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Adhesion behaviors on superhydrophobic surfaces Cite this: Chem. Commun., 2014, 50, 3900

Huan Zhu,ab Zhiguang Guo*ab and Weimin Liub The adhesion behaviors of superhydrophobic surfaces have become an emerging topic to researchers in various fields as a vital step in the interactions between materials and organisms/materials. Controlling the chemical compositions and topological structures via various methods or technologies is essential to fabricate and modulate different adhesion properties, such as low-adhesion, high-adhesion and anisotropic adhesion on superhydrophobic surfaces. We summarize the recent developments in both natural superhydrophobic surfaces and artificial superhydrophobic surfaces with various adhesions and also pay attention to superhydrophobic surfaces switching between low- and high-adhesion. The methods to regulate or translate the adhesion of superhydrophobic surfaces can be considered from two perspectives. One is to control the chemical composition and change the surface geometric

Received 11th October 2013, Accepted 5th December 2013

structure on the surfaces, respectively or simultaneously. The other is to provide external stimulations to

DOI: 10.1039/c3cc47818a

adhesion behaviors on solid–solid interfaces, such as the behaviors of cells, bacteria, biomolecules and

induce transitions, which is the most common method for obtaining switchable adhesions. Additionally, icing on superhydrophobic surfaces are also noticeable and controversial. This review is aimed at giving

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a brief and crucial overview of adhesion behaviors on superhydrophobic surfaces.

Introduction a

Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials and Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, People’s Republic of China. E-mail: [email protected]; Fax: +86-931-8277088; Tel: +86-931-4968105 b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China

Miss Huan Zhu joined Prof. Guo’s group at the University of Hubei in 2012 to pursue her PhD degree. Her current scientific interests are devoted to contrasting superhydrophobic surfaces with responsive or multifunctional properties, and exploring the interaction between surfaces and cell, understanding the relationship between surfaces and special properties. Huan Zhu

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In the past decade, superhydrophobic surfaces have gained increasing attention in both fundamental research and practical applications due to their excellent water-repellent and self-cleaning properties.1 With the development and extension of superhydrophobic surfaces, more and more multi-functionalities have been found, such as low-/high-adhesion, anisotropy, antibacterial, antiplatelet, chirality, icephobicity, anti-corrosion, drug reduction,

Professor Zhiguang Guo received his PhD from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS) in 2007. After that, he worked in University of Namur (FUNDP), Belgium, as a post-doctor. From Sep 2008 to Mar 2011, he worked in Funds of National Research Science (FNRS), Belgium, as a ‘‘Charge de Researcher’’. During Feb 2009 to Feb 2010, he worked in Department of Physics, Zhiguang Guo University of Oxford, UK, as a visiting scholar. Now he is a full professor in LICP financed by the ‘‘Top Hundred Talents’’ program of CAS.

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water-collecting, electro-wetting, anti-reflection, structural color, and transparency.2 Originally, a superhydrophobic surface was defined as having a large contact angle (41501) and a very low slide angle (o101). Subsequent discoveries of superhydrophobic surfaces with high-adhesion and directional-adhesion have broken the traditional cognition and have aroused great interest in special adhesion. The adhesion behaviors of water droplets on superhydrophobic surfaces can be divided into three types: low-adhesion, high-adhesion and anisotropic adhesion. Low-adhesive superhydrophobic surfaces, with droplets rolling off spontaneously, were initially inspired by the lotus leaf while high-adhesive superhydrophobic surfaces originated from gecko feet and rose petals, with droplets attaching on the surface vertically or even turn upside down. A butterfly wing is a typical example of an anisotropic adhesive superhydrophobic surface. Droplets on it can roll-off along one-orientation while clinging along the other orientation. As research into special adhesion has deepened, the mechanism has become clearer. Similar to extreme wettability, adhesion behaviors on solid–liquid interfaces are largely determined by the surface chemical composition and structural roughness. Both the scale and shape of surface structures have significant influence on its adhesion strength. Guided by this, switchable adhesion between low-adhesion and high-adhesion was developed. Various adhesion surfaces have been fabricated by adjusting the chemical composition or structural roughness. For example, by changing the immersion time to control the topographies of CuO surfaces, Li et al. tuned water adhesion ranging from extremely low to very high.3 Beyond this, external stimulation was also popularly utilized to achieve switching via introducing stimuli-response groups and materials, using light irradiation, pH, and magnetic fields.4 In addition to solid–liquid adhesions on superhydrophobic surfaces, solid–solid adhesions have also been a focus. Much attention has been paid to icephobicity, protein adsorption, cell behavior, and anti-bacterial properties on superhydrophobic surfaces.

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The amazing repellency of superhydrophobic surfaces makes it a new promising topic to reduce biomolecule adhesion and icephobicity. For example, Beltran et al. studied fibronectin (FN) adsorption and adhesion of MC3T3-E1 cells on superhydrophobic polystyrene (SH-PS) surfaces.5 Both FN adsorption and cell adhesion and proliferation were significantly lower on SH-PS as compared with PS. A natural model of inhibiting bacterial cells has also been found on Cicada wings,6 which kill bacteria due to nanopillar structures arranged on the surface. The cooperation of antimicrobial materials and superhydrophobic surfaces possessing unique structures improves the efficiency of bacteria prevention. However it is also controversial that superhydrophobic surfaces do not always take effect. These paradoxical results are attributed to different wettabilities, chemical compositions, roughness, cell types, experimental environments, and so on. In this review, we present natural and biomimetic superhydrophobic surfaces with special adhesion properties and discuss the multi-structure effects on them. We also pay attention to switchable adhesion between low-adhesion and highadhesion. In the second section, we summarize protein adsorption, cell behaviors and bacterial adhesion on superhydrophobic surfaces. Many factors including surface wettability, chemical composition, roughness, and cell types are taken into account to explain the difference in results. What’s more, we also focus on the icephobicity of superhydrophobic surfaces since this is vital to life and industry. Moreover, the mechanism of ice crystallization and methods of hindering icing are concerned. Finally, we make a conclusion and outlook on this field, pointing out the current problems and the potential research directions of superhydrophobic surfaces in the coming decade.

Liquid–solid adhesion of superhydrophobic surfaces Surfaces with special adhesion in nature

Professor Weimin Liu received his PhD with major of Lubricating Materials and Tribology from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences in 1990. After that, he joined State Key Laboratory of Solid Lubrication (LSL), Lanzhou Institute of Chemical Physics. From June 1993 to June 1994, he worked as a Visiting Scholar in Pennsylvania State University, USA. In 1996, he was Weimin Liu promoted to Professor, and in 2000, nominated as Head of the State Key Laboratory of Solid Lubrication. During 2005–2012, he was the Director of the LICP. In 2013, he was elected as the Academician of Chinese Academy of Sciences.

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Nature is a prodigious world with fascinating properties and functions, which gives us inspiration and important ideas to create valuable materials and devices. Biological surfaces provide multifunctional interfaces. Some of them are superhydrophobic and self-cleaning, with a large contact angle of more than 1501 and a low contact angle hysteresis (CAH) of less than 101, such as water strider legs (Fig. 1A)7 and lotus leaves.8 The lotus leaf is a typical example with a large contact angle above 1601 and a sliding angle below 31. Fascinated by this interesting phenomenon, Barthlott and Neinhuis observed thousands of leaves and concluded that this unique property was attributed to micrometerscale papillae (5–9 mm in diameter) on the rough surfaces and epicuticular wax.9 Further research discovered that the surface of a lotus leaf is a micro- and nanoscale hierarchical structure. The cilium-like micro-nanoscale (120 nm in diameter) hierarchical architectures superimpose on the micron-sized papillae (Fig. 1B).10 This self-cleaning surface property whereby droplets cannot stay on the surface and roll off immediately is termed the ‘‘lotus effect’’, showing the ability of low-adhesion.

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superhydrophobicity with a CA of 1571 and anisotropic adhesion due to the fact that the water droplets can roll more easily along the direction parallelling the rice leaf edge than along the perpendicular one (Fig. 1F).8 Biological research indicates that this unique structure is not only a crucial factor in achieving the special wettability, but is also closely related to the multifunction-like adhesion properties. Attracted by these fascinating phenomena of adhesive surfaces in nature, researchers have devoted a lot of effort to exploring these intrinsic structure–function relationships. Fig. 1 Photos of superhydrophobic surfaces in nature: (A) a water strider (macroscopic view) resting on the water surface and SEM images of a water strider leg with oriented needle-shaped microsetae and the grooved nanostructure on the seta.7a (Copyright 2007 American Chemical Society) (B) A lotus leaf (macroscopic view) and SEM images of a lotus leaf surface covered with micrometer-scale papillae and cilium-like micronanostructures on papillae.8,10 (Copyright 2007 Elsevier) (Copyright 2002 Wiley-VCH) (C) A gecko walking on a wall and SEM images of its leg covered with microscale setaes and spatula-shaped structures on the setae.11b (Copyright 2012 Royal Society of Chemistry) (D) Rose petals absorbing water and SEM images showing the periodic array of micropapillae and nanofolds on each papilla top.13a (Copyright 2008 American Chemical Society) (E) A butterfly and SEM images of its wing surface arrangement of overlapping micro-scales and lamella-stacking nano-stripes on the scales.14 (Copyright 2007 Royal Society of Chemistry) (F) Water rolling on a rice leaf and SEM images of the leaf covered with hierarchical papillae and nanoscale pins on each papillae.8 (Copyright 2007 Elsevier)

Conversely, the gecko is well-known its inverse property of climbing easily on almost any kind of surface in different orientations, indicating high-adhesion to the surface. This remarkable ability is correlative with the hierarchical microand nanostructures on its feet.11 A gecko’s foot has nearly five hundred thousand keratinous hairs or setae and each seta contains hundreds of projections terminating in 0.2  0.5 mm spatula-shaped structures (Fig. 1C). Apart from the solid–solid adhesion of gecko feet, its wettability and solid–liquid adhesion are also noted. It was found that gecko setae are superhydrophobic with a large contact angle (CA) of 160.9  1.41 and a high adhesive force towards water.12 Additionally, high-adhesive superhydrophobic surfaces are also observed on other biological surfaces in nature, such as the Chinese Kaffir lily, red rose and sunflower. The rose petal has a hierarchical structure with an array of micropapillae (about 16 mm in diameter) and nanofold structures at the top of each micropapillae, which provides sufficient roughness for superhydrophobicity with a CA of 152.41 and a high adhesive force for water (Fig. 1D).13 In addition to the low-/high-adhesion surfaces, nature also gives us another surprise—anisotropic adhesion. Droplets can easily roll off the surface of butterfly wings along the radial outward (RO) direction of the central axis of the body and are pinned tightly to this RO direction, owing to the directiondependent one-dimensional oriented arrangement of flexible nano-tips on the ridging nano-stripes and overlapped microscales (Fig. 1E).14 This interesting property has also been found in some plants. Rice leaves are covered with hierarchical papillae (about 5–8 mm diameter) in a one dimensional order of papillae and distributed numerous nanoscale pins, resulting in both

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Artificial superhydrophobic surfaces with special adhesion These fascinating phenomena inspired intensive research into the mechanism and two classical models have been proposed: Wenzel’s model15 and Cassie and Baxter’s model.16 Based on Young’s equation,17 the former discussed the impact of roughness and the latter further discussed the influence of porosity and heterogeneities, and introduced the liquid–air–solid composite interface. The discussion of the mechanism gives us inspiration and principles to design and fabricate smart surfaces with multi-functionalities. Biomimetic research indicates that the cooperation of the unique structures and chemical composition play a crucial role in achieving desired wettabilities and multi-functionalities. In other words, in order to obtain low-/ high-adhesion superhydrophobic surfaces, it is necessary to create appropriate multiscale geometrical structures and specific chemical components. Accordingly, various methods have been designed to fabricate low-/high-adhesive superhydrophobic surfaces. Researchers have made and summarized a lot about low-adhesion surfaces with a low CAH which can be prepared by hydrothermal reaction, electrochemical deposition, selfassembly, layer-by-layer, plasma etching, sol–gel, polymerization, chemical vapor deposition, electro-spinning, and so on.18 As a new focus, the exploration of high-adhesion surfaces has aroused great interest among researchers. Similar to low-adhesion surfaces, the most important factors for wettability and high-adhesion are chemical composition and surface geometric structure. The former is related to the surface energy. Materials with inherent hydrophobicity have a low surface energy which favors fabrication of superhydrophobic surfaces. Additionally, polar or hydrophilic-/hydrophobic-terminal groups can be used to control the magnitude of adhesion force efficiently. The latter is the other key design parameter. Roughness can not only amplify the hydrophobicity, but can also greatly influence adhesion properties. Generally, the hydrophobicity of a material is intrinsic, which is limited in some cases. For example, although a surface modified by –CF3 is known for an extremely low free energy, the CA can only reach a maximum of 1201.19 Therefore, there is no doubt that the geometrical structure plays a vitally important role in special adhesion. But how it contributes to the final adhesive performance is still unclear. Researchers have attempted to answer these questions. Campo et al. fabricated micropillar-like polydimethylsiloxane (PDMS) using a combination of two-step photolithography with SU-8 resist and subsequent soft lithography.20 Two-level structured surfaces with pillars of different

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dimensions were prepared and compared with one-level structures. Contrary to expectation, the hierarchical structures did not improve the adhesion compared to single-level pillars. This revealed that hierarchy does not improve adhesion against flat substrates, even though the effective stiffness decreases significantly upon addition of another level of hierarchy. However, a study by Bhushan and Lee showed that the adhesive forces of one- and two-level fibers are stronger than that of the flat surfaces due to an increase in the number of fibers on the surfaces.21 Jiang and coworkers discussed the effects of different nanoporous structures on adhesion properties by creating a nanopore array (NPA), a nanotube array (NTA), and a nanovesuvianite structure (NVS), which showed high adhesion, controllable adhesion and extremely low adhesion to water, respectively, resulting from the variation in the three contact line.22 Subsequently, their group also fabricated aligned polystyrene (PS) nanopillars terminating in flat or concave tips and nanotubes by a simple and reproducible method to explore the effect of tip geometry.23 The as-prepared surfaces were superhydrophobic and highly adhesive to water. But their adhesion was different. The surface with concave tips had the largest adhesive force to water, followed by the surface with flat tips, whereas the aligned nanotube surface had a relatively lower adhesion, indicating that the tip geometry play an important role in determining the adhesive properties. The influence of structural roughness is complex and various. Parameters like shape, density and size are of great importance and should be taken into consideration. As intensive research into the mechanism, and inherent relationship of adhesion and structure becomes clearer, the rapid development of fabrication methods accelerates. High-adhesive surfaces have emerged since Jiang et al. initially fabricated a superhydrophobic surface with high adhesive force, which was composed of aligned polystyrene nanotubes.24 After that, a great number of methods and techniques have been employed to make high-adhesive superhydrophobic surfaces. Among them, mimicking bio-surfaces is an efficient approach. The gecko’s foot is an important object to mimic as it distributes compact nanotube arrays and microcapillaries to generate superhydrophobicity and strong adhesion. Accordingly, analogous structures such as nano-/micro-tubes, fibrates, and pillars have been fabricated using various methods and techniques. Lai et al. prepared titanate nanotube films by electrophoretic deposition. After 1H,1H,2H,2H-perfluorooctyl-triethoxysilane modification, they displayed superhydrophobicity with strong adhesion.25 Jiang et al. produced fluorinated polyimide nanofibric mats with adhesive superhydrophobicity, the water contact angle on the mat reached as high as 157.81 and the adhesive force to a water drop was up to 98.3 mN.26 However, these structures with high aspect ratios or high number densities which lead to high adhesion have the risk of clustering, resulting in dramatic surface topology changes and even adhesion to water changing from sticky to slippery. Inspired by the lotus leaf structure, nanoparticles which are widely used to fabricate rough surfaces with low-adhesion can also be applied to construct high-adhesion surfaces. Ge et al.

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fabricated well-defined raspberry SiO2/polystyrene (SiO2/PS) particles with a submicron SiO2 core which was functionalized by methacryloxypropyltrimethoxysilane (MPS) via a radiation miniemulsion polymerization.27 After deposition to generate a dual-size rough surface topology, this film had a static water contact angle as high as 1511. Meanwhile, this film exhibited a large contact angle hysteresis of 1161 and strong adhesion to water. Li and Shen utilized a simple and time-saving two-step dipping process by combining two sizes of particles from a silica sol and silica microsphere suspension to generate a double-structure superhydrophobic surface.28 The high-adhesive property that causes a droplet to steadily hang on a 1801-rotated substrate can be obtained by controlling the dipping depth of the two-step process. Inspired by rose petals, Bhushan and Her fabricated superhydrophobic surfaces with high- and lowadhesion according to different rose petals with diverse rough structures.29 This shows that the pitch values of the microstructures and density of the nanostructures play an important role in real rose petals and artificial surfaces in controlling their adhesion properties. Mimicking different biological surfaces can lead to similar wettability and adhesion properties. Conversely, the same biological surfaces can be inspired to fabricate surfaces with different adhesion behaviors. There is no doubt that the fabrication and development of low/high adhesion superhydrophobic surfaces deepens and expands the potential applications. But this is monotonous and has limited control over the motion of droplets. Therefore, tunable adhesion which allows the manipulation of water droplets on superhydrophobic surfaces is expected to integrate superhydrophobicity and reversible switching of water mobility to construct more dense and complicated microfluidic devices due to the increasing need for controlled transport of small volumes of liquids in localized chemical reactions, biochemical separation, bio/chem analysis assay, single-molecule spectroscopy, and tissue engineering. Switchable liquid–solid adhesions on superhydrophobic surfaces Superhydrophobic adhesion switchable between low-adhesion and high-adhesion is emerging as a new goal for special superhydrophobic surface studies due to the great demand for smart materials. Taking chemical composition and the surface geometric structure into consideration, it is effective to control the adhesive forces by adjusting these factors respectively or simultaneously. Besides, providing external stimulations to conquer the energy barrier is also one of the most common methods for obtaining switchable adhesions. Controlling the chemical composition and structural roughness Chemical composition is important in determining surface wettability and adhesion. It seems that the interactions between surfaces and droplets play a decisive role. Polarity and hydrophility can enhance the interaction forces between surfaces and droplets. It is feasible to achieve switchable adhesion while maintaining superhydrophobicity by controlling the hydrophilic

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and hydrophobic composition. There are two approaches to achieve chemical-controlled adhesion. One is to introduce hydrophobic and hydrophilic groups on a micro-scale rough surface and adjust the ratio of the hydrophilic and hydrophobic groups, which is inspired by the desert beetle’s back that possesses hydrophilic and hydrophobic micro-domains to help it collect water for maintaining life activities. The other is to adjust the amount of amphiphilic molecules on the superhydrophobic surfaces. Amphiphilic molecules are sensitive to certain external stimulations, which can change the terminal groups on the outer surface to adjust the wettability and adhesion. For example, Lai et al. fabricated superhydrophobic sponge-like nanostructured TiO2 surfaces modified with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PTES) by a simple electrochemical and self-assembly method.30 A wide range of tunable adhesive forces can be markedly controlled by surface chemical components instead of surface structure through adjusting the nitrocellulose (NC) concentration (Fig. 2A), leading to substitution of the lower energy CF3 groups by more energetic NO groups on the film surface and a conformation change of the PTES layer resulting from the hydrogen bonds formed by NC nitro groups with the silanol groups on the TiO2 surface (Fig. 2B). Biomimetic studies and theoretical research have revealed that characteristic micro/nanostructures of biological surfaces are crucial factors in special wettability. Various geometric structures can result in different adhesion properties. Tailoring the surface topographies of solid substrates applies efficient ideas to design biomimetic materials and develop new methods. There are several parameters which can be used to control the surface topographies to tune adhesion behaviors, such as aspect ratio, high number density and the shape of micro-/nano-structures.31 Lei fabricated a superhydrophobic surface by combining microand nano-scale structures and low surface energy materials via a hydrothermal technique and the self-assembly of stearic acid.31a The adhesion was controlled by affecting the morphology of the microstructures by oxidizing the zinc substrates in different zinc salt aqueous solutions without altering the surface composition. The ZnO surface prepared, from a Zn(NO3)2 aqueous solution, densely composed of ZnO nanorods, showed a water CA of 158  11 and high adhesion to water resulting in the water droplet being firmly pinned to the surface without any movement, even if it is turned upside down (Fig. 3a and b). However, the ZnO surface, prepared from a Zn(CH3COO)2 aqueous solution, with flower-like

Fig. 2 (A) Illustration of surface chemical composition changes before and after PTES–NC mixed layer self-assembly on the hydroxylated spongelike TiO2 surfaces. (B) The effect of NC concentration to water CA and adhesive force on the spongelike structure TiO2 surface.30 (Copyright 2008 American Chemical Society)

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Fig. 3 FE-SEM images with different magnifications of ZnO surfaces: (a, b) ZnO nanorods prepared by Zn(NO3)2 aqueous solution with high water adhesion. (c, d) Flower-like ZnO nanosheets prepared by Zn(CH3COO)2 aqueous solution with low water adhesion.31a (Copyright 2012 Elsevier)

Fig. 4 Relationship of deposition time and contact angle/sliding angle.32 (Copyright 2012 Royal Society of Chemistry)

architectures composed of nanosheets showed a CA of 161  11 and a SA (sliding angle) of 31 (Fig. 3c and d). In addition to adjusting adhesion by controlling chemical composition and surface geometric structure, adjusting both of them to achieve tuned adhesion is also possible. Chen et al. prepared a transparent cross-aligned superhydrophobic TNB/ FAS film on a conducting glass substrate. Initially, the droplets showed strong adhesion to the surface even when the substrate was vertically aligned or upside down due to the sparse distribution and mostly horizontal alignment of the TNB particles and interaction between the water droplet and the partially uncovered hydrophilic substrate.32 As the deposition time continued, the adhesion decreased from ‘‘sticky’’ to ‘‘roll-off’’ which was attributed to the enhanced roughness and the pores generated by the cross-stacking of TNB particles and the increasing number of hydrophobic –CF3 groups (Fig. 4). External stimuli As we discussed regarding the mechanism, superhydrophobic surfaces may be in a metastable state with droplets partially wetting the surfaces. The energy barrier prevents the spontaneous transition.

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External stimuli, such as lighting, thermal treatment and pressure can realize the adhesion changes between the Cassie and Wenzel states. Light-stimulation is a common method to tune water adhesion and UV is the main light source. In many cases sharp adhesion switching after UV illumination is caused by a local chemical change. One effective strategy is modification with a chemical substance consisting of hydrophilic groups and hydrophobic groups on the rough surface.33 For example, Liu et al. fabricated a TiO2 nanotube film modified with a perfluorosilane monolayer.33a The adhesion can be switched between sliding superhydrophobicity and sticky superhydrophobicity by selective illumination through a mask and heat annealing (Fig. 5A) due to the formation of hydrophilic regions containing hydroxyl groups (Fig. 5B) which are surrounded by still superhydrophobic regions, resulting in adhesion changing dramatically from easy sliding to a highly sticky movement without sacrificing the superhydrophobicity (Fig. 5C). What’s more, the transition is easier and faster using heating treatment (Fig. 5D) than some transition methods like storing in the dark for several weeks.34 Another strategy is incorporating photosensitizers like azobenzene, spiropyran functional groups, or transition metal oxides such as TiO2 that change the trans/cis conformation under vis and UV illumination into rough surfaces.35 Jiang et al. prepared a micro-nanopost arrayed silicon substrate by photolithography and chemical etching and a side-chain azo-polymer (poly-4-(8-acryloyloxy) octyloxy-4 0 -hexyloxy azobenzene) was simply spin-coated onto it.35a With alternating irradiation of UV and visible light, the azo-polymer coatings show a trans/cis photoisomerization, which leads to reversible adhesion with a water droplet from sticking on the micro-nanopost array (average adhesion force (AF), 80.8  4.9 mN) (Fig. 6A and B) to rolling off (AF, 60.6  12.3 mN) (Fig. 6C and D). As with the photo-sensitive polymer introduced above, grafting stimuli-sensitive polymers is effective to fabricate smart surfaces

Fig. 5 (A) Switching photographs of a spherical water droplet with a SA lower than 11 (left), and a liquid drop suspended under the superhydrophobic TiO2 NT surface (right) before and after UV illumination, respectively. (B) Schematic of the switchable adhesive process. (C) Inter-relationship between CA and maximum adhesion volume with heat treatment time at 80 1C. (D) Relationship between annealing temperature and recovery time from adhesion to the sliding state of water droplets on the TiO2 NT surface.33a (Copyright 2009 Royal Society of Chemistry)

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Fig. 6 Photo-induced changes in superhydrophobic adhesion on a micronanopost array with an azo-polymer coating. (A) Azo-polymer coatings show a cis photoisomerization and an average CA of 147.71  0.91 after UV light irradiation. (B) Optical image of the maximum-deformed water droplet on the array after UV light irradiation. The surface is in the cis state, which shows high water adhesion (average AF, 80.8  4.9 mN). (C) Azo-polymer coatings show a trans photoisomerization and the average CA is 148.21  0.71 after visible light irradiation. (D) Optical image of the maximum-deformed water droplet on the array after visible light irradiation. The surface is in the trans state, which shows low water adhesion (average AF, 60.6  12.3 mN).35a (Copyright 2012 Wiley-VCH)

due to the flexibility of introducing methods and the diversity of the stimuli. Polymers responding to pH, temperature and ions are widely used. Among them, pH-sensitive polymers are typically based on ionized weak acid or base moieties such as PAA, P4VP. For example, Liu et al. grafted pH-responsive pentamethyldiethylenetriamine (PMDETA) brushes on a rough anodized alumina surface.4b The wettability and adhesion behaviors can be controlled by changing the pH values of the droplets to regulate the surface chemical reactions (Fig. 7A). The droplets with a

Fig. 7 (A) Schematic illustration of in situ transition of adhesion after NH3 reaction with the acidic droplet. (B) The corresponding CAs and SAs of droplets with different pHs on the PDMAEMA grafted substrate (In–PFOTS 1 : 15).4b (Copyright 2012 Royal Society of Chemistry)

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pH from 1 to 6 show pinning effects to surfaces due to a hydrophilic reaction between the acidic droplets and the amine groups of PDMAEMA while the SAs of the basic droplets with pHs larger than 7 are smaller than 251 and the droplets can easily slide off the surface as a result of the hydrophobic interactions between the basic droplets and the PDMAEMA chains (Fig. 7B). Temperature is also a stimuli-factor for switching the adhesion of droplets on superhydrophobic surfaces. The main way to realize this kind of transition is to introduce a temperature-sensitive polymer containing repeat units such as N-isopropylacrylamide (NIPA), oligo(ethylene glycol)methacrylate, 2-dimethylamino ethyl methacrylate, and hydroxypropyl acrylate into topological surfaces.36 For example, Zhu and coworkers prepared poly(acrylic acid)-block-polystyrene (PAA-b-PS) functionalized multiwall carbon nanotubes (MWNTs) by a nitroxide-mediated ‘‘living’’ free-radical polymerization and sprayed these MWNT-PAA-b-PS nanoparticles onto a copper substrate to fabricate superhydrophobic MWNT films.36a The surface can be switched between low adhesion and high adhesion due to the amphiphilic PAA-b-PS copolymer with hydrophobic PS chains and hydrophilic PAA chains (Fig. 8A). The reason is due to the part formation of hydrogen bonds with water molecules providing high adhesion between the MWNT-PAA-b-PS film and water droplet while immersion in water and formation of PS while heating, result in hydrophilic domains, which would be occupied by the hydrophobic PS chains again, inducing a CA increase to 1651 and the SA to recover to 201 (Fig. 8B). Great progress has been made in responsive smart surfaces that respond to a single stimuli due to extensive applications including sensing, biomedicine, tissue engineering, membranes, and self-cleaning surfaces. However, there are still big challenges to subtly master cooperative interactions. In order to solve these problems and achieve programmable response, many efforts over the last decade have focused on the design and synthesis of multi-responsive smart surfaces. Variable functional polymers and molecules that respond to stimuli such as temperature, pH, ionic strength, light, and electric or magnetic fields, chemical and biochemical stimuli have been introduced to prepare smart surfaces. Based on this, multi-responsive surfaces switching

Fig. 8 (A) The switching photographs of a spherical water droplet with high adhesion (right) to low adhesion (left) before and after heating, respectively. (B) The corresponding schematic of switching adhesion.36a (Copyright 2009 Wiley-VCH)

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Fig. 9 (A) Schematic illustration of corresponding switchable adhesion with thermal change. (B) Schematic illustration of corresponding switchable adhesion with pH change. (C) Schematic illustration of corresponding switchable adhesion with electrolyte change.37 (Copyright 2010 American Chemical Society)

between high-adhesion and low-adhesion superhydrophobic surfaces have also been researched. Zhou et al. grafted two stimuli-responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(dimethylamino)ethyl methacrylate (PDMAEMA) onto irregular micro/nanoscale Al2O3 substrates.37 The as-prepared surfaces can be switched between high-adhesion and low-adhesion while remaining in a highly hydrophobic state by changing the temperature, pH and electrolyte due to the collapse and stretching of polymer chains in the different conditions (Fig. 9A–C). In addition to introducing sensitive polymers, tuning adhesion of the superhydrophobic surface can also be realized via external forces or field effects like pressure, magnetic field and electrical field. By magnetizing and demagnetizing surfaces and droplets in a magnetic field to change the interaction forces, Jiang and coworkers fabricated an intelligent superhydrophobic iron surface to obtain a reversible transition between a high adhesive pinning state and low adhesive rolling state, which can be achieved by simply magnetizing and demagnetizing the surface alternately (Fig. 10A), showing adhesion ranging from ultralow (13.2 mN) to very high (60.9 mN) (Fig. 10B). This effect derives from the magnetic domains of the iron plate from disordered with no magnetic force to ordered with magnetic force, and the microdroplet from the Cassie state to the Wenzel state at the same time (Fig. 10C).4c Similar to the magnetic field, the electric field can be used to regulate wettability and adhesion due to its simplicity, efficiency, and fast response. Zhao et al. investigated the electrowetting of droplets on a rough superhydrophobic surface covered with MnO2 nanotube arrays (MTA) and found that the wettability

Fig. 10 The changes in superhydrophobic adhesion with magnetization and demagnetization: (A) a low adhesion before magnetization and high adhesion after magnetization. (B) The relationship between magnetic field intensity and adhesion force before and after magnetization. (C) Schematic diagram of transition between high adhesion and low adhesion.4c (Copyright 2008 Wiley-VCH)

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Fig. 11 (A) The equipment schematic of electro-controlled adhesion. (B) Relationship between voltage droplet for positive and negative bias voltages and CA, respectively. (C) Relationship between voltage droplet for positive and negative bias voltages and hysteresis angles, respectively.38 (Copyright 2011 Wiley-VCH)

and adhesion properties are related to the polarity of bias applied to the surfaces (Fig. 11A), which showed that the CA remained apparently unchanged with increasing voltage when a positive bias was applied to the MTA membrane (with the MTA membrane acting as an anode) while the CA was found to decrease slightly from 1621 to 1571 as the negative bias voltage applied to the MTA membrane (membrane acting as a cathode) increased and the droplet sank into the membrane (Fig. 11B).38 Moreover, the adhesion changed dramatically with a small decrease in CA (Fig. 11C). The hysteresis angle increased with voltage when the MTA was adopted as a cathode, while only a slight increase was observed when MTA was adopted as an anode due to the promoted continuity of TCL from a discontinuous ‘‘point contact’’ to a quasi-continuous ‘‘line contact’’ TCL (Fig. 12A and B). Curvature, as a factor of surface energy, has been taken account when fabricating special surfaces.39 A novel curvaturedriven reversible in situ adhesive superhydrophobic surface was prepared by Jiang and coworkers.40 A superhydrophobic poly(dimethylsiloxane) (PDMS) pillar-array film fabricated through the combination of interference lithography and soft lithography showed very high adhesion to water droplets. When the surface curvature was increased to a certain range, along with a slightly increased CA (up to 1601), the adhesion force and the SA decreased significantly (SA o 51) (Fig. 13A–C). A possible

Fig. 12 (A) Schematic of water droplets on nanotube structured surfaces before and after supplying voltage. (B) Local contact angles are different depending on different tilting angles of posts under certain Laplace pressures.38 (Copyright 2011 Wiley-VCH)

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Fig. 13 (A) Schematic model of the spherical surface of a pillar array and curvature calculation. (B) The relationship between the curvature and CA. (C) Relationship between adhesion force and distance between adjacent pillar tips (due to curvature). (D) Schematic of curvature-driven reversible adhesion.40 (Copyright 2011 Wiley-VCH)

reason for the unique change between pinned state and rolldown superhydrophobic state could be the curvature-induced air injection into the pillar arrays owing to the excellent flexibility of PDMS, and the pillar array film could be alternated between a flat and a curved surface by an external force (Fig. 13D). Anisotropic wettability, as an unusual superhydrophobic behavior, has attracted considerable attention.41 Malvadkar et al. prepared anisotropic structures with inclined poly( p-xylylene) nanorods, exhibiting high water contact angles and strong anisotropic adhesion due to the orientation of the nanorods.41a Mimicking the rice leaf, Lee et al. fabricated a hierarchical artificial rice leaf structure consisting of a constant wavy microstructure and a number of nanoporous multilayers (PAH/SN) by surface winkling and electrostatic layer-by-layer (LBL) assembly, respectively.41b As the number of PAH/SN bilayers on the wrinkled PDMS substrate increased, the surface root meansquare (RMS) roughness (Rq) varied from 21 to 138 nm and nanopores were formed at the same time (Fig. 14A–C), indicating that the roughness and nanoporosity can be controlled by the assembled number of PAH/SN bilayers, leading to both perpendicular (y>) and parallel (yJ) CAs increasing and reaching a saturation value (Fig. 14D). Meanwhile, the difference in contact angle hysteresis along the parallel and perpendicular directions decreased and the water droplet behavior changed from anisotropic/pinned to the anisotropic/rollable and isotropic/ rollable state (Fig. 14E and F). As shown above, the chemical composition and surface structure greatly influence the wettability and adhesion behaviors of droplets, which are essential to obtain smart surfaces with tunable adhesions. Responsive surfaces are an efficient choice as

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Fig. 14 (A–C) SEM images of nanostructured films with PDMS microwavy structures with a different number of deposition cycles; (A), (B), (C) have 2, 5, and 9 PAH/SN bilayers, respectively. (D–F) Relationship between static water contact angle (D), roll-off angle (E), and contact angle hysteresis (F) perpendicular (black squares) or parallel (white dots) with the roughness of the nanostructure (Rq).41b (Copyright 2013 Wiley-VCH)

they are versatile and can have a variety of external stimulations. However most methods are performed on different surfaces and in situ switching adhesion is limited. Additionally, multi-response surfaces are rare and desirable due to the great demand in applications.

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radius defined as the smallest radius of nucleation to remain stable. Besides, the liquid must be in a condition of undercooling DT, which was defined as the difference between actual crystallization temperature and equilibrium temperature. Typically, freezing in nature is heterogeneous, because the free energy of heterogeneous nucleation is theoretically smaller than that of homogeneous nucleation. Thus, the undercooling DT of heterogeneous nucleation is smaller compared to homogeneous nucleation. Moreover, surfaces in nature are always rough and contaminated, which makes for heterogeneous nucleation. Apart from temperature, other environmental factors have also been discussed.44 Jung et al. explored the mechanism of supercooled droplet freezing on surfaces in different humidities or gas flows. Their investigation showed that humidity or gas flow can unexpectedly change ice crystallization.44a Evaporative cooling generated by humidity and gas flow can cause ice crystallization by homogeneous nucleation instead of heterogeneous nucleation. What’s more, the adhesion of water on a superhydrophobic surface can also be affected by environmental conditions. All of these results reveal that environmental conditions have a great impact on icephobicity. Anti-icing properties of superhydrophobic surfaces

Ice formation and adhesion on superhydrophobic surfaces In addition to the internal research of dynamic adhesion, the multifunctional application of it is another important focus. The low adhesion of droplets on superhydrophobic surfaces provides a new idea to reduce ice formation. Ice formation, as a natural phenomenon, sometimes makes life difficult and can even cause huge damage. Ice accretion on exposed surfaces may lead to damage of electrical devices like wires, antennas, insulator failure, traffic accidents, aircraft stalling, and so on, which often cause big problems and enormous economic losses.42 Thereby, many researchers have contributed to fabricating antiicing surfaces. They have not only improved the efficiency of hindering ice formation, but also probed into the intrinsic mechanism of ice crystallization. Mechanism of ice crystallization According to nucleation theory, ice formation involves the formation of a crystal nucleus and its subsequent growth.43 The formation of the crystal nucleus occur in two ways: homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation occurs in water without the influence of surfaces and foreign particles or substances. Heterogeneous nucleation is prone to take place at heterogeneous nuclei which can be dirt, molecules, bacteria, or simply a container wall. It is crucial to generate a crystallite firstly which comes from the incessant shaking caused by thermal fluctuations and structural fluctuations in the liquid. However, most the crystallites are unstable and dissolve back into the liquid. Thermodynamic theory suggests that the growth of crystallites will decrease the Gibbs free energy of stable crystallite formation if the radius of the crystallites is larger than the critical

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Anti-icing surfaces have been studied for several decades, but there are still immense challenges in achieving efficient antiicing on surfaces. Superhydrophobic surfaces, due to their excellent water repellency, have been applied to anti-icing surfaces and aroused great interest of researchers. Generally, there are two ways to achieve anti-icing on superhydrophobic surfaces: one is to delay ice formation and accumulation. The other is to reduce ice adhesion strength. As for the former, it is difficult for drops to remain on the surface, which discounts and/or delays the possibility of ice formation. Jiang et al. found that Morpho nestira butterfly wings display robust water repellency at low temperatures and changeable relative humidities, which is attributed to the multi-level micro-/nanostructures on the wing that can trap air to suspend the three phase contact line, and accordingly effectively retard surface wetting induced by low temperatures.45 Inspired by this, they explored the role of multi-level micro-/nanostructures in designing anti-icing surfaces.46 They revealed that the antiicing properties of micro-/nanostructure surfaces are much better than other surfaces like nanostructured surfaces, microstructured surfaces, and smooth surfaces, with a delay time of 7000 s for ice formation. Experiments on icephobicity of superhydrophobic surfaces taken in nature also supported this. Gao et al. coated superhydrophobic nanoparticle–polymer composites on one side of an Al plate to achieve superhydrophobicity, while the other side was not treated, and left it for more than 1 week.47 After freezing rain, the side with the superhydrophobic composite had little ice compared with the untreated side which was completely covered by ice (Fig. 15A and B). This phenomenon has also been proved on commercial satellite dish antenna surfaces (Fig. 15C and D). All of these results indicate superhydrophobic surfaces have a positive effect on anti-icing.

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Fig. 15 Experiments of anti-icing properties of superhydrophobic surfaces in nature. (A) The side which was not treated with superhydrophobic composites is covered with ice after freezing rain. (B) The side coated with superhydrophobic composites has little ice. (C, D) The integrated view and close-up view of a satellite dish antenna both untreated (left) and treated (right), respectively.47 (Copyright 2009 American Chemical Society)

However, the delay of ice formation is limited. As time extends or temperature lowers significantly, ice formation is inevitable. Therefore, reducing ice adhesion strength is the more efficient way for anti-icing. While the ice adhesion force is small enough, it is possible that it will fall off automatically due to its own weight or natural wind. This is a feasible and efficient method to de-ice. Many reports indicated that superhydrophobic surfaces can reduce ice adhesion substantially. Kulinich and Farzaneh compared the ice adhesion strength on a flat hydrophobic and rough super-hydrophobic surface with different contact angle hysteresis and when both of which were coated with similar surface chemistry.48 The ice adhesion strength increased while the CAH became smaller and the adhesion strength of surfaces with low CAH was 5.7 times lower than that on flat surfaces, demonstrating that ice adhesion correlated with wetting hysteresis of the superhydrophobic surfaces. Nevertheless, there are also researchers who queried that superhydrophobic surfaces cannot reduce ice adhesion.49 Kulinich et al. prepared various superhydrophobic surfaces to explore their anti-ice performance under different conditions.49a The anti-icing properties recede gradually during icing–deicing cycles due to the collapse of surface functionality. What’s more, environmental factors like humidity have a considerable impact in ice-adhesion. The efficiency of superhydrophobic surfaces to reduce ice-adhesion strength is significantly lower in a humid atmosphere. Chen et al. investigated ice adhesion on model surfaces with different wettabilities ranging from superhydrophilic to superhydrophobic.49b The results showed that ice adhesion forces of superhydrophilic surfaces and superhydrophobic surfaces are almost the same, while hydrophobic surfaces and hydrophilic surfaces have much smaller ice adhesion forces.

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These inconsistent results suggested that the intrinsic mechanism of icephobicity on superhydrophobic surfaces is still unclear. Many factors have been taken into consideration to explain it, such as hydrogen bonds,50 heat transmission,51 rough structure,52 and mechanical interlocking.53 It was believed that hydrogen bonds play a role in influencing the strength of ice adhesion. A large amount of hydrogen bonds can be formed between a hydrophilic surface and ice due to the numerous hydroxyl groups on it. Conversely, a hydrophobic surface has few hydroxyl groups to form hydrogen bonds, resulting in weak adhesion. In terms of thermodynamics, the system of a solid– liquid–air interface gains heat from air by contact heat conduction and thermal radiation and loses heat to the cold surface through contact heat conduction and thermal radiation between the drop and rough structure. It is supposed that a superhydrophobic surface loses less heat due to the smaller contact area between drop and superhydrophobic surface, bringing about a smaller temperature change to delay ice formation. The impact of structural roughness has been expounded in two opposite aspects. On the one hand, rougher structures increase the rate of heterogeneous nucleation. On the other hand, the unique air-pockets in superhydrophobic surfaces reduce the contact area between the surface and drops, resulting in lower adhesion strength owing to the Cassie–Baxter model. It has also been reported that mechanical interlocking exists between the ice and the surface texture of the superhydrophobic and superhydrophilic surfaces to increase the adhesion strength while for smooth surfaces almost no mechanical interlocking forms. Although the icephobicity of superhydrophobic surfaces is dubious and controversial, it’s study is ongoing. Lately, Boinovich et al. fabricated a robust icephobic coating on stainless steel, which can maintain its superhydrophobicity and anti-icing properties after 100 icing–deicing cycles. Meanwhile, the coating was also tested outdoors, showing a good performance in reducing the accumulation of snow and ice.54 These inspiring results indicate that superhydrophobic surfaces still have potential in this field. Therefore, it is very necessary to clarify the influencing factors related to the inherent properties of these surfaces like wettability, and the external conditions, and make more efforts in this aspect.

Solid–solid adhesion of superhydrophobic surfaces Anti-icing, taking phase transformation into consideration, involves not only liquid–solid adhesion behaviors on superhydrophobic surfaces, but also solid–solid adhesion behaviors. On one hand, the water repellency decreases the possibility and volume of water gathering on surfaces. On the other hand, the ice crystallizing on the surfaces may easily wipe off. Beyond that, superhydrophobic surfaces were also expected to make a difference in the biological field. Many solid–solid adhesion behaviors in this area have been investigated, such as protein adsorption, cell adhesion, anti-bacterial and so on.

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Protein adsorption on superhydrophobic surfaces Proteins, as one of the most important actors in the life of an organism, play a key role at the interfaces involving reactions with implanted biomaterials, growth of soil bacteria, and mammalian cell growth in culture. Protein adsorption is the first step of the interaction between a material surface and cells or platelets. Generally, before cell contact with a surface, proteins adsorb onto the surface. Then the adsorbed proteins are recognized by ion channels or receptors at the cell membranes. Subsequently, receptors cluster into integrins, providing trans-membrane links between the proteins and the cytoskeleton. Finally, cells adhere on the surface. Hence, it is necessary to firstly explore the interaction between protein and biomaterials. In some areas, protein adsorption causes big problems. For example, the bio-fouling of biomedical devices and some pipes induced by protein adsorption can arouse considerable energy consumption, inducing the reduction of efficiency, and even breakdown.55 As for biocompatibility, protein adsorption on material surfaces like artificial blood vessels may result in a series of physiological responses, such as thrombus formation, hemolysis, and complement activation. Therefore, reducing protein adsorption is important in many areas such as microfluidic transport, biomedical devices, tissue engineering, and implant materials like artificial blood vessels.56 Protein adsorption is a dynamic and complex process and the interactions occurring in it are mostly noncovalent, such as H-bonding, electrostatic forces, and hydrophobic interactions and van der Waals forces.57 Leaving out the intrinsic characteristics of proteins, the surface properties are strongly related to protein adsorption, including chemical properties,58 surface charges,59 topological structures,60 and hydrophobicity/hydropholicity.61 The chemical properties have an effect on the interaction forces between proteins and material surfaces. Studies have revealed that hydrophilic groups, including hydroxyl, amino, acylamino, etc., can reduce protein adsorption on account of repulsive forces between these hydrophilic groups and amino acid residues distributed on the proteins. Analogously, surface charges influence the electrostatic forces between surfaces and proteins by changing the ion distributions in solution. Additionally, topological structures will determine the types, adsorbing capacities, and conformations of proteins, involving the roughness, size and distribution of porous structure, radius of curvature of nanoparticles, orientation and size of groove structure.62 Protein adsorption is also closely related to the surface wettability. Surfaces which are highly hydrophilic can reduce protein adsorption.63 Therefore, a hydrophilic surface has better biocompatibility. However, it has been suggested that superhydrophobic surfaces could also reduce protein adsorption due to their extremely low surface energy and small solid surface area at the liquid interface. For example, Tsougeni and Petrou created superhydrophobic surfaces to observe protein adsorption in dynamic microchannels. They found that no proteins adhered on superhydrophobic stripes while rough hydrophilic stripes had more proteins.56b In addition to surface wettability, protein adsorption is also affected by roughness, experimental conditions, and so on.

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Koc et al. fabricated superhydrophobic surfaces with different roughnesses to investigate the adhesion of proteins.64 It is clear that the number of proteins adhering on superhydrophobic surfaces with a micrometer scale roughness is larger than that on smooth and nanometer-scale rough surfaces. This result indicated that protein adsorption is related to surface roughness and superhydrophobic surfaces are not always favorable to reduce adhesion. They also found that proteins adsorbed on superhydrophobic surfaces removed a considerably larger proportion in flow than that on smooth surfaces, implying superhydrophobic surfaces have potential applications in antibiofouling pipelines. Leibner et al. investigated adsorption of human serum albumin (HSA) on superhydrophobic polytetrafluoroethylene (PTFE) at atmospheric and vacuum conditions by using radiometry or electrophoresis.65 Unlabeled HSA measured by the electrophoresis method was nearly 4-fold higher than that using radiometry (125I-labeled HSA) or electrophoresis. They also found that when adsorption was performed with degassed protein solutions under vacuum, the adsorption results were similar using the two methods. This suggests that air pockets are essential to hinder adsorption. Cell adhesion on superhydrophobic surfaces As the smallest independent unit and functional unit, the cell is the foundation of organism metabolism. Generally, when a cell contacts with a surface, it will adhere onto it and proliferate instinctively.66 But sometimes cell adhesion and proliferation on surfaces can be detrimental. Thus, restraining cell adhesion is necessary and controlling cell attachment and proliferation is fundamental research in various areas such as biomedicine, tissue engineering, microfluidics and high-throughput screening devices. Until now, the inherent mechanism of cell–biomaterial interaction is far from understood, due to the complex factors, such as different surfaces properties like wettability, polarity, charge, cell types and protein adsorption. It is generally known that surface wettability and topography are critical factors in influencing cell behavior on biomaterials.67 Superhydrophobic surfaces, owing to their special wettability and multifunctions, are expected to make a breakthrough in the field of biomedical materials. Recently, many efforts have been made in the research of cell behavior on superhydrophobic surfaces. But the results are inconsistent. Many reports concluded that there is less cell adhesion and proliferation on superhydrophobic surfaces while others have opposite opinions. For instance, Piret et al. fabricated superhydrophilic/superhydrophobic SiNW surfaces, showing opposite cell behavior with selective adhesion on superhydrophilic regions due to cell cytoplasmic projections penetrating the hydrophilic silicon nanowire layer.68 However, Cha et al. fabricated polystyrene substrates with a similar surface structure to the lotus leaf and tested the cell behavior. A higher cell attachment was found on the superhydrophobic surface compared with the flat PS surfaces but with no significant change to the cell proliferation rate.69 These controversial results might be on account of the differences in the cell types, surface chemical groups, roughness, and so on.

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Fig. 16 (A) BCA albumin adsorption on PS and PLLA surfaces after 24 h of immersion in 500 mg mL1 of BSA. Data are reported as mean  SD (n = 3) and significant differences were found for (*) p o 0.05 and (**) p o 0.01. (B) MTT quantification of MC3T3-E1 cell line and bovine articular chondrocytes (BCH) on the different surfaces and on tissue culture polystyrene (TCPS) as control after 1 day in culture. Data are expressed as mean % change compared to control  SD (n = 3). Significant differences between different surface types on the same culture day were found for (*) p o 0.05 and (**) p o 0.01.71 (Copyright 2012 Springer-Verlag)

Oliveira assessed the performance of three different cell lines (SaOs-2, L929 and ATDC5) on superhydrophobic surfaces in vitro, discovering that ATDC5 and SaOs-2 cell lines were not able to proliferate on such surfaces while L929 cells could proliferate.70 SaOs-2 cells have also been used to discuss the impact of different nanotextured roughnesses to cell adhesion. It was revealed that small nanoscale roughness reduces cell adhesion better than the larger scales. Lourenc et al. studied cell behavior on the superhydrophobic surfaces of polystyrene and poly(L-lactic acid) with different micro-/nano-topographies.71 Protein on superhydrophobic surfaces tended to be reduced compared to smooth surfaces (Fig. 16A). What is more, the cell viability/cytotoxicity on the as-prepared surfaces, which informed us of the cell viability at the rough surfaces, was generally lower than at the corresponding smooth surfaces, especially for BCH (Fig. 16B). All the results indicated the adsorption effect and cell attachment appear to be independent from polymer composition and surface topography on superhydrophobic surfaces. It was also demonstrated that cell behavior is dependent not only on surface properties but may be also affected by the cell types. Bacterial adhesion on superhydrophobic surfaces It is known that bacteria is everywhere. In some cases, a surface microbial infestation may result in serious infection and biomedical device failure. Therefore, anti-microbic materials have been required for a long time due to more and more extensive demands in various fields such as biomedical devices like catheters, textiles, daily supplies. The classic agents used to hinder bacteria can be divided into three main classes: (a) natural or mimic anti-bacterial chitosans, berberines, peptides, and so on. (b) Organic anti-bacterial agents: halogens, imidazoles, hyperoxides, organonitrogens and so on. (c) Inorganic anti-bacterial agents, e.g. heavy metals72 like Ag, Cu, and Zn, photocatalysis oxides like TiO2, ZnO, CdS, WO3, and SnO2. Among them, silver, well-known for is broad antibacterial activity has been followed with special interest. The antibacterial properties of silver work for both Gram-negative like Escherichia coli and Gram-positive like Staphylococcus aureus bacteria, even some fungus.

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However, with the increasing popularity of anti-bacterial materials, drug-resistant microbes have emerged and rapidly spread. Meanwhile, the antibacterial effects of Ag+ which is closely related to its dissolution property and releasing rate is confined by release time.73 The inherent toxicity towards mammalian cells also makes it dangerous to use.74 In order to ameliorate these problems, superhydrophobicity was introduced into inhibiting microbial adhesion due to its very good repellency, which can reduce the bacterial adhesion to surfaces instead of killing them. Inspired by the multi-structure of the lotus leaf, deposition of nanoparticles on rough surfaces has become a popular method to achieve superhydrophobicity. In addition, it has also been demonstrated that nano-structured silver has more efficient anti-bacterial properties.75 Therefore, using silver nanoparticles to fabricate superhydrophobic surfaces is a simple and potent approach to achieve antibacterial properties.76 For example, Abada and Yazdanshenasb prepared superhydrophobic surfaces on cotton textiles via cooperation between the multi-size roughness formed by producing silver nanoparticles (Ag NPs) on cotton fibers and modification with octyltriethoxy silane.76b The as-fabricated textiles, with a CA of 1511 for a 10 mL droplet and high adhesion such that water drop did not slide off even when upside down, exhibit potent antibacterial properties toward both Gram-positive (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli). Up to now, the popular method of fabricating superhydrophobic surfaces with anti-bacterial properties is to grow or embed antimicrobial silver ions or nanoparticles on substrates. But the releasing of antimicrobial particles or ions is uncontrollable, resulting in a limited short period of time to work. In order to solve this problem, controllable release has caught the attention of scientists.77 Liu et al.77c fabricated multilayered polyelectrolyte films which enclosed silver nanoparticles and subsequently modified them with fluoro-silane to engender superhydrophobic surfaces. The anti-bacterial investigation elucidated that silver antimicrobial activity is related to Ag+ ion release from the embedded nanoparticles into the surrounding aqueous medium. The superhydrophobic surface could control the release of Ag+ ions effectively (Fig. 17A and B), showing a potent and durable antibacterial activity. This indicates that controlling the chemistry and topology of surfaces is important to design surfaces which can efficiently prevent or reduce bacterial adhesion. However, reports have also suggested superhydrophobic surfaces cannot reduce bacterial and can even induce more adhesion.

Fig. 17 Schematic of preventing microbial adhesion and killing bacteria in seawater before and after silver release.77c (Copyright 2012 American Chemical Society)

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Sousa et al. discussed the capability of Staphylococcus aureus and Pseudomonas aeruginosa to adhere and colonize on a rough superhydrophobic PLLA surface and a smooth hydrophobic one, respectively, indicating that both bacterial strains colonized the SH surface significantly more than the hydrophobic one.78 What’s more, the adhesion strength on the superhydrophobic PLLA surface is larger. Currently, researchers still focus on investigating the phenomenon at interfaces without deeply discussing mechanism. Actually, it is difficult to clarify the mechanism of biological adhesion on superhydrophobic surfaces. On one hand, biological responses to material surfaces are complex and influenced by many factors. On the other hand, at present experimental and testing methods are limited and have no standard. The types of objects are also rare. For example, the conditions of investigated cells differ from one report to another. The tested bacteria are limited to Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Thus, the results vary and are difficult to compare with only the parameter of water contact angle. Therefore, much more efforts should be made in the field. More biological types need to be investigated and more factors should be discussed.

Conclusions and outlook This article reviews recent progress in the special adhesion behaviors on superhydrophobic surfaces. We divided it into two parts. The first part states the solid–liquid adhesion on superhydrophobic surfaces and highlights the switching adhesion between low-adhesion and high-adhesion. Due to the important influence of surface compositions and rough structure, they can be used to control adhesion efficiently. Additionally, environmental stimulations also provide an efficient method to modulate the adhesion by cooperating with proper surface roughness, such as light irradiation, thermal treatment, magnetfield, electric-field. The second part illustrates the solid–solid adhesion on superhydrophobic surfaces and underlines the study on interaction between biomolecules or cells/bacteria and bio material surfaces. It also introduced the icephobicity of superhydrophobic surfaces. Due to the immense potential of special adhesive surfaces in various fields, such as anti-icing, medical materials, antifouling, fluid transport, and so on, significant progress has been made. However, the discussion above still rests on limitations in practical applications that need to be overcome in the future. For example, the strength and durability of the prepared materials with objective surface wettability are not good enough. The relationships between superhydrophobicity and icephobicity or biomolecules are still unclear, even controversial. What is more, big differences between in vivo and in vitro make results intricate and dubious. Thus, there are big challenges which need be conquered in the future. In order to satisfy requirements in practice, firstly, more attention should be paid to improve the capability and durability of adhesive superhydrophobicity in various environmental considerations. Mechanical strength also needs to be taken

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into consideration. Then, it is necessary to clarify the mechanism and build an inherent relation between smart materials and biomolecules. For example, how do superhydrophobic surfaces interplay with biomolecules? The exact reasons why superhydrophobic surfaces can hinder biomolecules adhesion also needs to be discussed. Up to now, studies indicate that superhydrophobic surfaces with special adhesion are promising. They show highly controllable adhesion and response to external stimulations, making it feasible in different environments to expand application. In addition, they normally show inhibition to biomolecules, exhibiting blood compatibility and anti-bacterial properties, which are useful in medical devices and biomedicine. In future, further studies in these aspects are necessary.

Acknowledgements This work is supported by the National Nature Science Foundation of China (NO 11172301), the ‘‘Funds for Distinguished Young Scientists’’ of Hubei Province (2012FFA002), the ‘‘Western Light Talent Culture’’ Project, the Co-joint Project of Chinese Academy of Sciences and the ‘‘Top Hundred Talents’’ Program of Chinese Academy of Sciences and the National 973 Project (2013CB632300) for financial support. Finally, Dr Guo thanks Prof Zhongqun Tian, Xiamen University, for his fruitful discussions.

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Chem. Commun., 2014, 50, 3900--3913 | 3913

Adhesion behaviors on superhydrophobic surfaces.

The adhesion behaviors of superhydrophobic surfaces have become an emerging topic to researchers in various fields as a vital step in the interactions...
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