Superhydrophobic surfaces: a review on fundamentals,applications, and challenges

Superhydrophobicity is the tendency of a surface to repel water drops. A surface is qualified as a superhydrophobic surface only if the surface possesses a high apparent contact angle (>150°), low contact angle hysteresis (<10°), low sliding angle (<5°) and high stability of Cassie model state. Efforts have been made to mimic the superhydrophobicity found in nature (for example, lotus leaf), so that artificial superhydrophobic surfaces could be prepared for a variety of applications. Due to their versatile use in many applications, such as water-resistant surfaces, antifogging surfaces, anti-icing surfaces, anticorrosion surfaces etc., many methods have been developed to fabricate them. In this article, the fundamental principles of superhydrophobicity, some of the recent works in the preparation of superhydrophobic surfaces, their potential applications, and the challenges confronted in their new applications are reviewed and discussed.     

Preparation of a Superhydrophobic ZnO Film on ITO Glass via Electrodeposition Followed by Oxidation. Effect of the Deposition Time

This study investigates the fabrication of a stable superhydrophobic surface with low contact angle (CA) hysteresis using ZnO thin films prepared by cathodic electrodeposition and subsequent gaseous oxidation. The deposition time is a crucial factor in nanostructuring and producing surface roughness of the films. Cathodic electrodeposition for 60 s created a number of nanopillars, which exhibited the highest CA value, i.e., 167.9°. The rough ZnO surface displayed not only enhanced water repellency with low CA hysteresis but also excellent superhydrophobic stability. The application of the Cassie–Baxter model demonstrated that the ZnO nanostructure contributed to increasing the area of a water droplet in contact with air, leading to superhydrophobicity. Such a unique textured surface showed a great potential for the engineering of strong superhydrophobic coatings.     

Superhydrophobic antireflective coating with high transmittance

The aim of this study is to fabricate a superhydrophobic antireflective (AR) coating that can be deposited on the covering of a solar cell system. First, AR coatings were synthesized on glass substrates with an average transmittance over 96% by layer-by-layer deposition of polyelectrolyte. Superhydrophobic sol gel was prepared by hydrolyzing tetraethoxysilane and then reacting it with hexamethyldisilazane. The sol gel, aged at 20°C for 96 h, was used to spin-coat a superhydrophobic film with a water contact angle of 163° and a transmittance of ~91%. The superhydrophobic sol gel was spin-coated on the top of an AR coating to form a superhydrophobic AR coating on a glass substrate. The average transmittance, advancing contact angle, and contact angle hysteresis of the superhydrophobic AR coating, which was spin-coated from sol gel aged for 96 h or 168 h, were 94.5 ± 0.7%, 154.0° ± 1.5°, and 15.4° ± 0.3° or 96.4 ± 0.2%, 158.4° ± 4.4°, and 1.8° ± 0.3°, respectively. Strategies for obtaining a superhydrophobic AR coating are discussed herein.     

Preparation, characterization and wettability of porous superhydrophobic poly (vinyl chloride) surface

A porous superhydrophobic poly (vinyl chloride) surface was obtained by a simple approach. The water contact angle and the sliding angle of the superhydrophobic poly(vinyl chloride) surface were 154 ± 2.3o and 7o, respectively. The porous superhydrophobic PVC surface remained superhydrophobic property in the pH range from 1 to 13. When the superhydrophobic PVC surface was immersed in water with the temperatures ranging from 5 °C to 50 °C for 1 h to 30 days, the water contact angle remained higher than 150°. After outdoor exposure for 30 days, the contact angle still remained 150o.     

A stable porous superhydrophobic high‐density polyethylene surface prepared by adding ethanol in humid atmosphere

A stable porous superhydrophobic high‐density polyethylene (HDPE) surface with water contact angle of 160.0 ± 1.9° and sliding angle of 2.0 ± 1.6° was obtained by adding ethanol in humid atmosphere at 5°C. Soaked in water with temperatures ranging from 5 to 50°C for 15 days, even suffering compressive forces, and the water contact angles were still higher than 150°. After water flowed through the surface continuously for 30 min, even water droplets with a diameter of 4 mm dropped onto the HDPE surface from 30 cm high for 10 min, the water contact angles were also higher than 150°. A brief explanation to the formation of the porous superhydrophobic HDPE surface was put forward. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

微纳米结构对聚合物超疏水表面润湿性的影响

对聚合物超疏水表面疏水原因的理论分析及其超疏水表面常用的制备方法进行了总结,并从热力学角度评述了关于超疏水状态的两种理论模型:Wenzel模型和Cassie模型,分析了微纳米结构对聚合物超疏水表面表观接触角的影响.     

A simple method for the fabrication of silica-based superhydrophobic surfaces

The present article reports on a simple and convenient method for the fabrication of superhydrophobic surfaces based on silica particles by spraying the as-prepared silica suspension containing silica sol and silica microspheres on the substrate. The morphologies of the silica particulate coatings could be controlled by varying the silica microsphere concentration. The silica particulate coatings as prepared were exceptionally rough and superhydrophilic, with water contact angles less than 5°. The surface silanol groups of the hydrophilic coatings could be functionalized using 1H,1H,2H,2H-perfluorodecyltriethoxysilane to form hydrophobic groups. The resulting surface showed excellent superhydrophobic property with water contact angle up to 165.6 ± 0.9° and sliding angle of 3.5 ± 0.4°. In addition, the superhydrophobicity of the coating possessed a good stability after 3 months of exposure in air for a wide range of pH values.     

Preparation and characterization of superhydrophobic surface based on polydimethylsiloxane (PDMS)

Biomimetic superhydrophobic surfaces exhibit excellent self-cleaning properties due to their special micro/nano-scale binary structures. In order to prepare the superhydrophobic surface of the polydimethylsiloxane (PDMS), a facile fabrication method for replicating micro/nano-scale binary aluminium structures into PDMS is presented. The microscopic morphology, composition, surface roughness (Ra) and wettability of the sample surface were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, roughness measurement equipment and contact angle meter respectively. Based on the measurements of the contact angles of deionized water (DI water) and ethanediol, surface free energies of the coatings were estimated according to the Owens two-liquid method. The superhydrophobic PDMS exhibited lower surface free energy than flat PDMS with a DI water contact angle (WCA) of 165°. The surface roughness (Ra) increased with the increasing of etching time in the range 0–80?min, and then decreased with the change of etching time, similar to the variation of contact angle with etching time. Moreover, the prepared surface had different micro-morphologies and its wettability was changed by regulating the chemical etching time. In addition, the superhydrophobic PDMS also showed good self-cleaning properties and the bouncing effect of the water droplets.     

铜基超疏水表面防覆冰/抗霜冻特性分析

超疏水表面具有良好的防覆冰性能,有望改善低温条件下设备和设施的可靠性。本文采用氨气腐蚀法,制备具有微纳结构的铜表面,通过低表面能氟硅烷修饰后,金属铜表面表现出超疏水特性,其水接触角可达152.1°。利用电镜扫描、接触角测量、结冰和结霜实验分别对超疏水铜表面的表面结构、湿润性能和防覆冰性能进行研究。结果表明,超疏水表面的防覆冰/抗霜冻性能不仅与表面的粗糙度有关,还受液滴在固体表面的湿润状态的影响。当液滴在具有微-纳米结构的超疏水表面处于Cassie状态时,液滴与金属表面的接触面积小,液滴结冰速率较慢,金属表面同时具有较好的防覆冰和抗结霜性;而当液滴在金属疏水表面处于Wenzel状态时,霜晶与固体表面的接触面积增加,加快霜层的生长,金属表面的抗结霜性明显降低。     

Influence of the coating method on the formation of superhydrophobic silicone–urea surfaces modified with fumed silica nanoparticles

Effect of the coating method on the formation of superhydrophobic polydimethylsiloxane–urea copolymer (TPSC) surfaces, modified by the incorporation of hydrophobic fumed silica nanoparticles was investigated. Four different coating methods employed were: (i) layer-by-layer spin-coating of hydrophobic fumed silica dispersed in an organic solvent onto TPSC films, (ii) spin-coating of silica–polymer mixture onto a glass substrate, (iii) spray coating of silica/polymer mixture by an air-brush onto a glass substrate, and (iv) direct coating of silica–polymer mixture by a doctor blade onto a glass substrate. Influence of the coating method, composition of the polymer/silica mixture and the number of silica layers applied on the topography and wetting behavior of the surfaces were determined. Surfaces obtained were characterized by scanning electron microscopy (SEM), white light interferometry (WLI) and advancing and receding water contact angle measurements. It was demonstrated that superhydrophobic surfaces could be obtained by all methods. Surfaces obtained displayed hierarchical micro-nano structures and superhydrophobic behavior with static and advancing water contact angles well above 150° and fairly low contact angle hysteresis values.     

Preparation and electrowetting transitions on superhydrophobic/hydrophilic bi-layer structures

A simple method of preparing porous superhydrophobic materials using glass fiber materials, where hydrophobicity is provided by a variety of coatings such as self-assembled alkyl-silane monolayers and fluoropolymers such as Teflon is presented. Fibrous structures of the filter material provide for the modulation of “surface roughness” on the micro- and nano-scale, required for achieving a superhydrophobic state, with advancing contact angle of water on such surfaces close to 150 degrees. Such superhydrophobic structures are effective at separating water-octane mixtures by allowing only low-surface-tension component to go through the thickness of the material, while repelling the water (high-surface-tension component) and preventing it from permeating through the material. In addition, a bi-layer structure that combines a superhydrophobic surface with a highly hydrophilic bulk material is described. It is formed by subjecting superhydrophobic fiber material to a brief oxygen plasma treatment to remove the hydrophobic coating from one side of the material, whereas the opposite side is protected during treatment and remains superhydrophobic. Tunable properties of the superhydrophobic fiber material are demonstrated using electrowetting with PEDOT–PSS conductive polymer core, parylene as a dielectric and Teflon as a hydrophobic coating. Applicability of such bi-layer materials to microfluidic and energy storage micro-devices is discussed.     

Single-step prepared hybrid ZnO/CuO nanopowders for water repellent and corrosion resistant coatings

In this study, ZnO/CuO hybrid hydrophobic nanopowders were synthesized using a common single-step chemical precipitation route without using modifiers. Influence of initial ZnO:CuO precursor concentrations and alkaline agent type on the wettability behavior of the prepared samples were investigated. Wettability properties of the prepared samples were assessed by measuring the water contact angle and contact angle hysteresis values. Fourier transform infrared spectra, scanning electron microscope micrographs and X-ray diffraction patterns were applied to identify the surface chemistry and morphological features. Scanning electron microscope images of the synthesized ZnO/CuO nanocomposites indicated flower-like morphologies containing plenty of nano-needles, -rods, and -sheets with thicknesses lower than 90 nm. The sample prepared under the optimum conditions was superhydrophobic having water contact angle and contact angle hysteresis of 162.6°±1 and 2°, respectively. It was applied to coat the surface of stainless steel meshes by spray deposition method. The resultant superhydrophobic surface exhibited excellent self-cleaning (water repellency) property and a suitable stabilities under the ambient and saline solution (NaCl, 3.5%) media. Additionally, electrochemical corrosion tests confirmed that the corrosion resistance of the fabricated ZnO–CuO coating was higher than the initial bare mesh.     

Oleic acid-modified superhydrophobic ZnO nanostructures via double hydrothermal method

Superhydrophobic ZnO nanostructures (ZNs) are synthesized via double hydrothermal method without any surface chemical modification. The water contact angle (CA) was approximately ～150?±?2°. A subsequent chemical treatment with oleic acid (OA) contributed to a superhydrophobic surface with a water CA of 162?±?2°. X-ray diffraction pattern revealed high crystalline quality ZNs with hexagonal wurtzite phase. The crystallite size was approximately ～23?nm. Field-emission scanning electron microscopy images display nanosheet formations assembled in flower-, cauliflower-, and nanorod-like ZNs. The CA of ZNs surfaces increased steadily from 152?±?2° to 162?±?2° when the OA weight percentage increased from 2?mg to 10?mg. Furthermore, UV–Vis spectra of ZNs indicated that the absorption band of all samples is distinguished in the ultraviolet region. All ZnO samples were maintained at room temperature for 1?h to 5 months to examine the stability of the structure surface.     

Microsphere tensiometry to measure advancing and receding contact angles on individual particles

In this paper, a method to measure the advancing and receding contact angles on individual colloidal spheres is described. For this purpose, the microspheres were attached to atomic force microscope cantilevers. Then the distance to which the microsphere jumps into its equilibrium position at the air-liquid interface of a drop or an air bubble was measured. From these distances the contact angles were calculated. To test the method, experiments were done with silanized silica spheres (4.1 μm in diameter). From the experiments with drops, an advancing contact angle of 101 ± 4° was determined. A receding contact angle of 101 ± 2° was calculated from the jump-in distance into a bubble. Both experimental techniques gave the same contact angle. In contrast, on similarly prepared planar silica surfaces, a clear hysteresis was measured with the sessile drop method; contact angles of 104.5 ± 1° and 93.8 ± 1° were determined for the advancing and receding contact angles, respectively.     

Laser-Patterned Super-Hydrophobic Pure Metallic Substrates: Cassie to Wenzel Wetting Transitions

A femtosecond laser was used to create microstructures on very pure metal surfaces. The irradiated samples initially showed super-hydrophilic behavior. With time and exposure to ambient air the contact angle increased to about 160° with very low hysteresis. The surfaces supported the Cassie and Wenzel wetting states, depending on the technique used to deposit the water droplets. The created surface morphologies were idealized with a geometric model that is an assembly of densely packed cylindrical pillars with semispherical caps. Using this geometric model for calculation of the surface roughness, a theoretical Young contact angle of about 99° was calculated for all samples from the Wenzel and Cassie–Baxter equations. While the value of 99° significantly differs from the measured hydrophilic contact angles on the polished pure metallic samples, it indicates that a laser-induced surface reaction must be responsible for the evolution of contact angles to super-hydrophobic ones and that this phenomenon is independent of the type of metal.     

化学刻蚀法制备黄铜基超疏水表面

采用含有三氯化铁和盐酸的水溶液刻蚀金属黄铜表面，在黄铜表面上得到了一层由不规则块状结构和更细小的乳突状结构相结合的具有双重粗糙度的阶层结构。该表面经氟化处理后表现出超疏水性，水在该表面上的接触角达到了157°，接触角滞后为5°。考察了不同刻蚀时间对表面疏水性的影响，结果表明，刻蚀时间对表面上这种阶层结构的形成和水滴在表面上的接触角数据都有十分重要的影响。随着刻蚀时间的增加，表面上逐渐开始形成粗糙结构，接触角数据也不断增加，增加到一定数值后，接触角变化开始不明显。接触角滞后随着接触角的增加而减小。初步分析了这种阶层结构的形成机制，并用Cassie理论对表面的润湿性进行了分析。     

Superhydrophobicity of a three-tier roughened texture of microscale carbon fabrics decorated with silica spheres and carbon nanotubes

A stable superhydrophobic surface with low contact angle hysteresis using microscale carbon fabrics decorated with submicroscale silica (SiO₂) spheres and carbon nanotubes (CNTs) is created. Without any surface treatment, superhydrophobicity is achieved, and a microsized water drop can be suspended on the three-tier roughened surface, leaving an air film underneath the droplet. A modified Cassie–Baxter model analyzes that the combined effect of SiO₂ spheres and CNTs contributes a high area fraction of a water droplet in contact with air, leading to superhydrophobicity. Such a three-tier surface texture has robust superhydrophobic properties.     

One-pot fabrication of robust hydrophobia and superoleophilic cotton fabrics for effective oil-water separation

A one-pot sonochemical irradiation method was developed for the fabrication of superhydrophobic and superoleophilic cotton fabric from a solution consisting of branched silica nanoparticles and tetraethoxysilane-dodecyltrimethoxysilane sol. The silica/sol-coated cotton fabric could be wetted by liquids of low surface tension, but was water repellent with a water contact angle of 159 ± 1.2° and water shedding angle of 6 ± 0.8°. The as-prepared cotton fabric could be used as effective materials for the separation of oil from water with separation efficiency as high as 98.2% and maintained separation efficiency above 94% after 30 separation cycles for the kerosene-water mixture. Moreover, the superhydrophobic and superoleophilic cotton fabric could maintain stable superhydrophobicity after treatment with strong acidic and alkali solutions, and harsh mechanical damage. Therefore, this reported robust superhydrophobic cotton fabric exhibits encouraging practical application for oil-water separation.     

Durable,self-cleaning and superhydrophobic bamboo timber surfaces based on TiO2 films combined with fluoroalkylsilane

Decorative materials, including bamboo timber, have been proposed to exploit their superhydrophobic and self-cleaning properties, but a comprehensive appraisal of their environmental adaptability is still deficient. In this paper, a robust and durable superhydrophobic surface was formed on bamboo timber substrate through a process combining chemical solution deposition and chemical modification. The superhydrophobic surface resulted from micro-nanoscale binary-structured TiO₂ films and the assembly of low-surface-energy fluorinated components, which exhibited a water contact angle of 163±1° and a sliding angle of 3±1°. The surface maintained superhydrophobicity after mechanical abrasion against 1500 mesh SiC sandpaper for 800 mm at the applied pressure of 1.2 kPa, indicating good mechanical stability. Moreover, the superhydrophobic surface exhibited good chemical stability against both acidic and basic aqueous solutions (e.g., simulated acid rain). After exposure to atmosphere for more than 180 days, the obtained surface still maintained a contact angle of 155±2° and a sliding angle of 6±2°, revealing good long-term stability. In addition, the as-prepared superhydrophobic surface exhibited almost complete wet self-cleaning of dirt particles with water droplets. It is believed that the method presented in this study can provide a straightforward and effective route to fabricate a large-area, mechanically robust, anticorrosive and self-cleaning superhydrophobic surface on woody materials for a great number of potential applications.     

Control of morphology and surface wettability of anodic niobium oxide microcones formed in hot phosphate–glycerol electrolytes

We report the fabrication of superhydrophobic surfaces with a hierarchical morphology by self-organized anodizing process. Simply by anodizing of niobium metal in hot phosphate–glycerol electrolyte, niobium oxide microcones, consisting of highly branched oxide nanofibers, develop on the surface. The size of the microcones and their tip angles are controlled by changing the applied potential difference in anodizing and the water content in the electrolyte. Reduction of the water content increases the size of the microcones, with the nanofibers changing to nanoparticles. The size of microcones is also reduced by increasing the applied potential difference, without influencing the tip angle. The hierarchical oxide surfaces are superhydrophilic, with static contact angles close to 0°. Coating of the anodic oxide films with a monolayer of fluoroalkyl phosphate makes the surfaces superhydrophobic with a contact angle for water as high as 175° and a very small contact angle hysteresis of only 2°. The present results indicate that the larger microcones with smaller tip angles show the higher contact angle for water.