplets, respectively. Using this platform, we initiated and observed the rapid solidification of supercooled sulfur microdroplets upon crystalline sulfur touching, which confirms supercooled sulfur’s metastability at room temperature
In a follow-up work (see Publication 2 below), we expanded the above methodology to generate polybromide droplets, which is the charge product in zinc-bromine flow batteries, a promising technology for stationary energy storage. The liquid behavior was also confirmed by rapid merging and shape relaxation. The results provide insights into the future design of zinc-bromine flow batteries.
This presentation is based on the following two recent publications from my lab:
1. Proceedings of the National Academy of Sciences, 2019, 116 (3), 765-770
https://doi.org/10.1073/pnas.1817286116
2. Angewandte Chemie International Edition, 2019, 131, 1-7
https://doi.org/10.1002/ange.201906980

Nanogels at liquid-liquid interfaces are used for applications ranging from emulsion stabilization to interfacial catalysis and enhanced oil recovery. We present herein a dissipative particle dynamics (DPD) approach to model dynamics of polyacrylamide (PAAm) hydrogels at the oil/water interface. To develop the model, we first compare hydrogel swelling dynamics to continuum theory. We compare the distribution of end to end distances of polymer chains within a swollen hydrogel network to the Gaussian model predicted for polymers in a theta solvent. Next, we compare equilibrium swelling volume fraction of gels measured from the DPD simulations to continuum Flory-Rehner theory. Using this equilibrium swelling volume fraction, we estimate the elastic moduli of our model hydrogels to be within the range observed in experiments. Having established these comparisons, we next focus on the shape changes of the hydrogel as it adsorbs from the aqueous phase onto the oil/water interface. The nanogel undergoes reshaping and adopts several morphologies from spherical to pancake. We highlight the competition between interfacial energy (which tends to deform the gel) and the gel’s elastic energy (which opposes this deformation). We also investigate spreading dynamics and interactions of nanogel arrays initially placed in the aqueous phase. We investigate conditions at which saturation of the interface with these gels forces protrusion of gels into the aqueous phase. These findings provide data for optimizing the design of PAAm-based nanogels for various applications.

Anisotropic nanoparticles have gained significant interest in the past decade due to their enhanced properties (both physical and chemical). These properties can be tuned and are dependent not only on their size and morphology, but also on their composition. The use of anisotropic nanoparticles for enhanced surface plasmon resonance (SPR) effect is widely studied. In the present work we use droplet-based microfluidics for the synthesis of anisotropic gold nanoparticles (a-AuNPs) for applications in biology based on its SPR effect. Specifically, we use the Pico injection (a technique of introducing picolitre volume reagents into droplets by the application of electric field that destabilizes the surfactant in the droplets and thereby allowing the addition of the new solution) for the controlled synthesis of a-AuNPs. While there are several other potential applications for these a-AuNPs, we are interested in using these for the lysis of microbial cells in single-cell sequencing experiments. The current single-cell techniques predominantly focus on multicellular organisms and cannot be readily transferred to unicellular organisms due to the differences in their size and composition. The microbial cells are small (~1 μm) with concomitantly small amount of RNA and have a tough, adaptive cell wall that permits their survival in harsh environments (extreme pH or temperatures, anti-microbials, etc.); these characteristic features of the microbes together make their lysis challenging. Here, we use the a-AuNPs and their SPR effect (induced by laser irradiation) to achieve micron-scale heating when attached to the cell wall and thereby its lysis by disrupting the cell wall. Once lysed, the single-cell lysates will be sent for high-throughput single-cell RNA sequencing. Single-cell sequencing of microbial cells is critical in addressing some of the challenges in medicine and other environmental engineering applications.

Drying bio-colloidal drops have a wide range of applications from medical diagnostics to industries as the patterns at the dried state can be linked to the nature of the constituent particles. The drying process and the resulting patterns are found to be affected by the atmospheric conditions including temperature, humidity, and so on. It, therefore, demands a detailed investigation to understand the nature of atmospheric conditions on the drying states. Lysozyme is a well-known globular protein, and preparations of lysozyme in de-ionized water make the simplest bio-colloid which could be used to explore the atmospheric effects on the drying states. During the drying process, a drop containing lysozyme particles gets pinned to the substrate and undergoes a convective flow. In the next stage, a fluid front recedes in a stick-slip manner and deposits particles from the periphery to the central region of the drop. The particles get redistributed during the fluid front movement; form a "coffee-ring", and finally result in crack patterns like other bio-colloids. A mound-like structure which forms in the central region is believed to be the leftover lysozyme particles flowed along with the fluid front. It is considered as a fingerprint for any aqueous lysozyme drop. In this experimental work, the time evolution and the resulting morphology during the drying process at different initial concentrations (1-20 wt%) and temperatures (25-55 °C) are investigated using bright-field optical microscopy. The captured images are then analyzed by various image processing techniques in ImageJ. The initial concentrations of lysozyme solutions are divided into three regimes, diluted (1 wt%), concentrated (5-13 wt%) and ultra-concentrated (17-20 wt%) regimes. During the drying process, initially, the fluid front moves slowly and linearly and then shows a fast, non-linear movement in both dilute and concentrated regimes. With the increase of initial lysozyme concentration, this movement in the non-linear region slows down as the front carries and deposit more particles at each step. However, in the ultra-concentrated regime, the fluid front moves linearly throughout the drying process. The deposition of particles by the fluid front creates the “coffee-ring” and is found to increase with every initial concentration. A dimple is observed in the existing mound-structure, and both structures grow with the increase of the initial concentration. The width of the ring also increases monotonically with the concentration. The cracks which are only observed in the ring at dilute regime, are spread throughout the drop at concentrated and ultra-concentrated regimes. Besides these concentration effects, the temperature increases the drying rate at every regime. It provides almost no time for the convective flow of the particles during the early stage of the drying process. The dimple in the mound structure changes to an inflamed spot in the diluted and concentrated regimes, whereas, it diminishes in the ultra-concentrated regime when the temperature rises. Both concentration and temperature gradient within the drop leads to a surface tension gradient, affecting the drop morphology. Due to the presence of a large number of particles in the ultra-concentrated regime, the drop forms thickest film and experiences high mechanical stress resulting in smaller and delaminated crack domains. This study sets an example to the diverse assays of bio-colloidal drops and has the versatility and applicability for machine learning algorithms.