Superhydrophobic materials have attracted widespread attention due to their unique non-wetting properties, and have been rapidly developed in recent years. Multifunctional superhydrophobic surfaces suitable for different fields have emerged. Among them, transparent superhydrophobic materials are favored by people because of their special contributions in the optical field. The transparent hydrophobic coating technology is of great significance for practical applications. The transparent coating can not only meet the high light transmittance requirement of optical device protection, but also maintain the basic appearance of the protective body, and shows broad application prospects in the fields of self-cleaning, anti-fouling, anti-icing, anti-fog, anti-corrosion, etc. Here, we systematically elaborate the latest developments in the research progress of superhydrophobic surfaces and functional transparent superhydrophobic surfaces, surface design, manufacturing, and their applications. Although many significant progress has been made, the current durability of superhydrophobic materials still have many problems, such as easy to damage by mechanical external force, instability of superhydrophobic properties on the surface under extreme environments, and aging problems, which limits the wide range of applications of transparent hydrophobic coating technology. In future research, on one hand, relevant theoretical knowledge is to be further enriched to provide more theoretical support for the application of transparent hydrophobic coating technology. On the other hand, improving the transparency and mechanical durability of the coating is still the top priority of future research.
Contents
1 Introduction
2 Theoretical basis
2.1 Wettability related theories
2.2 The construction principle of transparent superhydrophobic surface
3 Preparation method of transparent superhydro-phobic surface
Super Hydrophobic Surface (SHS) is a kind of Surface composed of Hydrophobic substance with low surface energy and surface microstructure, which requires that the stable contact angle of water droplets on the surface should be greater than 150 ° and the rolling contact angle should be less than 10 °[1⇓⇓~4][5]. Many surfaces in nature are highly hydrophobic and have self-cleaning properties, and the initial study of superhydrophobic surfaces is inspired by nature. The most famous example is the lotus leaf (Figure 1A), which can "come out of the mud without being stained". There are many other examples in nature, such as roses (Figure 1b), beetle backs (Figure 1C), butterfly wings (Figure 1D)[6].
Thanks to the development of Scanning Electron Microscope (SEM) technology, the mystery of surface hydrophobicity of animals and plants in nature was solved in the 1960s. In the following decades, superhydrophobic surfaces have developed rapidly (Figure 2).
Fig. 3 is a scanning electron micrograph of the surface of a lotus leaf, wherein figs. 3A and B show SEM images of the lotus leaf at different magnifications, respectively. It can be clearly seen that numerous micrometer-sized waxy papillae are attached to the upper surface of the lotus leaf, with a size of about 20-40 μm (Fig. 3A)[11,12]. Further enlargement shows that a large number of nano-particles with similar structure and size of 70-100 nm are attached to the surface of each micron-sized papilla[12]. At the same time, many nanorods with an average diameter of about 50 nm were randomly distributed in the lower layer of the lotus leaf surface (Fig. 3C). Scientists named this structure as micro-nano composite structure, and it is the synergistic effect between this special hierarchical surface structure and waxy materials with hydrophobic properties, that is, low surface energy substances, that leads to the super-hydrophobic nature of lotus leaf surface. The micro-nano composite structure can generate an air layer when the water drop contacts with the solid surface (Figure 4), reduce the contact area, make the water drop not easy to stay on the lotus leaf surface, and when there is dust on the solid surface, the water drop can carry the dust while rolling down, giving the solid surface a certain self-cleaning function[17]. In addition to lotus leaves, researchers have also observed similar surface structures and wettability on rice leaves and taro leaves. The properties of superhydrophobic surface endow it with good self-cleaning performance, and it has a wide range of applications in anti-fouling, anti-icing, fluid drag reduction, anti-corrosion and so on[18].
In addition to the effective hydrophobicity of the layered structure of lotus leaves, inspired by the research on the imitation of pitcher plants, researchers injected lubricants into the surface of rough porous structures to form a continuous liquid layer, which could minimize the adhesion to droplets without an air layer, so that droplets on the lubricating layer could slide smoothly[19⇓~21]. Such surfaces are called slippery liquid-infused porous surfaces (SLIPS)[14,15,22,23]. Similarly, SLIPS surface also has excellent anti-fouling and anti-icing performance, even in relatively humid environment, small tilt angle, water droplets can easily slide off the surface[24].
In order to explain this phenomenon, the mechanism of hydrophobic performance of SLIPS will be introduced as follows: the surface in real life has problems such as chemical inhomogeneity or uneven roughness, which will cause contact angle hysteresis and affect the adhesion to water droplets[24]. When the low surface energy lubricant is injected into the rough structure, the surface will be nearly molecularly smooth, and the contact angle hysteresis will be effectively reduced, thus reducing the adhesion force of the surface to water droplets[25].
The results show that the micro-roughness of the surface and the type, viscosity and molecular weight of the lubricant have a great influence on the hydrophobic properties[26,27]. When constructing SLIPS, three criteria should be met: (I) the solid surface must be a coarse structure with micro-nano texture to provide a large space for lubricant absorption and storage[14]; (ii) the lubricating fluid is not miscible with the repellent fluid; (iii) The chemical affinity of the lubricating fluid for the solid matrix is higher than the chemical affinity of the repelling fluid for the matrix.
The injected lubricant surface has excellent performance in reducing the adhesion of water droplets, but two weaknesses of SLIPS still need to be solved. The first is the cost problem, researchers recommend the use of silicone lubricants, more environmentally friendly than fluorinated solutions, cheaper[28,29]; The second is the durability of SLIPS, because of high temperature evaporation or repeated use, the injected lubricating oil is easily exhausted, and ionic liquids with higher stability can be used instead of lubricating oil.Other researchers have combined the UV-cured polymer network with the porous surface injected with lubricant, which can effectively improve the oil-locking performance of the surface, thereby improving the service life of SLIPS[28,30,31][29]. In the future, lubricant-impregnated surfaces will show great potential for practical applications (Figure 5).
2 Basic theoretical knowledge
2.1 Correlation theory of wettability
Wettability is one of the important characteristics of solid surface, which refers to the ability or tendency of a liquid to spread on a solid surface, mainly determined by the chemical composition and microstructure of the surface, and expressed by the contact angle (θ) of water droplets on the surface[33,34]. When θ = 0 °, it indicates that the droplet is completely wetted on the solid surface, and the material is called a super-hydrophilic material; When 0 ° < θ < 90 °, it indicates that the droplet is partially wetted on the solid surface, and the material is called a hydrophilic material; When 90 ° < θ < 150 °, it indicates that the droplet is difficult to infiltrate on the solid surface, and the material is called hydrophobic material; When θ > 150 °, it indicates that the droplet is difficult to infiltrate on the solid surface, and the material is called superhydrophobic material. The magnitude of the contact angle (fig. 6) can be calculated from the Young's equation[7,35⇓~37].
Where γsg is the solid-gas interfacial tension (N/m);γsl is the liquid-solid interfacial tension (N/m);γlg is the liquid-gas interfacial tension (N/m); θ is the contact angle at gas/liquid/solid three-phase equilibrium.
The Young's equation is applicable to the ideal smooth homogeneous solid surface. However, there is no such surface in real life, so the use of the Young's equation has certain limitations. Based on the Young's equation, Wenzel model and Cassie model are proposed through unremitting research and experiments (Figure 7).
In real life, the surface of solid has a certain rough structure. Wenzel found that when a liquid contacts with a solid surface, assuming that the liquid can completely infiltrate into the interstices of the solid surface, the actual contact area between the solid and liquid will be larger than the apparent contact area between them, thus increasing the contribution of solid-liquid and gas-solid interfacial tensions to the free energy of the system[8]. Wenzel used the roughness factor to modify Young's equation and proposed an equation to calculate the Young's of contact angle on rough surface:[8]
As can be seen from equation (2), the rough structure of the surface amplifies the hydrophobic properties of the smooth surface. Given that R is always greater than 1, on a hydrophobic surface (θγ>90°), the contact angle increases with increasing roughness, and the surface of the material appears more hydrophobic; On the hydrophilic surface (θγ<90°), the contact angle decreases with the increase of roughness, and the surface of the material is more hydrophilic.
In real life, there is also a state in which the liquid does not infiltrate or partially infiltrate into the voids of the solid surface. Uch as a surface with a rough porous structure, when a droplet falls on such a surface, an air layer is created between the liquid-solid interface, and only the top of the microstructured surface is wetted[17]. At this time, the droplet is supported by a solid-gas composite surface composed of solid and air, in which the "air" part of the composite surface is considered to be completely non-wetting, greatly reducing the contact area between the droplet and the solid surface, giving the droplet low friction and high mobility. Cassie and Baxter carried out a thermodynamic analysis and obtained the Cassie equation:[9]
Where θC is the contact angle in the Cassie state, f1 and f2 are the area fractions of the liquid in contact with the solid surface and air, respectively, and θ1 and θ2 are the intrinsic contact angles of the liquid with the solid surface and air, respectively.
The hydrophobicity of a solid surface is one of the important factors that determine the morphology of a droplet on the surface. When the hydrophobicity of the solid surface is small, the droplet is easy to penetrate into the texture of the rough structure of the surface, and "line contact" occurs between the droplet and the surface, showing a Wenzel state. When the hydrophobicity is large, the droplet is difficult to penetrate into the surface texture, and will remain on the solid surface in the form of "point contact", showing Cassie state. The morphology of the droplets can be controlled by adjusting the hydrophobicity of the solid surface[39⇓⇓⇓⇓~44].
2.2 Construction Principle of Transparent Superhydrophobic Surface
Single superhydrophobic surface can no longer meet the needs of people, and many multifunctional superhydrophobic materials have emerged, among which transparent superhydrophobic materials have attracted the interest of researchers because of their great potential in industrial applications[25,26,45⇓⇓~48]. For some devices with special requirements in the field of optics (such as automobile windshields, smart windows, goggles, solar panels, camera lenses, etc.), they will be exposed to various environments during use, which will easily adhere to pollutants and affect the normal use of the devices. In the process of manual cleaning, firstly, it is easy to cause damage to the surface of the device, and secondly, the use of detergents will cause environmental pollution. Therefore, it is very necessary to develop a transparent super-hydrophobic coating with dust-proof and self-cleaning properties to solve practical problems.
In order to fabricate transparent superhydrophobic surface, the properties and surface structure of the material should be considered simultaneously. It has been shown that the light absorption of coatings and the light reflection of interfaces (bulk properties), as well as the light scattering caused by high surface roughness (surface properties), can reduce the transparency of surface coatings[49,50].
The use of low absorbance materials is the first step in the preparation of high transparency coatings, and the smaller the refractive index of the material, the higher the transmittance of light. Table 1 is the refractive index (n) of conventional transparent materials. Among these materials, silica has the lowest refractive index. Most transparent superhydrophobic coatings are prepared on the basis of silica[51].
In addition to the properties of the material itself, attention should be paid to the surface structure of the coating to further enhance transparency. Large-scale surface roughness can cause light scattering and reduce the transparency of the material. Different scales of roughness can cause different scattering behaviors, such as Mie scattering and Rayleigh scattering[46,47]. When the roughness of the surface is on the visible wavelength scale, there is a high probability of significant light scattering by Mie scattering, and the surface becomes opaque[46,49]. When the roughness is significantly smaller than the visible wavelength (typically less than one-tenth of the visible wavelength), Rayleigh scattering dominates the light scattering and Mie scattering does not occur[52].
It is assumed that the rough structure of the material surface is spherical particles and does not change the direction of the incident light quadratically. The intensity of Rayleigh scattering (I) can be expressed as follows:[49,53]
Where θ is the incident angle, S is the distance between the particle and the detector, λ is the wavelength of the incident light, d is the diameter of the surface roughness, n is the refractive index of the particle, and Ii is the intensity of the incident light. Typically, visible wavelengths range from 390 to 700 nm. Equation (4) shows that the intensity of Rayleigh scattering is affected by several factors, among which the effect of d on I is significant. If d is kept below 70 nm, the value of I is very small, with a magnitude around 10-14×Ii. In other words, light scattering can be practically neglected when Rayleigh scattering dominates the scattering behavior. We can weaken the light scattering by controlling the scale of the surface roughness below 70 nm.
In the preparation of transparent hydrophobic surface, the transparency of the material can be enhanced by reducing the scale of the rough structure (below 70 nm), while the realization of hydrophobicity requires a larger surface roughness (more than hundreds of nanometers), which are contradictory. This problem can be effectively solved when holes much smaller than the wavelength of visible light are introduced into the material[54,55]. By introducing pores into the material, even if the surface roughness is very small, enough air layer can be retained to make the surface of the material have a large contact angle.
According to the Lorentz-Lorenz model, the effective refractive index neff of a material with a porous structure can be expressed as:[55]
Where and φ is the porosity. Reducing the effective refractive index neff of a porous material can be achieved by increasing the porosity φ. Therefore, the introduction of holes much smaller than the wavelength of visible light into the material can not only maintain the super-hydrophobicity of the material, but also reduce the refractive index and further enhance the transparency of the material.
To sum up, there are two ways to endow materials with higher transparency: (1) using materials with low absorbance; And (2) controlling the surface microstructure of the material, reducing the micro-roughness of the surface of the material, and creating a porous structure on the surface of the material.
3 Method for preparing transparent superhydrophobic surfaces
Based on the preparation of superhydrophobic surfaces, researchers have carried out a lot of work. Onda et al. Used the self-assembly of alkyl ketene dimer (AKD) during the curing process to grow a superhydrophobic solid film with a thickness of about 100 μm on a glass plate, which was the first time that a superhydrophobic surface was synthesized artificially[10]. Since then, more and more methods have been used to synthesize superhydrophobic surfaces, such as vapor deposition, dry etching, colloidal lithography, self-assembled monolayers, electrochemical methods, sol-gel methods and so on. These methods improve the hydrophobicity of substances from two angles, one is to change the roughness to obtain hydrophobicity; The other is to reduce the surface energy by chemical modification and further increase the hydrophobicity. Next, the common methods of preparing superhydrophobic surface and transparent superhydrophobic surface will be introduced.
3.1 Vapor deposition method
Chemical vapor deposition (CVD) is an efficient and often preferred method for building micro and nano particles, nanorods, into ordered macrostructures. In the process of evaporating water from polyelectrolyte poly (sodium 4-styrenesulfonate) coated CNTs, Yan et al. Prefabricated large-size pyramid-like microstructure patterns through capillary-driven self-assembly, and the surface showed good superhydrophobicity[56]. Similarly, superhydrophobic vertically aligned, ultra-low density, large-diameter double-walled carbon nanotube arrays have also been studied[57]. The obtained coating also has excellent superhydrophobicity, but the transparent effect of the coating is not ideal. Deng et al. Used soot as a template to deposit a layer of silica nanoparticle shell on the outside of soot by ammonia-catalyzed hydrolysis of tetraethyl orthosilicate[58]. A transparent silica coating with a porous network structure was obtained by calcining at 600 ℃ to remove the soot, and then fluorosilane was deposited on the surface of the material by vapor deposition for lyophobic (water phase and oil phase) modification.Finally, the super-hydrophobic surface with high transparency was obtained, and the transparency at 500 nm was only 10% lower than that of the untreated bare glass. The obtained transparent layer also has excellent chemical stability and thermal stability, but its mechanical properties are poor, and the fine structure on the surface is not easy to exist stably, which affects the service life (Fig. 8).
3.2 Dry etching technology
Palumbo et al. Used oxygen plasma etching to produce a columnar structure with a width of tens of nanometers on the polymer, and then modified the surface with fluorine-containing silane to obtain a stable transparent superhydrophobic coating. More and more researchers use plasma etching treatment to prepare surfaces with different wetting properties[59]. By using the etching technology, the expected and regular microscopic patterns can be obtained. Moreover, the plasma etching technology can be applied to various materials and substrate surfaces. Uhm et al. Proved that carbon nanotubes (CNTs) are also hydrophobic by methane glow discharge plasma etching at low pressure. By plasma treatment of CNTs, the surface free energy is sharply reduced, and functional surfaces with specific hydrophilic/hydrophobic properties can be obtained[60]. Her et al. Etched the polymethyl methacrylate (PMMA) film with CF4 gas plasma to produce a certain roughness on its surface. After the metal fluoride on the surface was removed by hydrolysis, a layer of hexamethyldisiloxane (HMDSO) was vapor-deposited to obtain a stable coating (Figure 9)[61]. The contact angle of the obtained coating is larger than 160 degrees, and the transparency is higher than 95 percent.The method is simple to operate, is environment-friendly, has good practical application value, and simultaneously expands the application field of PMMA.
Similarly, Ghosh et al. Etched conical nanoarrays on the surface of perfluorosulfonic acid with low surface energy and high transparency by ion irradiation, and controlled the size of the nanoarrays to be much smaller than the wavelength of light to obtain a stable surface with a contact angle of more than 150 ° and a light transmittance of more than 95%[62]. The method has that advantage of simple operation, mild reaction conditions, controllable product structure and wide application range.
This method is also applicable to a variety of flexible polymer substrates, in addition to the substrates having a certain degree of stiffness described above. Gourgon et al. combined nanoimprinting and plasma technology to etch micro-nano layered structures on flexible films such as fluorinated ethylene propylene copolymer (FEP), polymethyl methacrylate (PMMA) and polyethylene terephthalate (PET), respectively, to obtain transparent super-hydrophobic films. This method can be used for large-area processing and has good application value[63]. Zhong et al. Used the femtosecond laser method to carry out fine structural processing on a stainless steel plate, and combined a micron-scale groove with a micropore array to form a template.Then the PDMS was cast on the template to obtain a PDMS film with a micron-sized cone-like array, and a highly transparent superhydrophobic film was obtained by further optimizing the diameter, height and other parameters of the conical array (Figure 10)[64]. The obtained film also has high durability, and can still maintain super-hydrophobicity and high transparency after long-time illumination, high-temperature treatment and abrasive paper polishing for a plurality of times.
3.3 Colloidal lithography
Colloidal lithography is an effective method to achieve substrate patterning by using submicron or micron-sized monodisperse spherical colloidal particles or arrays assembled by them as a mask, combined with other chemical and physical means. This method not only has low cost, but also the characteristic scale of the constructed pattern can easily reach the nanometer level, which has attracted people's attention. Kim et al. Coated a double-layer film of metal aluminum and photoresist on a silicon wafer, patterned it and anodized it to convert metal aluminum into aluminum oxide.The treated double-layer film is transferred to polyethylene terephthalate (PET) coated with ultraviolet glue, and the photoresist stained with a small amount of unreacted metal aluminum is removed by using an organic solvent to obtain the PET-ultraviolet glue alumina film[65]. Carry out chemical etching on that obtain PET-ultraviolet glue alumina film to obtain the transparent super-hydrophobic flexible film of which the alumina on the upper layer is arranged in a pyramid shape, wherein the characteristic scale of the alumina arranged in the pyramid shape is in a nanometer magnitude. The obtained film also has excellent mechanical properties, and the surface morphology and wettability of the film are basically unchanged after the film is subjected to 1500 times of folding treatment. However, the preparation process of the coating is complicated, and the transparency of the coating is not very ideal.
On the basis of the above experiments, Kim et al. transferred the periodic polymer microcolumn array onto the PET film by using PDMS with micropore array on the surface as a template, and then transferred the anodized alumina film onto the ultraviolet adhesive bonded with PET, and finally obtained the PET-ultraviolet adhesive alumina film with micro-nano two-stage rough structure on the surface after chemical etching (Fig. 11)[66]. The contact angle of the obtained film is as high as 170 °, and the transparency is increased to 82% compared with the previous work. The obtained film also has excellent mechanical properties, and the film still maintains good transparency and superhydrophobicity after being folded for 2000 times. As a protective layer of the solar cell, it can endow the solar cell with self-cleaning performance.
3.4 Self-assembled film forming method
Self-assembled monolayers (SAMs) is a very simple method to prepare micro-nano dual-scale superhydrophobic surface structures, which can be finely controlled and widely used. For example, Jiang et al. prepared conductive and superhydrophobic hollow polyaniline spheres by a self-assembly method in the presence of perfluorooctane sulfonate (PFOSA), which were used as dopants and soft templates[67]. Zhou et al. Constructed micro-nano hierarchical structures on cotton substrates through controlled assembly of carbon nanotubes to endow them with superhydrophobicity[68]. Lee et al. Prepared a superhydrophobic film of file-like slurry-like particles by using assembly of silica particles[69]. Cho et al. prepared a biomimetic superhydrophobic coating on a curved surface by modifying a single-layer polystyrene microsphere array with silver nanoparticles[70]. Rotello et al. introduced a new strategy to prepare hydrophobes into superhydrophobic surfaces by depositing about 2.5 nm iron-platinum alloy (FePt) nanoparticles with different degrees of fluorinated ligands on their surfaces[71]. This is the first time that magnetic materials have been introduced into superhydrophobic surfaces, which can help prevent oxidation and prolong the service life of magnetic media.
The self-assembly method can be used to prepare stable superhydrophobic surfaces, while the self-assembly of silica nanoparticles is often used to prepare transparent films. Zhang et al. prepared a superhydrophobic surface by depositing poly (diallyldimethylammonium chloride)/sodium silicate multilayer film layer by layer on a substrate coated with silica spheres, and then fluorinating it. However, due to the weak bonding force between the silica spheres and the substrate, the hydrophobicity of the surface will be reduced after a certain degree of friction[72]. Cohen et al. Showed that the layer-by-layer assembly method could realize the growth of nanoparticles of different sizes in multilayer films and control the aggregation state of particles[73]. A transparent and antireflective superhydrophobic film was prepared on silica nanoparticles of different sizes. The prepared transparent superhydrophobic surface has become a hot spot because of its wide industrial potential. Yang et al. also spin-coated 100 nm silica particles modified by fluorinated trichlorosilane on silicon wafers treated with propylsuccinic anhydride triethoxysilane (TESPSA) for self-assembly, and obtained a transparent superhydrophobic coating with appropriate roughness by adjusting the concentration of silica in the spin-coating solution[74]. The method for construct that transparent super-hydrophobic coating has simple operation, can be suitable for large-area production, and is also suitable for various substrate. Cheng et al. spin-coated three kinds of block copolymers (BCPs) with different molecular weights on a glass plate layer by layer, prepared a strong and transparent nanosilica coating through multilayer self-assembly, and then carried out fluorination treatment to further enhance hydrophobicity (Fig. 12)[75]. The contact angle of the prepared coating is about 155 degrees, the light transmittance is as high as 92.36%, the obtained coating also has excellent mechanical stability, and the surface morphology is almost unchanged after 100 times of adhesive tape stripping experiments, so the prepared coating has wide industrial potential.
3.5 Lectrochemical method
Electrochemical deposition has been widely used to construct biomimetic superhydrophobic surfaces, which is a multifunctional technique for the preparation of micro- and nano-scale structures, and is mostly used for metal substrates. Bell et al. prepared metallic superhydrophobic surfaces by electrodeposition using a very simple metal salt solution with a water contact angle as high as 173 °. This method can be used to coat objects of any size or shape[76]. Jiang et al. Prepared copper membranes with layered structure, which have super-hydrophobicity and super-lipophilicity through simple electrochemical deposition induced by long-chain fatty acids, and can be well applied to oil-water separation[77]. Compared with the photoetching electrochemical method, the non-photoetching electrochemical method is simple and effective. Li et al. Used hydrogen bubbles as a dynamic template for metal electrodeposition to prepare three-dimensional porous copper films with typical micro-nano structure integration[78]. Lai et al. obtained a uniform and stable titanium dioxide nanobelt (TNB) film by electrophoretic deposition, and then used fluorosilane (FAS) to self-assemble on the surface of the randomly stacked titanate film to form a monolayer, and prepared a super-hydrophobic film with high transparency and excellent chemical stability[42]. Xiang et al. immersed the activated low-carbon steel in ZnA-Ni electroplating solution by electroplating method, and then further treated with myristic acid to obtain a super-hydrophobic coating with excellent corrosion resistance. The contact angle of the coating was still greater than 150 ° after immersion in NaCl solution for 48 H, which could effectively prevent the corrosion of industrial metals (Fig. 13)[79].
3.6 Sol-gel method
The advantage of the sol-gel method to fabricate biomimetic superhydrophobic surfaces is that it is suitable for various solid substrates. For example, Huang et al. used hexamethylenetetramine and ethylene glycol as capping agents to construct a biomimetic superhydrophobic surface on a copper alloy engineering material, with Cu2+ and Fe2+ ions with high stability constants as capping agents[80]. Ordered porous indium oxide array films with controllable superhydrophobicity and lipophilicity were prepared from colloidal monolayer polystyrene by sol impregnation method. Tiemblo et al. Used p-toluenesulfonic acid as a catalyst to prepare superhydrophobic aerosols by refluxing methoxysilane in toluene[81]. Superhydrophobic polypropylene membranes with nanoribbon polyaniline structure were prepared by graft polymerization of acrylic acid on the surface of polypropylene membrane, followed by oxidative polymerization of aniline on the graft surface. However, the experimental process is complex and requires precise control of experimental conditions, which is not conducive to large-scale processing.
In addition to metal, plastic, silicon and other substrates, this method is often used on glass substrates to characterize the transparency of coatings. Zhang et al. prepared a mixed solution of carbon nanotubes and silica by a sol-gel method, sprayed the obtained mixed solution on the glass surface, calcined it at 600 ℃ to remove the carbon nanotube template, and then modified its surface by fluorination to give it low surface energy, and finally prepared a transparent amphiphobic coating with an average transmittance of more than 80% in the visible light range[82]. The obtained coating also has excellent high temperature resistance and chemical stability. Chen et al. mixed 20 nm silica particles and 1 H, 1 H, 2 H, 2H-perfluorooctyltriethoxysilane (PFOTES) into a sol matrix to obtain a PFOTES-SiO2 sol solution.After magnetic stirring and placing for a certain time, the coating is sprayed on the glass surface and cured to obtain a transparent hydrophobic coating. Similarly, the spraying method is not conducive to the connection between the silica particles and the substrate, and the hydrophobicity of the surface will be reduced after a certain degree of friction[83]. Xu et al. prepared silica nanoparticle colloids by a typical St Stöber sol-gel method, and added a certain amount of aminopropyltriethoxysilane to adjust the aggregation state of silica nanoparticles[84]. The obtained sol was then dip-coated onto a glass plate and further Hydrophobically modified with perfluorosilane to obtain a coating with superhydrophobicity and high transparency (fig. 14). This method is suitable for large-scale coating. During the study, they also found that the mechanical stability of the coating increased with the thickness of the silicon dioxide layer, but decreased the transparency.
3.7 Other methods
In addition to the above methods, many other methods have been applied to prepare transparent superhydrophobic surfaces. Xu et al. prepared organic-inorganic hybrid nanoporous coatings with high transparency and superhydrophobicity on glass surface by a simple coacervation-induced phase separation method using polysiloxane (PSO) containing — SiH and —SiCH CH2 bonds as the precursor and methyl-terminated PDMS as the porogen[85]. The average transmittance of the obtained coating in a visible light range is as high as 85%, the contact angle of water drops on the coating is 155 degrees, the rolling angle is less than 1 degree, and the coating also has certain thermal stability and mechanical stability and good application value. Bhushan et al. Used a simple dip-coating process to coat mixtures of silanized different nanoparticles (silica, zinc oxide, and indium tin oxide) with methylphenyl silicone resin on different transparent substrates (glass, polycarbonate, polymethyl methacrylate)[86]. The transparent super-hydrophobic coating can be finally obtained by regulating the ratio of the inorganic particles to the silicone resin. The obtained coating also has certain wear resistance. Yang et al. prepared silica nanoparticles with diameters of 20 nm and 100 nm, respectively, modified by 3-aminopropyltrimethoxysilane (APTS)[52]. A double-layer nano rough structure is constructed on the surface of a silicon wafer by simple continuous dip-coating, the stability between the particles and a matrix is improved by high-temperature calcination, and then the particles are hydrophobized by heptadecafluorodecyl trichloromethane to finally prepare a super-hydrophobic surface with a contact angle of about 160 degrees, a rolling angle of less than 5 degrees and a light transmittance of nearly 100%. Yoon et al. prepared an organosilane sol using methyltrimethoxysilane (MTMS), added methyl methacrylate (PMMA) and alumina particles to the sol to obtain a mixed solution, and electrostatically sprayed the mixed solution on indium tin oxide (commonly known as ITO) glass to finally obtain a stable coating with a transparency of up to 90% and a contact angle of greater than 162 °[87].
The preparation methods of superhydrophobic surface are rich and varied, but each method has its own characteristics. Table 2 analyzes and compares the common methods.
4 Application
4.1 Self-cleaning
Self-cleaning means that when a droplet rolls down on a surface to which contaminant particles are attached, the droplet can absorb or adhere to the contaminant and roll off the surface together. In 1997, Neinhuis and Barthlott observed the microstructure of lotus leaf surface and found that the self-cleaning property of lotus leaf was caused by waxy crystals on the surface and papillae on the rough surface[11]. In 2002, Jiang et al. Made an in-depth study on this basis and found that there were nano-structures on the papillae of the micro-structure on the surface of lotus leaves[12]. The composite structure composed of micro-structure and nano-structure leads to the reduction of the contact area between the pollutant particles and the surface and between the droplets and the surface, and the droplets only stay at the top of the micro-nano composite structure. As the droplets roll over the leaves, the pollution particles are absorbed by the liquid and carried away.
The number of contaminant particles removed from a smooth surface depends primarily on its wettability. On the surface with small water contact angle, the droplet rolls relatively slowly on the surface, and the contaminant particles contact the droplet and transfer to the side of the droplet, and then redeposit behind the droplet without being removed. In particular, the hydrophobic particles do not roll off with the droplets and tend to remain on these surfaces. This phenomenon can be explained by the interfacial tension between the particle and the surface[11]. On the surface with a large contact angle of water droplets, the adhesion of pollutant particles to water droplets is stronger than that of solid surfaces. Under a small force, spherical water droplets on superhydrophobic surfaces can carry dust and roll off the surface quickly (Fig. 15). Therefore, the superhydrophobic surface can clean itself when there is dust on it.
Superhydrophobic materials have a wide range of applications in national defense, industrial and agricultural production and daily life because of their good self-cleaning performance. For example, superhydrophobic technology can be used on the needle tip of microsyringes to prevent adhesion, blockage and reduce loss; It can be used in textiles and leather, and can also be made into waterproof materials, waterproof and anti-fouling clothing, etc.
4.2 Optical field
For many optical equipment and high-precision instruments, the surface often adheres to the dust particles in the air, or leaves sweat and grease on the surface when touching, which affects the normal use of the device. Manual cleaning is time-consuming and labor-intensive, and will inevitably cause damage to the surface, and the chemical cleaning agents used in cleaning will also cause damage to the environment. Uch as optical lenses, are often susceptible to damage to their optical properties during cleaning. Coating a layer of super-hydrophobic film on the surface of the device can maintain its normal use and will not damage the surface of the device.
It is of great research value to introduce superhydrophobicity and optical transparency on the same solid surface. Generally, the superhydrophobic surface has a large roughness, but the large roughness will cause light scattering and reduce the light transmittance. Therefore, how to optimize the surface roughness is the key factor for the preparation of transparent superhydrophobic surface. When holes much smaller than the wavelength of visible light are introduced into the material, the superhydrophobicity and high transparency of the surface can be effectively maintained[54,55]. Li et al. Dispersed hydrophobic fumed silica particles in ethanol solution and simply rubbed them on the surface of a glass plate. During the evaporation of ethanol, the hydrophobic fumed silica particles were self-assembled into a uniform nanoporous structure, and then fluorinated to obtain a super-hydrophobic coating with high transparency[89].
4.3 Dustproof
Transparent super-hydrophobic coating not only has certain self-cleaning, dust-proof and anti-fouling effects, but also has high transparency, which broadens its application in deep space exploration, solar panels and so on. In the actual use of solar panels, a layer of pollutant particles will gradually accumulate on the light-facing surface, which will reduce the transmittance of light, affect the photoelectric conversion efficiency and deteriorate the performance of solar cells. Traditional surface dustproof and antifouling technologies mainly include natural dust removal, mechanical washing, chemical cleaning, electric curtain dust removal and robot dust removal. These traditional dust removal methods often have the problems of low efficiency, high cost and environmental pollution, which can be well solved by preparing transparent super-hydrophobic dust-proof coating[90,91]. V Vüllers et al. fabricated a free-standing superhydrophobic nano-film with a reflectivity as low as 4% through a combination of hot embossing and hot drawing techniques, which can be applied to solar panels as a transparent superhydrophobic coating (Fig. 16)[92].
Research on the adhesion mechanism of particles on the surface of solar panels will help to improve the utilization efficiency of solar energy, which is expected to alleviate the global energy crisis. According to the main types of pollutants on the surface of solar panels and the geographical characteristics of solar panels, the design of dust-proof solar panels with good dust-proof effect and less energy consumption will be a hot research direction in this field in the future.
4.4 Anti-icing and anti-fogging
Road icing may cause serious traffic accidents. Freezing on the outdoor antenna may affect the communication and power consumption in daily life; Aircraft icing will increase energy consumption and endanger people's lives in serious cases. The freezing of water on ocean-going ships will cause the ship to lose its balance and affect its safety. The formation of frost in the refrigerator will reduce the heat transfer efficiency in the refrigerator and increase energy consumption. In addition, ice and frost covering the leaves of outdoor plants can cause loss of plant cell activity, resulting in plant death and agricultural losses. The preparation of effective anti-icing and deicing surfaces is of great significance to our production and life.
In low temperature and high humidity environment, water droplets or water vapor in the air will condense on the solid surface to form ice or frost, and the condensation process is closely related to heat exchange[93]. Common surfaces in life, such as stainless steel, glass, wood and so on, are hydrophilic surfaces. Water droplets or water vapor in the air will gather on the surface and spread out into a layer of liquid film. The existence of liquid film reduces the heat transfer efficiency between the solid surface and the vapor to be condensed, increases the temperature difference between gas and solid, reduces the critical nucleus radius, and accelerates the condensation process. On the hydrophobic surface, the condensed vapor contacts the solid surface in the form of small droplets, which greatly enhances the heat transfer efficiency between the solid surface and the condensed vapor.
The current super-hydrophobic anti-icing strategy is mainly reflected in three aspects (Figure 17):[23,30]
Reduce the contact between the droplet and the interface (the rebound of small droplets under supercooling conditions), delay the icing time (the self-jumping phenomenon of small condensed droplets), and reduce the adhesion of water droplets and ice to the surface (anti-icing of the sliding surface). Studies have shown that the phenomenon of small droplets rebound is common on superhydrophobic surfaces.When the small droplets hit the surface, their kinetic energy is not easy to convert into the internal energy of the superhydrophobic surface, and the small droplets will gather and rebound, which can effectively reduce the contact time between the droplets and the surface and achieve the purpose of anti-icing[94⇓~96][97,98]. When the roughness of the superhydrophobic surface is less than and infinitely close to the minimum radius value of the critical nucleus, the superhydrophobic surface has an icing delay effect[99][83,100]. Layered superhydrophobic surface prepared by low surface energy materials can effectively reduce the adhesion force of the surface to ice[101⇓⇓⇓⇓⇓⇓~108]. However, in a high humidity environment, the droplets may penetrate into the texture of the surface, and form a mechanical lock with the surface after ice formation, which increases the adhesion strength of the solid surface to ice. Moreover, in the subsequent mechanical deicing process, the micro-nano structure of the surface will be destroyed, which will affect the anti-icing performance of the surface. Inspired by Nepenthes, Aizenberg et al. prepared a smooth, liquid-infused porous surface that reduced ice adhesion even at high humidity[14]. The application of lubricating fluid injected into porous surface for anti-icing is also a future research trend.
4.5 Corrosion resistance of metal
High transparency is essential for the performance of optical devices and devices. Because they have been exposed to various environmental conditions, physical or chemical reactions occur between the metal and the environment, resulting in the migration of ions, resulting in changes in the properties of the metal, and the formation of pits or cracks on the surface. This pitting of the surface continues to spread throughout the material as long as the material remains in a corrosive environment. Therefore, it is highly desirable to develop a transparent coating capable of preventing corrosion. The super-hydrophobic coating provides a way to solve this problem because of its super hydrophobic characteristics. Based on the air layer theory of Cassie model and the capillary effect of micro-nano structure, there are a large number of air layers between the super-hydrophobic surface and the corrosive medium, which can prevent or inhibit the migration of ions and form a barrier between the metal substrate and the corrosive medium[9][109]. Xiang et al. prepared a super-hydrophobic ZNC anti-corrosion coating with a corrosion potential of -0.777 V and excellent anti-corrosion performance (Fig. 18)[79]. The research of Vazirinasab et al. Shows that the superhydrophobic surface can improve the positive shift of corrosion potential, increase the corrosion current density by at least two orders of magnitude, and improve the corrosion resistance of metals[109]. The hydrophobicity of superhydrophobic coating will decrease when it is exposed to corrosive medium for a long time, which will lead to the corrosion resistance of superhydrophobic coating. Therefore, the super-hydrophobic anti-corrosion coatings with self-healing properties will become the research trend in the future[111].
5 Conclusions and outlook
With the development of superhydrophobic technology, the demand for superhydrophobic materials is increasing in all walks of life. For example, because of its self-cleaning performance, it can be used to clean the outer surface of large buildings which are difficult to clean, as well as the outer surface of transportation equipment which causes waste of water resources in daily cleaning. For surfaces with special requirements in the optical field, the transparent superhydrophobic surface endows the surface with other properties such as self-cleaning, mechanical strength and the like without affecting the efficiency of equipment; The inner lining of the pipeline used for transporting the liquid can reduce the energy wasted for resisting the resistance in the transportation process; It is used for the surface of underwater submarines, etc., to reduce the friction resistance with water, thereby reducing energy consumption and increasing the running speed; Can also be used on clothing textiles to achieve the effect of waterproof, decontamination and self-cleaning. With the development of China's aerospace industry and the deepening exploration of the moon, the adhesion of lunar dust on the surface of spacesuits and spacecraft will cause many failures, including mechanical structure seizure, sealing layer failure, thermal control system failure and so on.The application of super-hydrophobic dust-proof technology in spacesuits and spacecraft can reduce the adhesion of lunar dust and protect the life and health of astronauts. In a word, the characteristics of superhydrophobic materials are destined to have a wide range of applications in scientific research and practical applications.
Among them, transparent superhydrophobic surface has stronger application demand and broader application prospect because of its high light transmittance. However, both superhydrophobic materials and transparent superhydrophobic materials still have some shortcomings, which make them unable to achieve a wide range of applications. For example, the problem of mechanical stability is that the rough structure on the surface is fragile and easy to be damaged by mechanical external forces such as impact and friction during processing and use, and the mechanical strength and transparency of the coating are always contradictory[18]. When preparing transparent hydrophobic surfaces, it is necessary to reduce the scale of the roughness to enhance the transparency of the material, which will make the roughness on the surface more fragile and reduce the mechanical strength of the surface. Superhydrophobic surfaces with self-healing properties have been prepared. When the hydrophobicity of the surface fails due to wear and other reasons, the superhydrophobicity of the surface can be restored by standing, heating, tetrahydrofuran and other treatments[112]. Another strategy to improve the mechanical durability of the coating is to use a primer similar to "glue" to "stick" the coating to the substrate, so as to increase the wear resistance as much as possible without affecting its transparency[113]. Deng et al. Proposed to put a layer of "armor" on the fragile rough structure to increase the wear resistance of the material. A dense inverted pyramid-shaped micron structure is etched on a silicon plate, and a cavity is filled with a hydrophobic nano structure, so that the problem that the superhydrophobic material is not durable is solved in theory[114]. For example, the hydrophobic performance of super-hydrophobic surface is unstable in extreme environments such as high temperature and high humidity, which affects the normal use of the device. Li et al. successfully fabricated a robust superhydrophobic surface through a one-step process. Firstly, the natural attapulgite (APT) with micro-nano porous structure was modified by fluorination, and the inorganic aluminum phosphate (AP) was selected as the binder to improve the bonding strength between APT and the substrate. Finally, the SLIPS with thermal stability and moisture resistance was successfully prepared by injecting silicone oil into the super-hydrophobic surface[115]. When the surface is exposed to the air for a long time, the oil in the environment may adhere to and gradually fill in the rough structure of the surface.As a result, the super-hydrophobic property of the surface gradually decreases and finally disappears, and in the process of cleaning oil stains, it will also lead to the destruction of rough structure, which requires the selection of anti-aging raw materials as far as possible.
At present, super-hydrophobic films with high transparency have been successfully prepared, and the light transmittance is more than 95%. Although the durability of the surface still needs to be improved, it can be applied to a small range of scenes with less friction, such as precision instruments, to solve practical problems. In the follow-up study, on the one hand, we need to make more improvements from the preparation scheme, hydrophobic effect and other aspects to obtain a durable transparent super-hydrophobic surface with low cost, simple preparation, green environmental protection and wide application. On the other hand, there is still a lack of theoretical knowledge related to transparent superhydrophobic coating technology, which needs to be further improved to provide a solid theoretical basis for application.
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