Superhydrophobicity in air

Our research began with lotus leaves, on which water slides off easily, and which show superhydrophobicity in air, with a contact angle larger than 150° and a sliding angle less than 5°.^{Reference Feng, Li, Li, Li, Zhang, Zhai, Song, Liu, Jiang and Zhu2} We found that there are many micropapillae (nipple-like structures) on the leaves and also large numbers of nanorods on the top of the micropapillae. These micro/nano two-tier structures, together with lower surface energy materials, are key for the superhydrophobicity of the liquid/gas/solid three-phase system. There are many more superhydrophobic interfaces in nature, including, for example, mosquito eyes, in which the periodically aligned micro-/nanostructure gives anti-fogging properties.^{Reference Gao, Yan, Yao, Xu, Zhang, Zhang, Yang and Jiang31} Another famous example is water strider legs. Water striders can walk freely on water due to nanoscale grooved structures on the oriented microscale setae or barbs.^{Reference Gao and Jiang32} Also, water droplets, such as those that adhere to rose petals, originate from periodic arrays of nanofolds covering micropapillae.^{Reference Feng, Zhang, Xi, Zhu, Wang, Xia and Jiang33}

In addition to isotropic superhydrophobicity in air, anisotropic superhydrophobicity also exists widely in nature. We discovered that rice leaves possess anisotropic wettability due to 1D ordering of the micro-/nanopapillae.^{Reference Feng, Li, Li, Li, Zhang, Zhai, Song, Liu, Jiang and Zhu2} Another example of anisotropic wettability is the butterfly wing,^{Reference Zheng, Gao and Jiang8} on which a water droplet can easily roll in one direction but becomes tightly pinned in the opposite direction. These phenomena are ascribed to the directional arrangement of multiscale structures.

From the superhydrophobic interfaces in nature, it can be seen that the cooperative effects of micro/nano two-tier structures and low surface energy materials play important roles in the formation of superhydrophobic properties. The arrangement and orientation of micro- and nanostructures may control the wetting state and directional tendency for motion of liquids. With this biological principle, bioinspired interfacial materials can be designed using a range of methods.

Superhydrophobic interfaces have been constructed using a template method and aligned polyacrylonitrile nanofibers, generating the first superhydrophobic plastic without any modification.^{Reference Feng, Li, Li, Zhai, Song, Jiang and Zhu34} Using the same method, another superhydrophobic plastic consisting of amphiphilic poly(vinyl alcohol) nanofibers has also been fabricated, as shown in Figure 4a.^{Reference Feng, Song, Zhai, Liu, Xu, Jiang and Zhu35} The template method can also be extended to prepare superhydrophobic surfaces with various materials, demonstrating that it is an effective and low-cost method for fabricating superhydrophobic materials. In addition, a porous micro-/nanostructured plastic film composed of polystyrene that resembles the lotus leaf has been prepared by combining phase separation and electrospinning.^{Reference Jiang, Zhao and Zhai36} A superamphiphobic film has also been easily synthesized using the phase separation method.^{Reference Xie, Xu, Feng, Jiang, Tang, Luo and Han37}

Figure 4.

Superwettability in two-dimensional systems, from nature to artificial materials to applications. (Middle): Biological system with superwettability from lotus leaves. Reproduced with permission from Reference 2. © 2014 Wiley-VCH. Mosquito eyes. Reproduced with permission from Reference 31. © 2007 Wiley-VCH. Water strider legs. Reproduced with permission from Reference 32. © 2004 Nature Publishing Group. Rose petals. Reproduced with permission from Reference 33. © 2008 American Chemical Society. Rice leaves. Reproduced with permission from Reference 2. © 2002 Wiley-VCH. Butterfly wings. Reproduced with permission from Reference 8. © 2007 Royal Society of Chemistry. Fish scales. Reproduced with permission from Reference 10. © 2009 Wiley-VCH. Fish skin. Reproduced with permission from Reference 39. © 2014 Wiley-VCH. (a) Superhydrophobicity in air: poly(vinyl alcohol) nanofibers prepared by a template method. Reproduced with permission from Reference 35. © 2003 Wiley-VCH. (b) Artificial fish scales prepared using hierarchical macromolecule–nanoclay hydrogels, which show superoleophobicity underwater. Reproduced with permission from Reference 38. © 2010 Wiley-VCH. (c) Superhydrophobic sponges show superaerophilicity underwater. Reproduced with permission from Reference 40. © 2012 Wiley-VCH. (d) Schematic of an underwater superaerophobic nanoporous MoS₂ electrode. Reproduced with permission from Reference 41. © 2014 Wiley-VCH. (e) Superamphiphobic textiles prepared by plasma treatment and fluorocarbon modification. (f) Water/oil separation film based on superhydrophilicity but superoleophobic underwater. Reproduced with permission from Reference 43. © 2011 Wiley-VCH. (g) Superoleophobic underwater-based printing.

Superoleophobicity underwater

Gaining inspiration from self-cleaning fish, we extended our system further from air-liquid-solid interfaces to liquid-liquid-solid interfaces. By directly observing fish scales, we found that these are patterned with many oriented micropapillae that are arranged in radial directions, and the surfaces of the micropapillae possess fine-scale roughness.^{Reference Liu, Wang, Wei, Song and Jiang11} The hierarchical structures on the fish scales render them superoleophobic underwater and superhydrophilic in air. Learning from nature, we developed a strategy to achieve artificial fish scales by constructing a surface of hierarchical macromolecule–nanoclay hydrogels (Figure 4b).^{Reference Lin, Liu, Chen, Chen, Ma, Han and Jiang38} In addition to being superoleophobic, the biomimetic surfaces also possess robust properties as compared to their biological counterparts. The artificial surfaces exhibit high compressive and tensile strengths, which is beneficial for industrial applications.

We have also recently found that the skin of filefish shows anisotropic oleophobicity underwater due to the oriented hook-like spines arrayed on the fish skin. Oil droplets tend to roll off in the head-to-tail direction but are pinned in the opposite direction.^{Reference Cai, Lin, Xue, Liu, Wang and Jiang39} Artificial fish skin with anisotropic oleophobicity underwater can be mimicked by soft lithography and plasma treatment with polydimethylsiloxane.

Superaerophilicity and superaerophobicity underwater

Lotus leaves are well-known for their superhydrophobicity in air. However, little research has focused on their superaerophilicity underwater. In 2009, we were the first to find that an air bubble could spread completely on submerged lotus leaves, where the spread was rapid due to the micro/nano two-tier structures on their surfaces.^{Reference Wang, Zheng, Nie, Zhai and Jiang12} Using photolithography combined with a wet-etching method, artificial lotus leaves that exhibited superaerophilicity underwater were obtained.^{Reference Wang, Zheng, Nie, Zhai and Jiang12}

Inspired by the above results, we produced materials which effectively absorb methane bubbles by endowing commercial sponges with superhydrophobicity.^{Reference Chen, Wu, Su, Wang, Song and Jiang40} As shown in Figure 4c, they possess superaerophilicity underwater and thus, a large number of marine methane bubbles could be absorbed, stored, and transported effectively underwater. This is a simple yet practical method for the reduction of marine methane.

The wettability of air bubbles has great significance in real applications, such as gas evolution electrodes. Recently, we constructed an underwater superaerophobic nanoporous MoS₂ electrode (Figure 4d) to which gas bubbles adsorb very weakly, such that they can easily leave the surface.^{Reference Lu, Zhu, Yu, Zhang, Li, Sun, Wang, Wang, Wang, Luo, Lei and Jiang41} These superaerophobic electrodes could effectively promote electrocatalytic performance, and we believe that the design based on superwettability would open up a new way for the development of gas evolution electrodes.

Superwettability-based applications

With all of these versatile methods of building surfaces with superwettability, some superhydrophobic interfaces could be produced on large scales for application in many industrial fields. Our group has developed an easy two-step method based on plasma treatment and fluorocarbon modification to produce superamphiphobic textiles (Figure 4e). This method is not only very easy but also does not change the other properties of the textiles.

Based on the properties comprising superhydrophobicity but superoleophilicity in air, and superhydrophilicity but superoleophobicity underwater, water/oil separation films have been fabricated through a simple spray-and-dry method using polytetrafluoroethylene^{Reference Feng, Zhang, Mai, Ma, Liu, Jiang and Zhu42} or hydrogel coated mesh (Figure 4f).^{Reference Xue, Wang, Lin, Chen, Liu, Feng and Jiang43} Both light and heavy oils could be separated from water using these two kinds of films. These materials based water/oil separation devices have been used in over 500 ships to remove condensed water in oil tanks.

Another technology based on underwater superoleophobicity was developed in recent years and has been termed “green printing.”^{Reference Tian, Song and Jiang44} The printing ink in this case spreads precisely on the superhydrophobic areas of the patterned plates in air and leaves the superhydrophilic part of the plates as a margin. Colorful images with fine resolution can be obtained using this method, as shown in Figure 4g.^{Reference Tian, Song and Jiang44}

Thermal responsive smart interfaces in air

Poly(N-isopropylacrylamide) (PNIPAM) is a thermally responsive molecule that has gained considerable attention due to its different characteristics below and above the lower critical solution temperature (LCST; the temperature below which the individual components of the polymer are completely miscible). Inspired by nature, we introduced a PNIPAM micro-/nanostructure onto silicon surfaces (Figure 5a).^{Reference Sun, Wang, Feng, Liu, Ma, Jiang and Zhu4} Thus, the roughness-enhanced wettability could be changed from superhydrophobic to superhydrophilic, while the difference in contact angles on their flat counterparts was less than 30°. These new interfaces with wettability that can be tuned from superhydrophobic to superhydrophilic not only pave a new way to smart interfacial materials with superwettability, but also could have great influence in potential applications, including intelligent microfluidic devices and controllable drug release.

Other stimuli-responsive smart interfaces in air

Combining micro-/nanostructured substrates and responsive molecules, a series of composite materials have been generated that possess two complementary properties of superwettability that can be switched using light, pH, electric, chemical, or multiple stimuli.

For instance, ZnO nanorods fabricated by hydrothermal methods show reversible wettability between superhydrophobic and superhydrophilic due to their photoresponsive properties.^{Reference Feng, Feng, Jin, Zhai, Jiang and Zhu48} Using electrochemical deposition methods, the as-prepared nanostructured WO₃^{Reference Wang, Feng, Yao and Jiang49} could also switch between superhydrophobic and superhydrophilic under the stimulus of light. Compared to the slow response times of inorganic materials, many kinds of organic molecules, such as azobenzene, also show great responsiveness to light.^{Reference Jiang, Wang, He, Wang, An, Song and Jiang50} A superwettability gate was obtained by binding an azobenzene-containing polymer to a laser-etched rough substrate.

The i-motif DNA (four-stranded DNA secondary structures that can form in sequences rich in cytosine) can undergo a transition from i-motif to a single-stranded structure under different pH conditions, as described in Figure 5b. By binding a fluoro group to the i-motif DNA, the wettability of the DNA integrated micro-/nanostructured surfaces can be changed from 8.8° at pH 4.5 to 148.3° at pH 8.5.^{Reference Wang, Liu, Liu, Ma, Fang and Jiang51}

The wettability of interfaces can also be tuned using chemicals such as ammonia.^{Reference Zhu, Zhai, Sun and Jiang52} Due to the reversible change of the surface structure and surface-free energy under an ammonia atmosphere, the wettability of indium hydroxide can be altered from superhydrophobic to superhydrophilic with good reproducibility.^{Reference Zhu, Zhai, Sun and Jiang52} When two or more stimuli are combined, their cooperative interaction will generate interfaces that have the potential to be responsive to multiple stimuli.

Electrowetting is an effective and traditional technique for transferring a water droplet from the Cassie state to the Wenzel state (descriptions of the two states are found in Reference 51); however, the voltage needed has always been relatively high.^{Reference Marmur53} In 2009, our group realized photoelectric cooperative wetting based on patterned ZnO nanorods film, as shown in Figure 5c.^{Reference Tian, Chen, Nie, Xu, Song and Jiang54} The wettability transition occurred around 3 V when ultraviolet light and electric fields both acted on the film, and the wetting region could be controlled precisely, leading to the desired pattern.^{Reference Tian, Chen, Nie, Xu, Song and Jiang54}

By grafting poly(N-isopropylacrylamide-co-acrylic acid) onto the rough silicon substrate, the film showed temperature and pH dual responsive properties (Figure 5d), and the wettability could be switched between superhydrophobic and superhydrophilic under the cooperative stimuli of pH and heat.^{Reference Xia, Feng, Wang, Sun, Song, Jiang and Jiang55} Similarly, integrating PNIPAM into pH- and glucose-responsive poly(acrylamidophenylboronic acid) molecules yielded multiple responsive smart surfaces, as shown in Figure 5e.^{Reference Xia, Ge, Hou, Sun, Chen, Zhang and Jiang56}

Smart interfaces underwater

Inspired by fish scales, we have transferred the smart design concept into the oil/water/solid three-phase system. For example, under the stimulus of temperature, oil adhesion underwater could be controlled reversibly on the surface of PNIPAM hydrogels. Oil adhesion varied from superoleophobicity and low adhesion below LCST to oleophobicity and high adhesion above LCST.^{Reference Chen, Liu, Lin, Zhang, Ma, Song and Jiang57} Using rough films of the conducting polymer polypyrrole as the substrate, an in situ dynamic wettability transition between partly wetting and superoleophobic characteristics could be achieved by alternately applying positive and negative voltages.^{Reference Liu, Nie, Wei, Song and Jiang58} By tuning the doping and electrochemical driving potential of the aligned polypyrrole nanotube, a reversible superoleophobicity to superoleophilicity transition could also be achieved.^{Reference Liu, Liu, Ding, Wei, Zhu and Jiang59}

The previously mentioned smart interfaces were all realized by tuning the chemical compositions or structures of the solid phase. Recently, a solid-phase-independent oil wetting transition was achieved by controlling the polar interactions at oil/water interfaces.^{Reference Liu, Xue, Liu and Jiang15} The wettability of polar oils on nonresponsive solid surfaces could switch from oleophilic to superoleophobic due to the Lewis acid–base interactions at different pH values in aqueous solution. This study provides a new strategy to construct smart, superwettability systems.

Smart bioinspired nanochannels and their applications

Nanoscale channels that exist in living organisms play important roles in many physiological processes. Most of the channels possess “smart gates” to selectively allow ions or small molecules in and out of the channels. Learning from nature, we have constructed two kinds of nanochannels, including bioinspired ion channels and bioinspired ion pumps, and we have also investigated their potential applications.

Bioinspired ion channels

Solid-state polymeric single nanochannels have been widely used due to their outstanding mechanical and chemical properties, which are also easy to modify.^{Reference Hou, Dong, Zhu and Jiang60–Reference Zhang, Hou, Yang, Yan, Li, Tian, Wang and Jiang62} We first studied bioinspired ion channels with conical shapes by modifying the surface of single polymeric nanochannels with pH responsive i-motif DNA.^{Reference Xia, Guo, Mao, Hou, Xue, Xia, Wang, Song, Ji, Qi, Wang and Jiang10} As an extension of our previous work on 2D surfaces, the molecules could also undergo conformation changes and thus, affect the physical radius and wettability of the inner face of the nanochannel. As a result, the gate of the nanochannels could open and close in response to pH variation, as shown in Figure 6a. This is the first reported bioinspired single-ion channel, and it has drawn significant attention from scientists in this field. Following this work, we constructed a series of smart single nanochannels by integrating functionalization into the nanochannels (Figure 6b–c) that respond to various external stimuli such as pH,^{Reference Hou, Liu, Dong, Yang, Li and Jiang63} ions,^{Reference Hou, Guo, Xia, Nie, Dong, Tian, Wen, Wang, Cao, Yang, Xue, Song, Wang, Liu and Jiang64–Reference Tian, Zhang, Wen, Ma, Zhang, Liu, Zhai and Jiang66} temperature,^{Reference Guo, Xia, Xia, Hou, Cao, Wang, Xue, Zhang, Song, Zhu, Wang and Jiang67} and small molecules.^{Reference Han, Hou, Zhang, Guo, Li and Jiang68,Reference Hou, Zhang, Xie, Xiao, Wen, Li, Tian and Jiang69} Nanochannel systems that respond to multiple stimuli can also be developed by modifying single nanochannels with copolymers or functionalizing the channels with multiple responsive molecules (Figure 6d).^{Reference Guo, Xia, Cao, Xia, Wang, Zhang, Song, Wang, Jiang and Zhu70–Reference Zhang, Hou, Wang, Tian, Fan, Zhai and Jiang72}

Figure 6.

Smart bioinspired nanochannels and their applications. (Middle) Bioinspired nanochannels with different shapes. (a) Conical single polymeric nanochannels (SPNs) modified by i-motif DNA; the gate of the nanochannels can be opened and closed in response to pH. Reproduced with permission from Reference 9. © 2008 The American Chemical Society. (b) Thermal-responsive smart bioinspired ion channels (SBICs) fabricated by modifying conical SPN with poly(N-isopropylacrylamide) (PNIPAM). Reproduced with permission from Reference 67. © 2010 Wiley-VCH. (c) Mercury(II)-gated SBIC constructed by Hg²⁺-sensitive 50-thiol-ended T-rich single-stranded DNA integrated into Au-coated conical SPN. Reproduced with permission from Reference 66. © 2013 Royal Society of Chemistry. (d) pH- and thermal-cooperative responsive SBIC constructed through asymmetric modification with poly(acrylic acid) (PAA) and PNIPAM at the two sides of hourglass-shaped SPN, respectively. Reproduced with permission from Reference 66. © 2013 Royal Society of Chemistry. (e) Bioinspired ion pump based on cigar-shaped SPN. The nanochannel has two separate small tips embedded on the two sides of the nanochannel. Reproduced with permission from Reference 73. © 2013 American Chemical Society. (f–i) Four kinds of applications of the bioinspired single nanochannels. (f) Retina-inspired energy-conversion system that could transfer the photo energy to electric signals constructed by the pH-driven i-motif DNA modified nanochannels and photo acid/base molecules. Reproduced with permission from Reference 75. © 2010, Wiley-VCH. (g) Electric eel inspired energy-conversion device that could transfer the ion-concentration-induced chemical energy to electrical energy by using the negatively charged solid-state single nanochannels. Reproduced with permission from Reference 76. © 2010 Wiley-VCH. (h) Transmembrane salinity-gradient-induced electricity prepared by mesoporous carbon combined with the anodic aluminum oxide. Reproduced with permission from Reference 77. © 2014 American Chemical Society. (i) Pressure-driven energy-conversion system based on the negatively charged two-dimensional layered graphene hydrogel membrane. Reproduced with permission from Reference 78. © 2013 Wiley-VCH. Note: PVP, poly(vinylpyridine); PET, poly(ethylene terephthalate).

Smart bioinspired fibers for directional liquid transport

Another 1D smart system is nanofibers that show smart superwettability and that transport liquid droplets directionally. These would have applications in microfluidics, water or oil collection, and smog removal. The arrangement of the micro-/nanostructures plays an essential role in their liquid transport ability.

Spider silks and their artificial counterparts

Inspired by the dew that hangs on spider silk, we investigated the smart superwettability of spider silk and developed a series of artificial spider silks. As shown in Figure 7a, under a misty environment, the fiber shows periodic spindle knots and joints, which result in a gradient of Laplace pressure (the pressure difference between the inside and the outside of a curved surface) and surface-free energy between the knots and joints. Water droplets can then move directionally along the fiber.^{Reference Zheng, Bai, Huang, Tian, Nie, Zhao, Zhai and Jiang13}

Figure 7.

Smart bioinspired fibers for directional liquid transport. Reproduced with permission from Reference 30. © 2014 American Chemical Society. (a) Microstructures of natural spider silk. Reproduced with permission from Reference 12. © 2010 Nature Publishing Group. (b) Artificial spider silks prepared by dip-coating method. Reproduced with permission from Reference 79. © 2010 Wiley-VCH. (c) Smart artificial spider silks prepared by modifying the spindle knots with thermal-responsive poly(N-isopropylacrylamide). Reproduced with permission from Reference 82. © 2013 Wiley-VCH. (d) Scanning electron microscopy images of a natural cactus spine. Reproduced with permission from Reference 83. © 2012 Nature Publishing Group. (e) Artificial cactus spines prepared by gradient electrochemical corrosion and gradient chemical modification. Reproduced with permission from Reference 84. © 2013 Wiley-VCH. (f) Arrays of artificial cactus spines for oil collection in oil/water/mixtures. Reproduced with permission from Reference 85. © 2013 Nature Publishing Group. Note: LCST, lower critical solution temperature.

Learning from the wettability of spider silk, we have fabricated a range of smart artificial fibers using methods such as dip coating (Figure 7b),^{Reference Bai, Tian, Zheng, Ju, Zhao and Jiang79} fluid coating,^{Reference Bai, Sun, Ju, Yao, Zheng and Jiang80} and electrospinning.^{Reference Dong, Wang, Wang, Bai, Wu, Zheng, Zhao and Jiang81} Like 2D surfaces, the wettability of fibers is also controlled by surface roughness and chemical compositions. Thus, we successfully prepared artificial spider silk and mimicked directional water transport by tuning these two factors of the nanofibers. Going beyond nature, we also made our structures stimulus-responsive by introducing thermal responsive molecules into the nanofibers, endowing the artificial spider silk with “intelligence” (Figure 7c).^{Reference Hou, Gao, Feng, Chen, Xue, Jiang and Zheng82}

Superwettability-based chemistry

Using superwettable substrates, as shown in Figure 8a, we performed diverse chemical reactions in different media. Since the droplets on the superhydrophobic substrates maintain a spherical shape due to the low surface energy of the substrates, such that molecules cannot easily escape from the droplets, they could be utilized as miniature reactors to perform chemical reactions at very small scales, such as inorganic reactions in air,^{Reference Su, Wang, Song and Jiang86} organic reactions underwater,^{Reference Su, Wang, Song and Jiang87} and heated reactions under oil.^{Reference Su, Wang, Song and Jiang88} The basic principles are shown in Figure 8b–c.^{Reference Su, Wang, Song and Jiang86–Reference Su, Wang, Song and Jiang88}

Figure 8.

Superwettability-based chemical reaction and fabrication of inorganic/organic materials. Reproduced with permission from Reference 17. © 2014 Wiley-VCH. (a) Interfaces with superwettability. (b–c) A chemical reaction system based on superwetting interfaces. One droplet (containing reagent A) on a superwetting interface could coalesce with another droplet (containing reagent B) assisted by superhydrophobic tweezers, yielding a mixed droplet (containing product C). (d–e) When a solution droplet makes contact with a high-adhesion superhydrophobic patterned pillar-structured interface, liquid bridges could be formed between the adjacent micropillars. If the solution viscosity is low, the liquid bridges will break, thus, ordered nanocrystal patterns would be generated after water evaporation. (f–g) If the solution is sticky, the liquid bridges will remain, and aligned nanowires of the functional materials would be prepared after liquid evaporation. (a–e) Reproduced with permission from Reference 96. © 2011 Wiley-VCH. (f–g) Reproduced with permission from Reference 97. © 2012 Wiley-VCH.

Superwettability-based fabrication of inorganic/organic functional materials

Recently, we used interfaces with superwettability characteristics to fabricate a series of inorganic/organic functional materials (Figure 8d–f).^{Reference Su, Wang, Wu, Chen, Song and Jiang89–Reference Su, Wang, Ma, Song and Jiang96} When placing an aqueous droplet upon the superhydrophobic pillar-structured surfaces, direction-controlled liquid bridge arrays could be generated. Following the inward shrinkage of the three-phase contact line of the droplet, strictly-oriented liquid bridges connecting adjacent micropillars would be generated. After water evaporation, these well-defined, 1D liquid bridges can restrict the alignment of dissolved suspended building blocks of functional materials. With this strategy, small molecules were used to fabricate nanowires and crystalline nanowires.^{Reference Su, Wang, Ma, Song and Jiang96} Polymer nanowires have thus been aligned, and silver nanoparticles or microspheres have been patterned using this method.^{Reference Su, Wang, Ma, Wu, Chen, Song and Jiang97} These studies are anticipated to boost the development of 1D micro-/nanostructured materials for potential application in optoelectronic nanodevices.