Ice repellency can be achieved on various hydrophilic and hydrophobic surfaces, although a surface that repels ice under all environmental scenarios remains elusive. Different strategies are reviewed with a focus on the recent development of superhydrophobic and lubricant-infused surfaces.

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Design of anti-icing surfaces: smooth, textured or slippery?

Fogging occurs when moisture condensation takes the form of accumulated droplets with diameters larger than 190 nm or half of the shortest wavelength (380 nm) of visible light. This problem may be effectively addressed by changing the affinity of a material’s surface for water, which can be accomplished via two approaches: i) the superhydrophilic approach, with a water contact angle (CA) less than 5°, and ii) the superhydrophobic approach, with a water CA greater than 150°, and extremely low CA hysteresis. To date, all techniques reported belong to the former category, as they are intended for applications in optical transparent coatings. A well-known example is the use of photocatalytic TiO2 nanoparticle coatings that become superhydrophilic under UV irradiation. Very recently, a capillary effect was skillfully adopted to achieve superhydrophilic properties by constructing 3D nanoporous structures from layer-by-layer assembled nanoparticles. The key to these two “wet”-style antifogging strategies is for micrometer-sized fog drops to rapidly spread into a uniform thin film, which can prevent light scattering and reflection from nucleated droplets. Optical transparency is not an intrinsic property of antifogging coatings even though recently developed antifogging coatings are almost transparent, and the transparency could be achieved by further tuning the nanoparticle size and film thickness. To our knowledge, the antifogging coatings may also be applied to many fields that do not require optical transparency, including, for example, paints for inhibiting swelling and peeling issues and metal surfaces for preventing corrosion. These types of issues, which are caused by adsorption of moisture, are hard to solve by the superhydrophilic approach because of its inherently “wet” nature. Thus, a “dry”-style antifogging strategy, which consists of a novel superhydrophobic technique that can prevent moisture or microscale fog drops from nucleating on a surface, is desired. Recent bionic researches have revealed that the self-cleaning ability of lotus leaves and the striking ability of a water-strider’s legs to walk on water can be attributed to the ideal superhydrophobicity of their surfaces, induced by special microand nanostructures. To date, the biomimetic fabrication of superhydrophobic microand/or nanostructures has attracted considerable interest, and these types of materials can be used for such applications as self-cleaning coatings and stain-resistant textiles. Although a superhydrophobic technique inspired by lotus leaves is expected to be able to solve such fogging problems because the water droplets can not remain on the surface, there are no reports of such antifogging coatings. Very recently, researchers from General Motors have reported that the surfaces of lotus leaves become wet with moisture because the size of the fog drops are at the microscale—so small that they can be easily trapped in the interspaces among micropapillae. Thus, lotuslike surface microstructures are unsuitable for superhydrophobic antifogging coatings, and a new inspiration from nature is desired for solving this problem. In this communication, we report a novel, biological, superhydrophobic antifogging strategy. It was found that the compound eyes of the mosquito C. pipiens possess ideal superhydrophobic properties that provide an effective protective mechanism for maintaining clear vision in a humid habitat. Our research indicates that this unique property is attributed to the smart design of elaborate microand nanostructures: hexagonally non-close-packed (ncp) nipples at the nanoscale prevent microscale fog drops from condensing on the ommatidia surface, and hexagonally close-packed (hcp) ommatidia at the microscale could efficiently prevent fog drops from being trapped in the voids between the ommatidia. We also fabricated artificial compound eyes by using soft lithography and investigated the effects of microand nanostructures on the surface hydrophobicity. These findings could be used to develop novel superhydrophobic antifogging coatings in the near future. It is known that mosquitoes possess excellent vision, which they exploit to locate various resources such as mates, hosts, and resting sites in a watery and dim habitat. To better understand such remarkable abilities, we first investigated the interaction between moisture and the eye surface. An ultrasonic humidifier was used to regulate the relative humidity of the atmosphere and mimic a mist composed of numerous tiny water droplets with diameters less than 10 lm. As the fog was C O M M U N IC A IO N

The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography: a superhydrophobic state with high adhesive force

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 mate...

Bio-inspired strategies for anti-icing.

Nanotextured superhydrophobic surfaces have received significant attention due to their ability to easily shed liquid drops. However, water droplets have been shown to condense within the textures of superhydrophobic surfaces, impale the vapor pockets, and strongly pin to the surface. This results in poor droplet mobility and degrades condensation performance. In this paper, we show that pinning of condensate droplets can be drastically reduced by designing a hierarchical micro-nanoscale texture on a surface and impregnating it with an appropriate lubricant. The choice of lubricant must take into account the surface energies of all phases present. A lubricant will cloak the condensate and inhibit growth if the spreading coefficient is positive. If the lubricant does not fully wet the solid, we show how condensate-solid pinning can be reduced by proper implementation of nanotexture. On such a surface, condensate droplets as small as 100 μm 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.

Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces

Engineered wettability is a traditional, yet key issue in surface science and attracts tremendous interest in solving large-scale practical problems. Recently, different super-wettability systems have been discovered in both nature and experiments. In this Review we present three types of super-wettability, including the three-dimensional, two-dimensional, and one-dimensional material surfaces. By combining different super-wettabilities, novel interfacial functional systems could be generated and integrated into devices for use in tackling current and the future problems including resources, energy, environment, and health.

Bioinspired Super-Wettability from Fundamental Research to Practical Applications

Understanding the mechanism of ice adhesion on surfaces is crucial for anti-icing surfaces, and it is not clear if superhydrophobic surfaces could reduce ice adhesion Here, we investigate ice adhesion on model surfaces with different wettabilities The results show that the superhydrophobic surface cannot reduce the ice adhesion, and the ice adhesion strength on the superhydrophilic surface and the superhydrophobic one is almost the same This can be rationalized by the mechanical interlocking between the ice and the surface texture Moreover, we find that the ice adhesion strength increases linearly with the area fraction of air in contact with liquid

Superhydrophobic surfaces cannot reduce ice adhesion

Hierarchically structured porous aluminum surfaces for the high efficient removal of condensed water microdroplets are prepared via simply immersing aluminum sheets in hot water followed by modification with a low surface energy chemical. A correlation between the work of adhesion with the self-removal of condensed water microdroplets is established.

Hierarchically structured porous aluminum surfaces for high-efficient removal of condensed water

Removing condensed water from a cold surface can improve the surface heat-exchange coeffi cient by at least one order of magnitude, compared with the case of condensed water staying on the surface, [ 1–3 ] which is very important for all cooling systems due to increasing energy concerns nowadays. Because of their excellent water-repellent properties, superhydrophobic surfaces with low adhesion to water are promising candidates for effi cient removal of condensed water microdroplets. [ 4–7 ] It has been reported that coalescing microdroplets can self-remove from superhydrophobic surfaces when powered by the released surface energy, [ 8 , 9 ] which has aroused interest. [ 10–13 ] Though the self-removal of condensed microdroplets is energy saving, its effi ciency depends on the growth rate and the coalescing frequency of condensed droplets. Generally, faster growth and more frequent coalescence will lead to higher self-removal effi ciency of condensed droplets. However, though hydrophilic surfaces are favorable for the quick nucleation and growth of condensed water, they are unfavorable for the self-removal of condensed microdroplets. [ 1 , 14 , 15 ] While the high nucleation energy barrier on hydrophobic or superhydrophobic surfaces slows down the growth of condensed droplets, [ 16 ] the uncontrollable distance between condensed microdroplets decreases the coalescing frequency, resulting in a low self-removal effi ciency. Thus, how to control the condensation and coalescence processes and accelerate the self-removal of condensed microdroplets remains a great challenge for developments of new antifogging, anti-icing materials and heat exchangers. Herein, inspired by the peculiar hydrophilic/hydrophobic structures on a beetle’s elytra, [ 14 , 17 , 18 ] a micro-/nanoporous superhydrophobic surface modifi ed with hydrophilic polymer was designed for effi ciently controlling microdroplet self-removal. The hierarchical micro-/nanoporous structure was fabricated on aluminum by integrating microcontact printing [ 19 , 20 ] and chemical bath deposition. [ 11 , 21 ] After modifying the bottom of

Hierarchical porous surface for efficiently controlling microdroplets' self-removal.

A series of surfaces with the similar morphology but different surface free energy were fabricated to achieve surfaces with distinct condensation modes. It was found that the freezing of condensed water formed via filmwise condensation occurred much more quickly and at a higher temperature than that of condensed water formed via dropwise condensation.

Condensation mode determines the freezing of condensed water on solid surfaces

In this paper we report a surface-mediated growth process for oriented anisotropic particles with tunable morphologies. The morphology of the anisotropic particles can be tuned and the generality of this process is checked. This process also provides a new avenue for constructing ensembles of anisotropic particles.

Xiping Zeng

Papers

Superhydrophobic surfaces cannot reduce ice adhesion

Hierarchically structured porous aluminum surfaces for high-efficient removal of condensed water

Hierarchical porous surface for efficiently controlling microdroplets' self-removal.

Condensation mode determines the freezing of condensed water on solid surfaces

Surface-mediated buckling of core–shell spheres for the formation of oriented anisotropic particles with tunable morphologies