Symposium SM06—Nano- and Microgels

The interplay between morphology, excluded volume, and adhesivity of colloidal-scale particles critically determines the physical properties of numerous materials such as gels, suspensions, emulsions, foams, and coatings. The structure-building and gel-forming abilities of colloids in liquid can be enhanced by particles with branched and fractal morphology. We will introduce a new class of soft dendritic colloids ("dendricolloids") that have a hierarchical morphology similar to molecular-scale polymer dendrimers, but two orders of magnitude larger in scale. The denricolloids are fabricated by a simple and scalable process of polymer precipitation in turbulently sheared liquid media. The ultralow interfacial tension of the polymer solution, combined with concurrent phase separation and precipitation, results in the formation of hierarchical fibrillar polymer microgels. The branched, microgel-like colloids are surrounded by a corona of polymer nanofibers spreading out in all directions. Thus, the dendricolloids combine the properties of two of the most fascinating and studied soft matter systems – the freely-suspended soft colloids have very large excluded volume, while in contact their nanofiber corona possesses the highly adhesive abilities of the nanofiber-padded gecko legs. We investigate and analyze the origin of these effects, which are closely associated with omnipresent van der Waals forces and the phenomenon of "contact splitting" that allows the legs of the gecko lizards to stick to any surface. The fractal branching and contact splitting phenomena of the denricolloids 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 into coatings and nonwoven sheets.

Man-made nanofibrillar hydrogels have recently emerged as a new class of biomimetic materials that reproduce the filamentous nature and properties of in-vivo extracellular matrices, act as scaffolds for three-dimensional cell culture and tissue engineering, and offer interesting properties in actuation and soft robotics. Our group has developed new “inks” for 3D printing of hydrogels that are derived from cellulose nanocrystals and different types of polymers. These inks offer a number of useful properties, including their shear thinning behavior, the biocompatibility of the resulting hydrogels and the capability to fine-tune hydrogel composition, structure and physical properties in a throughput manner.

At high concentration, microgels form glassy materials which behave as weak elastic solids at rest but yield and flow at high stresses. These materials are basic components of viscoplastic formulations used as high-performance coatings, solid inks, ceramic pastes, textured food, and personal care products. Synthetic chemistry offers a panel of strategies to tune the internal architecture of microgels and their interactions, which involve elastic repulsion and attractions of different origins. An important question concerns the relation between the particle elasticity, the phase diagram, the nature of the liquid to solid transition, and the macroscopic rheology [1].

Microgel elasticity can be tuned by varying the crosslink density, which appears to be one of the key parameters that determine the phase diagram and the rheology of microgel suspensions. Highly crosslinked microgels exhibit well-defined glass and jamming transitions and cross from liquids to entropic glasses and jammed glasses when the volume fraction is increased [2]. In the jammed glass regime, the flow properties exhibit universal properties. The macroscopic rheology is well described by a micromechanical model where microgels are described as purely elastic particles interacting under the combined action of elastic repulsive forces and viscous drag forces [3]. Weakly crosslinked microgels behave very differently. They solidify above a well defined volume fraction but the glass and jamming transitions can no longer be disentangled. Yielding is now time dependent and the flow curves exhibit distinct scaling properties.

These behaviors are representative of two universality classes of soft particles where the flow properties are controlled by the dynamics of the elastic cages formed by interlocked neihgboring particles and the viscoelastic deformation of individual particles, respectively.

[1] D. Vlassopoulos, M. Cloitre, “Tunable rheology of dense soft deformable colloids“, Curr. Opin. Colloid Interface Sci. 19, 561-574 (2014)
[2] C. Pellet and M. Cloitre, “The glass and jamming transitions of soft polyelectrolyte microgel suspensions”, Soft Matter 12, 3710-3720 (2016).
[3] T. Liu, F. Khabaz, R. T. Bonnecaze, M. Cloitre, “On the universality of the flow properties of soft-particle glasses”, Soft Matter 14, 7064-7074 (2018).

Dense microgel suspensions have been shown to exhibit a thermal glass-to-jammed glass transition, characterized by large yield stress and linear dependence of plateau modulus on concentration, as well as other fine differences in flow curves [1]. There appears a consensus suggesting that shape adjustment in the highly concentrated regime is at the origin of jamming [3]. On the other hand, hairy particles represent a distinct class of soft colloids, being essentially the counterpart of microgels, with the additional feature (and degree of freedom) being their dangling arms [4]. Hence, a natural question that emerges concerns the role of internal microstructure on the dynamic properties of soft colloids and in particular the possible glass to jamming transition at high concentrations. Here, we address this challenge by systematically investigating highly concentrated suspensions of well-characterized microgels and multiarm star polymers. Whereas both types of particles are able to shrink and deform under the action of osmotic pressure, star polymers can also interpenetrate their arms and, moreover, can be investigated throughout the entire concentration regime up to the dry melt state. We show that the difference in the internal particle microstructure is reflected in distinct rheological signatures of these soft colloids. We focus on linear and nonlinear rheological shear measurements and analysis with additional support from dynamic light scattering, and attempt at providing a generic guidelines for tuning the flow properties of soft colloids via knowledge of their microstrostructe. Subtle differences in flow curves and associated velocity profiles are critically discussed and linked to the different features that dictate particle softness. We show that these distinct universality classes of soft colloids possess flow properties with generic features which can be tailored at molecular level.

[1] C. Pellet, M. Cloitre, “The glass and jamming transitions of soft polyelectrolyte microgel suspensions”, Soft Matter, 12, 3710 (2016).
[2] G. M. Conley, P. Aebischer, S. Nöjd, P. Schurtenberger, F. Scheffold, Jamming and overpacking fuzzy microgels: Deformation, interpenetration, and compression, Sci. Adv., 3, e1700969 (2017).
[3] I. Bouhid de Aguiar, T. van de Laar, M. Meireles, A. Bouchoux, J. Sprakel, K. Schroën, “Deswelling and deformation of microgels in concentrated packings”, Sci. Rep., 7, 10223 (2017).
[4] D. Vlassopoulos, M. Cloitre, “Tunable rheology of dense soft deformable colloids“, Curr. Opin. Colloid Interface Sci., 19 (2014)

Passive microrheology is a powerful tool to study gels and their formation. Passive microrheology consists of using micron sized particles to measure the local deformation of a sample resulting from the thermal energy, the Brownian motion. Our technique is based on Diffusing Wave Spectroscopy, which is a multiple backscattering technique using a CCD camera to correlate the fluctuation of the backscattered light to the motion of the particles. By mathematical processing of the signal, the motion of the particles can be related to the viscoelastic properties of the sample, as particle speed and particle displacement are directly related to viscosity and elasticity. This work will give an overview of possible applications, with a focus on the Sol-Gel processes. Indeed, various functional and customized materials are prepared for a wide range of applications by the Sol-Gel process, and precise microrheological characterization could benefit to the optimization of preparation and properties of such materials properties. This work will show, how Rheolaser Master® can be used to study formation and degradation of the gel in time, the correlation of gel strength during formation with the mechanical properties after hardening. Different parameters are changed such as composition or temperature.

The rheological behaviors of poly(N-isopropylacrylamide) (PNIPAM) microgel suspensions in a glass-liquid-solid transition were investigated using a single-particle microrheometer based on total internal reflection microscopy (TIRM) and magnetic tweezers. A probe particle dispersed in the microgel suspension was actively oscillated with displacement varying from ~100 to several nanometers near a surface. The frequency and temperature dependence of the viscoelastic moduli (G’ and G") were investigated in a continuous phase transition between glass, liquid, and aggregation states of the microgel suspensions using one identical probe particle within the same sample. Below the lower critical solution temperature (LCST ~ 33 °C), the viscoelastic properties are solely governed by the effective volume fraction (Φ_eff) with a critical glass transition point Φ_glass ~ 0.7, and the relaxation scaling of G" ~ ω^0.5 indicates the glassy state of the swollen microgels ensembles as hard spheres with a continuous repulsive potential at high volume fractions. At a higher temperature near the LCST (T_gel), a liquid-solid transition is revealed by the second intersection of G’ and G", and a minimum value of G" followed by a sudden increase of both G’ and G" at T* (~33.5 °C). Finally, the different responses of T_gel and T* as a function of frequency illustrate the corresponding repulsive and attractive interactions between microgel particles and the different relaxation responses of the cages formed by their neighboring particles. Unlike conventional mechanical spectroscopy measurements, measurement by our single-particle microrheometer reveals more local and complete viscoelastic behavior during the phase transitions of PNIPAM microgels. We hope these findings will promote more experimental investigations of the physical mechanics in phase transitions on the single-particle level.

The bulk behaviour of colloidal suspensions depends crucially on the microscopic details of the particle-particle interaction[1]. For polymer-based building blocks, the interactions depend on a large number of parameters such as the particle microstructure, its composition and related physico-chemical properties (solvophobicity, charge density, etc.). Among the huge variety of available systems, stimuli-responsive microparticles built out of polymer networks, so-called microgels, have emerged as one of the most interesting class of soft particles, for both a theoretical and applicative standpoint[2].
Here we build upon a recently-developed method[3] to generate and simulate realistic in silico microgels. We first look at the single-particle mechanics by calculating the elastic moduli in the small-deformations regime. We then use Umbrella Sampling and a generalised Widom insertion method to accurately estimate the two-body effective interaction.
We show for the first time that the Hertzian theory works well for large separations, and that in this regime the single-particle elastic moduli can predict the amplitude of V(r) for a wide range of network topologies[4]. However, for smaller separations microgels start to strongly interact and change their shape and V(r) deviates from the predicted Hertzian behaviour, affecting the bulk behaviour of the suspension[4].
This work establishes a clear link between the microscopic network properties and the resulting microgel-microgel interactions, paving the way for a deeper understanding of the bulk behaviour of microgel suspensions.

[1] C. N. Likos, Phys. Rep., 348, 267 (2001)
[2] A. Fernandez-Nieves, H. Wyss, J. Mattsson, D. A. Weitz, Microgel Suspensions: Fundamentals and Applications; John Wiley & Sons (2011)
[3] N. Gnan, L. Rovigatti, M. Bergman, E. Zaccarelli, Macromolecules, 50, 8777 (2017)
[4] L. Rovigatti, N. Gnan, A. Ninarello, E. Zaccarelli, arXiv (2018)

Microgels are solvent-containing, cross-linked polymer networks of colloidal size that can reversibly swell or deswell in response to external stimuli. Ionic microgels in particular are highly sensitive to changes in environmental conditions such as temperature, solvent quality, polymer cross-linking, suspension ionic strength and particle concentration, which allows for controlling their size and effective interaction.
In this work, we theoretically study the effects of concentration-dependent de-swelling of weakly-crosslinked ionic microgels on structural and dynamic suspension properties [1]. We use and compare two different theoretical approaches, namely the Denton-Tang method based on a Poisson-Boltzmann cell model [2] and a thermodynamic perturbation method [3], to calculate the equilibrium microgel size. The state-dependent microgel size is a salient ingredient to the effective interaction potential for spherical ionic microgels derived by Denton [4] which underlies our calculation of static pair correlation functions and structure factors. The latter static quantities are used as input in our calculation of dynamic suspension properties including the hydrodynamic function, collective diffusion coefficient, and low- and high-frequency viscosities. For a quantitative assessment of deswelling effects, results for static and dynamic suspension properties are compared with corresponding findings for a reference suspension having constant microgel size.

References

[1] M. Brito, J. Riest, A. R. Denton and G. Nägele, to be submitted.
[2] A. R. Denton and Qiyun Tang, J. Chem. Phys. 145, 164901 (2016).
[3] T. J. Weyer and A. R. Denton, Soft Matter 14, 4530 (2018).
[4] A. R. Denton, Phys. Rev. E 67, 011804 (2003).

Polymer microgels are soft colloidal particles highly sensitive to variation of environmental conditions (e.g. temperature, pH, etc.) leading to the volume and shape transformations. It is well known that concentration of solid colloidal particles in suspensions can lead to their phase separation and/or crystallization. The latter means that the particles are spatially-ordered in the solution forming face-centered cubic (FCC) or hexagonal close-packed (HCP) structures. The natural question arises: What is going on with the soft particles upon concentration and crystallization? We can imagine that in addition to the spatial ordering, the shape and volume transformations of the microgels can occur upon concentration. In the current paper we give a detailed description of the structure and dynamics of concentrated suspensions of soft microgels. We explore the deformation and deswelling mechanisms of microgels of different architectures and cross-linking density in overcrowded conditions. We propose computer simulations of regular (continuous) and hollow microgels (with cavity) subjected to uniform compression (concentration) and uniaxial deformations (one- or two-dimensional solvent evaporation). We have demonstrated that final structure of the microgels and their ordering strongly depends on the cross-linking density, relative size of the cavity (hollow microgels) and compression degree. Uniform compression of the simulation box with continuous microgels beyond the overlap regime leads to their compression with further shape transformation. Ultimate shape of the microgels is close to the Wigner-Seitz cell of the fcc-structure and two-fold compression still keeps the fcc ordering. Quite different behavior is observed for the case of the hollow microgels. The uniform compression leads to the spontaneous buckling of the microgels and cavity collapse. This is accompanied by breaking of the fcc-ordering. Uniaxial deformation of the simulation box with continuous microgels leads to reordering of the microgels. At the same time homogeneous deswelling, volume reduction and self-similar shape transformation was observed. Surprisingly the highly cross-linked hollow microgels under applying the uniaxial strain to the system could form liquid crystal phases. Selected strain direction provides overall uniaxial deformation of the hollow microgels in such a way that they from disk-like objects which exhibit liquid crystalline behavior. We have used the Q-tensor approach for nematic liquid crystalline systems and found conditions for formation of highly ordered phases where orientation order parameter approaches the values close to 1.

Increasing the particle density of a suspension of microgel colloids above the point of random-close packing, must involve deformations of the particle to accommodate the increase in volume fraction. By contrast to the isotropic osmotic deswelling of soft particles, the particle-particle contacts give rise to a non-homogeneous pressure, raising the question if these deformations occur through homogeneous deswelling or by the formation of facets. Here we aim to answer this question through a combination of imaging of individual microgels in dense packings and a simple model to describe the balance between shape versus volume changes. We find a transition from shape changes at low pressures to volume changes at high pressures, which can be explained qualitatively with our model. Whereas contact mechanics govern at low pressures giving rise to facets, osmotic effects govern at higher pressures, which leads to a more homogeneous deswelling. Our results show that both types of deformation play a large role in highly concentrated microgel suspensions and thus must be taken into account to arrive at an accurate description of the structure, dynamics and mechanics of concentrated suspensions of soft spheres.

In classical polymerization of small molecules, network structures for gelation can be synthesized by introducing multifunctional monomers during polymerization. With colloidal nanoparticles as building blocks, chain- or network-like superstructures can be produced by connecting well-defined patches on the nanoparticles. Particularly, nanoscale micelles of diblock copolymers can have distinct patches on their surface so that they can be assembled into linear or crosslinked superstructures depending on the number of patches on the nanoparticles. In this presentation, we investigated the formation of multiple patches on diblock copolymer micelles on a solid substrate as well as in a solution. Especially, nanoscale micelles having multiple patches coated on a substrate can be utilized for ultimate generation of two-dimensional network-like superstructures. For this purpose, spherical micelles after crosslinking their cores were first coated on a substrate and then the patches were induced by exposing them to a core-preferable solvent. The number of patches on the micelles depending on the block ratio of copolymers was characterized by SEM and AFM. We also explored the confinement effect on the patch formation by restraining spherical micelles in localized spaces.

Predicting gelation in polymer systems is difficult because of the long-time dynamics and complex components that characterize them. In this work we perform coarse-grained molecular dynamics simulations of reacting epoxies to access longer times and relevant volumes for observing up to 100nm features, gelation, and glass transitions. Here we focus on calculations that connect nanostructure to transition temperatures. We demonstrate that gelation and glass transitions depend on the degree of cure in epoxy systems, and quantify the degree to which coarse-grained models are predictive of experiments. We find that these coarse-grained models are quantitatively predictive of glass and gelation transitions for simple epoxy thermosets. Using GPU-accelerated simulations we demonstrate the potential for using coarse-grained models of polymer gels to provide a window into how processing protocols can be used to tailor the structure and properties of gels synthesized by crosslinking.

Light and temperature dual responsive microgels system based on spiropyran modified poly (N-vinylcaprolactam)-vinylamine copolymer was designed and built up. The novel and applicable microgels synthesis route involved first preparing spiropyran modified poly (N-vinylcaprolactam)-vinylamine copolymer via RAFT polymerization, hydrolysis and coupling, and then being crosslinked to microgels through W/O mini-emulsion. Light and temperature responsiveness of the synthesized microgels were investigated. The UV-vis spectrum demonstrated the reversible light-responsive behavior of microgels in aqueous solution, which comes from the spiropyran photoisomerization under different irradiation. DLS results show that the microgels swell in darkness due to the-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. DLS results also show the spiropyran modified microgels have temperature responsiveness: the microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature and the VPTT decreased comparing the non spiropyran modified microgels due to the hydrophobic spiropyran units.

Microgels or nanogels are cross-linked polymer networks that can respond to environmental change like temperature or pH. When triggered, they collapse or swell in solvents, while preserving their 3D structure. Micron-sized microgels can have plenty of interesting applications in crystallization studies, microlensing or mimicking cells. Microgels with the size above 10 µm can be synthesized using microfluidics [1]. However, very low yields and specific equipment requirements lead to high demand on synthesis of microgels via dispersion polymerization approaches. Moreover, the typical size of gels synthesized using standard batch polymerization reactions is in nanometer scale. Thus, particles ranging from 1 to 10 µm are hardly accessible, and means to obtain them need further investigation. Micron-sized poly(N-Isopropylacrylamide) (pNIPAm) microgels have been obtained previously [2]; however, known toxicity and suspected cancerogenity of pNIPAm disqualifies the usage of these gels in medical applications.

In contrast to pNIPAm, poly(N-Vinylcaprolactam) (pVCL) is non-toxic and biocompatible [3], making it a great candidate in medical and biomaterial fields. Here, for the first time, we have synthesized temperature-responsive pVCL microgels with large diameter (1 -5 µm) via aqueous, surfactant-free precipitation polymerization. The size control was achieved by employing programmed temperature ramp during the nucleation stage of polymerization. In this work, the influence of various parameters such as monomer, cross-linker and initiator concentration, rate of temperature ramp, start and end temperature as well as dosing of the reagents was investigated. We were able to obtain microgels in different size ranges, depending on parameters altered.


[1] R. K. Shah, J. Kim, J. J. Agresti and L. Chu, Soft Matter, 2008, 4, 2303–2309.
[2] Z. Meng, M. H. Smith and A. Lyon, Colloid and Polymer Science, 2009, 287, 277-285.
[3] H. Vihola, A. Laukkanen, L. Valtola, H. Tenhu and J. Hirvonen, Biomaterials, 2005, 26, 3055–3064.

Microfluidic chips enable reproducible generation of complex droplets and particles, but have a low throughput that blocks their industrial use. Furthermore, the resulting particles cannot be readily used as building blocks for solid (meta)materials, as they are contained by a liquid phase. Here we address these limitations by ejecting a train of droplets into the air and manipulating their properties by impact of a second liquid jet. The surface tension of the jet was reduced, so that it encapsulates the droplet by Marangoni spreading. Subsequent on-the-fly manipulations, including solidification and controlled deformation, enable fabrication of monodisperse emulsions, particles, and fibers with diameters of 20 to 300 µm at rates that are 10 to 100 times higher than chip-based droplet microfluidics. Deposition of the partly-solidified droplets onto a solid substrate enables one-step manufacture of three-dimensional (3D) multiscale materials. For example, we demonstrate an artificial pancreatic tissue in which each solidified droplet forms a controlled micro-environment for MIN6-cells that maintain their capacity to proliferate and produce insulin.
In-air microfluidics, as we name this approach, enables fabrication of controlled multi-material particles and capsules at industrially relevant rates (up to 3 ml/minute/nozzle). I will present recent results including the fabrication of multifunctional Janus particles for enzymatic cascade reactions, and sketch how In-air microfluidics may contribute to one-step fabrication of multi-scale materials with controlled mechanical, biological, or thermal properties.

Microgels are macromolecular networks swollen by the solvent they are dissolved in. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles or vesicles. When swollen, they are soft and have a fuzzy surface with dangling chains and the presence of cross-links provides structural integrity - in contrast to linear and (hyper-) branched polymers. Finally, microgels reveal interface activity without being amphiphilic. Due their properties, microgels can be used to tune the colloid-to-polymer transition.
We will discuss properties of microgels of different architecture (as e.g. ultra-low crosslinked, hollow, multi-shell) both in aqueous solution and at interfaces. These microgels respond to various stimuli as, e.g. temperature and pH, and enable uptake and release applications. The structure of microgels and complexes with guest species are investigated by means of scattering methods, especially exploiting the technique of contrast variation in small angle neutron scattering. The experimental results are be compared to computer simulations. In addition, superresolved fluorescence microscopy is employed.
Microgel adsorb to fluid and rigid interfaces surfaces and their response to external stimuli allows preparing responsive emulsions and coatings which can be used in biocatalysis and as sensors. Interfacial properties are probed via compression isotherms as well as by scanning force and fluorescence microscopy.


Plamper, F. A.; Richtering, W. Functional Microgels and Microgel Systems. Accounts of Chemical Research 2017, 50, 131–140.
Keidel, R.; Ghavami, A.; Lugo, D. M.; Lotze, G.; Virtanen, O.; Beumers, P.; Pedersen, J. S.; Bardow, A.; Winkler, R. G.; Richtering, W. Time-Resolved Structural Evolution During the Collapse of Responsive Hydrogels: the Microgel-to-Particle Transition. Science Advances2018, 4, eaao7086
Sigolaeva, L. V.; Gladyr, S. Y.; Mergel, O.; Gelissen, A. P. H.; Noyong, M.; Simon, U.; Pergushov, D. V.; Kurochkin, I. N.; Plamper, F. A.; Richtering, W. Easy-Preparable Butyrylcholinesterase/Microgel Construct for Facilitated Organophosphate Biosensing. Anal Chem2017, 89(11), 6091–6098.
Gelissen, A. P. H.; Oppermann, A.; Caumanns, T.; Hebbeker, P.; Turnhoff, S. K.; Tiwari, R.; Eisold, S.; Simon, U.; Lu, Y.; Mayer, J.; Richtering, W.; Walther, A.; Wöll, D. 3D Structures of Responsive Nanocompartmentalized Microgels. Nano Lett.2016, 16(11), 7295–7301

We investigate the drainage dynamics of thin liquid foam films containing PNiPAM microgels suspensions with two cross-linking densities (1.5 % or 5% mol BIS) and at two microgel concentrations (0.1 and 1% wt). For this purpose we use a thin film pressure balance apparatus that can apply a controlled and sudden hydrostatic pressure on the film and record the subsequent film thinning as a function of time. Once the film thickness has reached a stationary value, we test the adhesion between the interfaces of the film by reducing the pressure and measuring the angle between the film and the meniscus. This angle increases on reduction of pressure for adhesive films, which resist the separation of their interfaces. Non-adhesive films separate easily, and the meniscus angle stays constant. At low microgel concentration, the more densely cross-linked microgels (5% mol BIS) tend to drain into more adhesive films than the more loosely cross-linked particles (1.5% mol BIS). The adhesion results from particles that bridges the two air-water interfaces constituting the film, i.e. particles being shared between both interfaces of the films. In those cases, the film, which is initially stabilized by a bilayer of microgel particles rearrange to a state where the particles are shared by them. These results are discussed and compared with previous studies at low concentration of microgels, which have shown that emulsions stabilized with densely crosslinked microgels are more adhesive and less resistant to mechanical stresses than those obtained with lower cross-linking densities. In addition micron-scale depleted zones with no microgels are observed in the films stabilized with the 5% mol BIS particles, which eventually lead to the rupture of the film. At 1% wt, the films drain slowly, are not adhesive and have the thickness of a bilayer of microgels while at 0.1% wt, the films have the thickness of a monolayer of microgels, are adhesive and show bridging. From the thin liquid foam film thicknesses we extract a rough estimation of the radii of adsorbed particles in the thick-films before applying the pressure. Our results are consistent with particles being adsorbed in a spread conformation for the 0,1% wt sample and in a compressed conformation for the 1% wt sample. In line with previous studies on emulsions, we conclude that a larger surface coverage may reduce rearrangements, thus preventing bridging.

A distinctive feature of microgel particles is that of being soft colloids with an internal polymeric architecture. At liquid interfaces, in particular, they deform and flatten significantly due to the balance between surface activity and internal elasticity. For this reason, recent experimental studies have shown that they are a valuable choice to stabilize smart emulsions in contrast to common rigid colloids [1, 2]. Despite the clear potential this system could bring, a detailed theoretical understanding of the phenomenon is still lacking. In fact, commonly accepted models in which the internal structure is neglected or diamond-like, where polymer chains have a fixed length, are not able to reproduce this behavior, especially by the fact that the corona would not flatten out. Conversely, we have now provided a realistic model that reproduces in silico the polymeric network of PNIPAM microgels [3], even in the presence of an explicit solvent [4].

By placing the microgel at liquid-liquid interfaces, we can now directly compare the morphology of the microgels resulting from simulations with in-situ cryo-electron microscopy measurements and AFM imaging after Langmuir-Blodgett depositions [5]. The correct modelling of the internal degrees of freedom allows for a qualitative comparison with the experimental outcomes and to report a consistent trend as a function of the fraction of crosslinkers. By varying the parameters of the monomer-solvent potential, we are able to selectively tune the surface tension, as expected for different combinations of solvent at the interface. This opens up the possibility to evaluate single-particle elastic properties and effective interactions among microgels, that can be used to predict their macroscopic behavior. These are also essential to develop predictive power for the use of microgels in a broad range of applications.

[1] W. Richtering, Responsive Emulsions Stabilized by Stimuli-Sensitive Microgels: Emulsions with Special Non-Pickering Properties, Langmuir 28 (2012)
[2] K. Geisel, L. Isa, W. Richtering, Unraveling the 3D Localization and Deformation of Responsive Microgels at Oil/Water Interfaces: A Step Forward in Understanding Soft Emulsion Stabilizers, Langmuir 28 (2012)
[3] N. Gnan, L. Rovigatti, M. Bergman, E. Zaccarelli, In silico synthesis of microgel particles, Macromolecules 50 (2017)
[4] F. Camerin, N. Gnan, L. Rovigatti, E. Zaccarelli, Modelling realistic microgels in an explicit solvent, Scientific Reports 8 (2018)
[5] F. Camerin et al., A realistic model for microgels at interfaces, in preparation

The structural properties of microgels adapt to external stimuli by swelling or shrinking in response to changes in temperature, pH, ionic strength of the solution, or solvent composition [1]. This renders them ideal candidates for a broad range of applications including encapsulation for drug delivery, sensing, and food additives. The possibility to change the microgel composition during the synthesis opens an avenue for novel applications. An example is the synthesis of core−shell microgels, which are typically comprised of polymers with distinctly different properties, e.g., their hydrophobicity, which allows for the control of the microgel morphology, and opens an avenue for novel applications [2].

Computer simulations are ideally suited to resolve structural and dynamical properties of microgels on length and time scales often inaccessible by experiments. Here, we present results of hybrid simulations combining the multiparticle collision dynamics method [3] for the fluid with molecular dynamics simulations for the microgel to systematically characterize the morphologies of core−shell microgels [2]. With increasing hydrophobic interaction of the shell polymers, we observe drastic morphological changes of the microgel from a core-shell structure to an inverted microgel, where the core is turned to the outside, or a microgel with a patchy surface of core polymers directly exposed to the environment. A phase diagram is established of the various morphologies.

Moreover, the morphologies of a microgel are analyzed during collapse from a swollen soft, deformable networks with a fuzzy surface to a colloid with homogeneous density and a sharp surface, which is achieved by solvent exchange [1]. Time-resolved small-angle x-ray scattering experiments and computer simulations unambiguously reveal a two-stage process: In a first, very fast process, collapsed clusters form at the periphery, leading to an intermediate, hollowish core-shell structure that slowly transforms to a globule. This structural evolution is independent of the type of stimulus and thus applies to instantaneous transitions as in a temperature jump or to slower stimuli that rely on the uptake of active molecules from and/or exchange with the environment. The fast transitions of size and shape provide unique opportunities for various applications as, for example, in uptake and release, catalysis, or sensing.

[1] R. Keidel, A. Ghavami, D. M. Lugo, G. Lotze, O. Virtanen, P. Beumers, J. S. Pedersen, A. Bardow, R. G. Winkler, and W. Richtering, Sci. Adv. 4, eaao7086 (2018)
[2] A. Ghavami and R. G. Winkler, ACS Macro Lett. 6, 721 (2017)
[3] G. Gompper, T. Ihle, D. M. Kroll, and R. G. Winkler, Adv. Polym. Sci. 221, 1 (2009)

For fabrication of stimuli responsive coatings one challenge is to generate stable films which are still mobile and sensitive to outer parameters. One option is to functionalize surfaces by deposition of stimuli sensitive hydrogel microgels. Beside coatings of solid interfaces microgel layers become more and more interesting as stabilizers for foams and emulsions [1,2]. It was found that they spread at the interface which is strongly related to their structure and mechanical properties.
During the last decades polymeric superstructures like particles or coatings made of N-isopropylacrylamide (NIPAM) have attracted much interest. They show thermoresponsive behaviour and can therefore be classified as “smart” materials. By copolymerisation with organic acids such as acrylic acid or allylacetic acid the charge and the hydrophobicity can be varied which leads to temperature shifts of the volume phase transition as well as to changes of the swelling ratio. Our work focuses on the fabrication of stimuli responsive films and on the effect of geometrical confinement on the phase volume transition of these microgel particles. It will be discussed how the structure and the swelling behaviour are related to the mechanics and rheology of the microgel particles [3]. The mechanics is characterized by indentation experiments with an AFM. For rheology studies dynamic AFM measurements are carried out. Therefore the cantilever is excited to vibrations and changes in phase and amplitude give information the storage and loss part of the shear modulus. Due to using a sharp tip the mechanical and rheological properties can be resolved on a length scale of several tens of nanometers. This gives the opportunity to monitor e.g. the degree of homogeneity within single microgels.
Usually one would expect that stronger swelling leads to softer particles and therefore to a lower E-modulus. Recent measurements on co-nonsolvency effects (water/ethanol) showed that the relation is not that simple [4].
Adsorption at the interface affects the volume phase transition due to the compression of the gel structure. In this context we studied the structure and the dynamics of the gels in bulk and at the interface with SANS and NSE in bulk and under grazing incidence (GISANS, GINSE).

[1] Pinaud, F.; Geisel, K.; Masse, P.; Catargi, B.; Isa, L.; Richtering, W.; Ravaine, V.; Schmitt, V. Soft Matter 2014, 10, 593.
[2] Picard, C.; Garrigue, P.; Tatry, M.-C.; Lapeyre, V.; Ravaine, S.; Schmitt, V.; Ravaine, V. Langmuir 2017, 33, 7968.
[3] S. Backes, R. von Klitzing, Polymers, 2018, 10, 978.
[4] S. Backes, P. Krause, W. Tabaka, M. U. Witt, R. von Klitzing, Langmuir 2017, 33, 14269.

By combining microgels with other colloidal particles, for example lattices and emulsions, the beneficial properties of both elements can be obtained in these now composite colloidal materials. In this talk, I will discuss two examples, a promising biomedical material and a modified food product which both employ this principle, and in addition discuss confocal microscopy experiments on model materials to elucidate the relationship between the microscopic mobility and macroscopic properties of such composites. Solid silica particles and collagen microgels have recently been combined to elicit both high elasticity, due to the rigidity of the silica particles, and self-healing, due to the deformable nature of microgels, into a single promising biomedical material for wound healing. Food thickeners are often either highly swollen polymers or in fact colloidal microgels; their addition to edible colloidal emulsions induces perceived macroscopic consumer benefits but can also cause undesirable consequences such as syneresis limiting product stability. Linking these advantageous or adverse macroscopic phenomena to microscopic interparticle interactions requires a detailed analysis of the heterogeneous particle dynamics within the colloidal composite materials by developing model confocal microscopy experiments and macroscopic rheology combined with computer simulations.

Glasses formed from nano- and microparticles form a fascinating testing ground to explore and understand the origins of vitrification. For atomic and molecular glasses, a wide range of fragilities have been observed; in colloidal systems, these effects can be emulated by adjusting the particle softness. The colloidal glass transition can range from a superexponential, fragile increase in viscosity with increasing density for hard spheres to a strong, Arrhenius-like transition for compressible particles. However, the microscopic origin of fragility and strength remains elusive, both in the colloidal and in the atomic domains. Here, we propose a simple model that explains fragility changes in colloidal glasses by describing the volume regulation of compressible colloids in order to maintain osmotic equilibrium. Our simple model provides a microscopic explanation for fragility, and we show that it can describe experimental data for a variety of soft colloidal systems, ranging from microgels to star polymers and proteins. Our results highlight that the elastic energy per particle acts as an effective fragility order parameter, leading to a universal description of the colloidal glass transition.

Raman spectra have been referred to as molecular fingerprints and used for molecular identification. In particular, Raman intensity can be dramatically enhanced by metal nanostructures that intensify electric field on the surface through surface plasmon resonance; this enhancement is known as surface-enhanced Raman scattering (SERS). Nevertheless, it is still difficult to use SERS in the direct molecular analysis of biological fluids because adhesive macromolecules passivate the metal nanostructures and prevent the access of small target molecules. Therefore, it is a prerequisite to purify and concentrate target molecules from the samples before molecular analysis.
To preclude the necessity of sample pretreatment and provide ease of analysis, we have designed microgels containing metal nanoparticles using water-in-oil emulsion drops as a template. As the hydrogel is a water-swollen network of hydrophilic polymers, the microgel matrix is highly permeable to small molecules while being impermeable to the macromolecules larger than its limiting mesh size. Moreover, as the average mesh size is controllable by adjusting the molecular weight of gel-forming monomers, the cut-off threshold of permeation is tunable. Taking advantages of the hydrogel for SERS analysis, we microfluidically prepare monodisperse microgels containing metal nanoparticles. The microgels exclude adhesive macromolecules while allowing infusion of small target molecules to the surface of metal nanoparticles. Therefore, Raman intensity for the target molecules can be selectively enhanced with a negligible interruption from the large molecules. Moreover, the Raman intensity can be further increased by forming agglomerates of metal nanoparticles in the microgels. The agglomerates have a high density of metal nanogaps in the interstitial regions, which provide strong localization of electric field and high SERS activity. In addition, charged microgels can concentrate oppositely-charged target molecules through electrostatic attraction, which also increases the Raman intensity.
Using the SERS-active microgels, we have demonstrated the direct detection of small molecules dissolved in complex biological fluids. For example, small molecules dissolved in a milk can be detected without a loss of measurement precision. In addition, Aspirin spiked in whole blood can be also detected without pretreatment. More interestingly, it is possible to directly detect insecticide dissolved in the egg. Recently, fipronil sulfone, a metabolite of toxic insecticide had been detected in eggs and the insecticide-contaminated eggs spread in Europe, South Korea, and other countries. The insecticide causes a headache and organ damage, threatening the health. With the positively-charged microgels, we successfully detect fipronil sulfone dissolved in the egg as the microgel effectively excludes adhesive proteins in eggs such as ovalbumin, ovoglobulin, and ovomucoid. We believe this SERS-active microgel platform can be used for fast on-site molecular analysis in various applications.

The treatment outcomes for malignant peritoneal mesothelioma are poor and associated with high co-morbidities due to suboptimal drug delivery. Thus, there is an unmet need for new approaches that concentrate drug at the tumor for a prolonged period of time yielding improved antitumor efficacy and improved metrics of treatment success. A paclitaxel-loaded pH-responsive expansile nanoparticle (PTX-eNP) system is described that addresses these unique challenges to improve the outcomes for peritoneal mesothelioma. The synthesis of the eNPs is described first followed by several particle characterization techniques, including qNano, DLS, SEM, and TEM that measure particle size as a function of pH and swelling time. We next quantify the unique ability of drug-loaded eNPs to act as drug depots for paclitaxel within the cell as well as eNPs to enter the cell via macropinocytosis. Moreover, the rate of tumoral uptake of eNPs is an order of magnitude faster than rate of uptake in healthy cells; and, subsequent disruption of autophagosomal trafficking leads to prolonged intracellular retention of eNPs. Thus, following intraperitoneal administration, eNPs rapidly and specifically localize to tumors within 4 hr of injection with persistent intratumoral retention for >14 days. Second, the high tumor-specificity of PTX-eNPs leads to delivery of 100-1000 times higher concentrations of drug in tumors compared to PTX alone and this is maintained for at least seven days following administration. As a result, overall survival of animals with established mesothelioma more than doubled when animals were treated with multiple doses of PTX-eNPs compared to an equivalent dose of PTX (standard of care) or non-responsive PTX-loaded nanoparticles.

Deoxyribonucleic acid (DNA) offers many advantages in the design of responsive hydrogels due to the large extent of control over its higher order structure and interactions with other molecules. In the present work, we focus on microgels based on a polyacrylamide network with additional crosslinks made up by partially hybridized DNA strands. The hydrogels respond by swelling to the presence of a target DNA strand, due to a DNA competitive displacement reaction where the target strand opens the dsDNA crosslink in the gel. It is often advantageous to reduce the size of the hydrogel, as this decreases the response time, and for molecule sensitive hydrogels also reduces the amount of target molecule necessary. A small size brings about the challenge of accurately monitoring the hydrogel’s response with meaningful resolution. Our group has been using a fiber optic based interferometric readout with a resolution of 2 nm, that can measure the size of quasi hemispherical microgels with characteristic dimensions of about 50 micrometers. The observed swelling is the result of the cascading processes migration, binding of the target ssDNA and the crosslink opening leading to local change in crosslink density. Their interplay influences the rate of hydrogel’s overall swelling. Our focus was on the effect of the toehold length on the hydrogel swelling, target migration and binding. Using a combination of interferometry and confocal laser scanning microscopy (CLSM), we have shown that faster binding (longer toehold) increases the binding rate but slows down the migration of the target, thus resulting in only moderate increase in the swelling rate compared to the shorter toehold.

We present a scale-overarching approach to construct cross-linked microgel particles [1] and to understand the experimentally observed behavior of concntrated solutions of the same under the influence of an external, alternating electric field [2]. We consider self-avoiding polymer chains modelled using bead-spring model and crosslink them inside a spherical cavity using a spatial proximity criterion. Applying this approach, we construct microgels with different crosslinking degrees and provide a comprehensive analysis of the polymer network to understand the internal structure. We also study the behaviour of these microgels in different solvents: in good and poor solvents, and in a solvent, where electrostatic interaction is taken into account by the Yukawa potential, obtaining swelling behavior similar to that observed in, e.g., thermoresponsive PNIPAM particles. We further consider the behavior of concentrated microgel suspensions under the influence of linearly polarized, alternating electric fields of varying sterngth and frequency, studied by a combination of complementary experimental and theoretical techniques [2]. We develop a coarse-grained description, in which effective interactions among the charged microgels are induced by both equilibrium ionic distributions and their time-averaged hydrodynamic responses to the applied field. These contributions are modeled by the buildup of an effective dipole moment at the microgels backbones, which is partially screened by their ionic double layer. We show that this description is able to capture the structural properties of this system and the buildup of self-assembled chains, allowing for very good agreement with the experimental results.

[1] E. S. Minina, S. S. Kantorovich and C. N. Likos, submitted (2018).

[2] T. Colla, P. S. Mohanti, S. Nöjd, E. Bialik, A. Riede, P. Schurtenberger, and C. N. Likos, ACS Nano 12, 4321 (2018).

Using dissipative particle dynamics (DPD), we develop a mesoscale model of microgel particle and use it to probe the underlying mechanics and kinetics of microgels throughout the volume-phase transition. By applying non-linear fitting to Flory-Rehner model, we establish a direct connection between our simulations and experiments. We find good agreement of microgel swelling kinetics with Tanaka’s theory. The simulations reveal that gel swelling proceeds significantly slower than gel deswelling. Deswelling is characterized by high network inhomogeneity due to polymer chain bundling, which accelerates the network collapse. We probe the effect of crosslink density on the volume transition and show that its decrease leads to a sharper volume phase transition near the critical point. Finally, we examine the bulk modulus of compressed microgel suspensions at different packing fractions and solvent conditions. We evaluate the degree of particle-particle penetration to establish its relation to the micromechanics of compressed microgel suspensions.

Droplet based microfluidic is a versatile tool to generate confined volumes in form of an emulsion. In contrast to bulk emulsification techniques, microfluidics produces highly uniform droplets, which act as templates for chemical reactions. Microfluidic emulsification has been used in particular to create functional and soft colloidal structures.¹ In the microfluidic drop-maker, these particles are generated consecutively one-after-the-other and the yield over time is low with a single drop-maker.

To overcome this challenge, several approaches were taken by parallelizing in ladder- or tree- type structures using PDMS based systems and soft-lithography, resulting in large footprint devices with large dead volumes. We could show that by producing microfluidic devices by rapid prototyping we can stack drop-makers on top of each other and minimize the device footprint and overall channel length.² Furthermore, such 3D printed microfluidic devices enable new design strategies, which are impossible to obtain using common soft-lithographic approaches. To showcase the freedom of design in 3D prototyped devices we designed a helical channel structure device where all drop-makers disperse into a single continuous channel. This design leads to significantly increased droplet concentrations of the dispersed phase. Compared to individual drop-makers the volume fraction of droplets is up to 300% higher. Double emulsions have also received a high interest in biomedical applications because of their potential to produce monodisperse capsules and facile loading protocols with diameters on the micrometer scale.³ Classical (glass capillary and PDMS) devices lack durability and required surface modification while parallelization strategies remain complicated. We present a new system based on a 3D printed device that utilizes the advantages of rapid prototyping to produce double emulsions in three or more parallelized channel geometries for high throughput reaction containers for polymer and microgel capsules.

References
(1) A. Jans et al., Biomacromolecules, 2017, 18 (5), pp 1460–1465.
(2) T. Femmer, A. Jans et al., ACS Appl. Mater. Interfaces, 2015, 7(23), 12635-12638.
(3) L. B. P. Guerzoni, J. Bohl, A. Jans, J. C. Rose, J. Koehler, A. J. C. Kuehne, L. DeLaporte, Biomater. Sci., 2017, 5, 1549

Macroscopic supramolecular assembly (MSA) investigates multivalent interactions between building blocks larger than 10 micrometer and modified with numerous interactive motifs. MSA has provided a platform for fundamental understanding of various interfacial phenomena such as underwater adhesion, self-healing etc. Because large interactive interfaces of macroscopic building blocks have induced more kinetic possibilities, the assembled structures are normally poorly-ordered, which is not favorable for practical applications. To address the problem of macroscopic self-assembly processes being insensitive to errors and having poorly aligned assemblies, we have proposed two strategies for precise self-assembly by (1) pre-orientation of building blocks or (2) post-method of self-correction. The first strategy applies long ranged forces (capillary forces, magnetic forces) by anisotropic surface modification of the building blocks and may ameliorate the precision to some degree. However, the essence of insensitivity to errors still leads to some defects. Therefore, we used the second strategy of self-correction to realize precise MSA: through suitable control over the disassembly kinetics, the poorly and precisely aligned structures could be self-identified and selectively disassembled the poorly aligned structures, which led to re-assembly of the building blocks; after cyclic iteration of disassembly and re-assembly, we could achieve precise MSA of all interactive pairs.
References
[1] M. Cheng, F. Shi et al. Angew. Chem. Int. Ed. 2018, 57, 14106.
[2] G. Ju, M. Cheng et al. Angew. Chem. Int. Ed. 2018, 57, 8963.
[3] G. Ju, M. Cheng et al. Adv. Mater. 2017, 29, 1702444.
[4] M. Cheng, F. Shi et al. Adv. Funct. Mater. 2015, 25, 6851.
[5] M. Xiao, F. Shi, et al. Angew. Chem. Int. Ed. 2015, 54, 8952.
[6] M. Cheng, F. Shi, et al. Adv. Mater. 2014, 26, 3009.

Over the last years, anisotropic microgels became a spotlight subject in the research fields of material design and tissue engineering. They have the ability to function as crucial microscopic building blocks for macroscopic injectable constructs¹. Due to their geometry, magneto-responsive, rod-shaped microgels can align into 3D anisotropic hybrid hydrogels in a low-invasive manner, resulting in oriented cell growth and extension of functional nerves^2-3. Ideally, monodisperse microgels with a large range in stiffness, dimensions, aspect ratio’s, and (bio)chemistries should be produced in a high-throughput, continuous manner. However, currently, none of the existing methods fulfills all these requirements.
To overcome these limitations, we developed a microfluidic gelation system operable in both droplet and in-jet gelation modes to produce anisometric rod-shaped microgels with adjustable size and mechanical properties. The microgels are crosslinked via a light trigger, employing a pulse with modulation (PWM) light controller with variable pulse length, frequency, and output intensity of any light source coupled to the PWM. The systems presented here are equipped with a UV-LED (365 nm) for droplet gelation or a laser (405 nm) in the case of in-jet gelation. As precursor solution, poly (ethylene oxide)-diacrylate (PEG-DA), supplemented with the water soluble photoinitiator lithium phenyl-2,4,6-trimethylbenzoyl-phosphinate (LAP), is used. Characteristically, droplet gelation is limited to the fabrication of microgels with a diameter equal to the channel diameter of the microfluidic chip; the length and thus aspect ratio is varied by the ratio of the flow rates of the inner and outer phases. Here, microgels with various polymer concentrations, ranging between 50 and 9 wt% PEG-DA, are produced, leading to elastic moduli between 300 kPa and 3 kPa, respectively. Via a single pulse irradiation method, we elucidated that the required gelation time increased from 0.8 s to 70 s, respectively.
As ultra-fast gelation is possible after droplet formation, the system was further developed to in-jet gelation in order to achieve rod-shaped microgels with a diameter smaller than the channel diameter of the chip. Stable microfluidic jets are realized with diameters 10 times smaller as the channel diameter by adjusting the flow ratio of the inner and outer phases. A continuous laser signal results in continuous jet gelation, leading to hydrogel fibers with a diameter equal to the jet diameter. To produce short, anisometric microgels, the 405 nm laser is operated at different pulse modes to crosslink alternating compartments of the jets. The non-crosslinked compartments dissolve after leaving the chip, while the crosslinked parts maintain their shape and swell, depending on the polymer concentration. For a 80 µm channel diameter, ~ 500 microgels/s are produced with a rod diameter of ~ 8 µm, while the length is adjusted between 2200 and 64 µm by varying the pulse length. In order to achieve oscillated gelation in the jet without losing jet stability a non-reactive, a linear filler (400 kDA PEG-OH) is mixed with the PEG-DA.
In summary, as promising alternative to established methods, a modular light controller is combined with either UV-LED droplet gelation or laser-initiated in-jet gelation to realize continuous fabrication of rod-shaped microgels with adjustable diameters, aspect ratios and stiffness. The mechanical properties, swelling behavior, can microgel dimensions are adjusted by changing the prepolymer concentration, flow ratio, channel size, and pulse length. This method offers the opportunity to fabricate elevated amounts of tailored anisometric microgels from different prepolymer systems and crosslinking chemistries, to be used in a wide variety of applications.

References
1. Krüger, A.J.D et al, Chem Comm 2018, 54 (50), 6943-6946.
2. Rose, J. C et al, Nano Lett 2017, 17 (6), 3782-3791.
3. Rose, J. C et al, Biomater 2018, 163, 128-141.

Recently, it was proposed to use soft, adaptive polymer microgels for the emulsion stabilization. As compared to the rigid colloids, polymer microgels possess the ability to swell/collapse and the penetrability for low-molecular-weight substances. Being responsive to pH, temperature, and solution ionic strength, these particles can serve as a tool controlling droplet size and emulsion stability. Alternation of external condition affecting microgel charge and degree of swelling may result in breaking of emulsions on demand. The sensitivity to external stimuli as well as microgel permeability makes the microgel-stabilized emulsions promising for many applications including biocatalysis. In this study we demonstrate a number of new effects. The microgels can serve as compatibilizers of immiscible molecules.^1,2 In particular, we demonstrate that two initially immiscible liquids, A and B (oil and water), can partially or fully be mixed within the microgel adsorbed at their interface. If the incompatibility of the liquids is relatively low, they form a homogeneous mixture within the whole microgel particle being segregated outside. As the incompatibility grows, separation into two (micro)phases within the microgel occurs. We demonstrate this effect for homopolymer^1,2 and amphiphilic³ (AB-copolymer) microgels.
Adsorption of the microgels on a solid surface leads to their flattening: the shape of the microgel is determined by a balance between the gain in the energy of adsorbed monomer units and the penalty in the elastic free energy of the subchains subjected to lateral stretching. Despite a strong interaction with the surface, the adsorbed microgels are able to swell and collapse in response to environmental changes (pH and temperature) varying the size on the surface. We have demonstrated a peculiar behavior of the microgels adsorbed on a patterned surface (planar surface with cylindrical holes or pores).⁴ It turns out that a microgel can enter and exit a narrow cylindrical pore under external stimuli leading to collapse and swelling of the microgel. Attractive interactions between the microgel and the pore surface stimulate the microgel entering upon its collapse. The entering is driven by a gain in the surface energy: the area of the microgel-pore contacts is maximized within the pore. Swelling of the microgel within the pore of a finite size is thermodynamically favorable if the pore thickness exceeds a certain threshold value. Otherwise, the swelling leads to the microgel exit. The physical reason for this is a gain in the elastic free energy of the subchains which are less stretched outside the pore. We systematically study swelling and collapse of the microgel within the pore. Both longitudinal size and radial concentration profiles are calculated for different strength of interactions of the beads with each other and the pore surface. We predict an intra-microgel “phase” coexistence leading to the formation of a dense adsorbed layer near the pore surface and highly swollen central part of the microgel. Furthermore, the permeation of nanoparticles, whose size is smaller than the mesh-size of the microgels was simulated under different swelling and adsorption degrees. It is demonstrated that the microgel can slow down and completely stop the permeation of nanoparticles through the pore.

Acknowledgement. Financial support of the Russian Science Foundation, project # 15-13-00124 is gratefully acknowledged.

References:


¹A. M. Rumyantsev, R. A. Gumerov, I. I. Potemkin, Soft Matter 2016, 12, 6799—6811.
²R. A. Gumerov, A. M. Rumyantsev, A. A. Rudov, A. Pich, W. Richtering, M. Moeller, I. I. Potemkin, ACS Macro Letters 2016, 5, 612-616.
³R. A. Gumerov, S. A. Filippov, W. Richtering, A. Pich, I. I. Potemkin, in preparation.
⁴I. V. Portnov, M. Moeller, W. Richtering, I. I. Potemkin, Macromolecules 2018, 51, 8147–8155.

Microgels are soft and deformable colloidal particles which can be swollen by a solvent and adsorb at liquid interfaces. The well-known poly(N-isopropylacrylamide) (pNIPAM) microgels are thermo-sensitive and exhibit a volume contraction when the temperature is raised above the volume phase transition temperature (VPTT). These particles have shown high potential as Pickering emulsions stabilizers: emulsions could be stable at temperatures below the VPTT and be destabilized on-demand above it [1]. Understanding microgel adsorption at model interfaces gives better insight into the link between microgels conformation, mechanical properties of the interface and properties of the resulting emulsions [2, 3].
In this work, we highlight the role of structural parameters such as the cross-linking density i.e. the microgel deformability, their size or the presence of charges through the use of pH sensitive groups or in-situ (pH, electrolyte concentrations). For that, a systematic approach is adopted using two complementary methods: the spontaneous adsorption and the forced compression. Both equilibrium and kinetics states are studied: by the pendant drop method, we examine the microgel interfacial properties, measuring the dynamic surface tension, pressure and the dilatational visco-elasticity [4]. By the Langmuir film technique, the conformation and packing of microgels as a function of surface pressure could be discussed [3].
Moreover, microgels deformation and adsorption at the liquid interface have consequences on emulsion macroscopic properties (stability, flocculation, etc.). Despite the complexity of comparison, spontaneous adsorption of microgels at model interfaces can be correlated with the microgels conformation in emulsions or foam films in which microgels are locked into various conformations and with the emulsions properties. Taking into account the kinetics of adsorption at the interface and processing conditions, we could control emulsification pathways from high energy methods to microfluidics.
Finally, on the basis of this knowledge, we show how the concept can be extended to new designed microgels that are responsive to other (bio)-stimuli such as glucose [5].

[1] Destribats M., Lapeyre V., Wolfs M., Sellier E., Leal-Calderon F., Ravaine V., Schmitt V., Soft Matter, 2011, 7, 7689.
[2] Pinaud F., Geisel K., Massé P., Catargi B., Isa L., Richtering W., Ravaine V., Schmitt V., Soft Matter, 2014, 10, 6963.
[3] Picard C., Garrigue P., Tatry M.-C., Lapeyre V., Ravaine S., Schmitt V., Ravaine V., Langmuir, 2017, 33, 7968–7981.
[4] Tatry et al., in preparation.
[5] Tatry et al., in preparation.

Macroscopic supramolecular assembly (MSA) is a recent progress in supramolecular chemistry regarding assembly events between building blocks larger than ten micrometer. Researches on this topic have provided both a deep fundamental understanding of interface-interface molecular recognition in a multivalent manner and a novel methodology for the fabrication of supramolecular materials. To address the problems of “what kind of building blocks could achieve macroscopic assembly?”, we have established a general design rule of MSA that assembly probability decreases with the increasing elastic modulus of building blocks. Moreover, we have proposed the concept of “flexible spacing coating” to achieve MSA of rigid materials. To promote the practical applications of MSA, we have fabricated biocompatible 3D structures with targeted chemical modification, which provide a novel strategy to address the current challenges in fabricating complex 3D tissue scaffolds with localized protein for further cell differentiation.

Liquid crystalline elastomers (LCEs) represent a realizable physical system that can exhibit a large, non-linear response to an environmental stimulus. By adding light-sensitive moieties to the mesogens responsible for liquid crystalline order, one can create elastomers that will change shape in response to ultraviolet light. This provides a basis for a “write once, read many times” (WORM) memory. Information is encoded in an array of LCE microposts through a magnetic field during cross-linking, and then read out by introducing a light source. The system will return to its initial state upon removal of the stimulus, allowing the reading process to be repeated without altering the system. We developed a finite element simulation code to study components of such a system. Using our simulation method, we can predict the micropost deflection as a function of the preset nematic director and the incident angle of the light. We make comparisons to available experimental results and describe new findings that reveal how light can be used regulate the structure of an array of multiple, interacting LCE microposts. These studies point to new ways of utilizing the LCE arrays for technological applications.

2D binary colloidal alloys are useful structures as colloidal models and for nanofabrication applications. Up to now, only close-packed binary colloidal assemblies on a substrate have been reported. We instead report a versatile sequential deposition method using differently sized PNIPAM based microgels. We are able to produce complex assemblies with decoupled density gradients for the two particles and with fine control over the interparticle distance. The delicate interplay between capillary attraction and steric repulsion at an oil-water interface allows obtaining a broad range of crystalline microstructures. After deposition on a silicon wafer, such assemblies can be further used as masks for wet etching to obtain Vertically Aligned Nanowires (VA-NWs) with applications in photonics, cell transfection or superhydrophobic surfaces. We synthesized microgels and used a Langmuir trough to deposit the microgels from the water/hexane interface onto silicon substrates. In order to get the binary assemblies, sequential depositions of the microgels of different sizes were performed. This protocol allows the independent control of the interparticle distance for each particle type, e.g. achieving a gradient of interparticle distance for the small microgels while the big microgels are kept at a fixed interparticle distance. Moreover, through the fine-tuning on the microgel architecture and of the deposition parameters, square lattices can be obtained thanks to a fine balance between capillary quadrupolar attraction and steric repulsion at the water/hexane interface. AFM images were taken to characterize the transferred assemblies and HF wet etching enabled producing VA-NWs using the binary colloidal assembly as masks for metal-assisted chemical etching. Such VA-NWs have interesting optical properties, ranging from iridescence to structured colors and wavelength modulation, depending on the geometry of the VA-NWs, which we characterized with reflectivity measurements and compared to simulations.

The programmable spatiotemporal release of molecular outputs is a prerequisite for creating new classes of stimuli responsive biomaterials capable of executing sensing and computational programs. Here we demonstrate the ability to fabricate heterogeneous micromaterials that implement sequentially activated molecular cascades with programmed timescales of activation and release from a hydrogel. These materials use DNA-based strand-displacement to direct the sequential release of short oligonucleotides from spatial domains. Maskless photolithography enables the spatial sequestration of acrylate-modified short oligonucleotides for local hybridization reactions within hydrogels at sizes of tens of microns. To control temporal release, we used a toehold mediated DNA strand displacement reaction cascade that allows for the controlled release of oligonucleotides at 8-hour intervals. This system could be used to direct chemo-mechanical actuation within soft-robots or as a local chemical clock to sequentially coordinate movement. The programmed release of DNA oligonucleotides from hydrogel substrates enables the scalable development of DNA-based reaction-diffusion systems that regulate the availability of oligonucleotides at different points in space and time.

Microgels are well-known swellable colloidal gel particles that have been studied for more than 80 years. They have excellent colloidal stability in their swollen (microgel) state due to their inherently low effective Hamaker constant and this is augmented by steric and electrostatic repulsion. Microgels also have excellent synthetic versatility which enables water- or organic-swellable microgels to be synthesised in a scalable manner (to the multi-tonne scale). In less than a decade the power conversion efficiency of solution processable perovskite solar cells (PSCs) has, remarkably, increased from a few percent to greater than 23%. This incredible rate of efficiency increase is unprecedented in the history of solar cell research. PSCs are remarkably tolerant to additives and are scalable to, potentially, multi-megawatt deployment scale at low cost. In this discussion an overview of our recent work using microgels as additives to improve the performance of PSCs is discussed. Polystyrene microgels enable lower concentrations of expensive conjugated polymers to be used for the hole-transport matrix part of PSCs and also improve the stability[1]. Poly(N-vinylformamide) microgels enable micropatterning of the perovskite capping layer of PSCs to produce an unexpected disordered inverse opal morphology which improved the efficiency compared to control devices[2]. Recent results obtained using microgels in other parts of the solar cells to enhance PSC performance will also be presented. Because microgels and PSC technologies can be combined in a variety of ways to give performance benefits and both are potentially low cost this hybridisation of old and new technologies has a bright future.

[1] Chen et al, Nanoscale, 9, 10126, 2017.
[2] Dokhan et al, Phys. Chem. Chem. Phys. 2018, DOI: 10.1039/C8CP05148H

Stable, inexpensive and high-performance CO2 separation membranes are of significant interest as a key materials to sequester CO2 from exhaust gases of fire plants. In order to achieve the membranes procedure to create defectless CO2 permeable membranes which thickness is smaller than micrometer scale have to be developped. We have reported that temperature responsive hydrogel-particles containing amine groups can reversiblly absorb large aount of CO2 in response to the temperature dependent phase transition of hydrogels (JACS, 134, 18177, 2012. Angew. Chem. 53, 2654, 2014). In this presentation, we report that ultrathin hydrogel membranes of thickness below 100 nm can easily be prepared by deposition of the microgel particles on the top of porous films. The membranes showed CO2 selective permeability against nonacidic gases such as N2.

In this work, we discuss different characterization and modeling techniques employed to develop a predictive numerical model for swelling of micro-sized superabsorbent polymer gel particles. We use custom superabsorbent polymer, poly N,N-butylphenylacrylamide (NBPA) to develop a smart, adaptive hazmat suit for protection against target hazardous chemical aerosols.¹Upon impact with the chemical droplet, the polymer absorbs and swells restricting the permeation of the chemical. To achieve this, we integrate micro-beads of this polymer with a typical hazmat suit fabric with comparable pore size to achieve faster swelling response. We develop a numerical model to capture the droplet induced swelling kinematics of these microbeads to perform design optimization of the composite fabric.²To develop this high-fidelity predictive model, we characterize the polymer followed by mathematical implementation of its behavior under external chemical stimuli. Specifically, we measure crystallinity of the polymer microstructure, viscoelastic properties-G’, G”and relaxation time constant τand permeation properties of different target chemical simulants. We note that the glassy, amorphous dry polymer upon absorbing the solvent, transitions into soft, rubbery state showing significant drop in glass transition temperature as it swells. We quantify this transition by quantifying the motion of solvent front in polymer matrix with time via optical imaging and Magnetic Resonance Imaging (MRI). Next, to mechanically characterize the polymer, we perform indentation test where a swollen sample of NBPA is compressed locally using spherical indenter probe. Based on the indentation tests we calculate shear modulus, relaxation time constant, mesh size and diffusion coefficient using non-Hertzian contact mechanics theory.³In addition, we also verify the viscoelastic properties of gel by performing small amplitude oscillatory shear on commercial viscometer. We finally insert these properties into a finite element model based on poroviscoelasticity theory. We approximate case II like fluid permeation through glassy, amorphous polymer as the rate limiting process. The fluid permeation is coupled with a viscoelastic constitutive recipe⁴for the rubbery region to estimate the transient deformation of the micro-gel. We finally verify and benchmark the model against the experimental data and use it to predict the swelling behavior of micro sized polymer beads in the composite fabric.
References:
1. Manning, K. C., Phadnis, A., Simonet, D., Burgin, T. P. & Rykaczewski, K. Development of a Nonelectrolytic Selectively Superabsorbent Polymer. Ind. Eng. Chem. Res.acs.iecr.8b02710 (2018).
2. Phadnis, A., Manning, K. C., Sanders, I., Burgin, T. P. & Rykaczewski, K. Droplet-train induced spatiotemporal swelling regimes in elastomers. Soft Matter14,5869–5877 (2018).
3. Hu, Y., Chan, E. P., Vlassak, J. J. & Suo, Z. Poroelastic relaxation indentation of thin layers of gels. J. Appl. Phys.110,(2011).
4. Chester, S. A. A constitutive model for coupled fluid permeation and large viscoelastic deformation in polymeric gels. Soft Matter8,8223–8233 (2012).

Cross-flow inside-out filtration is a pressure-driven separation and enrichment process of microgel and colloidal suspensions where the feed suspension is continuously pumped tangentially through a membrane pipe. The transmembrane pressure (TMP) causes permeate (solvent) to flow out through the membrane, while the microgel particles are retained inside the pipe. Consequently, a particle-enriched diffuse layer is formed near the membrane wall which reduces the filtration efficiency. This so-called concentration-polarization (CP) layer is due to the balance of flow advection of particles towards and gradient diffusion away from the membrane. The resulting CP layer osmotic pressure operates against the TMP and counteracts the permeate flow.

To model the CP layer with its physical effects in the ultrafiltration regime where Brownian motion dominates flow convection, it is crucial to determine the gradient diffusion coefficient, suspension viscosity, and osmotic pressure of concentrated microgel suspensions. In this study, we calculate these properties for non-ionic microgels treated as solvent-permeable and softly interacting colloidal particles. The suspension flow is described by the effective Stokes equation, and the permeate flux by Darcy’s law. Pore-blocking of the membrane is assumed to occur uniformly throughout its surface, with the sole result of increasing the overall hydraulic membrane resistance. We solve the coupled advection-diffusion equations quantifying of our ultrafiltration model by using a finite element method (FEM). The FEM results are compared with the standard similarity solution in boundary layer approximation [1]. As a reference, we also compare with simplifying analytic solutions for pure solvent flow, and for suspension flow where the microgel transport properties are taken as concentration independent [2]. Furthermore, we generalize the FEM analysis to a segmented membrane with impermeable rings to observe the weakening of the CP layer [3].

References
[1] R. Roa, E. Zholkovskiy, and G. Nägele, “Ultrafiltration modeling of non-ionic microgels,” Soft Matter 11, 4106–4122 (2015).
[2] G. W. Park and G. Nägele, work in progress.
[3] G. W. Park, M. Brito, E. Zholkovskiy, and G. Nägele, work in progress.