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.     

Stable superhydrophobic surface based on low‐density polyethylene/ethylene‐propylene‐diene terpolymer thermoplastic vulcanizate

Stable superhydrophobic surface based on low‐density polyethylene (LDPE)/ethylene–propylene–diene terpolymer (EPDM) thermoplastic vulcanizate (TPV) was successfully fabricated by using etched aluminum foil as template. The etched aluminum template consisted of micropores and step‐like textures, was obtained by the metallographic sandpaper sanding and the subsequent acid etching. The surface morphology and hydrophobic properties of the series molded TPV surfaces were researched by varying the weight ratio of the LDPE/EPDM TPV. The superhydrophobic LDPE/EPDM TPV surfaces exhibited the microstructures consisting of step‐like textures obtained via molding with etched aluminum template and a large number of fiber‐like structures resulted from the plastic deformation of LDPE matrix. The obtained TPV (LDPE/EPDM weight ratio = 70/30) surface exhibited the remarkable superhydrophobicity, with a contact angle of 152.0° ± 0.7° and a sliding angle of 3.1° ± 0.8°. The molded TPV surface had excellent environmental stability when the pH of water solution was in the range of 1 to 14; moreover, the surface also showed the excellent resistance to various organic solvents. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46241.     

Fabrication of Polypropylene Super-Hydrophobic Surface Using PTFE-Coated-Sieves Template via Templating and Splitting Process

Super-hydrophobic Polypropylene surfaces were prepared employing combination method of replicating an artificial template and subsequent lifting off the master via a splitting way. The super-hydrophobic polypropylene surface, which was prepared using a 500-mesh PTFE-coated sieve, has gradient structures that microbumps with typical height and radius of approximately 20 µm and nano bundles on top of the bump surface. This surface exhibits the same self-cleaning property as the lotus leaf with water contact angle of 152.2 ± 2° and sliding angle less than 5°. The whole preparation process is simple, efficient, large-scale-prepare and environmentally friendly without organic solvents pollution.     

Fabrication and Properties of Superhydrophobic Surfaces Based on Low-Density Polyethylene/Ethylene–Propylene–Diene Terpolymer Thermoplastic Vulcanizate

A simple, effective, and inexpensive method has been developed to fabricate superhydrophobic surfaces based on low-density-polyethylene/ethylene–propylene–diene terpolymer thermoplastic vulcanizate. Field emission scanning electron microscope research showed that the rough microstructure could be fabricated successfully in the thermoplastic vulcanizate surface where metallographic sandpaper was used as molding template; moreover, the micrometer scale plastic deformation of thermoplastic vulcanizate matrix could be observed obviously, leading to the increasing surface roughness. Wettability test showed that the series rough thermoplastic vulcanizate surfaces that were molded with sandpapers had strong hydrophobic property; furthermore, the sliding angle value was less than 10°. Metallographic sandpaper template exhibited well re-used property.     

A simple and cost-effective method for fabricating lotus-effect composite coatings

This paper reports the fabrication of a lotus-effect coating by grafting epoxy (EP) resin on the surface of microsilica and nanosilica, respectively, and subsequent spraying. The coating shows the same structure and capability as lotus leaves, and shows a static contact angle as large as 165° and a sliding angle as small as 2.5°. SEM analysis shows that the hydrophobic capability depends on the surface structure of the coatings. This method may be suited for processing large scale or irregular surfaces.     

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.     

基于HDPE/EPDM TPV的超疏水表面的构建及性能

以高密度聚乙烯(HDPE)/三元乙丙橡胶(EPDM)热塑性硫化胶(TPV)为原材料,采用金相砂纸为模板,通过模压法在TPV表面构建出超疏水表面。FE-SEM的观察表明,以金相砂纸为模板可在TPV表面获得具有较高保真度的微米级粗糙结构,模压过程中TPV表面发生的塑性变形,使得TPV表面具有比砂纸表面更为复杂的粗糙结构;润湿性测试结果表明,基于TPV的粗糙表面具有良好的疏水性,当磨料目数高于W14号砂纸时,模压后TPV表面与水的接触角可超过150°,且滚动角在10°以内,符合Cassie模型;采用W10号砂纸为模板制备的TPV表面具有最佳的超疏水性能。     

Hydrophobic and superhydrophobic surfaces fabricated by plasma polymerization of perfluorohexane,perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene)

Fluoropolymer films were deposited on silicon (1 0 0) wafers, glass, epoxy, and hierarchical dual-sized filler epoxy composite surfaces by plasma polymerization of perfluorohexane, perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene). The procedure involved continuous wave plasma-enhanced deposition, followed by a discharge-off period, with the monomer gas feed maintained. Silanization of silicon wafers and glass surfaces with triethoxyvinylsilane was employed to improve plasma fluoropolymer bonding to these substrates. The presence of double bonds in perfluoro(2-methylpent-2-ene) and perfluoro(4-methylpent-2-ene) was found to influence fluoropolymer coating topography, thereby increasing surface roughness in modified glass and epoxy substrates. All fluorocarbons provided a similar level of hydrophobization of flat substrates, exhibited by water contact angles (WCA) of about 110°. Hydrophobization of nanocomposite hierarchical surfaces by plasma polymerization provided superhydrophobic surfaces, with WCA of 160° and contact angle hysteresis below 8°.     

Coatings to prevent frost

The ability of hydrophobic, organic–inorganic hybrid coatings to decelerate frost propagation was investigated. Compared to a bare aluminum surface, the coatings do not significantly reduce the freezing probability of supercooled water drops. On both surfaces, the probability for ice nucleation at temperatures just below 0°C, for example at ?4°C, is low. Freezing of a single drop on aluminum leads, however, to instant freezing of the complete surface. On hydrophobic coatings, such a freezing drop is isolated; the frozen area grows slowly. At ?4°C surface temperature in a +12°C/90% relative humidity environment, on surfaces providing a water contact angle hysteresis of about 10° and receding water contact angles higher than 90°, a rate for the growth of the average radius of the frozen area of about 2 µm/s was observed. Submitting the surface to an airflow of 1 m/s led to faster frost spreading in flow direction. Although the airflow compromised the anti-ice properties to some extent, the application of the hydrophobic coating in a heat recovery ventilation experiment extended the time interval between defrosting cycles by a factor of 2.3.     

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.     

Fabricating translucent polydimethylsiloxane (PDMS) super-hydrophobic surface greenly by facile water-dissolved fillers

In this study, a polydimethylsiloxane (PDMS) colloid was cast onto a template made of recycled polypropylene for structural duplication to fabricate translucent super-hydrophobic surfaces greenly without the use of any complicated method. The surface structure of the template was formed by hot embossing using salt grain fillers dissolved in water. The resulting contact angle (CA) values of fabricated PDMS surfaces were 153.4 ± 0.6°, 152.3 ± 1.8°, 152.2 ± 0.8°, and 152.0 ± 1.6° and the associated slide angle (SA) values were 5.8 ± 0.6°, 8.7 ± 1.5°, 8.6 ± 1.1°, and 16.3 ± 1.9° for filler grain size categories A, B, C, and D, respectively. The surfaces with grain sizes A, B, and C exhibited super-hydrophobic conditions, where CA > 150° and SA < 10°; on the other hand, the surface with grain size D exhibited an SA value greater than 10°. The measured percentages of light transmittance in the visible wavelength range of 400–800 nm were 75–80% for grain sizes B, C, and D and 85–90% for grain size A. Furthermore, a green laser light was diffused from a spot of 0.5–7.7 cm in diameter; related glare was also eliminated.     

Preparation and anti‐icing property of a lotus‐leaf‐like superhydrophobic low‐density polyethylene coating with low sliding angle

A lotus‐leaf‐like superhydrophobic low‐density polyethylene (LDPE) coating with low sliding angle was prepared by a facile method. The water contact angle and sliding angle of the as‐prepared superhydrophobic LDPE coating were 156 ± 1.7° and 1°, respectively. The anti‐icing property of the as‐prepared LDPE coating with low sliding angle was investigated in a climatic chamber with a working temperature of ?5°C. The results showed that the superhydrophobic LDPE coating with low sliding angle can largely prevent ice formation on the surface, showing excellent anti‐icing property. The as‐prepared superhydrophobic LDPE coating with good anti‐icing property will be perfectly desirable for outdoor equipments to reduce ice formation on their surfaces in cold seasons. This work will provide a new way to fabricate anti‐icing coating and thus find applications in a variety of fields. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Fabrication of mechanically robust superhydrophobic aluminum surface by acid etching and stearic acid modification

The aluminium surface with multi-scale structure has been fabricated via a facile and rapid solution-phase etching method by HCl/H₂O₂ etchants. After modification with stearic acid solution, the wettability of the etched aluminum surface turns into superhydrophobicity with an optimal water contact angle of 160° ± 2° and a sliding angle of 4° ± 1°. The processing conditions, such as the etching time, modifier types and the concentration of H₂O₂ are investigated to determine their effects on the surface morphology and wettability. As a result, the obtained sample shows excellent anti-adhesion property and bouncing phenomenon of water droplet. It can withstand mechanical abrasion for at least 100 cm under 12.3 kPa, or hydrostatic pressure under 24 ± 1 kPa without losing its superhydrophobicity, suggesting superior mechanical durability. Moreover, the surface also remains superhydrophobicity even after contacting corrosive liquids or long-term exposure in air over 100 days. Such a mechanically durable superhydrophobic aluminum surface can provide a promising practical application in various fields.     

荷叶表面的复刻及微纳结构对疏水性能的影响

结合改进的模板法和ZnO水热生长法在环氧树脂基底上得到了荷叶仿生超疏水结构,该方法工艺流程简单、制作成本低廉,可以实现微观结构的快速复刻。研究了模板法对天然表面复刻的适用范围,其对荷叶和水稻等具有突起类微观结构表面的复刻效果良好,并研究了水热法中ZnO生长液浓度对纳米结构的影响。同时为了研究不同微观结构对表面疏水性能的影响,制作了光滑表面、纳米结构表面和仿荷叶微米结构表面,并测试了表面的疏水性能。结果表明,粗糙结构能够提高低能表面的疏水性能,微纳复合结构更有利于表面形成超疏水;增加表面的粗糙结构能够增加液滴与固体接触面上的气-液占比,进而使得液滴在表面的接触角增加。     

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.     

Ultra-repellency of Al surfaces: design and evaluation

Aluminum (Al) surfaces with ultra-repellency as well as desirable robustness were designed and fabricated. With photolithographic patterning of a thick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were prepared. After derivatization with perfluoroalkyl phosphoric acid (FPA), the textured Al surfaces showed ultra-repellency for a wide variety of liquids. The contact angles (CAs) of deionized (DI) water, hexadecane and dodecane were larger than 150°, and those of methanol and ethanol were larger than 100°. The sliding angles (SAs) of DI water, hexadecane and dodecane were 5°, 10°, and 10°, respectively, showing excellent superamphiphobicity. The SAs of methanol and ethanol were in the range of 20°–30°. The robustness of the ultra-repellent Al surface was evaluated by three parameters: robust height (H*), robust angle (T*) and robust factor (A*). For the DI water probing, the values of the parameters are H* ≈ 403, T* ≈ 119 and A* ≈ 92, respectively, indicative of a desirable robustness. We clarified that only re-entrant structures can support composite liquid–solid–vapor interfaces when the corresponding Young’s CAs are smaller than 90°, and the function of the nanometer structures of the hierarchical textures which were widely adopted to fabricate superamphiphobic surfaces is to help construct re-entrant structures. FPA derivatization is effective in lowering the surface energy of Al surfaces, combining with re-entrant textures to provide a simple and high throughput approach to ultra-repellency for a wide variety of liquids.     

Novel,solvent-based method for the production of polymer sheets with a superhydrophobic surface

We have introduced a novel solvent-based method to roughen polymer surfaces and characterized treated polypropylene (PP). The method consists of three main steps: solvent treatment, drying, and peeling. We investigated the effects of process parameters such as time of immersion in the solvent, solvent temperature, and drying temperature on the surface morphology created. The structure formed on the surface is mainly influenced by solvent temperature and drying temperature. We also characterized the wetting behavior of the surfaces. The patterned surfaces exhibit superhydrophobic characteristics with a high water contact angle (CA) (>155 °) and low water contact hysteresis (<5°). Adding an effective nucleating agent to PP makes it possible to generate outstanding CAs (>160°) and tailor spherulite sizes. The method is simple and scalable, therefore this superhydrophobic material is easy to mass-produce.     

Fabrication of superamphiphobic-textured surfaces with reversibly switchable wettability

To realize switchable wettability for water and oils, we firstly design a facile approach to achieve superamphiphobic-textured surfaces that display the apparent contact angles greater than 150° and the sliding angles smaller than 10° for various liquids, such as water and hexadecane. Then, plasma treatment and surface fluorination were performed on the superamphiphobic surface. The tunability of the chemical composition, along with hierarchical rough surface structure, gave rise to reversibly switchable wettability between superamphiphobicity and superamphiphilicity. The wettability of the textured surface was demonstrated by visible experimental results and measurements. The reason of switchable wettability can be confirmed by X-ray photoelectron spectroscopy analysis. These results give a useful attempt in simultaneously and reversibly switching wettability for water and oils, which may have potential application in functional superamphiphobic materials.     

Simple method for preparing ZnO superhydrophobic surfaces with micro/nano roughness

In this paper, a facile, inexpensive, and environment-friendly method is developed to construct a superhydrophobic surface with hierarchical micro/nanostructures on the steel substrates. The superhydrophobic surface was fabricated by magnetic agitation of a mixture of micro and nanosized Zinc oxide (ZnO) suspensions on a substrate, after being modified with a low-surface energy monolayer of stearic acid, the as-prepared coating exhibits self-cleaning properties with a water contact angle of 162° and a sliding angle of 6°, and shows the good corrosion resistance. It is believed that the rapid and cheap technique have a promising future application for fabricating superhydrophobic surfaces on steel materials.     

Superhydrophobic and oleophilic micro‐nano hierarchical Pd‐decorated SiO2 layers

This paper reports a novel fluorinated micro‐nano hierarchical Pd‐decorated SiO₂ structure (hereafter called Pd/SiO₂), which was formed by the deposition of Pd nanoparticles (NPs) on SiO₂ microspheres. The SiO₂ layers with microscale roughness were fabricated by electrospraying a solution prepared using the sol‐gel process. Subsequently, the Pd NPs were deposited using an ultraviolet reduction process. The resulting surfaces exhibited a micro‐nano hierarchical morphology. After fluorination, the micro‐nano hierarchical surface exhibited outstanding water repellency with a water contact angle (WCA) of 170° and a sliding angle <5°, indicating excellent superhydrophobic properties. The layers exhibited good long‐term durability and excellent ultraviolet resistance. Interestingly, the surface was oleophilic (CA of oil ~10°). These results show the potential of employing superhydrophobic fluorinated Pd/SiO₂ layers in smart devices, such as self‐cleanable surfaces and intelligent water/oil separation systems.