Silane surface modification is an important tool applied to improve and/or change the properties of various substrates which find applications in various industries including medicine, transportation, construction, energy, biotechnology etc. Maintaining the integrity of the treated surface is very important to ensure the longevity and performance of the coating; however this is affected strongly by the wetting interactions at the interface.
Dipodal silanes possess two silicon atoms that can covalently bond to a surface (whereas conventional silane coupling agents only offer one such silicon atom for surface attachment). Dipodal silanes offer a distinctive advantage over conventional silanes in terms of maintaining the integrity of surface coatings, adhesive primers and composites in aqueous environments through improved ‘wet&’ durability. Such improved durability is associated with an increased crosslink density of the interphase formed across the surface, and its resistance to hydrolysis. Calculations based on the equilibrium constant for dissociation of the Si-O-Si bonds to silanols suggest that dipodal silanes may have 100,000 times greater resistance to hydrolysis than conventional silanes. Hydrolytic stability data for dipodal silanes with both hydrophobic alkyl functionality and hydrophilic PEG functionality will be compared to that obtained for conventional silanes, titanates and phosphonates with similar functionality. New dipodal silanes with “pendant” rather than “bridged” functionality will be introduced together with their stability data in aqueous environments. Significantly, in neutral and mildly acidic environments, dipodal silanes clearly demonstrate improved resistance to hydrolysis compared to conventional silanes, titanates and phosphonates, with several hydrophobic “pendant” dipodal silanes providing the best performance.

Electrochemical oxidation lithography is a scanning force lithography mode which demonstrated to be a useful tool to fabricate nanostructures with nanometer resolution. Applications of this technique include the fabrication of split-ring resonator arrays for plasmonic applications, as well as the fabrication of nanoelectronic device features, i.e., nanometric gap structures,[1] carbon nanotube assemblies,[2] single particle placement among others. The functionalization steps involved in the fabrication of such structures strongly depend on the wetting properties of the templates as well as on the utilization of tailor-made chemical interactions. In particular for optical applications there is a strong desire to utilize substrates others than silicon wafers for the fabrication of nanostructures. Up to now these possibilities were not at hand. Here we report on the extension of the range of suitable substrates for the electrochemical oxidation and functionalization process. Electrochemical oxidation lithography is based on the local electrochemical oxidation of a self-assembled monolayer consisting of n-octadecyltrichlorosilane (OTS) by means of a negatively biased conductive SFM tip. The self-assembly of OTS is however not limited only to the functionalization of silicon but can be applied also to a number of other technologically relevant materials, i.e., indium tin oxide, titanium dioxide and metal oxides like aluminum oxide. We investigated the peculiarities of the self-assembly of OTS on these substrates and studied in detail the characteristics of the electro-oxidation process. Essential parameters for the self-assembly as well as for the oxidation process are presented in a comparative study. In depth Scanning Kelvin Probe investigations were performed with the aim to elucidate the chemical nature of the formed species in the oxidation process. As a main result the strong shielding performance of the OTS monolayer could be demonstrated which effectively screens the surface potential of the underlying substrates; a result which has significant impact in the design and manipulation of electronic properties of nanostructured surfaces.[3]
These results open on the one hand the spectrum of applicable substrates for the fabrication of functional nanostructures and pave on the other hand also the way towards new applications since for the first time also transparent substrates could be used to combine the advantages of electro-oxidative SFM lithography with applications which essentially require transparent substrate materials.
[1] T.S. Druzhinina, S. Hoeppener, U.S. Schubert, Small 2012, 8, 852-857.
[2] T.S. Druzinina, C. Höppener, S. Hoeppener, U.S. Schubert, Langmuir 2013, doi.org/10.1021/la4000878.
[3] D. Meroni, S. Ardizzone, U.S. Schubert, S. Hoeppener, Adv. Funct. Mater. 2012, 20, 4376-4382.

The iCVD method is a powerful technology for engineering surfaces which utilizes the richness organic chemistry. In many cases, covalent bonding (grafting), can be achieved at the interface between the substrate and the film. The resultant conformal polymeric coatings, deposited without solvents, and at low substrate temperature (~room temperature), represent an enabling technology in many different fields of application.
For iCVD, one or more monomers species are feed simultaneously with an initiator into a vacuum chamber (~0.1 to 1 torr). The initiator, typically tert-butyl peroxide, thermally cracks over resistively heated filament wires (~250 to 300°C). The radicals produced initiated chain growth polymerization of the monomers on cooled surface (~25°C).
Surface Energy Control:
For poly(perfluorodecyl acrylate), p(PFDA), the degree of crystallinity and the orientation of the crystallites, parallel or perpendicular to the substrate, and the surface morphology, can be tuned using the iCVD process parameters (initiator to monomer flow rate ratio, filament temperature, and substrate temperature). Grafting is achieved by covalently bond of trichlorovinylsilane vapor to a silicon wafer. Polymerization through the surface vinyl groups creates anchoring point for synthesizing grafted polymer chains, resulting in durable superhydrophobic and oleophobic surfaces.
Surface Texturing Control:
Trichlorovinylsilane has been used as an adhesion promoter between polydimethyldisiloxane (PDMS) and heavily crosslinked, hard, poly(ethylene glycol diacrylate) p(EGDA) films deposited by iCVD. This system is buckled to construct highly ordered herringbone patterns through a new sequential wrinkling strategy.
Prevention of Biofouling on Reverse Osmosis Membranes:
The amine groups present in RO membranes with maleic anhydride (MA) at the interface between a zwitterionic polymer and substrate to form covalent bonds. Without MA grafting, zwitterionic films delaminated from the membrane when placed in water. The ultrathin (~20 nm) iCVD films showed anti-fouling properties against proteins and microbes without significantly impairing water flux or salt rejection performance.
Responsive iCVD films for biotechnology:
Longitudinal tissue constructs were formed inside the microgrooves of PDMS conformally coated with iCVD poly(N-isopropylacrylamide), p(NIPAAm). Its temperature response produces both swelling and a hydrophilicity change, allowing retrieval of the tissue construct from the microgroove, useful for 3D cell culture applications in tissue engineering and drug discovery

In nature, an electrical potential generated by an ionic gradient across the cell membrane plays a vital role in regulating the behaviors of membrane proteins for functioning. Examples of such proteins include certain G-protein-coupled receptors that undergo a conformational change upon electrical perturbation in the cell membrane and alter their binding affinities towards agonist ligands. Inspired by this notion, we designed a smart surface featuring a voltage-sensing molecular monolayer of an ionic headgroup-appended oligoether. By a conformational change in response to an applied electric field, this monolayer can capture and release a dendritic ionic guest molecule. When the conformational transition is restricted by a congested molecular design, the monolayer is not operable electrically in guest release. This finding not only paves the way for new electroactive materials affording high responsiveness and broad control scopes, but also provides implications for understanding the essential role of membrane potential in the function of membrane proteins.

Superhydrophobic surfaces, such as lotus leaves [1], exhibit extraordinary water repellent and self-cleaning properties. Recently, superhydrophobic surfaces were combined with photocatalytic materials to produce multifunctional surfaces. Reports on the preparation of superhydrophobic photocatalytic surfaces are limited and the techniques are problematic. Either they were created under extreme experimental conditions or the catalyst particles were embedded in the polymer matrix with reduced surface contact area, leading to lower photocatalytic activity. Scaling these techniques to produce large areas would be difficult. In addition, some films were not robust and lost their superhydrophobicity upon light irradiation.

Here we present a novel method for preparing superhydrophobic surfaces composed of photocatalytic particles. No chemical modification of the particle surfaces was required to achieve superhydrophobicity. Superhydrophobic surfaces were fabricated by printing polydimethylsiloxane (PDMS) into arrays of cylindrical cones that measure 400 µm base diameter, 25 µm tip diameter and 1000 µm tall on 500 µm pitch[2]. Catalyst particles of silicon phthalocyanine dispersed in a glass matrix, which we have shown to generate singlet oxygen (1O2) under 669nm light irradiation[3], were adhered onto the printed PDMS posts. Superhydrophobicity can be maintained, even with hydrophilic catalytic particles, due to this significant hierarchical structure. The surface has a water contact angle of 160° and maintains superhydrophobicity in contact with water under visible irradiation from a diode laser.
Another important feature of the high aspect ratio primary roughness is that it enables easy access to the plastron, i.e. the stable layer of air under the solid-liquid interface. Printing the PDMS posts on a porous membrane, and supporting the membrane over a plenum, provides a means to control the gas composition of the plastron and thus study catalysis at the solid-liquid-gas interface. The generation of 1O2 in the plastron region and the trapping of this short-lived reactive species after it travels across gas-liquid interface and is solvated in the supported solution were demonstrated. Because 1O2 generates no waste or byproducts, the fabricated superhydrophobic and photocatalytic composite surface can be used in water purification and disinfection, such as oxidizing toxic organic molecules and deactivating bacteria[4], as well as the synthesis important intermediates. Quantification of 1O2 as a function of plastron gas composition and flow rate was achieved using 9, 10-anthracene dipropionic acid as the trapping agent.
References
1. W. Barthlott, C. Neinhuis. Planta 1997, 202, 1-8.
2. M. Barahman, A. M. Lyons, Langmuir, 2011, 27, pp 9902-9909.
3. D. Bartusik, D. Aebisher, B. Ghafari, A. M. Lyons, A. Greer, Langmuir 2012, 28, 3053.
4. D. Bartusik, D. Aebisher, A. M. Lyons, A. Greer, Environ. Sci. Technol. 2012, 46, 12098.

Hydrophilicity is desirable for membranes in the water sector. Hydrophilic surfaces are less susceptible to fouling. They also improve water flux by facilitating more complete wetting of membrane structures to increase their effective porosity. However, most membranes are made from relatively hydrophobic polymers selected for their superior stability, mechanical integrity, and compatibility with traditional phase inversion fabrication techniques. Methods to hydrophilize hydrophobic membrane surfaces (e.g., plasma treatment, polymer chain grafting, or nanoparticle attachment) often suffer from lack of effectiveness, stability, or scalability. Additionally, they may compromise membrane performance by obstructing membrane pores or damaging structures that provide membrane selectivity.
In light of these challenges, a facile method for fabricating hydrophilic electrospun mats was developed. Stable nanofibers were electrospun from a solution of polysulfone (PSf) and polysulfone-block-poly(ethylene glycol) (PSf-b-PEG) in N,N-dimethylformamide. The ratio of PSf-b-PEG to PSf homopolymer in the electrospinning solution was varied, along with the total polymer concentration, to obtain nanofiber mats of differing composition and comparable structure. The nanofiber mats were then annealed in warm deionized water to drive the hydrophilic poly(ethylene glycol) (PEG) blocks toward the nanofiber surfaces. The water-insoluble PSf block and PSf homopolymer anchor the hydrophilic PEG block to the rest of the fiber, preventing its dissolution into aqueous environments.
As-spun and annealed fibers and fiber mats were characterized for morphological and material properties. Scanning electron microscopy shows that the fibers are cylindrical and mostly smooth with mean fiber diameters of 200 to 300 nm. Water contact angle decreased as annealing time increased, with drops from over 140° to less than 10° occurring with high PSf-b-PEG content and long annealing times. X-ray photoelectron spectroscopy revealed an increase of C-O bonds relative to C-C bonds as contact angle decreased, indicating that the reduction in contact angle was due to enrichment of PEG blocks at the fiber surface. Transmission electron micrographs and elemental mapping of the low contact angle fibers show that the fiber surface retains some PSf; the PEG enrichment that occurs does not form an exclusive PEG layer. Organic carbon concentration in the water used to anneal the fibers was low but increased with annealing time. When tested as a microfiltration membrane under gravity-driven flow, nanofiber mats rejected latex particles while allowing high water flux.
The use of PSf and PSf-b-PEG in this work demonstrates the potential this fabrication method has as a platform for producing high performance membranes. The bulk and surface properties of nanofibers can be tailored through custom pairing of other homopolymers and block copolymers to achieve application-specific membrane design requirements.

Osmosis describes the flow of water across semipermeable membranes powered by the chemical free energy contained in salinity gradients. It is a fundamental transport process for water in all living systems, and its applications are countless. While osmosis can fundamentally be expressed in simple terms via the van&’t Hoff ideal gas formula for the osmotic pressure, it is a complex phenomenon taking its roots in the subtle interactions occurring at the scale of the membrane nanopores.
I will first discuss some molecular views of osmosis, allowing to rationalize the osmotic pressure across nanochannels. I will then show that specific designs of nanofluidic devices allow to build an osmotic diode, leading to water flow rectification. This osmotic diode functionality opens new opportunities for water purification and complex flow control in nanochannels.
I will then report experiments on fluid transport at the nanoscales, in particular across nanopores, nanochannels and nanotubes. I will first demonstrate osmotically driven flow in nanochannels, as well as flow rectification in asymmetric nanochannels in line with the theoretical predictions for the osmotic diode.
Finally I will focus on the study of osmotic transport inside a single Boron-Nitribe nanotube. This trans-membrane nanofluidic device was developped using nanomanipulation tools using nanoscale building blocks. Experiments show unprecedented energy conversion from salt concentration gradients. Applications in the field of osmotic energy harvesting will be discussed.
References: see http://www-lpmcn.univ-lyon1.fr/~lbocquet
laquo; Giant osmotic energy conversion measured in a single transmembrane boron-nitride nanotube raquo;, A. Siria, P. Poncharal, A.-L. Biance, R. Fulcrand, X. Blase, S. Purcell, and L. Bocquet, Nature 494 455-458 (2013)
laquo; Nanofluidic osmotic diodes raquo;, C. Picallo, S. Gravelle, L. Joly, E. Charlaix and L. Bocquet, submitted (2013)
laquo; Soft nanofluidic transport in a soap film raquo;, O. Bonhomme, O. Liot, A.-L. Biance, and L. Bocquet, Phys. Rev. Lett. 110 054102 (2013)
laquo; Nanofluidics, from bulk to interfacesraquo;, L. Bocquet , E. Charlaix, Chemical Society Reviews 39, 1073 - 1095 (2010)

Materials with superhydrophobic properties are usually generated by covering the surfaces with hydrophobic nanoscale rough features. A major problem, however, for any practical application of such strongly water-repellent surfaces is the mechanical fragility of the nanostructures. Even moderate forces caused by touching or rubbing the surfaces are frequently enough to destroy the nanostructures and lead to the loss of the superhydrophobic properties. In this paper we study the mechanical stability of superhydrophobic surfaces with three different topographies i.e. with nano- and microscale features and surfaces carrying a combination of both. The surfaces are generated by silicon etching and subsequent coating with a monolayer of fluoropolymer (PFA). We perform controlled wear tests on the different surfaces and discuss the impact of wear on the wetting properties of the different surfaces.
A second system exhibits two roughness levels: silicon microcones surrounded by superhydrophobic silicon nanograss. The fabrication process of the surface structure is a mask-free process where both microcones and nanograss are simultaneously fabricated by the Deep Reactive Ion Etching (DRIE) technique in the overpassivation regime. Varying the process parameters, microcones of different size and density were fabricated, while the nanograss size and distribution was kept constant.
When strong shear stress is applied to such substrates, the microcones take the load and prevent contact of the shearing surface with mechanically instable silicon nanostructures. As result, the surfaces can withstand strong shear forces without noticeable loss in superhydrophobicity. Even when the shear stress is so very large that large scale mechanical damage happens the surface remains superhydrophobic (albeit with slight pinning caused by released hydrophilic areas from broken microstructures. Higher densities of microstructures make the surfaces much stronger against high shear forces, while causing slightly stronger pinning. Thus designing the surface structure allows to precisely tailor the mechanical and wetting properties of such nanostructures surfaces.

The wettability of surfaces is an important aspect of modern materials engineering. The ability to control the wetting behavior is a key feature in fields like surface coating, adhesives and microfluidic applications. We present a new method using femtosecond-laser micromachining to produce controlled wettability in a one-step process on metallic surfaces. Previously separated processing steps have been merged into a single-step, which results in better precision and a drastic reduction of process time. Thereby, we benefit from the femtosecond laser&’s property to alter both the surface topology and the chemical reactivity of a solid surface. Our method bases on the phenomenon that surfaces, which are chemically activated by femtosecond-laser micromachining, react with oxygen containing gases or gas mixtures to satisfy the oxygen deficit that originates from the exposure to femtosecond-laser irradiation. Modifying the surface chemistry consequently increases or decreases the original contact angle of the metal.
We have conducted the micromachining process on titanium (99.9%) under controlled conditions in different background media like oxygen, carbon dioxide and water, which serve not only as chemical reactant for the activated metallic surface but also contribute to the formation of the microstructure. Thereby, the background media influence the effective laser energy exposure of the target surface. Environmental parameters like temperature and pressure have been varied to study their influence on the outcome of the experiments. We have been able to render surfaces either hydrophilic or hydrophobic to different extents. Furthermore, we have controlled the formation of the nano- and microstructure via the parameters of the laser micromachining process, which has allowed us to reach superhydrophobic and superhydrophilic wetting behavior.
With this method we open a new field of possibilities to customise the wetting behavior of metals for various engineering applications to reduce consumption of energy and raw materials.

Electrospinning is a simple and useful method for producing various fiber assemblies by controlling the polymer solution and electrospinning parameters. The fibers can be spun into nonwoven structures having high porosity, small pore size and high surface-to-volume ratio. These electrospun membranes more hydrophobic than the corresponding cast films due to the air trapped in the voids of the membranes. High porosity combined with high hydrophobicity is desirable for membrane distillation desalination, in which water is transported through the membrane preferentially as vapor, due to a modest temperature differential across the membrane. In this work, various polymer fiber membranes are fabricated by the electrospinning technique. The membrane structure, porosity, hydrophobicity, and the effect of post-spin treatment, such as thermal annealing and modification of surface chemistry by initiated chemical vapor deposition, are studied. The electrospun fiber membranes are tested for desalination using the air gap membrane distillation configuration. The effect of membrane properties and the operating conditions on the membrane distillation performance are discussed.

The contact angle for a surface-liquid combination is governed by energy minimized states, also known as wetting states. One indicator of a wetting state is the penetration depth of the liquid inside the valleys of a rough surface. A clear quantification of the wetting thermodynamics, and explicit formulations for the contact angle have been limited to wetting states with complete penetration (Wenzel) and no penetration (Cassie). Over the past decade, several theories have been established which hint at an intermediate wetting state characterized by a partial penetration of the liquid inside the roughness valleys. This work aims at understanding the occurrence and thermodynamics of an intermediate wetting state. Assuming forced wetting arising at the apex of the roughness valley, the occurrence of a wetting state is realized by balancing the pressures acting on and away from the given surface with a square pillar topology. It is found that impact below a threshold velocity leads to the formation of an intermediate state. The feasibility of one wetting state over the other is governed by the surface characteristics. The principle of energy minimization is used to introduce the general equation of wettability, which computes the contact angle for any wetting state. A wetting parameter is incorporated, which forms the fingerprint of a wetting state. The wetting parameters corresponding to zero and complete penetration respectively return the Cassie and Wenzel equations. The intermediate wetting state is expressed by an implicit solution that is in good agreement with energy minimization. The physics of an intermediate wetting state is revisited, and a mathematical tool is provided which promises significant convenience in contact angle calculations.

With 15% CuInGaSe2(CIGS) and 11% efficiency Cu2ZnSn(S,Se)4(CZTS) solar cells reported, hydrazine-based spin coating technique has proven to be an effective method for fabricating Cu chalcogenide thin films and devices. During the solution-based process, drops of hydrazine solution interact with the substrate and, ideally, wet, spread, and form a coherent film as the solvent evaporates. One of the crucial factors that affect the film quality and device performance is the wettability of the hydrazine-CZTS solution on the substrate. Here we report a study on the wetting and spreading of hydrazine-CZTS slurries on various solid surfaces. Using surface tension calculations as our guide, we were able to alter the wetting behavior by controlling the solution composition and varying the hydrophobicity of the substrate surface through thin film addition. As predicted by our model, we were able to achieve improved wettability and, subsequently, smoother films.
To characterize the wettability and adhesion of hydrazine-CZTS slurries on various substrates, the surface tension was measured using a static contact angle method. 8 µL hydrazine-CZTS slurry was placed on various surfaces including soda-lime glass, Si, transparent conducting oxides (ITO, SnO2, ZnO), wide band gap chalcogenides (CdS, In2S3, ZnS), metals (Mo, Cu, Au, Al), carbon single-walled nanotubes (SWNTs), and CZTS thin films.The contact angle was measured macroscopically by using image analysis of a droplet on the substrate surface. The work of adhesion between the solid-liquid interface and the spreading coefficient for the droplets were determined using Young&’s equation. The work of adhesion and spreading coefficient for Mo and the CZTS slurry interface were measured to be 133.2 mJ/m2 and -0.37 mJ/m2, respectively. In comparison, those values for soda-lime glass and the CZTS slurry interface were 124.5 mJ/m2 and -9.24 mJ/m2. These results indicate that Mo surface is more easily wet by the CZTS slurry than the glass surface. A simple 2-D numerical model was developed to simulate the static wetting. The model indicates that the wettability of the surface can be increased by reducing the surface tension through surface modification, meaning a partially-wetting material such as glass can be become completely wetting. This was experimentally verified by adding a SWNT or metal film to the glass substrate. Additionally, the wettability between the CZTS slurry and the substrate can be controlled by changing the equilibrium of wetting through modifification of the CZTS slurry composition.
In order to investigate the effect of wettability on the quality of deposited thin films, surface roughness and morphology of CZTS films deposited on various substrates were characterized using AFM and SEM. Consistent with expectation, smoother films were found for surfaces with higher wettability. These results will provide valuable guidance for the solution-processed deposition of hydrazine-based CZTS thin films.

The spreading of a liquid on a solid surface, also known as wetting, usually takes place isotropically. However, if a surface can cause wetting to occur in certain specific directions only, benefits such as reduction in drag forces can arise. This has important implications for applications in the field of microfluidics, biosensing etc. and is the reason behind the heightened interest in surfaces that can produce directional wetting in recent years. Previous attempts at engineering surfaces that can induce directional wetting had mainly focused on the use of structurally anisotropic micro-/nano-structures such as bent or slanted nanowires. In this study, it is shown that directional wetting can also be achieved with nanoscale surface energy anisotropy. This surface energy anisotropy was generated by depositing a metal at an oblique angle onto an array of polymeric nanostructures fabricated with laser interference lithography and plasma etching. Because the polymer is more hydrophobic than the deposited metal, each nanostructure in the array had a face that was more hydrophilic than the other. When a water droplet was placed on such a surface, it was always found to wet preferentially in the direction of the hydrophilic face. Depending on the shape of the nanostructure, which can be controlled by the fabrication process, wetting can be made uni-, bi- or tri-directional. A model is proposed to suggest the basis for directional wetting observed in this study and is validated against measurements of contact angles and spreading anisotropy displayed by the droplet. The insights obtained in this study contribute to the understanding of wetting on heterogeneous surfaces and expand the capability of researchers to engineer functional surfaces for the control of wetting directions.

Microscale Polymethyl Methacrylate (PMMA) pillars with different height, spacing and molecular weight (MW) were fabricated on a quartz crystal microbalance (QCM) surface using nanoimprinting lithography (NIL) technology with Polydimethylsiloxane (PDMS) as the molding material. The patterned surfaces were treated with oxygen plasma and POTS (Perfluorinated Octyl Trichlorosilane) vapor to modulate surface wettability. The wettability of micropillars was initially characterized by contact angle measurement. Thereafter, a technique using laser scanning confocal microscope (LSCM) with immersion lens was developed to accurately measure the interface between pillars and liquid. The QCM impedance spectra for different water-micropillar interactions were obtained and correlated with their respective hydrophobicity and pillar parameters. Water was found always in the Wenzel state (fully penetrating) for micropillars of low height, and gradually shifts to the Cassie state (non-penetrating) with increasing pillar height and reduced pillar spacing. The results also show that hydrophobic surfaces in Wenzel state have similar responses to the hydrophilic surfaces while a significantly weaker response from QCM was found for the hydrophobic surfaces. Furthermore, the responses for hydrophobic surfaces are highly dependent on the height and spacing of micropillars, as well as the penetration depth of water. More importantly, the vibrating QCM surface results in micropillar vibration on the surface and, in turn, leads to changes in interactions between pillars and water. The effects of pillar height and molecular weight on surface hydrophobicity were studied in detail to clarify this interesting phenomenon.

Here we report a mussel-inspired, material-independent surface chemistry with controlled adhesive properties in a molecular-level for developing a platform to deliver the smallest therapeutic molecule, nitric oxide (NO). Poly(dopamine) (pDA) and poly(norepinephrine) (pNE) coating is both an emerging mussel-inspired surface functionalization techniques that can be applicable to a variety of materials regardless of their chemical environment of surfaces with its unique material-independent adhesion properties.[1,2] The surface modification mechanism of pDA and pNE are known as following the similar 5,6-dihydroxyindole (DHI) derivative chemical pathway,[3,4] and the secondary amine involved in DHI can be useful for an excellent NO-loading molecular platform. We prepared surface conjugated diazeniumdiolates groups (NO precursor) onto the secondary amines involved in the respective pDA or pNE layers, and the spontaneous dissociation of one diazeniumdiolates releases two molecules of NO gas molecules when water molecule is accessed to the surfaces.[5] We found that significant large amount of NO was attached and released from the pNE modified surface compared to pDA modified one. The difference in molecular adhesion of NO precursor is arising from the existence of a unique by-product called 3,4-dihydroxybenzaldehyde (DHBA), unavoidably generated during the pNE surface functionalization. DHBA can interact with NE by Schiff-base formation followed by auto-reduction of imine bond, resulting DHBA-NE dimer. This key leads to the nanoscale-smooth coating morphology of pNE, which can be a big advantage over pDA, especially in applications involving nano-scale substrates. Not only for controlling the morphology, the integration of DHBA-NE on pNE coating layer is essential that additional secondary amine involved in DHBA-NE can generate an excess amount of NO-loading molecular platform for NO storage and delivery, which can be potentially useful for biomedical applications.
[1] H. Lee, S. Dellatore, W. Miller, P. B. Messersmith, Science 2007, 426-430.
[2] S. M. Kang, J. Rho, I. S. Choi, P. B. Messersmith, H. Lee, J. Am. Chem. Soc. 2009, 131, 13224-13225.
[3] S. Hong, Y. S. Na, S. Choi, I.-T. Song, W. Kim, H. Lee, Adv. Funct. Mater. 2012, 22, 4711-4717.
[4] M. d'Ischia, A. Napolitano, A. Pezzella, P. Meredith, T. Sarna, Angew. Chem., Int. Ed. 2009, 48, 3914-3921.
[5] S. Hong+, J. Kim+, Y. S. Na, J. Park, S. Kim, K. Singha, G.-I. Im, W. J. Kim, H. Lee, Angew. Chemie. Int. Ed. 2013, accepted, DOI: 10.1002/anie.201301646.

Nature is a great source of inspiration for creating unique structures with special functions. The representative examples of water-repelling surfaces in nature, such as lotus leaves, rose petals, and insect wings, consist of an array of bumps (or long hairs) and nanoscale surface features with different dimension scales. On the other hand, low optical transparency due to the Mie scattering from micro-roughness with different dimension scales has recently been considered as a crucial issue for recent interest in as eye glasses, solar cell panels and car windows which need both transparency and superhydrophobic property. Herein, we introduced a method of realizing highly transparent multi-dimensional hierarchical structures inspired by termite wings along with water-repellancy of the surfaces with different drop impact scenarios. The multi-dimensional hierarchical structures were fabricated by soft imprinting method with TiO₂ nanoparticle pastes. In order to achieve the enhanced hydrophobicity, fluorinated moieties were adsorbed to the patterned surfaces to lower the surface energy. As a result, super-hydrophobic surfaces (above 176° in water contact angle with a hysteresis less than 2°) with high transparency (above 90 % in transmittance) in visible region were realized. Moreover, it has been shown that UV light was effectively blocked by TiO₂ and different dyes were also easily incorporated inside the mesoporous structure, resulting in photocromic functional films.

Stainless steel is widely known corrosion resistant metal. Its resistance to corrosion and staining makes it an ideal material for many applications such as architecture, industrial establishments and medical instruments. The corrosion resistance of stainless steel comes from sufficient chromium to form a passive film of chromium oxide. With its intrinsic corrosion resistance, the wettability of metal surface is also important property which depends on the chemical compositions and geometrical microstructures. Even though the fabrication of superhydrophobic surface has been researched for a long time, it is difficult to fabricate an engineering superhydrophobic surface on stainless steel due to the chemical degradation of surface and its difficulty to fabricate microstructure on the surface. Recently, superhydrophobic surfaces on stainless steel have been studied by sol-gel coating, dip-coating, electroless plating, Teflon-like coating with plasma and laser machining. The structure made by etching method is sturdy while coated films can be fallen off from the stainless steel substrate. With laser machining, very small structure can be fabricated but it is not cost effective due to its high cost and low throughput.
In this study, a method was developed to fabricate a superhydrophobic surface on stainless steel. Chemical wet etching and electrochemical etching were used to make nano scale roughness and micro structure on the surface. FeCl3 which is generally used as an etchant for stainless steel was used for chemical wet etching. Electrochemical etching process uses electrochemical reaction at the interface between metal and electrolyte. With this process, it is possible to fabricate micro structure without burr on the surface and to control the dimension of structure. SAM (self assembled monolayer) method was also used to deposit hydrophobic layer on the micro/nano hierarchical structure. A low surface energy of SAM layer on the structure makes the surface superhydrophobic. The fabricated stainless steel was analyzed by FE-SEM, 3D-profiler and FT-IR spectrometer. The characteristics of superhydrophobic surface were analyzed by measurements of the static/dynamic contact angle and sliding angle.
The superhydrophobic surface of stainless steel has been successfully developed with hierarchical structure and liquid SAM coating. The effect of dimension to hydrophobicity of metal surface was calculated with different microstructure which has various pattern size and pitch.

The development of multilayer thin films with nanopores containing biomolecules has attracted much attention in material and surface science as one of the optical applications and surface wettability control via LBL (Layer-by-Layer) self-assembly method. Also, chitin nanofibers (CHINFs) have attracted much attention because of their high mechanical strength. The shells of crustaceans are expected to be particularly useful for materials applications, because they are made from mineral salts, protein, and chitin; it is known that mineral salts can be removed using HCl, and proteins can be removed using NaOH. The CHINFs surface is transformed from chitin to chitosan by deacetylation, which results in a positive charge on the CHINFs, due to the presence of amine groups. In this study, we fabricated antireflective film with nanopores for robust functional surface composed bio-based polymer and added hydrophobic performance by gas-phase process. To fabricate a low refractive index film, therefore, porosity was introduced into a thin film. The porous thin film was obtained by increasing the number of airspaces inside the membrane. Then, by depositing the deacetylated CHINFs, Poly (acrylic acid) (PAA) and SiO2 nanoparticles using the LBL method, the lower refractive index layer was fabricated. (CHINFs/PAA) films indicated that the highest transmittance was 96% and the lowest refractive index was 1.29. The transmittance decreased by only 1.3% after abrasion of 200 g/cm2 with cotton. (CHINFs/SiO2) films showed that the highest transmittance was 97% and the lowest refractive index was 1.20. After adding hydrophobic performance by gas-phase process, the film indicated that the highest transmittance was 97%, the lowest refractive index was 1.23 and the water contact angle is 153°. In addition, we fabricated transparent SLIPS (Slippy Liquid Infused Surface) film by putting a drop of lubricant oil (Krytox 103) on the films and the highest transmittance of this SLIPS film was 98%.

Polymers bearing azobenzene functionality have been gaining increasing attention because of their ability to alter physical properties or create mechanical action of bulk polymers in response to changes in environmental conditions. Generally, azobenzenes are attached to the side chains of polymeric precursors followed by casting to generate thin films for further studies. Our approach presented here differs from the previous works in that it utilizes self-polymerization of hydroxyl and/or acrylate functionalized azobenzene moieties and assembly of stimuli-responsive materials using electrospinning technique. We hypothesize that increased surface area and porosity of electrospun nanofibers of azobenzene functionality will greatly improve the photomechanical response factor of the resulting materials. The polymerization reaction was carried out without using a crosslinker in inert atmosphere at 70 °C for 24 hours. Reaction progress and the product was monitored using 1H NMR. The resulting polymer was then dissolved in DMF at approximately 75% wt% which was determined to be have the ideal viscosity for electrospinning. The dissolved polymer was loaded into a syringe and electrospun with an applied voltage of 15kV at 2 mu;L/m for 30 minutes. The resulting deep orange mat exhibits reversible changes in hydrophobicity and color when exposed to 365 nm light. The resulting materials were examined using TGA, DSC, 1H NMR, wettability , viscosity, and gel chromatography.

Interfaces between metal and metal oxides are encountered in several applications, including surface protection, catalysis and electronics. In each one of these applications, the interfacial mechanical strength plays a decisive role in the stability and reliability of the systems. However, there are several factors that affect this property including interfacial composition, atomic level structure and processing conditions (temperature and pressure). Several theoretical and experimental methods have been developed to determine the interfacial mechanical strength. Among the theoretical methods, density functional theory (DFT) is a popular technique in the calculation of the work of separation (W_sep) of metal-metal oxide interfaces. However, DFT only provides information at 0 K situations, which implies that the direct quantitative comparison of the computed results with the experimental W_sep is not always possible.
In this work, we used DFT methods to calculate the W_sep of the Pt-HfO₂ interface at different interfacial O coverages (0, 0.25, 0.5, 0.75 and 1 OML) and several cleavage planes of the heterostructure. Using first principles thermodynamics, we combined the information of the lowest W_sep of the Pt-HfO₂ system for different O configurations and propose a parameter-free method to determine the average work of separation as a function of temperature and O pressures. The average work of separation obtained for the Pt-HfO₂ heterostructure is in excellent agreement with the experimental value reported in the literature, measured at high temperatures and low O pressures. The methodology used in this work can be extended to different metal-metal oxide interfaces to determine the adhesive properties as a function of temperature and pressure.

In recent years, nanocomposite materials fabricated by metal nanoparticles (NPs) and thin metal nano-grained films deposited on or embedded in soft polymeric substrates have emerged in the developing nanotechnonolgy applications (i. e. organic transistors, light-emitting diodes, solar cells etc.). To generate metal NPs or nano-grained metallic films on or near a polymer surface, ion implantation, evaporation, and sputtering methods have been utilized. To precisely control the surface properties of polymer nanocomposite films, it is essential to understand how NPs interact with the surface. In the present work, we report on an atomic force microscopy study of the wetting, adhesion and embedding kinetics properties (as a consequence of the long-range mobility of the polymeric chains above the glass transition temperature), under thermal processes, of Au and Ag NPs sputter-deposited on two amorphous soft polymeric layers: poly(methyl methacrylate) (PMMA) and polystyrene (PS). We studied in details the embedding process as a function of the annealing temperature and time so to evaluate the embedding rate both for Au and Ag in PMMA and PS. Also the NPs embedding statistics was studied analyzing the number of the embedded NPs as a function of the annealing time. The quantification of all these parameters, allow us to propose the embedding mechanism of deposited metal NPs on soft polymeric films, as an effective way to generate, in a controlled way, metal/polymer nanocomposites.

Since the creation of the Lifshitz theory of van der Waals-London dispersion (vdW-Ld) forces, accurate knowledge of dielectric response functions has been the key requirement for reliable computation. The connections between material properties, their processing into a form appropriate for computation, and -most important- the propagation of spectral details into specifics of vdW-Ld forces have no general solutions. They must, necessarily, be carefully recognized case-by-case for different materials, media, and geometries. Among the many electronic properties requiring careful attention are excitonic many-body effects for which it is not clear how the changes in optical spectra will affect the overall magnitude of the vdW-Ld interaction between materials.
Excitonic effects in low-dimensional systems, such as graphene and carbon nanotubes, tend to introduce additional absorption peaks in the energy range below 5 eV and to shift slightly the positions of some other peaks. An important question in this respect is how the changes in optical spectra due to excitons affect the overall Hamaker coefficients and the trends in the vdW-Ld interaction between different materials. Because the vdW-Ld interaction is non-local in frequency, being a functional (or more precisely a discrete sum over the Matsubara frequencies) of the frequency-dependent dielectric response, it is reasonable to expect that the interaction will depend more on the global properties of the dielectric response set by sum rules rather than on specific details of spectra.
A related problem is connected with refractive index-matching, sometimes used to modify the inter-particle interactions through their vdW-Ld component. In the index-matching method, one assumes that the vdW-Ld interaction stems only from the optical frequency regime; if this part of the spectra of interacting bodies and the intervening medium is made to coincide, its contribution to the vdW-Ld interaction will vanish identically. Again, because of the non-local nature of the vdW-Ld interaction, one should expect that even for refractive-index-matched situations the vdW-Ld interactions will depend primarily on the global properties of the dielectric response and much less on the presence or absence of a specific peak.
We have embarked on a detailed investigation of how changes in the spectral properties of the dielectric response function over a finite interval of frequencies alter the strength of vdW-Ld interactions expressed as the corresponding Hamaker coefficient. We consider dielectric spectra that differ only by the presence or absence of a single (exciton) peak and observe the consequence of this difference on the Hamaker coefficient for the interaction of two planar semi-infinite regions. We analyze specifically the changes of all the Matsubara components to the interaction free energy in order to monitor the effect of the dielectric response variation on the vdW-Ld interaction energy functional.

Abstract:
Membrane Distillation (MD) is a thermally-driven separation process, in which only vapor molecules transfer through a microporous hydrophobic membrane [1]. In sea water desalination using MD, it is highly desired that the membrane structure has a low affinity for water, which requires the membrane to exhibit high hydrophobicity to minimize water adhesion [1]. The extensive use of PTFE (polytetrafluoroethylene) and PVDF (polyvinylidenefluride) in MD has raised the attention to their environmental impact due to membrane disposal. Due to its biodegradability and hydrophobicity, polylactic acid (PLA) is a potential candidate for MD desalination [2, 3]. In this study, PLA membrane is fabricated by electrospinning. The morphology of the membrane can be controlled by adjusting the electrospinning process parameters and by applying post-heating treatments. Furthermore, the thickness of the membranes and the presence of beaded fibers, fused fibers and porous fibers can be adjusted. The morphology of the obtained membrane is investigated by means of scanning electron microscope. The static and dynamic contact angle (CA) is measured by sessile drop technique with water as the probe liquid. To enhance the hydrophobicity of the PLA membrane and obtain improved water repelling properties, a fluorinated silane (trichloro(1H,1H,2H,2H-perfluorooctyl)silane) is coated on the membrane under modest vacuum (asymp;10 kPa) by vapor deposition. The treated membranes are found to have unaffected morphology and structural characteristics and higher CA compared with those of the untreated membrane. Thus treating membranes of different thickness and morphology with fluorinated silane yields even higher hydrophobicity[4] which can be favorable for applications like anti-wetting and self-cleaning, etc. Meanwhile, keeping in mind that CA measurement only yields the macroscopic hydrophobicity characteristics of the membrane, the microscopic water repelling property inside the pores in the membrane is investigated by means of force spectroscopy in atomic force microscopy (AFM) [5].
References:
1. Alkhudhiri, A., N. Darwish, and N. Hilal, Membrane distillation: A comprehensive review. Desalination, 2012. 287: p. 2-18.
2. Gross, R.A. and B. Kalra, Biodegradable polymers for the environment. Science, 2002. 297(5582): p. 803-807.
3. Li, L., R. Hashaikeh, and H.A. Arafat, Development of eco-efficient micro-porous membranes via the electrospinning and annealing of poly (lactic acid). Journal of Membrane Science, 2013.
4. Buruaga, L., et al., Production of hydrophobic surfaces in biodegradable and biocompatible polymers using polymer solution electrospinning. Journal of Applied Polymer Science, 2011. 120(3): p. 1520-1524.
5. Santos, S., et al., Measuring the true height of water films on surfaces. Nanotechnology, 2011. 22(46): p. 465705.

In this work, we investigate the effect of various surface treatments on the bonding performance of initiated chemical vapor deposition (iCVD) polyglycidylmethacrylate (PGMA) thin film adhesives.
iCVD is a conformal, solvent-less, vapor phase polymerization and deposition technique that eliminates surface tension effects related to solvent phase processing. Moreover, iCVD has several advantages compared to traditional vapor-phase polymerization techniques like plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD). It is a low-energy technique that results in higher retention of polymer functional groups compared to high-energy PECVD and HWCVD processes. Furthermore, iCVD has minimal influence on the substrate chemistry, unlike PECVD which involves constant bombardment of surface with high-energy ions.
The gas-phase processing, substrate compatibility, and retention of reactive group functionality, enable use of iCVD-PGMA films as thin film adhesives. In semiconductor processes potential applications include thermo compression bonding for wafer thinning, 3D integration, and packaging of MEMS/ NEMS devices. In preliminary work we studied iCVD-PGMA for use in 300 mm silicon wafer bonding and characterized the adhesion properties using a four point bending technique. We noted that the bond failure was primarily adhesive in nature, indicating the polymer-silicon interface was the weakest link. Altering the surface chemistry should allow manipulation of the adhesion properties to enable both permanent and temporary wafer bonding. This lead to the work presented here, which investigates the influence of surface treatment on iCVD-PGMA/substrate adhesion strength.
Wafer bonding was accomplished by thermocompression above the glass-transition temperature (60 °C) for iCVD PGMA. Subsequently an anneal step between 90-175 °C was performed. The surface property of Si was manipulated by molecular vapor deposition (MVD) of amine, epoxide, alkane and fluoro-terminated silanes with untreated and plasma treated substrates as control. The epoxide groups on PGMA function as anchoring points to graft polymer chains to amine and epoxide silanes under moderate conditions where result indicated improved bond strength at increasing temperature. Furthermore, iCVD PGMA films bonded with hydrophobic fluoro-silane modified surfaces lead to reduced adhesion strength. The bond interface was investigated using scanning acoustic microscopy which showed excellent bond qualities with up to > 98% bond area. iCVD PGMA films were successfully demonstrated as thin film adhesives for 300mm Si wafer bonding where the adhesion strength can be manipulated to enable permanent and temporary wafer bonding applications.

We present an approach to create fully transparent, omniphobic surfaces with remarkable liquid repellency properties by a bioinspired design mimicking the slippery surface of Nepenthes pitcher plants referred to as slippery liquid-infused surfaces (SLIPS).[1] In contrast to the lotus effect that inspired the design superhydrophobic surfaces by introducing roughness features to create a solid/air composite surface that enables liquid droplets to roll off with ease, our design is based on the creation of a fluid/fluid interface between a lubricant firmly locked into a porous surface structure and a non-miscible liquid to be repelled. If properly held in place, the lubricant itself then acts as the repellent surface. The exchange from a solid/liquid to a liquid/liquid interface effectively eliminates pinning points and leads to super-repellent surfaces with extremely low contact angle hysteresis and sliding angles (i.e. the minimum tilting angle required for a drop to slide off the substrate) - even for liquids with low surface tensions. Additionally, the fluid nature of the interface inherently possesses self-healing characteristics as the lubricant can wick back into damaged parts of the surface.
We introduce a transparency and damage tolerant coating based on lubricant infusion coating by applying colloidal monolayers to structure the surface and create the roughness necessary to support stable SLIPS conditions.[2] The small size of the colloidal nanostructures does not interfere with the optical properties of the substrate material and enables the design of fully transparent surfaces with remarkable wetting characteristics. We show that such surfaces repel various liquids, possess superior transparency, are self-healing and tolerate mechanical damage, are stable over extend periods of time, prevent adhesion of liquid-borne contaminants, including blood serum proteins and drastically reduce the adhesion of ice. The highly ordered structure of the coating enables us to understand and predict the stability or failure of repellency as a function of lubricant layer thickness and defect distribution based on a simple geometric model. The method can be applied to curved substrates and to photolithographic processes to pattern the repellent parts of the substrate, thus opening opportunities to spatially confine low surface tension liquids or complex fluids as blood.

[1] T.S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, and J. Aizenberg, Nature 2011, 477, 443
[2] N. Vogel, R. Belisle, B. Hatton, T.S. Wong and J. Aizenberg, Nature Comm., accepted for publication

Developing adaptive systems that can reversibly change one or more properties in response to external stimuli not only generates fundamental insights into the relationship between microscopic structures and macroscopic functions, but also has immense technological relevance in diverse areas from creating dynamic interfaces with biological tissues to designing energy-efficient buildings. Recent efforts to mimic biological adaptive systems have led to the design of hierarchical multicomponent hybrid composites that are made responsive by introducing an active material into a multiscale architecture. While existing materials can change individual properties such as wetting, adhesion or friction separately, no existing material platform can combine all three switchable properties in a single material. Here we show a topographically responsive composite made by infiltrating a porous matrix with colloidal dispersions of magnetic nanoparticles or ferrofluid, and demonstrate how a unique multiscale coupling of magnetic and capillary forces reconfigures local surface topography and yields dynamic wetting and switchable adhesive and frictional properties, as well as the abilities to remotely control fluidic pumping, transport and mixing. We provide physical insights into the mechanism of these magnetically actuated properties and functions. We envision our new dynamic surfaces would not only offer opportunities to study fundamental phenomena such as fluid transport on topographically responsive surfaces, but also find applications in areas such as microfluidics, responsive coatings, and dynamic material interfaces with cells and tissues.

We report highly-sensitive measurements of pressure-induced water infiltration into hydrophobic nanostructured surfaces using transmission small angle x-ray scattering. We have developed new methods of patterning arbitrarily-large area nanostructured surfaces with ~10nm feature size and ~10^10/cm^2 feature density by combining block-copolymer self assembly and plasma-based etching. Control over processing conditions allows us to generate uniform surfaces of posts, lamellae, or tapered cones in order to relate the water infiltration characteristics to the nanotexture geometry. We study details of water penetration into the nanostructures under controlled hydrostatic pressure, determining the infiltration filling fraction from the intensity of the diffracted signal. In all structures, significant infiltration occurs irreversibly above a critical pressure depending on the texture size and geometry. We present the details of the infiltration isotherms, which are well modeled by accounting for the specific shape of the nanotexture's geometry.

The ability to control liquid behaviour at surfaces is highly relevant for a number of application areas, including for example the performance and reliability of inkjet nozzles. Generally, the wettability of solid substrates can be modified both by morphological micro- and nanostructuring as well as by chemical functionalization. We investigate the potential of chemically defined patterns consisting of hydrophobic self-assembled monolayers (fluoroalkylsilane) on hydrophilic (pristine SiO2) substrates to control dynamic liquid behaviour on morphologically flat surfaces.
We review our results on anisotropic spreading and evaporation of droplets on such chemically striped surfaces. The equilibrium shapes of asymmetric droplets, arising from patterns of alternating hydrophilic and hydrophobic stripes with dimensions in the low-micrometer range, are investigated in relation to the stripe widths. Owing to the well-defined small droplet volume, the equilibrium shape exhibit unique scaling behaviour. Additionally, high-speed camera measurements reveal the importance of kinetics involved in the formation of the asymmetric droplets. Similarly, evaporation of droplets on such surfaces also exhibits a directional anisotropy.
We also present experiments investigating the motion of the liquid from surface areas with low macroscopic wettability towards areas with a higher wettability. The density of self-assembled fluoroalkylsilane monolayers as defined by the chemical pattern proves to be of paramount importance. Both linear and radial patterns are presented, which induce liquid movement across chemically defined gradients in surface energy.
To provide a benchmark for the analysis of our results, we use two different numerical tools. In the first method the minimum energy state of a droplet on a chemically striped patterned surface is determined using Surface Evolver. Simulated results agree well with experiments of high viscosity liquids, while it does not match the result for low viscosity liquids, such as water. The effect of kinetic energy during deposition of a droplet on such a pattern will be discussed. In the limit of low kinetic energy, either by using a controlled way of depositing or by using highly viscous liquids, the droplet is close to the minimum energy state and the calculations for the minimal energy state match the experiments.
To enable modelling of dynamic behaviour of low viscosity liquids, we use the multicomponent multiphase lattice Boltzmann method (LBM). With this method it is possible to include actual dynamics of the droplet leading to different final shapes on the surfaces. The directional spreading behaviour is reproduced including the contribution of kinetic effects, and also the anisotropic evaporation characteristics emerge from the simulations. Finally, the LBM simulations also allows investigation of the gradient-induced motion, enabling straightforward modification of pattern designs prior to their actual fabrication.

We discuss recent numerical results, based on a lattice Boltzmann solution of a hydrodynamic phase field model, investigating how drops roll and bounce on superhydrophobic surfaces.

Recent years have witnessed intense interest in multifunctional surfaces that can be designed to switch between different functional states with various external stimuli including electric field, light, pH value, and mechanical strain. The present paper is aimed to explore whether and how a surface can be designed to switch between superhydrophobicity and superhydrophilicity by an applied strain. Based on well-established theories of structure buckling and solid-liquid contact, we show that this objective may be achieved through a hierarchically wrinkled surface. We derive general recursive relations for the apparent contact angle at different levels of the hierarchical surface and investigate the thermodynamic stability of different contact states. Our study may provide useful guidelines for the development of multifunctional surfaces for many technological applications.

Working equations that describe wetting phenomena can be derived in a variety of ways, by starting from capillary forces, Laplace pressure, or solid surface energies. We examined the relative importance of the contact line and interfacial areas in the capillary rise inside small diameter glass tubes. A series of simple experiments demonstrate that this wetting phenomenon is controlled by interactions in the vicinity of the contact line.

Liquid dielectrophoresis (L-DEP) allows a bulk force to be applied to a dielectric liquid using a non-uniform electric field. In our work we have developed a method using surface fabricated interdigitated electrodes whereby the effects of L-DEP can be localized to the liquid-vapor or solid-liquid interfaces. In the first case, which applies to thin films of liquid, this allows the liquid surface to be shaped and a voltage programmable diffraction grating to be created.¹ In the second case, which applies to droplets with a thickness greater than the decay length of the electric field, this allows the equilibrium contact angle to be controlled in a similar manner to electrowetting.² The ability to adjust the final equilibrium contact angle by choosing a voltage also creates the possibility of performing dynamic contact angle measurements over a full range from partial wetting to complete wetting on the same substrate.³ In this work we describe a modified Hoffman-de Gennes law for the edge speed-contact angle dependence and derive a theory for the dynamic spreading of droplets that predicts three regimes depending on whether the applied voltage is below, at or above a threshold voltage for complete wetting. These three regimes are: i) an exponential approach to a finite equilibrium contact angle, ii) a (Tanner&’s) power law approach to complete wetting, and iii) a superspreading regime The characteristic times for partial wetting and the exponents for spreading and superspreading are derived for both axisymmetric droplet spreading and the spreading of stripes. Experimental results for the spreading of stripes of 1,2 propylene glycol are found to be in good agreement with the theory in all three regimes. This approach provides an alternative to the use of surfactants for the study of superspreading.
We acknowledge funding from the UK EPSRC (EP/E063489/1). NS acknowledges funding from Nottingham Trent University.
¹Brown, C. V., Wells, G. G., Newton, M. I., McHale, G. (2009). Voltage-programmable liquid optical interface. Nature Photonics3, 403-405.
²McHale, G., Brown, C.V., Newton, M.I., Wells, G.G., Sampara, N. (2011). Dielectrowetting driven spreading of droplets. Phys. Rev. Lett.107, art. 186101.
³McHale, G., Brown, C. V. and Sampara, N. (2013). Voltage induced spreading and super-spreading of liquids, Nature Communications4, art. 1605.

Non-wetting surfaces containing micro/nanotextures impregnated with lubricating liquids can exhibit remarkable non-wetting properties and robustness compared to superhydrophobic surfaces that rely on stable air-liquid interfaces. In this talk, we examine the fundamental physicochemical hydrodynamics that arise when droplets, immiscible with the lubricant, are allowed to move along these surfaces. We find that these four-phase systems show novel contact line morphology comprising a finite annular ridge of the lubricant pulled above the surface texture and consequently as many as three distinct 3-phase contact lines. Depending on the spreading coefficients, the lubricant film can completely cloak the droplet, which can have important consequences for lubricant longevity and phase transitions. We show that these distinct morphologies not only govern the contact line pinning that controls droplets' initial resistance to movement but also the level of viscous dissipation and hence their sliding velocity once the droplets begin to move. Additionally, these unique four-phase systems can have up to three different 3-phase contact lines, giving up to 12 different thermodynamic configurations, which we describe in a thermodynamic regime map. Next we examine phase transitions such as condensation and freezing on these surfaces. By examining the microscopic characteristics in an Environmental SEM (ESEM) we obtain insights on the nucleation and growth processes, which are significantly influenced by the spreading characteristics of the lubricant. On surfaces optimized for condensation we find that condensate droplets smaller than 100 mu;m become highly mobile and move continuously at speeds that are several orders of magnitude higher than those on identically textured superhydrophobic surfaces. This remarkable mobility produces a continuous sweeping effect that clears the surface for fresh nucleation and results in enhanced condensation. In the case of frost formation, we find that lubricant spreading onto the growing ice can lead to depletion of lubricants from the surface, which can potentially compromise the icephobic properties of the surface. Given these findings, we conclude that lubricant-impregnated surfaces need to be carefully designed in order to deliver optimal non-wetting properties.

Learning from nature and starting from the superhydrophobic lotus leaves, we revealed that a super-hydrophobic surface needs the cooperation of micro- and nanostructures. Further studies have proved that the arrangement of micro/nano structure can directly affect the wettability and water movements. Recently, we found that hydrophilic compositions together with micro/nano structures endow the fish scale with superoleophobicity underwater. Inspired by this, artificial fish scales with robust mechanical strength have been fabricated.
Based on the micro/nano structured interfaces with special wettability, kinds of basic chemical reactions could be done within a small water drop. Crystal arrays could also been prepared, and also small molecule, polymer, silver NPs and microspheres can be arrayed in one direction.
Under certain circumstances, a surface wettability can switch between superhydrophilicity and superhydrophobicity. Besides the 2D interface, we recently extended the cooperation concept into 1D system. Artificial ion channels with smart gating properties have been fabricated by integrating smart molecules into the single nanochannels. These intelligent nanochannels could be used in energy-conversion system. The other one dimensional system is the artificial spider&’s silk. The periodic spindle knots on the spider&’s silk can drive liquid drops in a specific direction that can collect water from moist air. Further, we prepared artificial spider&’s silk and droplets of water on the artificial spider&’s silk behaved similarly to those on its biological counterparts. Most recently, inspired by the cactus surviving in the most drought desert, we probed into the relationship of the structure-function of cactus and found that the cactus had evolved a multi-structural and multi-functional integrated continuous fog collection system.
Learning from nature, the constructed smart multiscale interfacial materials system not only presents new knowledge, but also has great applications in various fields, such as self-cleaning glasses, water/oil separation, anti-biofouling interfaces, and water collection system.

The terms “ultrahydrophilic” and “superhydrophilic” were defined in 1996 for UV-irradiated TiO2 coated glass surfaces displaying contact angles theta; < 10° [1]. Ultra-/superhydrophilic metallic biomaterials were reported by us in 2001 [2]. However the phenomenon of superhydrophilicity is still poorly understood and lacks a consensual definition [3,4]. This appears due to the fact that measurements of hydrophilicity are impaired by the zero degree limit of the Young equation. Even if higher wettabilites are obtainable, dynamic contact angles for cos theta; > 1.0 are classed as “undefined”. It was found, that the dogma stating cos theta; > 1.0 as undefined" is false. The solution is to extend hydrophilic dynamic contact angles into imaginary number space (Θ), with this wettability termed as “hyperhydrophilic” [3,4]. Thus the “Lotus Effect” found on rough surfaces on the hydrophobic side of the wettabilty scale is now complemented by the “Inverse Lotus Effect” (i.e. “roughness induced hyperhydrophilicity”) on the hydrophilic side of the scale [3,4]. Examples for extending the analysis of wettability beyond the superhydrophilic into the hyperhydrophilic range will be shown for medicinal titanium surfaces [5,6] with roughness values (Ra) between 3-30 µm yielding advancing and receding imaginary contact angles in the range of Θ ~ 2i°-20i°. On the application side it has been shown that endosseous titanium implants with hyperhydrophilic surfaces result in an enhanced osseointegration in the sheep rib [7].
[1] Hayakawa, M., Kojima, E., Norimoto, K., Machida, M., Kitamura, A., Watanabe, T., Chikuni, M., Fujishima, A., Hashimoto, K., & Itoh, H. (1996) PCT Publ.No. WO96/29375, pp. 1-112
[2] Jennissen, H.P. (2001) Ultra-hydrophile metallische Biomaterialien, [Ultra-hydrophilic Metallic Biomaterials]. Bionanomaterials (formerly: Biomaterialien), 2, 45-53.
[3] Jennissen, H.P. (2011) Redefining the Wilhelmy and Young Equations to Imaginary Number Space and Implications for Wettability Measurements. Materialwiss. Werkstofftech. (Mater. Sci. Eng. Technol), 42, 1111-1117.
[4] Jennissen, H.P. (2012) Hyperhydrophilic Rough Surfaces and Imaginary Contact Angles. Materialwiss. Werkstofftech. (Mater. Sci. Eng. Technol), 43, 743-750.
[5] Lueers, S., Seitz, C., Laub, M., & Jennissen, H.P. (2013) On the Utility of Imaginary Contact Angles in the Characterization of Wettability of Rough Medicinal Hydrophilic Titanium. In Advances in Contact Angle, Wettability and Adhesion., Vol. 1, (Mittal,K.L., ed), pp. 155-172. Wiley-Scrivener, Salem, MA.
[6] Lueers, S., Laub, M., Kirsch, A., & Jennissen, H.P. (2013) Large Scale Preparation and Analysis of Hyperhydrophilic Dental Implants with µSLA Titanium Surface. Biomed. Tech. (Berl), 57, in print.
[7] Lueers, S., Lehmann, L., Laub, M., Schwarz, M., Obertacke, U., & Jennissen, H.P. (2011) The Inverse Lotus Effect as a Means of Increasing Osseointegration of Titanium Implants in a Gap Model. Bionanomaterials (formerly: Biomaterialien), 12, 34.

The dynamic contact angles of Newtonian and non-Newtonian fluids were measured on both hydrophobic and superhydrophobic surfaces using a modified Wilhelmy plate experiment. For the Newtonian case, water and aqueous solutions of low molecular weight polyethylene oxide solutions were studied in order to vary the liquid&’s viscosity. For the viscoelastic case, a series of solutions of high molecular weight polyacrylamide solutions were studied with varying relaxation time and viscosities. The Wilhelmy plates consisted of hydrophilic acrylic, hydrophobic Teflon, and an acrylic surface sprayed with a commercially available paint to make it superhydrophobic. In all cases, the advancing and receding contact angle were measured as a function of plate velocity, fluid viscosity and fluid elasticity. For the Newtonian fluids, the advancing and receding contact angle on the hydrophobic Teflon surfaces were found to obey the expected scaling trends with capillary number. Specifically, costheta;_s - costheta;_d ~ Ca^2/3. The response of the dynamic contact angle on the superhydrophobic painted surfaces was quite different. The advancing contact angle was not found to change with velocity, but remain constant at theta;_a =160°. More interesting, the receding contact angle on the superhydrophobic surface was found decay with increasing capillary number while obeying a new scaling relation, costheta;_s,r - costheta;_d,r ~ Ca¹. The origins of this new scaling will be discussed. In addition, a series of viscoelastic solutions were formulated to investigate the role of elasticity on the dynamic contact angle. Measurements were performed on both hydrophilic and hydrophobic plates. Our measurements show that the dynamic contact angle depends not only on capillary number, but Weissenberg number as well.

The self-cleaning function of superhydrophobic surfaces is conventionally attributed to the removal of contaminating particles by impacting or rolling water droplets, which implies the action of external forces such as gravity. Here, we demonstrate a unique self-cleaning mechanism whereby the contaminated superhydrophobic surface is exposed to condensing water vapor, and the contaminants are autonomously removed by the self-propelled jumping motion of the resulting liquid condensate, which partially covers or fully encloses the contaminating particles. The jumping motion off the superhydrophobic surface is powered by the surface energy released upon coalescence of the condensed water phase around the contaminants. The jumping-condensate mechanism is shown to spontaneously clean superhydrophobic cicada wings, where the contaminating particles cannot be removed by gravity, wing vibration, or wind flow. Our findings offer insights for the development of self-cleaning materials. [K.M. Wisdom et al, PNAS, vol. 110, pp. 7992-7997 (2013).]

The goal of this work is to study the key parameters affecting water droplet movement on a metallic substrate due to an underlying surface tension gradient and the effects that micro-structural roughness and anisotropic surface wettability have on water drainage. In air-conditioning applications, current research has already shown the tremendous potential of using surfaces that exhibit improved condensate management. This benefit arises because water retention on the air-side surface of metallic heat exchangers can reduce the air-side heat transfer coefficient, increase core pressure drop, and provide a site for biological activity. In refrigeration systems, the accumulation of frost on metallic fins requires periodic defrosting and reduces energy efficiency. When water is retained on these surfaces following the defrost cycle, ice is more readily formed in the subsequent cooling period, and such ice can lead to shorter operational times before the next defrost is required. Thus the management and control of water droplets on heat-transfer and air-handling surfaces is vital to energy efficiency, functionality, and maintenance of air-cooling systems.
Despite the proliferation of surface wettability research over the past few decades, the author is not aware of any published work that has successfully created a surface tension gradient using topography alone (i.e. no chemical coating) to move water droplets in a preferred direction along a metallic surface, with the goal of using those gradients to improve condensate distribution and drainage. With respect to this aim, we recently demonstrated the promise of this idea by creating a gradient in copper that resulted in water droplet movement of 1-2 mm along a horizontal plane— something that (to the best of our knowledge) had never been demonstrated before on a metallic surface without the use of chemistry. To date, however, only a single gradient pattern has been studied and no optimization has been performed. In this work, additional surface tension gradient patterns will be studied including radial and triangular designs. These patterns will be created using either laser etching or metal deposition techniques. The use of a hydrophobic self-assembled monolayer—namely, heptadecafluoro-1-decanethiol (HDFT)—will also be explored. The innovation of this research is the development and testing of novel, anisotropic surfaces that not only reduce water retention, but also facilitate the distribution of condensate droplets in preferred locations on the surface for the purpose of enhancing air-side heat transfer. Because heat exchangers are key components in many systems, both from a cost and performance point of view, it is expected that the fundamental science outcomes from this study could have a profound impact on many industries.

Dew (also called “Breath Figures” in the laboratory) is the dropwise condensation of water vapor on a substrate. In many regions of the world, dew water could serve as an additional water source, supplementing rain and fog water collection. An important concern is to improve water collection from the condensing surfaces. Wipers can be used, however it appears that natural wipers are present in the dew process. When one carefully observes the runoff of water drop condensing on windows in a room supersaturated with water vapor, one realizes that the drops mostly start to flow from the top edge of the window.
The detachment of droplets by gravity is a competition between gravity and contact line pinning forces. A drop will detach from an inclined surface when its diameter exceeds a critical size that depends on the inclination angle. In this event, the effect of boundaries seems thus to exhibit a special role in allowing the drop growing at an edge faster than in the middle of the surface.
The growth of droplets obeys general growth laws. A pattern of droplets of dimensionality D on a substrate of dimensionality d grows according to two mechanisms. (i) Nucleation and diffusive growth of individual drops with radius R_isim;(t/t₀)^α with α a growth exponent and t₀sim;F^-1/2p^-1 a typical growth time that depends on the water vapor flux F near the drops and the supersaturation p. (ii) Coalescence events, which rescale the growth, giving a mean radius <R_i>sim; (t/t₀)^β, with β = αD/(D-d). The exponent α depends on the shape of the concentration gradient around the drops. For a unique drop with a 3-dimensional vapor gradient, α = 1/2; for a surface gradient, α asymp; 1/3. The relative contribution of a surface or volume gradient depends on the thermal conditions inside the drop. However, when drops are in a pattern, they have to share the vapor gradients. It results a mean linear vapor profile, directed perpendicular to the surface, with the growth exponent α = 1/3.
Dedicated experimental investigation of droplet growth in a dew condensation chamber equipped with optical microscope and video camera shows that droplets obey the same growth law [<R_i>= At^β, with β asymp; 1] on edges, grooves, scratches and in the middle of the condensing surface. The prefactor A, however, is larger at the substrate edge than at the groove edge, itself larger than in the middle of the substrate. In contrast, A is lower inside the groove, close to the edge. The scratch gives very little effect. Drops near an edge thus grow faster because they collect more water vapor, experiencing less competing effects from the neighboring droplets than in the middle of the surface.

Controlled transport of aqueous volumes on surfaces with wettability patterns or gradients has strong relevance to engineering devices whose operation relies on dropwise condensation (e.g. heat exchangers and fog capture devices) or fluidic movement (lab-on-a-chip). An ideal surface for high-rate dropwise condensation should possess a large contact angle (to reduce solid-liquid contact area) and small contact angle hysteresis, so that the condensed droplets are highly mobile on the cooled surface, thus exposing fresh areas for further droplet nucleation and growth after droplets are swept away by gravity. However, this mechanism has two major limitations. First, droplet removal cannot take place on a horizontal surface. Second, the minimum droplet size for such removal, as designated by the capillary length, is ~2.7 mm for water. This large size leads to considerable delays for liquid removal by gravity, and a lowering of overall condensation heat transfer coefficient (the persisting liquid raises thermal resistance). Biphilic and super-biphilic [1] surfaces feature sharp wettability gradients —by creating spatial regions of high- and low-surface energy— on a heat transfer substrate. Ideally, dropwise condensation is favored on a superhydrophobic surface where the droplets form and grow due to further condensation and coalescence. However, sustained gravity-independent removal of the droplets from the superhydrophobic domains requires specially designed superhydrophilic channels, which drain the liquid through capillary forces. The smallest droplet size for drainage dictates the overall condensate removal rate; the smaller the size, the more efficient is the surface in removing the liquid. The dropwise condensation heat transfer rate depends on the wettability difference between the phobic and philic domains and their geometrical distribution on the substrate. The underlying physics can be unraveled by studying the transport dynamics of condensate droplets across sharp wettability steps. We fabricate and study large-area biphilic surfaces with alternate phobic and philic regions of prescribed size, shape and wettability jump for sustained capillary drainage of droplets from the phobic sections on the substrate. The dynamic motion of mm-sized droplets across phobic/philic boundaries of the substrate is visualized by high-speed imaging. The performance of the biphilic surfaces is quantified in terms of liquid removal rate and evaluated with respect to droplet size, surface energy differences and other salient geometrical parameters of the surface patterns. The results are used to optimize the surface engineering parameters for maximizing dropwise condensation and condensate removal rates.
1. A. R. Betz, J. Xu, H. Qiu, and D. Attinger, “Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling?,” Applied Physics Letters, vol. 97, p. 141909, 2010.

Wetting surfaces such as metals must be treated by a non-wetting modifier to yield dropwise condensation, but these hydrophobic promoters are often not durable in a steam environment. We report sustained dropwise condensation on an ultra-thin copolymer film grafted to a metal surface by initiated chemical vapor deposition (iCVD). These films exhibit low hysteresis under ambient conditions, and the diameter of departing drops during condensation is significantly smaller than seen on other hydrophobic surfaces such as gold ad a fluorinated silane layer. Using the low-power solvent-free iCVD method, we demonstrate how these copolymer films can be applied to complex geometries to lead to robust dropwise condensing surfaces with applications in power generation and desalination.

Icing on solid surfaces is commonplace in nature, technology and everyday life, bringing with it at times catastrophic consequences. Development of a passive, energy saving approach that exploits surface treatment or texturing to produce unfavorable icing conditions is of fundamental importance. Superhydrophobic surfaces, with micro- , nano-, or hierarchical roughnesses, have excellent water repellency marked by very high droplet contact angles (>150°). In addition, they also show low adhesion to water (characterized by low contact angle hysteresis) down to temperatures near and even below the freezing point. However, a rational framework for designing surfaces with predictable and extraordinary resistance to ice formation and adhesion remains a challenge. Here we introduce a thermodynamically-guided approach leading to surfaces with extremely long ice nucleation delay times combined with very low and robust nucleation temperatures. The classical nucleation theory of thermodynamics offers initial guidance on the design of an icephobic surface. For example, it predicts that nucleation is depressed on convex bumps of a nanoroughness and is promoted in concave nanoscale pits. As a consequence, the size and density of nanoscale pits on a surface should critically affect the nucleation temperature of a sessile supercooled droplet. The effect of the nanoscale pits explains the generally assumed trend of nucleation temperature increase for increasing nanoscale roughness. In this work, in a strong departure from the above plausible results of classic heterogeneous nucleation theory, we show that the presence of an interfacial quasiliquid layer on the ice in contact with a surface, counteracts the ice nucleation-promoting effect of nanoscale pits. We refer to this as interface confinement effect in the nanoscale pits. The confinement effect of the quasiliquid layer strongly suppresses the stable formation of ice nuclei. This critically important theoretical framework, guides our design and fabrication of nanostructured icephobic surfaces with extraordinaty nucleation suppression capability. These surfaces exhibited extremely low nucleation temperatures of ~-24 °C combined with extreme time delays in nucleation. The robustness in the value of ice nucleation temperature was demonstrated with the insensitivity of this temperature to roughnes RMS for over three orders of magnitude in RMS roughness (~0.1 to ~100 nm). Such nanotextures were also applied on top of micropillar structures to harvest the additional benefit from, high liquid repellency and low ice adhesion. The hierarchical morphology surfaces were tested around their nucleation temperatures to determine the time it took to freeze supercooled sessile droplets. When such surfaces, rated at -24 °C for immediate nucleation, were tested at a minimally higher temperature (-21 °C), they delayed the freezing of a sessile supercooled water droplet at the same temperature by a remarkable 25 hours.

The problems due to ice accretion and frost formation have been prevalent for decades in a number of systems including aircrafts, wind turbines and power lines. The state-of-the-art techniques are either inefficient or expensive making them challenging to apply in practical applications. The recent development in the field of lubricant-impregnated textured surfaces has offered an alternative to address this problem. This presentation will focus on the design of the texture and the lubricant used in these surfaces. Ice adhesion strength measurements and cryo-FIB/SEM imaging show the dependence of ice adhesion on the texture density. We report that the optimized lubricant impregnated surfaces perform better than the state-of-the art low-energy solid coatings in lowering the ice adhesion strength.

This presentation will explore the design and properties of a new type of layer-by-layer assembled polymer coating with zwitter-wettable behavior. A zwitter-wettable coating is capable of rapidily absorbing a large amount of non-freezing water from the vapor phase but appears to be hydrophobic when probed with water droplets. Since the water absorbed is in a non-freezing state, the coating exhibits anti-fog and anti-frost capabilities even when conditioned at sub-freezing temperatures and subsequently exposed to warm humid air. Under these conditions, typcial glass surfaces will experience extreme frosting. The coatings are fabricated from aqueous solutions of poly(vinyl alcohol) and poly(acrylic acid) at very specifc pH values. Subsequent thermal crosslinking renders the layer-by-layer assembled coating insoluble in water and more mechanically robust.

Ice formation and accumulation dramatically impact the efficiency and durability of industrial components and machines, can have very detrimental effects on wind turbines and airplanes and strongly affect power transmission lines during winter. In recent years many reports were published on using superhydrophobic surfaces as anti-icing coatings and by this decreasing ice adhesion as well as delaying frost formation. However, very little attention has been paid on the question how condensed water freezes on superhydrophobic surfaces. We report on the generation of superhydrophobic surfaces, which significantly delay the freezing of condensed drops under subzero conditions. The surfaces in our studies comprise an array of silicon nano-needles “nanograss”, coated with fluoropolymer monolayers. The fabrication process of nanograss is mask-free and only consists of a single dry etching step.
When the surface is cooled down below the dew-point temperature, water starts to condense on the surface and eventually forms drops in the Cassie state. The condensed drops spontaneously move on the surface due to release of surface free energy upon coalescence of two drops (so-called mobile condensation). Drop at the edge of the substrate jump off. This results in continuous water removal from sample and therefore considerably lower surface coverage with water if compared with condensation on surfaces possessing immobile condensation.
At subzero temperatures the freezing of condensed drops is mainly governed by heterogeneous nucleation mechanism. Here the ice embryo is preferably formed on water-solid contact area, which dimension determines the nucleation rate of the drop at certain conditions. The solid fraction of nanograss (i.e. the fraction of drop footprint in contact with the solid) was calculated to be 0,01%. Thus, low solid fraction combined with poor surface coverage with water, dramatically decrease the freezing probability at conditions leading to water freezing on hydrophobic surfaces or on surfaces with condensation of drops which are immobile.
As a result, the freezing experiments show that no freezing happened on the nanograss surface held at -10 °C in room environment (40% RH, 22 °C) even for up to 8 hours provided that freezing of sample edges could be neglected. However, when sample was cooled to -20 °C we observed several independent freezing events. This shows that despite the low solid fraction and poor surface coverage with water, the freezing temperatures are considerably higher than ones corresponding to homogeneous freezing. Ways to further improve the anti-freezing properties of surface might be further lowering the solid fraction of nanostructures and enhancing water removal from surface by tilting, shaking or exposing to continuous air flow.

Frost formation and accumulation on cold surfaces adversely affect the operational performance in aircrafts, refrigerators, wind turbines and power lines.[1,2] Engineering icephobic surfaces that can retard the frost formation and accumulation is of scientific and practical importance. Despite the promising potential in engineering icephobic surfaces, current materials are limited by the breakdown of their icephobicity in the condensation frosting environment. In particular, the frost formation over the entire surface is inevitable as a result of undesired inter-droplet freezing wave propagation initiated by the sample edges, which are associated with low heterogeneous frost nucleation energy barrier owing to their geometric singularity.[3,4] Moreover, without a rational design, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process.[5, 6] Thus, an ideal icephobic surface should not only suppress the onset of individual droplet freezing and inter-droplet freezing wave propagation during the condensation frosting process, but also promote the efficient ice removal at the defrosting stage. Here, we report a hierarchical surface that is capable of removing condensate droplets before any heterogeneous ice nucleation could occur, retarding frost formation through the suppression of inter-droplet freezing propagation over the entire surface, as well as promoting frost removal efficiently by self-lubrication. We find that the enhanced performances are mainly owing to the activation of the microscale edge effect in the hierarchical surface, which increases the energy barrier for ice bridging as well as engendering the frost integrity and liquid lubrication during the defrosting process. We envision that the concept of harnessing surface morphology to achieve superior performances in two distinctively opposite phase transition processes (frosting/defrosting) might open up a new avenue for the development of efficient materials for various applications ranging from anti-icing, dropwise condensation and water harvesting.
1. Meuler, A. J., McKinley, G. H. & Cohen, R. E. Exploiting topographical texture to impart icephobicity. ACS Nano 4, 7048-7052 (2010).
2. Stone, H. A. Ice-phobic surfaces that are wet. ACS Nano 6, 6536-6540 (2012).
3. Boreyko, J. B. & Collier, C. P. Delayed frost growth on jumping-drop superhydrophobic surfaces. ACS Nano 7, 1618-1627 (2013).
4. Guadarrama-Cetina, J., Mongruel, A., González-Viñas, W. & Beysens, D. Percolation-induced frost formation. Europhys. Lett. 101, 16009 (2013).
5. Chen, J. et al. Superhydrophobic surfaces cannot reduce ice adhesion. Appl. Phys. Lett. 101, 111603 (2012).
6. Nosonovsky, M. & Hejazi, V. Why superhydrophobic surfaces are not always icephobic. ACS Nano 6, 8488-8491 (2012).

Zwitterionic chemistries have shown great potential as ultra-low fouling surface functionalities, which resist the adhesion of a large variety of molecules and organisms and maintain exceptional cleanliness in practical environments such as sea water, bacterial culture(1) or human serum(2). This resistance to surface adhesion originates from the extreme hydrophilicity of the zwitterionic moieties. Almost perfect wetting occurs when a zwitterionic surface is in contact with water(3). The replacement of surface-bound water molecules by foulants is enthapically unfavorable and thus the surface is anti-fouling.
The extreme hydrophilicity renders zwitterionic polymers soluble in water and therefore the incorporation of cross-linker is necessary to make zwitterionic coatings viable and applicable to various substrates. However, reduced hydrophilicity and impaired fouling resistance have been observed as results of cross-linking. Although the water uptake of a copolymer film is reduced only by 30-50% with 50% cross-linker content(4), the fouling resistance can be reduced to a much greater extent because of the surface chain reorganizations upon film casting(1). The hydrophobic segments introduced by cross-linker tend to migrate to the uppermost surface to minimize the interface energy during the casting in open air.
Here, we report a two-step synthesis scheme that produces surface-concentrated zwitterionic moieties with cross-linked and mechanically robust bulk films(1, 5, 6). A precursor film containing tertiary amine groups is deposited via initiated chemical vapor deposition (iCVD) using commercially available monomers. Then a reaction with 1,3-propanesultone is carried out under diffusion-limited conditions to convert tertiary amine to zwitterion. The diffusion-controlled conditions are the key to high surface concentration of zwitterionic moieties. This method allows for the tuning of bulk film properties independently of the surface fouling resistance. Virtually any surface can be rendered anti-fouling with this substrate-independent and benign method(7). Ultra-thin (30nm) zwitterionic coatings have been applied directly to the surfaces of reverse osmosis desalination membranes with minimal impact on performance in terms of the salt rejection or water permeation rate. An in situ grafting method has been developed to adapt the coated membranes for long-term operations.
1. Yang, R.; Gleason, K. K. Langmuir 2012, 28, 12266-12274.
2. Jiang, S.; Cao, Z. Adv. Mat. 2010, 22, 920-932.
3. Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Angew. Chem. Int. Ed. 2006, 45, 1770-1774.
4. Zhang, L.; Cao, Z.; Bai, T.; Carr, L.; Ella-Menye, J.-R.; Irvin, C.; Ratner, B. D.; Jiang, S. Nat. Biotechnol. 2013, 31, 553-556.
5. Yang, R.; Xu, J.; Ozaydin-Ince, G.; Wong, S