Figure 6. Typical biological materials with superwettability and corresponding multiscale structures. (a) Lotus leaves demonstrate low adhesive, superhydrophobic, and self-cleaning properties, due to randomly distributed micropapillae covered by branch-like nanostructures. (b) Rice leaf surfaces possess anisotropic superhydrophobicity arising from the arrangement of lotus-like micropapillae in one-dimensional order. (a and b) Reproduced with permission from ref 7. Copyright 2002 Wiley. (c) Butterfly wings exhibit directional adhesion, superhydrophobicity, structural color, self-cleaning, chemical sensing capability, and fluorescence emission functions due to the multiscale structures. Reproduced with permission from ref 45. Copyright 2007 American Chemical Society. (d) Water strider legs have robust and durable superhydrophobicity arising from directional arrangements of needlelike microsetae with helical nanogrooves. Reproduced with permission from ref 46. Copyright 2004 Nature Publishing Group. (e) Mosquito compound eyes demonstrate superhydrophobic, antifogging, and antireflection functions due to HCP microommatidia covered by HNCP nanonipples. Reproduced with permission from ref 47. Copyright 2007 Wiley. (f) Poplar leaves possess superhydrophobic and antireflection properties originating from dense hairs with the hollow fibrous structure. Reproduced with permission from ref 48. Copyright 2011 The Royal Society of Chemistry. (g) Gecko feet present superhydrophobic, reversible adhesive, and self-cleaning functions due to the aligned microsetae splitting into hundreds of nanospatulae. Reproduced with permission from ref 49. Copyright 2012 The Royal Society of Chemistry. (h) Red rose petals exhibit superhydrophobicity with high adhesion and structural color arising from periodic arrays of micropapillae covered by nanofolds. Reproduced with permission from ref 31. Copyright 2008 American Chemical Society. (i) Salvinia leaves demonstrate the superhydrophobic and air-retention properties due to the Salvinia Effect. Reproduced with permission from ref 50. Copyright 2010 Wiley. (j) Fish scales present drag reduction, superoleophilicity in air, and superoleophobicity in water due to oriented micropapillae covered by nanostructures. Reproduced with permission from ref 33. Copyright 2009 Wiley. (k) Clam shell shows low adhesive superoleophobicity underwater arising from the surface multiscale structures and special chemical composition. Reproduced with permission from ref 51. Copyright 2012 Wiley. (l) Peanut leaves exhibit high adhesive superhydrophobicity and fog capture properties originating from the special surface multiscale structures and chemical composition. Reproduced with permission from ref 52.

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.[1] To date, all techniques reported belong to the former category, as they are intended for applications in optical transparent coatings. [1][2][3][4][5][6][7] A well-known example is the use of photocatalytic TiO 2 nanoparticle coatings that become superhydrophilic under UV irradiation. [3,4] Very recently, a capillary effect was skillfully adopted to achieve superhydrophilic properties by constructing 3D nanoporous structures from layer-by-layer assembled nanoparticles. [1,5,6] 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, [7] 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 [8][9][10] and the striking ability of a water-strider's legs to walk on water [11] can be attributed to the ideal superhydrophobicity of their surfaces, induced by special micro-and nanostructures. To date, the biomimetic fabrication of superhydrophobic micro-and/or nanostructures has attracted considerable interest, [12][13][14][15][16][17] 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, [1] 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 mic...

In this review we focus on recent developments in applications of bio-inspired special wettable surfaces. We highlight surface materials that in recent years have shown to be the most promising in their respective fields for use in future applications. The selected topics are divided into three groups, applications of superhydrophobic surfaces, surfaces of patterned wettability and integrated multifunctional surfaces and devices. We will present how the bio-inspired wettability has been integrated into traditional materials or devices to improve their performances and to extend their practical applications by developing new functionalities.

Nature is a school for scientists and engineers. After four and a half billion years of stringent evolution, some creatures in nature exhibit fascinating surface wettability. Biomimetics, mimicking nature for engineering solutions, provides a model for the development of functional surfaces with special wettability. Recently, bio-inspired special wetting surfaces have attracted wide scientific attention for both fundamental research and practical applications, which has become an increasingly hot research topic. This Critical Review summarizes the recent work in bio-inspired special wettability, with a focus on lotus leaf inspired self-cleaning surfaces, plants and insects inspired anisotropic superhydrophobic surfaces, mosquito eyes inspired superhydrophobic antifogging coatings, insects inspired superhydrophobic antireflection coatings, rose petals and gecko feet inspired high adhesive superhydrophobic surfaces, bio-inspired water collecting surfaces, and superlyophobic surfaces, with particular focus on the last two years. The research prospects and directions of this rapidly developing field are also briefly addressed (159 references).

Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported 1-3 . Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film's transparency and its ability to continuously manipulate various low-surface-tension droplets from free-sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.

Artifi cial superhydrophobic surfaces [1][2][3][4][5][6][7][8][9][10] with water contact angles (CAs) greater than 150 ° have been intensively investigated due to their unique "anti-water" property that could be utilized in a wide range of applications. [11][12][13] Recent development of intelligent devices, such as microfl uidic switches and biomedicine transporters, makes strong demands on surface wettability control, therefore, responsive surfaces have become a signifi cant issue for superhydrophobic studies. Up to now, various smart surfaces have been successfully developed as reversible switches for wettability control through a micronanostructured surface on a responsive material. [14][15][16][17][18][19][20][21][22][23][24][25] These unique tunings of surface wettability greatly contributed to refi ned control of surface wettability. With the thorough understanding of superhydrophobic phenomenon, superhydrophobic surfaces have been classifi ed into fi ve states [ 26 ] according to the details of CA hysteresis, which have been well verifi ed on different samples based on experimental results. [ 1 , 8 , 27-29 ] Superhydrophobic surfaces in different states show distinctive advantages in varied applications. Hence, efforts have also been devoted to precise tuning between different superhydrophobic states. For example, Lai et al. [ 23 a] investigated superhydrophobic surfaces with controlled adhesion to water droplets by using different kinds of rough surfaces. Li et al. [ 23 b] observed reversible switching between a transitional state (sliding angle of 75 ° ) and the Wenzel superhydrophobic state (high adhesion force) by changing the temperature. This inspired no-loss microdroplet transfer and trace-liquid reactor applications, [ 15 ] which usually need precise control of water droplet movement on the same surface from "roll-down" to "pinned" superhydrophobic states. Nevertheless, this no-loss transfer of a given water droplet requires a sensitive in situ tuning of surface wettability. Jiang et al. have reported an in situ control of magnetic droplet movement using extra magnetic fi eld, where the tuning was based not on pure water droplets, but on magnetic liquids. [ 27 ] From the practical point of view, it is still worth pointing out that the above-mentioned tuning approaches usually depend on harsh tuning conditions, such as UV irradiation, [ 18 ] electrical current, [ 19 , 21 ] a wide range of temperature, [ 23 ] or treatments by chemical solvents. [ 22 , 25 ] They may be not suitable for mild condition applications. For example, enzymes or biological cells in microfl uidic devices would be seriously affected under UV irradiation, temperature change, or addition of chemical substances. In addition, most of these tunable surfaces are based on artifi cially introduced material compositions or particular material species, [18][19][20][21][22][23][24][25] such as azobenzene and metal oxides, which suffer from poor biocompatibility. Therefore, it is urgently desirable to fi nd a simple, environmentally...

Surfaces directing fluid flows Although surfaces can be made to attract or repel liquids using coatings, surface textures with specific curvature can also be used to achieve the same effect. However, fluid transport is usually limited by the specific pattern that only drives flow at the surface itself. Feng et al . created a dual-reentrant surface that has an asymmetric profile so that fluids spread out at the surface and subsurface layers. Furthermore, these surfaces can be designed so that different liquids will naturally steer in opposing directions simply because of their specific interactions with the surface. —MSL

The ability to control drops and their movements on phobic surfaces is important in printing or patterning, microfluidic devices, and water-repellent materials. These materials are always micro-/nanotextured, and a natural limitation of repellency occurs when drops are small enough (as in a dew) to get trapped in the texture. This leads to sticky Wenzel states and destroys the superhydrophobicity of the material. Here, we show that droplets of volume ranging from femtoliter (fL) to microliter (μL) can be self-removed from the legs of water striders. These legs consist of arrays of inclined tapered setae decorated by quasi-helical nanogrooves. The different characteristics of this unique texture are successively exploited as water condenses, starting from selfpenetration and sweeping effect along individual cones, to elastic expulsion between flexible setae, followed by removal at the anisotropic leg surface. We envision that this antifogging effect at a very small scale could inspire the design of novel applicable robust water-repellent materials for many practical applications.water strider | water repellency | self-propulsion | antifogging | breath figure W hen a vapor condenses on a rough hydrophobic surface, liquid nuclei naturally appear within the roughness, forming tiny droplets stuck in the texture (1-3). Without the assistance of external forces, dew tightly adheres to the surface in the so-called Wenzel state, which most often destroys the superhydrophobicity of the material (4, 5). An applicable robust water-repellent surface thus requires an additional mechanism to expel these condensates from the texture (6). It has been reported in the literature that nanotextures (such as found on the surface of lotus leaves or cicada wings) might generate adhesion forces small enough to permit an efficient conversion of surface energy (coming from droplet coalescence) into kinetic energy, so that droplets can be mobilized (7-9). However, contact angle hysteresis effects are generally dominant at a small scale, so that a key challenge for antifogging materials lies in the generation of a force able to expel droplets from microtextures.Water striders (Gerris remigis, Fig. 1A) living at the water surface in a highly humid environment offer a remarkably simple solution to this problem. Without any external force, tiny condensed droplets in the range of femtoliters (fL) to microliters (μL) get removed from striders' legs, owing to the presence of oriented conical setae. A Gerris leg is a centimeter-size cylinder (of typical diameter 150 μm) decorated by an array of inclined tapered hairs characterized in Fig. 1 B and C by micro X-ray computed tomography (XCT) and scanning electron microscopy (SEM). Individual setae have a length L = 40-50 μm, a maximum diameter of ∼3 μm, and an apex angle of ∼5°. They make regular arrays with a mutual distance of 5-10 μm, and are tilted by an angle β = 25-35°to the base of the leg (Fig.