Liquid-infused nanostructured surfaces with extreme anti-ice and anti-frost performance

Extremely durable biofouling-resistant metallic surfaces based on electrodeposited nanoporous tungstite films on steel

Formation of unwanted deposits on steels during their interaction with liquids is an inherent problem that often leads to corrosion, biofouling and results in reduction in durability and function. Here we report a new route to form anti-fouling steel surfaces by electrodeposition of nanoporous tungsten oxide (TO) films. TO-modified steels are as mechanically durable as bare steel and highly tolerant to compressive and tensile stresses due to chemical bonding to the substrate and island-like morphology. When inherently superhydrophilic TO coatings are converted to superhydrophobic, they remain non-wetting even after impingement with yttria-stabilized-zirconia particles, or exposure to ultraviolet light and extreme temperatures. Upon lubrication, these surfaces display omniphobicity against highly contaminating media retaining hitherto unseen mechanical durability. To illustrate the applicability of such a durable coating in biofouling conditions, we modified naval construction steels and surgical instruments and demonstrated significantly reduced marine algal film adhesion, Escherichia coli attachment and blood staining.

Improved Icephobic Properties on Surfaces with a Hydrophilic Lubricating Liquid

Slippery liquid-infused porous surfaces were developed recently for icephobic surface applications. Perfluorinated liquids, silicone oil, hydrocarbon, and water were used as lubricating liquids to form a continuous layer on a suitable substrate to prevent icing. However, ice accretion performances of these surfaces have not been reported previously depending on the type of the lubricant. In this work, fluorinated aliphatics, polyalphaolefin, silicone oil, and decamethylcyclopenta siloxane were used as hydrophobic lubricants; water, ethylene glycol, formamide, and water-glycerine mixture were used as hydrophilic lubricants to be impregnated by hydrophobic polypropylene and hydrophilic cellulose-based filter paper surfaces; ice accretion, drop freezing delay time, and ice adhesion strength properties of these surfaces were examined; and the results were compared to those of the reference surfaces such as aluminum, copper, polypropylene, and polytetrafluoroethylene. An ice accretion test method was also developed to investigate the increase of the mass of formed ice gravimetrically by spraying supercooled water onto these surfaces at different subzero temperatures ranging between -1 and -5 °C. It was determined that hydrophilic solvents (especially a water-glycerine mixture) that impregnated hydrophilic porous surfaces would be a promising candidate for anti-icing applications at -2 °C and 56-83% relative humidity because ice accretion and ice adhesion strength properties of these surface decreased simultaneously in these conditions.

Formation, characterization and stability of oil nanodroplets on immersed substrates

Nanoscale oil droplets locating at solid-liquid interfaces significantly impact the interfacial properties, which are concerned in both industry applications and fundamental studies. This review article presents an overview of the current progress in nanodroplet research. We will start from the characterization of interfacial nanodroplets and the formation of interfacial nanodroplets by direct adsorption from emulsions and by the solvent exchange protocol. Then we will review the experimental and theoretical studies on the evolution of oil nanodroplets including spreading, dissolution, and detachment. We will also cover the emerging applications of the interfacial nanodroplets in the fields of surface functionalization and nanostructure engineering, and particularly, highlight the potential application as capping agents to obtain architectures on microparticle surface. Finally we propose the challenges and the opportunities in this area. In our opinion, the nanodroplets have not only of high relevance to practical applications, but also serve as a model system for understanding many interfacial phenomena, such as phase separation and wetting on a microscopic scale.

A Drosera-bioinspired hydrogel for catching and killing cancer cells

A variety of bioinspired materials have been successfully synthesized to mimic the sophisticated structures or functions of biological systems. However, it is still challenging to develop materials with multiple functions that can be performed synergistically or sequentially. The purpose of this work was to demonstrate a novel bioinspired hydrogel that can interact with cancer cells, functionally similar to Drosera in catching and killing prey. This hydrogel had two layers with the top one functionalized with oligonucleotide aptamers and the bottom one functionalized with double-stranded DNA. The results show that the top hydrogel layer was able to catch target cells with high efficiency and specificity, and that the bottom hydrogel layer could sequester doxorubicin (Dox) for sustained drug release. Importantly, the released Dox could kill 90% of the cells after 1-h residence of the cells on the hydrogel. After the cell release, this bifunctional hydrogel could be regenerated for continuous cell catching and killing. Therefore, the data presented in this study has successfully demonstrated the potential of developing a material system with the functions of attracting, catching and killing diseased cells (e.g., circulating tumor cells) or even invading microorganisms (e.g., bacteria).

Controlled Fabrication of Polypyrrole Surfaces with Overhang Structures by Colloidal Templating

Here we present the fabrication of polypyrrole (PPy) surfaces with a controlled overhang structure. Regularly structured PPy films were produced using interfacial polymerization around a sacrificial crystalline colloidal monolayer at the air/water interface. The morphology of the final inverse colloidal PPy film is controlled by the amount of monomer, the monomer: oxidant ratio and polymerization time. The PPy films exhibit an overhang structure due to depth of particle immersion in the water phase. As a result of the overhang structure, the PPy films are made hydrophobic, although the material itself is hydrophilic. The apparent contact angle of water on the structured surfaces is 109.5°, which is in agreement with the predicted contact angle using the Cassie-Baxter equation for air-filled cavities. This fabrication technique is scalable and can be readily extended to other systems where controlled wettability is required.

The springtail cuticle as a blueprint for omniphobic surfaces

Omniphobic surfaces found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies. One example is the water and even oil-repellent cuticle of springtails (Collembola). The wingless arthropods evolved a highly textured, hierarchically arranged surface pattern that affords mechanical robustness and wetting resistance even at elevated hydrostatic pressures. Springtail cuticle-derived surfaces therefore promise to overcome limitations of lotus-inspired surfaces (low durability, insufficient repellence of low surface tension liquids). In this review, we report on the liquid-repellent natural surfaces of arthropods living in aqueous or temporarily flooded habitats including water-walking insects or water spiders. In particular, we focus on springtails presenting an overview on the cuticular morphology and chemistry and their biological relevance. Based on the obtained liquid repellence of a variety of liquids with remarkable efficiency, the review provides general design criteria for robust omniphobic surfaces. In particular, the resistance against complete wetting and the mechanical stability strongly both depend on the topographical features of the nano- and micropatterned surface. The current understanding of the underlying principles and approaches to their technological implementation are summarized and discussed.

Creating Anti-icing Surfaces via the Direct Immobilization of Antifreeze Proteins on Aluminum

Cryoprotectants such as antifreeze proteins (AFPs) and sugar molecules may provide a solution for icing problems. These anti-icing substances protect cells and tissues from freezing by inhibiting ice formation. In this study, we developed a method for coating an industrial metal material (aluminum, Al) with AFP from the Antarctic marine diatom, Chaetoceros neogracile (Cn-AFP), to prevent or delay ice formation. To coat Al with Cn-AFP, we used an Al-binding peptide (ABP) as a conjugator and fused it with Cn-AFP. The ABP bound well to the Al and did not considerably change the functional properties of AFP. Cn-AFP-coated Al (Cn-AFP-Al) showed a sufficiently low supercooling point. Additional trehalose coating of Cn-AFP-Al considerably delayed AFP denaturation on the Al without affecting its antifreeze activity. This metal surface–coating method using trehalose-fortified AFP can be applied to other metals important in the aircraft and cold storage fields where anti-icing materials are critical.

Investigation into the Formation and Adhesion of Cyclopentane Hydrates on Mechanically Robust Vapor-Deposited Polymeric Coatings

Blockage of pipelines by formation and accumulation of clathrate hydrates of natural gases (also called gas hydrates) can compromise project safety and economics in oil and gas operations, particularly at high pressures and low temperatures such as those found in subsea or arctic environments. Cyclopentane (CyC5) hydrate has attracted interest as a model system for studying natural gas hydrates, because CyC5, like typical natural gas hydrate formers, is almost fully immiscible in water; and thus CyC5 hydrate formation is governed not only by thermodynamic phase considerations but also kinetic factors such as the hydrocarbon/water interfacial area, as well as mass and heat transfer constraints, as for natural gas hydrates. We present a macroscale investigation of the formation and adhesion strength of CyC5 hydrate deposits on bilayer polymer coatings with a range of wettabilities. The polymeric bilayer coatings are developed using initiated chemical vapor deposition (iCVD) of a mechanically robust and densely cross-linked polymeric base layer (polydivinylbenzene or pDVB) that is capped with a covalently attached thin hydrate-phobic fluorine-rich top layer (poly(perfluorodecyl acrylate) or pPFDA). The CyC5 hydrates are formed from CyC5-in-water emulsions, and differential scanning calorimetry (DSC) is used to confirm the thermal dissociation properties of the solid hydrate deposits. We also investigate the adhesion of the CyC5 hydrate deposits on bare and bilayer polymer-coated silicon and steel substrates. Goniometric measurements with drops of CyC5-in-water emulsions on the coated steel substrates exhibit advancing contact angles of 148.3 ± 4.5° and receding contact angles of 142.5 ± 9.8°, indicating the strongly emulsion-repelling nature of the iCVD coatings. The adhesion strength of the CyC5 hydrate deposits is reduced from 220 ± 45 kPa on rough steel substrates to 20 ± 17 kPa on the polymer-coated steel substrates. The measured strength of CyC5 hydrate adhesion is found to correlate very well with the work of adhesion between the emulsion droplets used to form the CyC5 hydrate and the underlying substrates.

Fabrication of liquid-infused surfaces using reactive polymer multilayers: principles for manipulating the behaviors and mobilities of aqueous fluids on slippery liquid interfaces

The design of slippery liquid-infused porous surfaces (SLIPS) using nanoporous and chemically reactive polymer multilayers is reported. This approach permits fabrication of slippery anti-fouling coatings on complex surfaces and provides new means to manipulate the mobilities of contacting aqueous fluids. The results expand the range of tools that can be used to manipulate the behaviors of SLIPS and open the door to new applications of this emerging class of soft materials.

Physics of icing and rational design of surfaces with extraordinary icephobicity

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

Simple approach to superhydrophobic nanostructured Al for practical antifrosting application based on enhanced self-propelled jumping droplets

Frost formation can cause operational difficulty and efficiency loss for many facilities such as aircraft, wind turbines, and outdoor heat exchangers. Self-propelled jumping by condensate droplets on superhydrophobic surfaces delays frost formation, so many attempts have been made to exploit this phenomenon. However, practical application of this phenomenon is currently unfeasible because many processes to fabricate the superhydrophobic surfaces are inefficient and because self-propelled jumping is difficult to be achieved in a humid and low-temperature environment because superhydrophobicity is degraded in these conditions. Here, we achieved significantly effective anti-icing superhydrophobic aluminum. Its extremely low adhesive properties allow self-propelled jumping under highly supersaturated conditions of high humidity or low surface temperature. As a result, this surface helps retard frost formation at that condition. The aluminum was made superhydrophobic by a simple and cost-effective process that is adaptable to any shape. Therefore, it has promise for use in practical and industrial applications.

"Insensitive" to touch: fabric-supported lubricant-swollen polymeric films for omniphobic personal protective gear

The use of personal protective gear made from omniphobic materials that easily shed drops of all sizes could provide enhanced protection from direct exposure to most liquid-phase biological and chemical hazards and facilitate the postexposure decontamination of the gear. In recent literature, lubricated nanostructured fabrics are seen as attractive candidates for personal protective gear due to their omniphobic and self-healing characteristics. However, the ability of these lubricated fabrics to shed low surface tension liquids after physical contact with other objects in the surrounding, which is critical in demanding healthcare and military field operations, has not been investigated. In this work, we investigate the depletion of oil from lubricated fabrics in contact with highly absorbing porous media and the resulting changes in the wetting characteristics of the fabrics by representative low and high surface tension liquids. In particular, we quantify the loss of the lubricant and the dynamic contact angles of water and ethanol on lubricated fabrics upon repeated pressurized contact with highly absorbent cellulose-fiber wipes at different time intervals. We demonstrate that, in contrast to hydrophobic nanoparticle coated microfibers, fabrics encapsulated within a polymer that swells with the lubricant retain the majority of the oil and are capable of repelling high as well as low surface tension liquids even upon multiple contacts with the highly absorbing wipes. The fabric supported lubricant-swollen polymeric films introduced here, therefore, could provide durable and easy to decontaminate protection against hazardous biological and chemical liquids.

Bioinspired engineering of thermal materials

In the development of next-generation materials with enhanced thermal properties, biological systems in nature provide many examples that have exceptional structural designs and unparalleled performance in their thermal or nonthermal functions. Bioinspired engineering thus offers great promise in the synthesis and fabrication of thermal materials that are difficult to engineer through conventional approaches. In this review, recent progress in the emerging area of bioinspired advanced materials for thermal science and technology is summarized. State-of-the-art developments of bioinspired thermal-management materials, including materials for efficient thermal insulation and heat transfer, and bioinspired materials for thermal/infrared detection, are highlighted. The dynamic balance of bioinspiration and practical engineering, the correlation of inspiration approaches with the targeted applications, and the coexistence of molecule-based inspiration and structure-based inspiration are discussed in the overview of the development. The long-term outlook and short-term focus of this critical area of advanced materials engineering are also presented.

Controlled electro-coalescence/non-coalescence on lubricating fluid infused slippery surfaces

Aqueous drops on lubricating oil infused substrates depict non-coalescence or spontaneous coalescence which can be controlled using an external electric field.

Multiple sheet-layered super slippery surfaces based on anodic aluminium oxide and its anticorrosion property

A key challenge for this paper is to discuss the effects of porosity on the properties of substrates. Co₃O₄grown on AAO super slippery surfaces with high porosity possessed good anticorrosion properties and super slippery properties by comparison.

Direct observation of drops on slippery lubricant-infused surfaces

Water droplet resting on a slippery surface.

Functional map of biological and biomimetic materials with hierarchical surface structures

The properties and functions of both biological and artificial materials with hierarchical surface structures are reviewed to establish the functional map of various hierarchical surface structures.

Mechanism of delayed frost growth on superhydrophobic surfaces with jumping condensates: more than interdrop freezing

Delayed frost growth on superhydrophobic surfaces (SHSs) with jumping condensates has been found by many researchers recently. However, the mechanism of this phenomenon has not been elucidated clearly. In this study, copper SHSs with or without jumping condensates were selected as the substrates for observing condensation icing at a relative humidity (RH) of 60%. The results showed that only SHS with jumping condensates showed delayed condensation icing. Moreover, when such SHSs were placed upward and the surface temperature was held at -10 °C, some discrete frozen drops first appeared on the SHSs. The following icing mainly occurred on these discrete global crystals and then expanded around them until covering the entire surface. Little macroscopic interdrop freezing phenomenon was found. The growth of the frost front is mainly dominated by jumping freezing (the condensed droplets jumped onto the ice crystals and were frozen) or direct vapor-ice deposition. Using microscopy, we found interdrop freezing occurred, in addition to the two mechanisms mentioned above. By placing the SHS downward at -10 °C and intentionally introducing or eliminating tiny dusts, we confirmed that there were no superhydrophobic defects on our SHSs. The discrete frozen drops first appearing on the SHSs were triggered by tiny dusts falling on the surface before or during condensation icing. The key approach in delaying or resisting frost growth on SHSs with jumping condensates is to retard initial ice crystal formation, e.g., eliminating the edge effect and keeping the SHSs clean.

Multiscale effect of hierarchical self-assembled nanostructures on superhydrophobic surface

In this work, we describe self-assembled surfaces with a peculiar multiscale organization, from the nanoscale to the microscale, exhibiting the Cassie-Baxter wetting regime with extremely low water adhesion: floating drops regime with roll-off angles < 5°. These surfaces comprise bundles of hierarchical, quasi-one-dimensional (1D) TiO2 nanostructures functionalized with a fluorinated molecule (PFNA). While the hierarchical nanostructures are the result of a gas-phase self-assembly process, their bundles are the result of the capillary forces acting between them when the PFNA solvent evaporates. Nanometric features are found to influence the hydrophobic behavior of the surface, which is enhanced by the micrometric structures up to the achievement of the superhydrophobic Cassie-Baxter state (contact angle (CA) ≫ 150°). Thanks to their high total and diffuse transmittance and their self-cleaning properties, these surfaces could be interesting for several applications such as smart windows and photovoltaics where light management and surface cleanliness play a crucial role. Moreover, the multiscale analysis performed in this work contributes to the understanding of the basic mechanisms behind extreme wetting behaviors.

Bioinspired materials: from low to high dimensional structure

The surprising properties of biomaterials are the results of billions of years of evolution. Generally, biomaterials are assembled under mild conditions with very limited supply of constituents available for living organism, and their amazing properties largely result from the sophisticated hierarchical structures. Following the biomimetic principles to prepare manmade materials has drawn great research interests in materials science and engineering. In this review, we summarize the recent progress in fabricating bioinspired materials with the emphasis on mimicking the structure from one to three dimensions. Selected examples are described with a focus on the relationship between the structural characters and the corresponding functions. For one-dimensional materials, spider fibers, polar bear hair, multichannel plant roots and so on have been involved. Natural structure color and color shifting surfaces, and the antifouling, antireflective coatings of biomaterials are chosen as the typical examples of the two-dimensional biomimicking. The outstanding protection performance, and the stimuli responsive and self-healing functions of biomaterials based on the sophisticated hierarchical bulk structures are the emphases of the three-dimensional mimicking. Finally, a summary and outlook are given.

Thickness dependence of ice removal stress for a polydimethylsiloxane nanocomposite: Sylgard 184

Minimizing adhesion of ice has been the subject of extensive studies because of importance to applications such aircraft wings, spacecraft, and power transmission wires. A growing interest concerns coatings for wind turbine blades and refrigeration. Herein, a new laboratory test was employed to obtain the thickness dependence of ice adhesion for Sylgard 184-a filled polydimethylsiloxane elastomer. A correlation between ice adhesion and coating thickness (t) was found that follows a relationship developed by Kendall over 40 years ago for removal of a rigid object from an elastomer. With a 0.05 mm/s probe speed a nearly linear relationship between peak removal stress (Ps) and 1/t(1/2) was obtained with Ps ∼ 460 kPa for an 18 μm coating, decreasing to ∼120 kPa for 533 μm. Preliminary results suggest that below ∼10 μm Ps departs from the 1/t(1/2) correlation while above ∼500 μm a limiting value for Ps may be reached. We previously reported that probe speed has negligible effect on the glassy polymer PMMA. In contrast, probe speed is identified as an important variable for testing ice release on elastomeric Sylgard 184 coatings. While work of adhesion, which is related to surface free energy, is recognized as an important factor that can affect ice release, the results reported herein show that coating thickness can override this single parameter for elastomeric substrates.

Anti-ice coating inspired by ice skating

Accumulation of ice to surfaces brings dangerous and costly problems to our daily life. In this paper, an anti-ice coating inspired by ice skating is reported. Hyaluronic acid is used in the anti-ice coating to form aqueous lubricating layer benefitting from its high water absorbing property. Dopamine, the main component of the mussel adhesive protein, is introduced to anchor the hyaluronic acid to the solid surfaces to render the coating applicable to all types of solid surfaces. At the same time it serves as the crosslinking agent for hyaluronic acid, thus the thickness of the water collecting film could be easily varied. Ice adhesion strength on surfaces coated with such kind of coating could be more than one order of magnitude lower than that of uncoated ones. The results indicate that this anti-ice coating with the aqueous lubricating layer has great potential for fighting against icing problems.

Interfacial material system exhibiting superwettability

Engineering the wettability of solid materials is a traditional, yet key issue in surface science and attracts tremendous interest by researchers in diverse fields. Recently, different superwetting phenomena have been discovered in both nature and experimental results. Therefore, in this review, various superwetting states, leading to a "superwettability" system, are summarized and predicted. Fundamental rules for understanding superwettability are discussed, mainly taking superhydrophobicity in air as an example. Then, some recent application progress of individual members of this "superwettability" system are introduced. Notably, several novel application fields, mainly gas, water, oil and/or other liquid environments, are presented in the following section. By combining different members of this "superwettability" system, new interfacial functions can be generated, allowing unexpected applications, such as in environmental protection, energy, green industry, and many other important domains. Finally, the future development of this interesting "superwettability" system is discussed.

Surface micro/nanotopography, wetting properties and the potential for biomimetic icephobicity of skunk cabbage Symplocarpus foetidus

Lotus (Nelumbo nucifera) is known for its two remarkable properties: superhydrophobicity and thermogenesis; however, the relationship between these two properties remains obscure. Most botanists agree that thermogenesis helps to attract pollinators, while non-wetting helps to catch pollinators and prevents contamination. Here we investigate the surface micro- and nanotopography and wetting properties of eastern skunk cabbage (Symplocarpus foetidus), another thermogenic plant, which is known for its ability to melt snow. The skunk cabbage leaves are hydrophobic but not superhydrophobic, and they have high contact angle hysteresis (similar to the rose petal effect). We develop a heat transfer model to relate icephobicity with heat transfer and discuss the biomimetic potential that both thermogenic and superhydrophobic plants may have for icephobicity in soft materials.

Drag reduction using lubricant-impregnated surfaces in viscous laminar flow

Lubricant-impregnated surfaces (LIS), where micro/nanotextured surfaces are impregnated with lubricating liquids, have received significant attention for their robust, superslippery properties. In this study, we systematically demonstrate the potential for LIS to reduce drag in laminar flows. We present a scaling model that incorporates the viscosity of the lubricant and elucidates the dependence of drag reduction on the ratio of the viscosity of the working fluid to that of the lubricant. We experimentally validate this dependence in a cone and plate rheometer and demonstrate a drag reduction of 16% and slip length of 18 μm in the case where the ratio of working fluid viscosity to lubricant viscosity is 260.

Optical phenomena and antifrosting property on biomimetics slippery fluid-infused antireflective films via layer-by-layer comparison with superhydrophobic and antireflective films

Sophisticated material interfaces generated by natural life forms such as lotus leaves and Nepenthes pitcher plants have exceptional abilities to resolve challenges in wide areas of industry and medicine. The nano- and microstructures inspired by these natural materials can repel various liquids and form self-cleaning coatings. In particular, slippery liquid-infused surfaces are receiving remarkable interest as transparent, nonfouling, and antifrosting synthetic surfaces for solar cells and optical devices. Here we focus on the transparency of lubricant-infused texture on antireflective films fabricated by layer-by-layer self-assembly that decrease light scattering, which is important to maintain device properties. A slippery fluid-infused antireflective film composed of chitin nanofibers less than 50 nm in diameter prevented light scattering at the long-wavelength side by Rayleigh scattering to achieve 97.2% transmittance. Moreover, films composed of the same materials demonstrated three different morphologies: superhydrophilicity with antireflection, superhydrophobicity, and omniphobicity, mimicking the biological structures of moth eyes, lotus leaves, and pitcher plants, respectively. The effect of thermal changes on the ability of each film to prevent frost formation was investigated. The slippery fluid-infused antireflective film showed effective antifrosting behavior.

Drop impact dynamics on oil-infused nanostructured surfaces

We experimentally investigated the impact dynamics of a water drop on oil-infused nanostructured surfaces using high-speed microscopy and scalable metal oxide nano surfaces. The effects of physical properties of the oil and impact velocity on complex fluid dynamics during drop impact were investigated. We show that the oil viscosity does not have significant effects on the maximal spreading radius of the water drop, while it moderately affects the retraction dynamics. The oil viscosity also determines the stability of the infused lubricant oil during the drop impact; i.e., the low viscosity oil layer is easily displaced by the impacting drop, which is manifested by a residual mark on the impact region and earlier initiation of prompt splashing. Also, because of the liquid (water)-liquid (oil) interaction on oil-infused surfaces, various instabilities are developed at the rim during impact under certain conditions, resulting in the flower-like pattern during retraction or elongated filaments during spreading. We believe that our findings will contribute to the rational design of oil-infused surfaces under drop impact conditions by illuminating the complex fluid phenomena on oil-infused surfaces during drop impact.

Perspectives on surface nanobubbles

Materials of nanoscale size exhibit properties that macroscopic materials often do not have. The same holds for bubbles on the nanoscale: nanoscale gaseous domains on a solid-liquid interface have surprising properties. These include the shape, the long life time, and even superstability. Such so-called surface nanobubbles may have wide applications. This prospective article covers the basic properties of surface nanobubbles and gives several examples of potential nanobubble applications in nanomaterials and nanodevices. For example, nanobubbles can be used as templates or nanostructures in surface functionalization. The nanobubbles produced in situ in a microfluidic system can even induce an autonomous motion of the nanoparticles on which they form. Their formation also has implications for the fluid transport in narrow channels in which they form.

From superhydrophobicity to icephobicity: forces and interaction analysis

The term “icephobicity” has emerged in the literature recently. An extensive discussion took place on whether the icephobicity is related to the superhydrophobicity, and the consensus is that there is no direct correlation. Besides the parallel between the icephobicity and superhydrophobicity for water/ice repellency, there are similarities on other levels including the hydrophobic effect/hydrophobic interactions, mechanisms of protein folding and ice crystal formation. In this paper, we report how ice adhesion is different from water using force balance analysis, and why superhydrophobic surfaces are not necessary icephobic. We also present experimental data on anti-icing of various surfaces and suggest a definition of icephobicity, which is broad enough to cover a variety of situations relevant to de-icing including low adhesion strength and delayed ice crystallization and bouncing.

Anti-icing coating with an aqueous lubricating layer

In this paper, an anti-icing coating with an aqueous lubricating layer is reported. This anti-icing coating can be directly applied to various substrates, and the ice adhesion strength on the coated surfaces can be lowered greatly as compared to uncoated substrates. We demonstrate for the first time that the formed ice on this anti-icing coating can be blown off by a wind action in the wind tunnel with a controlled temperature and wind velocity. Moreover, the low ice adhesion of the anti-icing coating can be maintained even when the temperature is lowered to -53 °C. The robustness and durability of the anti-icing coating are proved by the icing/de-icing experiments. The results show that the anti-icing coating with an aqueous lubricating layer is of great promise for practical applications.

Air-stable droplet interface bilayers on oil-infused surfaces

Droplet interface bilayers are versatile model membranes useful for synthetic biology and biosensing; however, to date they have always been confined to fluid reservoirs. Here, we demonstrate that when two or more water droplets collide on an oil-infused substrate, they exhibit noncoalescence due to the formation of a thin oil film that gets squeezed between the droplets from the bottom up. We show that when phospholipids are included in the water droplets, a stable droplet interface bilayer forms between the noncoalescing water droplets. As with traditional oil-submerged droplet interface bilayers, we were able to characterize ion channel transport by incorporating peptides into each droplet. Our findings reveal that droplet interface bilayers can function in ambient environments, which could potentially enable biosensing of airborne matter.

Transparent slippery surfaces made with sustainable porous cellulose lauroyl ester films

In recent years, liquid repellent surfaces have attracted considerable attention because of their wide array of potential applications. In the present study, slippery surfaces were fabricated using novel sustainable, nanoporous cellulose lauroyl ester (CLE) films and slippery lubrication fluid. The nanoporous CLE films were obtained after spray-coating target surfaces using a nanoparticle suspension of CLE that was prepared via nanoprecipitation. After the deposition of the slippery liquid within the porous network, the obtained slippery surfaces exhibit both excellent liquid repellency upon liquid impact and anti-icing properties (by significantly retarding the icing time). Three-dimensional droplet manipulation was also achieved on these surfaces by taking advantage of the materials' low contact angle hysteresis and low adhesion property.

Superhydrophilic polyelectrolyte brush layers with imparted anti-icing properties: effect of counter ions

This work demonstrates the feasibility of superhydrophilic polyelectrolyte brush coatings for anti-icing applications. Five different types of ionic and nonionic polymer brush coatings of 25-100 nm thickness were formed on glass substrates using silane chemistry for surface premodification followed by polymerization via the SI-ATRP route. The cationic [2-(methacryloyloxy)ethyl]trimethylammonium chloride] and the anionic [poly(3-sulfopropyl methacrylate), poly(sodium methacrylate)] polyelectrolyte brushes were further exchanged with H+, Li+, Na+, K+, Ag+, Ca2+, La3+, C16N+, F-, Cl-, BF4-, SO4(2-), and C12SO3- ions. By consecutive measurements of the strength of ice adhesion toward ion-incorporated polymer brushes on glass it was found that Li+ ions reduce ice adhesion by 40% at -18 °C and 70% at -10 °C. Ag+ ions reduce ice adhesion by 80% at -10 °C relative to unmodified glass. In general, superhydrophilic polyelectrolyte brushes exhibit better anti-icing property at -10 °C compared to partially hydrophobic brushes such as poly(methyl methacrylate) and surfactant exchanged polyelectrolyte brushes. The data are interpreted using the concept of a quasi liquid layer (QLL) that is enhanced in the presence of highly hydrated ions at the interface. It is suggested that the ability of ions to coordinate water is directly related to the efficiency of a given anti-icing coating based on the polyelectrolyte brush concept.

Direct three-dimensional imaging of polymer-water interfaces by nanoscale hard X-ray phase tomography

We report three-dimensional (3D) direct imaging of complex surface-liquid interfaces by hard X-ray phase contrast tomography as a non-destructive approach for the morphological characterization of surfaces at the micro- and nanoscale in contact with water. Specifically, we apply this method to study the solid-air-water interface in hydrophobic macroporous polymethacrylate surfaces, and the solid-oil-water interface in slippery liquid-infused porous surfaces (SLIPS). Varying the isotropic spatial resolution allows the 3D quantitative characterization of individual polymer globules, globular clusters (porosity) as well as the infused lubricant layer on SLIPS. Surface defects were resolved at the globular level. We show the first application of X-ray nanotomography to hydrated surface characterizations and we anticipate that X-ray nanoscale imaging will open new ways for various surface/interface studies.

Rational nanostructuring of surfaces for extraordinary icephobicity

Icing of surfaces is commonplace in nature, technology and everyday life, bringing with it sometimes catastrophic consequences. A rational methodology for designing materials with extraordinary resistance to ice formation and adhesion remains however elusive. We show that ultrafine roughnesses can be fabricated, so that the ice nucleation-promoting effect of nanopits on surfaces is effectively counteracted in the presence of an interfacial quasiliquid layer. The ensuing interface confinement strongly suppresses the stable formation of ice nuclei. We explain why such nanostructuring leads to the same extremely low, robust nucleation temperature of ∼-24 °C for over three orders of magnitude change in RMS size (∼0.1 to ∼100 nm). Overlaying such roughnesses on pillar-microtextures harvests the additional benefits of liquid repellency and low ice adhesion. When tested at a temperature of -21 °C, such surfaces delayed the freezing of a sessile supercooled water droplet at the same temperature by a remarkable 25 hours.

Electrically conductive PEDOT coating with self-healing superhydrophobicity

A self-healing electrically conductive superhydrophobic poly(3,4-ethylenedioxythiophene) (PEDOT) coating has been prepared by chemical vapor deposition of a fluoroalkylsilane (POTS) onto a PEDOT film, which was obtained by electrochemical deposition. The coating not only maintained high conductivity with a low resistivity of 3.2 × 10(-4) Ω·m, but also displayed a water contact angle larger than 156° and a sliding angle smaller than 10°. After being etched with O2 plasma, the coating showed an excellent self-healing ability, spontaneously regaining its superhydrophobicity when left under ambient conditions for 20 h. This superhydrophobicity recovery process was found to be humidity-dependent, and could be accelerated and completed within 2 h under a high humidity of 84%. The coating also exhibited good superhydrophobicity recovering ability after being corroded by strong acid solution at pH 1 or strong base solution at pH 14 for 3 h.

Bio-inspired strategies for anti-icing

Undesired ice accumulation leads to severe economic issues and, in some cases, loss of lives. Although research on anti-icing has been carried out for decades, environmentally harmless, economical, and efficient strategies for anti-icing remain to be developed. Recent researches have provided new insights into the icing phenomenon and shed light on some promising bio-inspired anti-icing strategies. The present review critically categorizes and discusses recent developments. Effectively trapping air in surface textures of superhydrophobic surfaces weakens the interaction of the surfaces with liquid water, which enables timely removal of impacting and condensed water droplets before freezing occurs. When ice already forms, ice adhesion can be significantly reduced if liquid is trapped in surface textures as a lubricating layer. As such, ice could be shed off by an action of wind or its gravity. In addition, bio-inspired anti-icing strategies via trapping or introducing other media, such as phase change materials and antifreeze proteins, are discussed.

Strong anti-ice ability of nanohairs over micro-ratchet structures

A strong anti-ice property of nanohairs over micro-ratchet surfaces is observed. A long freezing delay of more than 185 min is achieved for a droplet on the nanohairs over ratchet structure with a period of ∼290 μm under -10 °C, which is attributed to the effective cooperation of the nano- and microstructures.

Antifogging and icing-delay properties of composite micro- and nanostructured surfaces

A composite micro/nanostrucutred (MN) surface was designed using poly(vinylidene difluoride) (PVDF) polymer in combination with ZnO materials via heat-pattern-transfer and crystal-growth techniques. The surface, composed of ZnO nanohairs over PVDF microratchets (i.e., ZP-MN), displays excellent antifogging and icing-delay properties. Condensed water droplets can be easily shed from the ZP-MN surface at -5 °C for ∼1600 s via a slight wind or tilting. The droplets do not completely freeze on the ZP-MN surface at -10 °C until ∼7360 s. This investigation offers a way to design a structured surface that possesses anti-icing ability, which is significant because it can be extended to fields such as microdevices, engineering systems, and engines that operate in a cold or humid environment.

Transparency and damage tolerance of patternable omniphobic lubricated surfaces based on inverse colloidal monolayers

A transparent coating that repels a wide variety of liquids, prevents staining, is capable of self-repair and is robust towards mechanical damage can have a broad technological impact, from solar cell coatings to self-cleaning optical devices. Here we employ colloidal templating to design transparent, nanoporous surface structures. A lubricant can be firmly locked into the structures and, owing to its fluidic nature, forms a defect-free, self-healing interface that eliminates the pinning of a second liquid applied to its surface, leading to efficient liquid repellency, prevention of adsorption of liquid-borne contaminants, and reduction of ice adhesion strength. We further show how this method can be applied to locally pattern the repellent character of the substrate, thus opening opportunities to spatially confine any simple or complex fluids. The coating is highly defect-tolerant due to its interconnected, honeycomb wall structure, and repellency prevails after the application of strong shear forces and mechanical damage. The regularity of the coating allows 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.

Activating the microscale edge effect in a hierarchical surface for frosting suppression and defrosting promotion

Despite extensive progress, current icephobic 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. Moreover, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process. Here, we report a hierarchical surface which allows for interdroplet freezing wave propagation suppression and efficient frost removal. 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 liquid lubrication during the defrosting process. We believe the concept of harnessing the surface morphology to achieve superior performances in two opposite phase transition processes might shed new light on the development of novel materials for various applications.

High-transparency, self-standable gel-SLIPS fabricated by a facile nanoscale phase separation

Slippery liquid-infused porous surfaces (SLIPSs) that were both highly transparent and free-standing (self-standability) were fabricated by an extremely simple process using non-solvent-induced phase separation (NIPS) of a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/di-n-butyl phthalate solution. We call these "Gel-SLIPS" because the porous PVDF-HFP film fabricated using the NIPS process has been used as a gel electrolyte in a lithium-ion battery. In previous reports, SLIPS fabrication required complex processes, high annealing temperatures, and drying. Gel-SLIPS can be fabricated from the adjusted solution and the lubricant at room temperature and pressure in 5 min by squeegee, cast, or dip methods. NIPS is based on a quick phase separation process in situ, and reduction of the surface energy is not required because of the considerable fluorine in PVDF-HFP. Moreover, because of the flexible nanonetwork structure of PVDF-HFP, Gel-SLIPS exhibited self-standability and high transmittance (>87% at 600 nm). Gel-SLIPS is thus highly versatile in terms of the fabrication process and film characteristics.

General frost growth mechanism on solid substrates with different stiffness

Preventing or delaying frost formation on surfaces is of significant importance in many aspects of our daily life. Despite many efforts and improvements recently achieved in the design of new icephobic materials and substrates, not all proposed solutions are universally applicable and frost formation still remains a problem in need of further flexible solutions. In this respect, we propose to take benefit from the tunable viscoelastic properties of soft polymer gel substrates, since they are known to strongly influence the dropwise condensation process of water, and to investigate condensation frosting on them. Using polymer gels with different stiffness and a hard substrate as a reference, we demonstrate their ability to delay frost formation compared to recent results reported in the literature on other solid substrates and in particular on superhydrophobic surfaces. By investigating the frost front propagation we singled out a general behavior of its dynamic evolution consisting of two processes presenting two different time scales. This general growth appears to be independent of experimental conditions as well as substrate stiffness.

Peanut leaf inspired multifunctional surfaces

Nature has long served as a source of inspiration for scientists and engineers to design and construct multifunctional artificial materials. The lotus and the peanut are two typical plants living in the aquatic and the arid (or semiarid) habitats, respectively, which have evolved different optimized solutions to survive. For the lotus leaf, an air layer is formed between its surface and water, exhibiting a discontinuous three-phase contact line, which resulted in the low adhesive superhydrophobic self-cleaning effect to avoid the leaf decomposition. In contrast to the lotus leaf, the peanut leaf shows high-adhesive superhydrophobicity, arising from the formation of the quasi-continuous and discontinuous three-phase contact line at the microscale and nanoscale, respectively, which provides a new avenue for the fabrication of high adhesive superhydrophobic materials. Further, this high adhesive and superhydrophobic peanut leaf is proved to be efficient in fog capture. Inspired by the peanut leaf, multifunctional surfaces with structural similarity to the natural peanut leaf are prepared, exhibiting simultaneous superhydrophobicity and high adhesion towards water.

Fabrics coated with lubricated nanostructures display robust omniphobicity

The development of a stain-resistant and pressure-stable textile is desirable for consumer and industrial applications alike, yet it remains a challenge that current technologies have been unable to fully address. Traditional superhydrophobic surfaces, inspired by the lotus plant, are characterized by two main components: hydrophobic chemical functionalization and surface roughness. While this approach produces water-resistant surfaces, these materials have critical weaknesses that hinder their practical utility, in particular as robust stain-free fabrics. For example, traditional superhydrophobic surfaces fail (i.e., become stained) when exposed to low-surface-tension liquids, under pressure when impacted by a high-velocity stream of water (e.g., rain), and when exposed to physical forces such as abrasion and twisting. We have recently introduced slippery lubricant-infused porous surfaces (SLIPS), a self-healing, pressure-tolerant and omniphobic surface, to address these issues. Herein we present the rational design and optimization of nanostructured lubricant-infused fabrics and demonstrate markedly improved performance over traditional superhydrophobic textile treatments: SLIPS-functionalized cotton and polyester fabrics exhibit decreased contact angle hysteresis and sliding angles, omni-repellent properties against various fluids including polar and nonpolar liquids, pressure tolerance and mechanical robustness, all of which are not readily achievable with the state-of-the-art superhydrophobic coatings.

One-step fabrication of robust and optically transparent slippery coatings

An optically transparent slippery surface was fabricated from a blend of Perfluoropolyether and (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane by electrospraying and subsequent low temperature curing.