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.     

Fabrication of robust self-cleaning superhydrophobic coating by deposition of polymer layer on candle soot surface

A facile and inexpensive way have been developed to fabricate robust self-cleaning superhydrophobic coating by depositing polymer layer on candle soot (CS) surface using dip coating method. The stability and robustness of the CS deposited superhydrophobic surface is much weaker due to weak interaction and nonchemical bonding with the substrate. We introduced a thin layer of polystyrene, polyethylene, polypropylene (PP), and polyvinylidene fluoride on CS surface in order to improve its mechanical properties. The fabricated surfaces with the use of respective polymers exhibited water contact angles of nearly 170, 174, 175, and 171° with sliding angles of 2, 1, 1, and 3°, respectively. Although, all the polymers used in this work exhibited excellent superhydrophobic and self-cleaning surface property, we found that the PP deposited CS surface exhibit better stability against water jet hitting and water drop impact tests. The PP deposited CS surface almost maintained their surface properties even after 50 cycles of sandpaper abrasion and 20 cycles of adhesive tape peeling tests. The mechanical durability tests confirmed that PP is a better polymer to improve the long-term durability of CS surface. Therefore, this simple, time saving, and inexpensive method for fabricating superhydrophobic coating can be used for potential industrial application.     

Superhydrophobic coatings for aluminium surfaces synthesized by chemical etching process

In this paper, the superhydrophobic coatings on aluminium surfaces were prepared by two-step (chemical etching followed by coating) and one-step (chemical etching and coating in a single step) processes using potassium hydroxide and lauric acid. Besides, surface immersion time in solutions was varied in both processes. Wettability and surface morphologies of treated aluminium surfaces were characterized using contact angle measurement technique and scanning electron microscopy, respectively. Microstructures are formed on the treated aluminium surfaces which lead to increase in contact angle of the surface (>150°). Also on increasing immersion time, contact angle further increases due to increase in size and depth of microstructures. Additionally, these superhydrophobic coatings show excellent self-cleaning and corrosion-resistant behavior. Water jet impact, floatation on water surface, and low temperature condensation tests assert the excellent water-repellent nature of coatings. Further, coatings are to be found mechanically, thermally, and ultraviolet stable. Along with, these coatings are found to be excellent regeneration ability as verified experimentally. Although aforesaid both processes generate durable and regenerable superhydrophobic aluminium surfaces with excellent self-cleaning, corrosion-resistant, and water-repellent characteristics, but one-step process is proved more efficient and less time consuming than two-step process and promises to produce superhydrophobic coatings for industrial applications.     

Ice adhesion on different microstructure superhydrophobic aluminum surfaces

The adhesion of ice to high -voltage overhead transmission lines should be small to ensure ease of ice shedding under small external forces. In this work, we studied the influence of the microstructure of superhydrophobic surfaces on the strength of ice adhesion at a working temperature of ?6?°C. Compared to a bare aluminum surface, the microstructure superhydrophobic aluminum surfaces did decrease ice adhesion strength. The superhydrophobic aluminum surfaces with a larger number of micro-holes produced the lowest strength of ice adhesion; its ice adhesion strength was ～163.8 times lower than that for the bare aluminum samples. Furthermore, such microstructure aluminum surfaces had water contact angles larger than 150° and water sliding angles of less than 8.2° even at a working temperature of ?6?°C. The low values of the ice adhesion strength of the above samples were mainly attributed to the superhydrophobic property, which was obtained by creating a structure of micro-nanoscale holes on the aluminum surface after treatment with a low- surface-energy fluoroalkylsilane (FAS).     

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.     

CNCs增强相分离法构建PVDF/PDMS超疏水表面

采用非诱导相分离法，通过将纤维素纳米晶（CNCs）与两种聚合物（聚偏二氟乙烯和聚二甲基硅氧烷）进行结合，利用CNCs之间的静电排斥力及其高比表面积特性，有效降低相分离过程中聚合物的聚集，减小了颗粒尺寸，增强了颗粒分散性，在棉布、木板和玻璃表面构造了精细均匀的微纳米粗糙结构。采用FTIR、SEM、AFM、接触角测量仪、3D光学轮廓仪对CNCs和超疏水表面的形貌、结构和超疏水性进行表征。研究发现，棉布、木板和玻璃表面的水接触角分别最高可达158 °、156.8 °和153.8 °，滚动角最低分别为2 °、2.7 °和3.4 °，呈现出明显的超疏水特征。经过机械摩擦（500余次）、酸碱处理（pH 1～13）、温度变化（–40～40°C）以及紫外光照射（0～320 h）后，基材表面仍具有较好的超疏水性。此外，基材具有优异的自清洁性和油水分离效率，超疏水棉布的最高分离效率可达98.4%。     

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.     

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。     

Superhydrophobic durable coating based on UV‐photoreactive silica nanoparticles

Superhydrophobic surfaces can be obtained by tailoring both the chemistry and roughness topography, mimicking the Lotus leaf characteristics. Most of the synthetic superhydrophobic surfaces reported have been composed of micro and nanoparticles (NPs) embedded in polymer‐based coatings. The particles which tailor the topography are bonded to the base polymers by weak secondary forces. Consequently, the topography integrity is highly affected by handling and surface drag making them unsuitable for long term applications. This work is focused on promoting covalent bonding between the NPs and the base polymer to obtain durable superhydrophobic surfaces. The rough topography was achieved by ultraviolet (UV) curing of SiO₂ NPs containing a photoreactive benzophenone moiety in addition to methylated fumed silica NPs which can bind covalently to the polymer base coating, on UV radiation. The hydrophobic chemistry was obtained by fluoroalkylsilane top coating. Coating durability was evaluated using surface air drag and accelerated weathering conditions (UV radiation, humidity and temperature). Results indicated that the proposed approach resulted in superhydrophobic surfaces having high contact angle (>150°) and low sliding angle (<10°) with improved long term durability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41122.     

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.     

Superhydrophobic to superhydrophilic wettability transition of functionalized SiO2 nanoparticles

The superhydrophobic and superhydrophilic surfaces and their transitions are of great interest for the production of self-cleaning, anti-biofouling, or corrosion-resistant materials. This work reports the wettability transition from superhydrophobic to superhydrophilic SiO₂ nanoparticles functionalized with 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (POTS) and induced by temperature. The functionalization of these nanoparticles was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy. The functionalization of SiO₂ nanoparticles with POTS resulted in superhydrophobic surfaces with water contact angles up to 157°. A sudden transition to superhydrophilic behavior with water contact angles (WCA) below 5° was observed when the sample was heat-treated at 500 °C, despite the presence of fluorine on the surface of these nanoparticles, as confirmed by XPS and transmission electron microscopy. XPS suggested that the transition was caused by the change in orientation of the fluoroalkyl molecules and its partial decomposition due to the loss of the –CF₃ group, resulting in shorter chains with a tail-end group with C–O bonds, which promoted the superhydrophilicity.     

Reversibly light-switchable wettability between superhydrophobicity and superhydrophilicity of hybrid ZnO/bamboo surfaces via alternation of UV irradiation and dark storage

Functional bamboo surfaces with reversibly tunable wettability have become much sought after because of their usefulness in sustainable material protection strategies and industrial applications. In this paper, the hybrid ZnO/bamboo surfaces with reversibly light-switchable wettability between superhydrophobicity and superhydrophilicity were successfully prepared via a hydrothermal method at low temperature. The bamboo substrates served as adhesion, and the well-aligned ZnO nanosheet arrays (WZNA) were deposited on the bamboo surfaces after a hydrothermal process. A subsequent chemical treatment with octadecyltrichlorosilane (OTS) led to a superhydrophobic surface with a water contact angle (WCA) up to 153°. Under UV irradiation, the WCA decreased gradually, and the surface eventually became superhydrophilic because of hydroxyl absorption on the ZnO surfaces. The wetting behavior of the WZNA can be reversibly switched between superhydrophilic and superhydrophobic via alternation of UV exposure for 12 h and dark storage for 10 days.     

High stability superhydrophobic glass-ceramic surface with micro–nano hierarchical structure

Inspired by the surface structure of lotus leaves, micro–nano hierarchical surface structures have been widely used for designing superhydrophobic surfaces. However, the conventionally designed superhydrophobic surface structures are fragile. In this study, a layer of micron-sized mullite whiskers was grown using molten salt on the surface of BaAl₂Si₂O₈ (BAS) glass ceramics. Subsquently, SiO₂ nanoparticles modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane were sprayed onto the whisker layer to form a superhydrophobic surface. The nanoparticles exhibit superhydrophobicity, which is protected by the whisker layer containing pores and bulges. This prohibits direct contact between the nanoparticles and external objects. Contact and rolling angle tests indicated that the surface contact angle of the micro–nano hierarchical structure is 158° and the rolling angle is less than 10°. The stability of the superhydrophobic surface was tested through ultraviolet light, long-time immersion in solutions with various pH values, water scouring, and sandpaper abrasion. The results showed that the contact angle is greater than 150°. This study is expected to provide a simple and effective method for fabricating superhydrophobic surfaces on ceramics on a large scale.     

基于单宁酸制备可喷涂超疏水材料

通过正硅酸乙酯水解合成了二氧化硅纳米粒子并形成凝胶颗粒,加入单宁酸以优化其形貌,以六甲基二硅氮烷为表面改性剂,合成了具有低表面能的超疏水喷涂材料。并用动态光散射仪(DLS)与扫描电镜(SEM)对其表征。将其分散于乙醇,并对纸张、玻璃、铝箔、木板、棉质纺织物、塑料泡沫等常见表面进行喷涂,均在短时间内构成了超疏水表面,水接触角均在150.0°以上。随后,考察了所制备超疏水涂层在受外力破坏后的自修复性与耐磨性。结果显示:1 g/L的喷涂液仅需喷三层即可构建超疏水表面,得到的涂层具有良好的透明性;超疏水涂层在受外力损坏后可用有机溶剂进行快速简易的自修复;且喷涂后的玻璃片在砂纸上负重磨损距离达到1000 mm后,接触角从153.5°降至105.5°,再喷一层即可恢复到154.0°。     

Towards superhydrophobic coatings made by non-fluorinated polymers sprayed from a supercritical solution

The objective of this study was to create a superhydrophobic surface using polymers that are non-fluorinated and applying them to a surface via rapid expansion of a supercritical solution (RESS). Solubility studies of poly(ɛ-caprolactone) (PCL) and a statistical copolymer of vinyl acetate and vinyl pivalate (P(VAc-VPi))in supercritical carbon dioxide (scCO₂) were carried out using an extraction procedure. It was found that the most suitable process parameters for spraying these polymers using the RESS technique were 30 MPa, 40 °C and 10% (v/v) acetone as a co-solvent. The surfaces produced were characterized in terms of their morphology and hydrophobic properties by scanning electron microscopy and contact angle measurements, respectively. The most hydrophobic surfaces were obtained by spraying the P(VAc-VPi) copolymers, giving advancing water contact angles in the range of 120–155° due to the hydrophobic character of the polymer and the microstructure formed with the RESS technique. These results show great promise for the creation of superhydrophobic surfaces using non-fluorinated polymers applied to surfaces via RESS technique.     

Synthesis of hydrophilic to superhydrophobic SU8 surfaces

In published literature, it is widely reported that the plasma treatment and funtionalization with Octadecyltrichlorosilane (OTS) self‐assembled monolayer (SAM) can individually alter the wetting properties of SU8 surface. A combination of the two approaches gives better results and the synergism of the two approaches produces a superhydrophobic SU8 surface, which is presented in this work. We have investigated various composition of plasma for treatment of SU8 surfaces and permuted the treated SU8 surfaces with deposition of OTS SAM. In all such synergized experiments, we obtained water contact angle higher than 150°, which is much higher than the one that can be obtained with individual application of the two approaches. The combined approach presented in this work is suitable for bulk production of superhydrophobic surface, and is a mask‐less process, which makes it cost effective. The surface topography, wetting, and chemical properties of SU8 surfaces were characterized using the contact angle goniometry, atomic force microscopy, FTIR, Raman, and XPS spectra. The superhydrophobic SU8 surfaces were observed to be stable even after five months. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41934.     

Mechanochemical durability and self-cleaning performance of zinc oxide-epoxy superhydrophobic coating prepared via a facile one-step approach

Research efforts have intensified on developing superhydrophobic ZnO nanoparticle-based surfaces as they can impart desired hierarchical rough structures and properties. However, the widespread use of these surfaces is impeded by the limitations: complex fabrication procedures, weak adhesion, and limited durability performance. Most of these fabrication processes involved multiple treatment steps included the pre/post-treatment process to modify the textured surfaces. Herein, a convenient and effective one-step strategy is used to synthesis superhydrophobic ZnO-based coating with the introduction of both epoxy resin (EP) as the adhesive promoter and stearic acid (SA) as the low-energy modifier into an aqueous solution containing ZnO nanoparticles and ethanol via solution coating deposition method that binds all the components in a sole system. In the dense network, EP is responsible to render mechanical strength and coating-substrate adhesion, meanwhile, whereas SA functions to reduce the surface energy and preventing the EP to fully envelop the ZnO nanoparticles, thereby achieving a robust hierarchical rough structure, creating a layer of trapped air pockets. The resultant coating exhibits high water repellency, low water adhesion, and excellent self-cleaning ability with water contact and a sliding angle of 160.24° and 3°, respectively. Besides that, the superhydrophobic coating exhibited good mechanical durability after subjected to sandpaper-abrasion for a 20-m distance long and tape-peeling for 200 cycles. Furthermore, the coating still retained its superhydrophobicity after immersed in the corrosive bath with pH concentration ranged from 1 to 13 for 3 h, as well as the direct exposure to the temperature changes from 0 to 150 °C, manifesting its good chemical and thermal stability. This straightforward yet effective approach to develop a superhydrophobic coating with good durability and self-cleaning performance will inspire the scalable fabrication of multifunctional surfaces for practical applications in self-cleaning.     

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.     

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