1 Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology,Wuhan 430074, China
2 CRRC Qingdao Sifang Co., Ltd, Qingdao 266111, China
Superhydrophobic surface has broad applications in daily life and industries owing to its unique wettability. However, the micro-nano structures and low surface energy substance of surface are vulnerable to mechanical damage and chemical erosion, often prone to lose their superhydrophobicity. It is of great practical significance to construct durable superhydrophobic surface through morphologic design and performance optimization for its commercialization. In this review, based on the surface wettability model, including classical theory, metastable theory and contact line theory, the development history of superhydrophobic theoretical model is first reviewed, as well as their key guiding roles in the design of superhydrophobic durability. Then, the preparation strategies for durable superhydrophobic surface, such as micro-nano structure design, adhesive + coating, armor protection, self-healing, air cushion replenishment or replacement, are summarized. And the advantages and limitations of each strategy are reviewed. In addition, according to mechanical and chemical stability, the evaluation methods of superhydrophobic durability are illustrated. Finally, the problems and prospects of durable superhydrophobic surface are summarized for the future research of durable superhydrophobic coating.
Contents
1 Introduction
2 Theoretical model of wettability
2.1 Classical wetting theory
2.2 Metastable wetting theory
2.3 Contact line theory
3 Preparation strategy of durable superhydrophobic surface
3.1 Micro-nano structure regulation
3.2 Adhesive+coating
3.3 Armor protection
3.4 Self-healing policy
3.5 Air cushion replenishment or replacement
4 Evaluation methods for durable superhydrophobic surface
The super-hydrophobicity of materials is a special surface wetting state in which the contact angle of water droplets on the surface is more than 150 ° and the rolling angle is less than 10 °. After more than 20 years of development, superhydrophobic surfaces have been widely used in the fields of anti-fouling, de-icing, oil-water separation, self-cleaning and anti-corrosion, and have been combined with other functional properties of materials to develop superhydrophobic surfaces with corrosion resistance, electrical conductivity, electromagnetic shielding, antibacterial and flame retardant properties, which significantly improves the commercial value of superhydrophobic materials[1][2][3][4][5][6].
At the beginning of the 21st century, researchers found that the surface micro-nano hierarchical structure and low surface energy are the key factors to obtain superhydrophobicity[7,8]. Micro-nano structures are prone to stress concentration and damage under mechanical wear, and low surface energy substances contact with corrosive media, or degrade under ultraviolet and salt spray erosion, resulting in the loss of superhydrophobicity of the surface, which has become a major obstacle to its commercial application[9]. Therefore, the preparation of superhydrophobic materials with excellent durability is particularly urgent. The premise of constructing durable superhydrophobic surface is to clarify the existing surface wettability model, and to design the surface structure based on the theoretical model to improve the service life of superhydrophobic surface under different working conditions. The commonly used preparation strategies of durable superhydrophobic surface include micro-nano hierarchical structure design, adhesion + coating strategy and armor means to enhance the adhesion and outer protection of micro-nano structure.After the super-hydrophobic surface layer fails, the damaged micro-nano structure and the low-surface-energy substance can be recovered by a self-repairing method, and the damaged air layer in the microstructure can be supplemented or replaced based on the intrinsic durability theory. The establishment of superhydrophobic durability evaluation criteria is also an important link to promote the commercial application of superhydrophobic materials, in which mechanical stability and chemical stability are the key.
In this paper, the research progress of durable superhydrophobic surfaces is reviewed from three aspects, namely, the theoretical model of surface wetting, the preparation strategy of durable superhydrophobic surfaces and the evaluation method of superhydrophobic durability. Finally, the current challenges and future development directions of durable superhydrophobic materials were prospected, in order to provide a reference for the development of high durable superhydrophobic materials.
2 Surface wetting theory model
2.1 Classical infiltration theory
The wetting behavior of solid surfaces is a common natural phenomenon and several classical theoretical models have been developed (Fig. 1). As early as 1805, Young and Laplace studied the surface tension and capillary effect in the wetting phenomenon of solid surface, and first introduced the concept of contact angle. Young believed that when a liquid wets an ideal smooth solid surface, the surface tension of the three phases of solid, liquid and gas will eventually reach equilibrium at the interface, showing a stable contact angle on the macro level, and put forward the famous Young's equation.
The is the surface tension of solid-gas, solid-liquid and gas-liquid interfaces respectively, and the θ0 is the contact angle of the solid surface in a smooth and flat state, also known as the intrinsic contact angle or Young's contact angle. Although Young's equation can qualitatively describe the macroscopic wetting state of solid surface from the point of view of energy balance, it also has obvious defects. Because the microscopic topography of the actual solid surface is rough and uneven, rather than the ideal smooth plane assumed by Young's equation. Therefore, on the basis of Young's equation, Wenzel introduced roughness factor R in 1936, that is, the ratio of the area of the rough solid surface actually wetted by liquid to its projected area, and established Wenzel's equation[10].
Where the is the Wenzel contact angle and the value of R is greater than 1. The theoretical derivation of the Wenzel equation shows that R can be regarded as an amplification factor in the mathematical relationship.When the intrinsic contact angle is , the is larger, and when the intrinsic contact angle is , the is smaller.The wetting state is that the rougher the intrinsic hydrophobic surface is, the stronger the hydrophobicity will be, and the rougher the intrinsic hydrophilic surface is, the stronger the hydrophilicity will be. The Wenzel equation is a description of a surface with a rough microstructure that is completely wetted by a liquid, also known as the Wenzel state. In 1944, Cassie and Baxter found that when the porous structure was introduced into the hydrophilic fabric surface, the surface became hydrophobic, which was contrary to the Wenzel model. It was found that besides the solid-liquid interface, there was also a part of gas when the fabric surface contacted with the liquid, and the gas phase in the wetting interface was the fundamental reason for the transformation of hydrophilic surface into hydrophobic surface. Therefore, based on the Wenzel model, Cassie and Baxter developed the Cassie-Baxter equation (Cassie equation for short) by considering the contributions of both gas and solid components[11].
Where the is Cassie contact angle, the is the ratio of the contact area of solid-liquid and gas-liquid components to the total wetted area, and the sum of the two is 1; the is the intrinsic contact angle of solid-liquid contact, and the is the intrinsic contact angle of gas-liquid contact. When the liquid is water, the is 180 °, and Cassie equation can be simplified as
Unlike the Wenzel state, the Cassie model assumes that the microstructure of a solid surface is not completely wetted by a liquid. Because of the existence of gas between the microstructures, the liquid can only be suspended on the top of the surface microstructures, which is called Cassie state. When the solid surface is in the Cassie state, the presence of the gas phase can reduce the total free energy of the wetted surface, and increasing the gas-liquid component ratio can increase the apparent contact angle to more than 150 ° and reduce the rolling angle to less than 10 °, reaching the superhydrophobic state. Through the geometric design of the surface microstructure, the apparent contact angle can even approach 180 °. The most important difference between Wenzel state and Cassie state in macroscopic performance is whether the liquid can be pinned on the contact surface. In Wenzel state, the liquid completely infiltrates the microstructure and produces interfacial bonding force with the solid contact surface, so the droplet can hang upside down on the contact surface. The typical natural phenomenon is the adhesion of rose petals to the droplet, so Wenzell state is also called rose effect. In the Cassie state, the droplet is only suspended at the top of the microstructure, and the droplet can roll on the contact surface after a small angle of inclination. A similar phenomenon can be observed on the surface of a lotus leaf, also known as the lotus leaf effect[12]. At present, Wenzel state and Cassie state, as the two most classical theories, are widely used to explain the wetting behavior of superhydrophilic and superhydrophobic surfaces, and are also an important basis for the classification of different surface wetting States.
2.2 Metastable wetting theory
After the Wenzel state and Cassie state are established, whether the energy relationship between them can be transformed into each other and whether there is an intermediate state have become the main research directions in the theoretical circles. In the course of the experiment, the researchers also observed the phenomenon that the same surface changed from Cassie state to Wenzel state many times. In 1999, Bico et al. Pressed a droplet in Cassie state and found that the contact angle of the droplet decreased from 170 ° to 130 °, indicating that the transition process from Cassie state to Wenzel state occurred[13]. The transition mechanism between the two States focuses on two aspects: one is explained from the energy point of view, that is, the Wenzel and Cassie States are in the energy minimum state, and there is an energy barrier between the two States, which requires external energy to cross the barrier before the state transition occurs[14,15]. Secondly, from the point of view of mechanical equilibrium, the critical pressure at the moment when the droplet suspends on the top of the surface microstructure and becomes unstable is used as the criterion for whether the Wenzel state and Cassie state can be transformed into each other[16,17]. Patankar established a theoretical model based on energy balance to demonstrate the possibility of metastability in the process of two-state transition[18]. By extending the model, Marmur calculates the free energy field of the droplet on the surface, quantifies the free energy of the Wenzel and Cassie States and the energy barrier between them[19]. Subsequently, the researchers captured the dynamic development process of metastability with laser confocal microscope, which proved the existence of metastability experimentally, as shown in Figure 2[20]. Lafuma and Qu Quéré put forward the concept of critical contact angle on the basis of experiments, unified the Wenzel equation and Cassie equation, and established the critical contact angle equation:[21]
The is the critical contact angle, and the surface may be in a metastable state when the θ0<θc occurs, so the θc can be regarded as a criterion for metastability. According to the criterion, even for the rough surface, when the gas escapes from the microstructure, the increases, and eventually the Cassie state will change to the Wenzel state. In general, the Wenzel state is thermodynamically more stable than the Cassie state. The escape of gas trapped by surface microstructures needs to overcome a certain energy barrier.Under the stimulation of external energy such as droplet impact, mechanical vibration and droplet evaporation, the local defects in surface microstructures will cause the transition from Cassie metastable state to the more thermodynamically stable Wenzel state, which is also the fundamental reason that hinders the application of superhydrophobic surface materials in Cassie state[22,23]. Therefore, in order to prepare superhydrophobic surfaces with stable Cassie state, it is necessary to reduce the θc, that is, to obtain a larger and more uniform surface roughness through geometric design, to stabilize the air layer in the microstructure, and to reduce the of solid-liquid component area ratio, which provides a theoretical basis for the preparation of durable superhydrophobic surfaces[24].
2.3 Contact Line Theory
Wenzel and Cassie's theory explains the static wetting behavior of solid surface from the thermodynamic point of view, and the contact line theory is more advantageous to explain the dynamic wetting process of droplets on the surface. According to the contact line theory, the solid-liquid surface is formed by the linear contact of different polar groups, and the contact line length and density of these polar groups determine the size of the contact angle. However, due to the limitation of conditions, the length of the contact line is difficult to define, and the theory has been excluded from the mainstream for a long time. In order to prove the applicability of the contact line theory, McCarthy et al. Designed different contact line lengths or densities on surfaces with the same , and the wetting behavior of droplets on different surfaces was also significantly different, which was contrary to the results predicted by Cassie equation and consistent with the contact line theory[25]. Extrand further developed the contact line theory, and successively established the linear calculation model of the contact line density function and the advance/retreat contact angle[26]. It was found that the higher the contact line density, the better the surface hydrophobicity, which was consistent with the prediction of contact line theory. The contact line theory can explain the dynamic process of the formation of the contact angle of a droplet on the surface, which is complementary to the Wenzel and Cassie theory. The process of Wenzel and Cassie's state transition can also be explained by the contact line theory. The contraction or forward inclination of the contact line of the local liquid, together with the escape or supplement of the air layer, will lead to the transition between the two States[27,28]. Compared with the first two theories, the contact line theory started late, and the corresponding experimental evidence is not enough, so it needs further development and improvement.
3 Preparation Strategies of Durable Superhydrophobic Surface
The key to the industrialization of super-hydrophobic surface is to improve its mechanical and chemical stability and prolong its service life, so the preparation of durable super-hydrophobic surface has become a key research direction. This section summarizes several commonly used strategies for the preparation of durable superhydrophobic surfaces, as well as the interrelationships and synergies between each strategy. The template method is used to design a rough surface with a micro-nano level structure to capture more air and improve the energy barrier of air escape, so as to obtain a super-hydrophobic surface with longer service life. However, the micro-nanostructures obtained by the template method can only be constructed on a specific substrate, and the binding force with the substrate is poor, so in recent years, the adhesive + coating strategy has been developed, which can be applied to a variety of hard or soft substrates by combining the surface micro-nanostructures with the substrate through adhesives or resins, thus expanding the application scope of superhydrophobic surfaces. The armor strategy is to use materials with excellent mechanical properties to directly protect the fragile micro-nano structures, which can significantly improve the mechanical damage resistance of superhydrophobic surfaces. For the damaged superhydrophobic surface, the self-repair strategy can restore the performance of the superhydrophobic surface, and the repair methods are various, which can prolong the service life of the superhydrophobic surface. In addition, based on the principle of intrinsic durability of superhydrophobic surface, better superhydrophobic repair effect can also be provided by supplementing or replacing the surface air cushion. The advantages and limitations of the above strategies are highlighted below.
3.1 Micro-nano structure regulation
In order to obtain a superhydrophobic surface with stable Cassie state, it is necessary to reduce the critical contact angle θc, increase the surface roughness, and reduce the solid-liquid component contact area ratio, which can be met by constructing micro-nano hierarchical structures on the surface[29,30]. Compared with the simple microstructure, the micro-nano hierarchical structure is similar to the "terrace", which can capture and store more air, reduce the contact area between the droplet and the solid surface, and improve the stability of the liquid-gas interface, thus greatly reducing the apparent surface free energy of the material and inhibiting the transition from Cassie state to Wenzel state[31][32][33][34,35]. According to the Cassie equation, the super-hydrophobic property with large contact angle and small rolling angle can be realized. In addition, the micro-nano hierarchical structure has better mechanical wear resistance, thereby improving the service life of the superhydrophobic surface[36,37].
Jung et al. Blended and sprayed natural paraffin-modified carbon nanotubes (CNT) and epoxy resin onto the micron structure to obtain a micro-nano hierarchical structure with a static contact angle and a contact angle hysteresis of 173 ° and 1 °, respectively, approaching the contact angle limit of Cassie's theoretical model, showing an excellent superhydrophobic effect[38]. Thanks to the good mechanical properties of CNT and the good bonding ability of epoxy resin, the wear resistance and impact resistance of the superhydrophobic surface are significantly improved. According to Cassie's theoretical model, Liu et al. Constructed biconcave structures on various hydrophilic substrates only by geometric morphology design without adding low surface energy substances (Fig. 3)[39]. The structure effectively reduces the solid-liquid contact area ratio, has good repelling effect on fluorine-containing solvents with low surface energy, and shows excellent super-amphiphobic property.
The template method is often used to directly control the micro-nano hierarchical structure on the substrate, and the geometric morphology with excellent superhydrophobic characteristics is designed according to the bionic concept, which has advantages in theoretical exploration and verification[40]. However, the micro-nano hierarchical structure has poor adhesion to the substrate and weak resistance to mechanical damage, which limits the practical application of superhydrophobic surfaces[41].
3.2 Adhesion + coating
In order to reduce the dependence of superhydrophobic surface on the substrate and strengthen the interfacial bonding force, the strategy of adhesive + coating was developed to prepare superhydrophobic surface. The surface super-hydrophobic layer and the substrate are connected by an intermediate layer, and the intermediate layer can anchor a micro-nano structure of the surface and also can be used as a shielding layer to additionally protect the substrate, so that the mechanical property of the super-hydrophobic surface is obviously improved, and a durable super-hydrophobic surface can be prepared on various substrates. There are two main choices for the middle layer, one is organic materials, such as commercial adhesives, rubber, resin, etc. Second, inorganic materials, such as inorganic adhesives, ceramic oxide layers and so on.
The intermediate layer of organic materials has been developed rapidly because of its mild preparation conditions and strong adaptability[42,43]. Lu et al. firstly proposed the strategy of "adhesion + coating" to construct a durable super-hydrophobic surface. Firstly, the commercial adhesive was coated on the substrate surface, and the nano-TiO2 was modified by fluorosilane with low surface energy. Then, the modified nano-TiO2 was dispersed in ethanol solution and sprayed on the adhesive to obtain a super-hydrophobic layer[44]. The super-hydrophobic layer can be well adhered to the substrate by the adhesive, and the surface contact angle is still maintained at more than 160 degrees even after finger wiping, water droplet impact and 40 times of abrasive paper cycle abrasion, showing good durability. Adhesives have good adhesion on both hard substrates (metal and glass) and soft substrates (fabric and paper), so the adhesion + coating strategy can prepare superhydrophobic surfaces on a variety of substrates, which greatly expands its application scope. Li et al added alkylsilane-modified nano-SiO2 as a filler to the suspension of epoxy resin and polydimethylsiloxane (PDMS), and then sprayed it on the Mg substrate to prepare a super-hydrophobic coating[45]. Epoxy resin with excellent adhesion and PDMS with low surface energy were used in the coating.The superhydrophobic property of the coating was stable after scraping and taping, and the coating still showed superhydrophobic property after 120 min and 60 min in boiling water and liquid nitrogen at 100 ℃, respectively. Wu et al. Used fluorosilane modified epoxy resin as primer, and blended nano-Al2O3/ modified epoxy resin as topcoat[46]. The super-hydrophobic surface with the best durability can be obtained by adjusting the pre-curing time of the primer. After 600 times of tape peeling and 550 times of linear wear, the coating still maintained good superhydrophobicity, which was mainly attributed to the excellent adhesion between epoxy resin and substrate and filler, as well as the wear resistance of the Al2O3 itself.
The organic interlayer is difficult to withstand the high temperature environment and has certain limitations, while the inorganic material as the interlayer can make the superhydrophobic surface applied in extreme high temperature conditions. Liu et al. Used aluminum phosphate as an inorganic adhesive, blended with nano-TiO2 and long-chain alkyl silane, and sprayed them on various substrates to obtain super-hydrophobic surfaces[47]. Thanks to the chemical stability, high temperature resistance and good adhesion of aluminum phosphate, the contact angle of the superhydrophobic layer is stable at more than 150 ° after repeated abrasive paper abrasion and sand impact, and the superhydrophobic layer still maintains superhydrophobicity after being immersed in 80 ℃ oil, water and 50 ℃ acetone for 24 H. Wang et al. used plasma high temperature liquid pulse technology to grow a papillate inorganic ceramic skeleton on the surface of the alloy, and then used crosslinking reaction to load polytetrafluoroethylene (PTFE) nanoparticles with intrinsic low surface energy on the surface of the ceramic skeleton to construct a micro-nano rough structure, thus obtaining an organic/inorganic composite super-hydrophobic layer with a multi-layer structure (Figure 4)[48]. The ceramic skeleton can be used as an intermediate layer to firmly combine the surface super-hydrophobic layer and the alloy substrate together, and in addition, the ceramic skeleton has excellent mechanical properties and high temperature resistance, and PTFE has chemical stability, so that the composite super-hydrophobic material can still maintain a super-hydrophobic state under extreme conditions such as friction and wear, strong acid, strong alkali, simulated seawater, high temperature of 500 deg C and the like.
The gluing + coating strategy has wide selectivity to the substrate, remarkably improves the bonding force between the super-hydrophobic layer and the substrate, and can be used for preparing the super-hydrophobic surface on a large scale by a spraying method, thereby reducing the production cost. This strategy has been commercialized, but it also has some limitations, such as the super-hydrophobic layer is damaged by external mechanical friction and chemical fouling, which greatly shortens its service life.
3.3 Armor Protection
Armor protection strategy is to use materials with excellent mechanical properties to protect the surface micro-nano structure, similar to the function of armor, now mainly using nano-armor and micro-armor two methods. Sheng et al. Proposed the concept of nanoscale armor protection, using a bottom-up preparation strategy, that is, growing a hard nano-thin layer on the fragile micro-nano structure of the superhydrophobic surface[49]. The existence of the nano-thin layer does not change the micro-nano rough structure and does not affect the super-hydrophobicity of the coating, and can avoid the direct contact between the fragile micro-nano structure and the external mechanical scratch. The strategy was as follows: firstly, the honeycomb-like micro-nanostructures were fabricated on the copper substrate by electrodeposition, and then the superhydrophobic samples were immersed in aluminum dihydrogen phosphate (ADP) solution for many times by dip-coating.The transparent and hard ADP was used as an inorganic adhesive to adhere to the surface of the micro-nanostructures on the copper substrate, followed by heat treatment at 120 ℃/2 H and 240 ℃/1 H. It is found that the microhardness of the surface protected by ADP is 10 times higher than that of the original surface, and the ability to withstand sand impact is 2.5 times higher. The finite element stress analysis shows that the transverse deformation and stress of the micro-nano structure are reduced after the armor protection. Wang et al. adopted the idea of micron-level armor protection, adopted a top-down strategy, comprehensively considered the two major factors of surface wettability and mechanics, and through simulation analysis and practical verification, summed up three geometric design principles of continuous structure, low solid contact area and large sidewall angle (Figure 5)[50]. On this basis, micro-scale continuous structures were constructed on silicon, metal, ceramic and glass substrates by photolithography and cold and hot processing technologies, and fragile hydrophobic nanoparticles were filled between the micro-scale structures. In the cyclic friction and wear test, tape peeling test and jet impact test, the continuous and stable micron-scale structure can protect the nanoparticles from the damage of external scratch and wear, and maintain a stable Cassie state. Finally, they also proposed a strategy for large-scale preparation of armored superhydrophobic surfaces by roll-to-plate imprinting technology, which is expected to promote the industrialization of superhydrophobic surfaces.
The armor protection strategy provides a new idea for the preparation of durable super-hydrophobic surface, which can effectively protect the micro-nano hydrophobic layer at the bottom and has broad prospects in large-scale production. However, the preparation method of nanoscale armor is complex, and microscale armor is mainly used on large-size hard substrates, which is difficult to play a role in the preparation of flexible substrates or small-size surfaces. At present, the research on armor protection strategy is still in its infancy, and further exploration is needed.
3.4 Self-repairing
The above strategies are mainly used to prepare wear-resistant superhydrophobic surfaces, while the self-repair strategy focuses on the restoration of superhydrophobic function after damage and prolongation of its service life. Surface superhydrophobicity is mainly determined by two factors, one is the micro-nano hierarchical structure, the other is the low surface energy material. In view of these two points, the self-repair strategies can be subdivided into the following categories: self-repair of low surface energy materials, self-repair of micro-nano structures, double self-repair of low surface energy/micro-nano structures, and self-complementary repair of the body. These strategies are introduced one by one as follows.
Low surface energy substances on superhydrophobic surfaces are degraded under the conditions of strong acid, strong alkali, UV irradiation and heating, or contaminated by other high surface energy substances, resulting in the increase of surface energy and the loss of superhydrophobic properties, so it is necessary to repair the damaged low surface energy substances in time[52]. Wang et al. First used silicone oil as a template to construct mesoporous hollow SiO2 microspheres by one-pot method, filled with mineral oil, and coupled the core-shell structure with KH-550[53]. When the microspheres are added into waterborne coatings such as polyurethane as fillers, excellent superhydrophobic effect can be obtained. When the hollow spheres on the surface of the coating are damaged by external wear, hydrophobic silicone oil is released from the hollow spheres to repair the damaged surface and quickly restore the superhydrophobicity of the surface (Figure 6). Wang et al. Used lotus leaf as a template to prepare PDMS inverted mold, and then blended paraffin and PDMS to form a film. Due to the low surface energy of paraffin and PDMS and the micro-nano structure constructed by the template method, the contact angle of the film reached 160 °[54]. Plasma is used for simulating and damaging the surface of the super-hydrophobic film layer, the film layer is changed into a super-hydrophilic state after a substance with low surface energy is damaged, and paraffin molecules can migrate from the inside of a body to the surface of the film layer at 25 deg C by utilizing the low boiling point property of paraffin and the three-dimensional structure of PDMS and the characteristic of high molecular mobility,Without human intervention, the contact angle can be restored to 160 ° within 20 min, and the superhydrophobic property of the film remains stable after 8 cycles of self-repair.
The surface micro-nano hierarchical structure can capture and stabilize the air layer to obtain the super-hydrophobic effect. However, under the external mechanical wear, the friction pair contacts with the top of the micro-nano structure in a small area, which leads to stress concentration, the micro-nano structure is easy to collapse and wear flat, and the stored air layer escapes and loses superhydrophobicity, so the effective self-repair of the micro-nanostructure becomes the key to prolong the superhydrophobic service life. Zhang et al. used the immersion method to restore the micro-nano structure, and added fluorosilane-modified SiO2 as a filler to the fluorocarbon resin. When the content of SiO2 exceeded 25 wt%, the coating showed superhydrophobicity[55]. The superhydrophobicity of the coating is lost after the micro-nano structure of the coating surface is abraded by external mechanical abrasion. However, the coating was soaked in a dilute solution of butyl acetate for 15 min and then dried at room temperature for 24 H. Through the selective dissolution of the diluent, the internal SiO2 nanoparticles were exposed from the matrix, the scratch defects on the coating surface disappeared, the micro-nano structure basically recovered to the original morphology, and the contact angle of the coating surface increased to more than 150 °. Guo et al. Utilized the characteristics of shape memory polymer to restore the micro-nano structure of the damaged superhydrophobic surface, deposited epoxy resin and PDMS shape memory polymer by chemical vapor deposition method at 290 ~ 350 ℃ to form a micro-nano hierarchical structure, combined with the low surface energy characteristics of PDMS itself, to obtain a superhydrophobic surface with a contact angle of more than 160 °[56]. When the worn surface of the micro-nano rough structure loses its superhydrophobicity, the mobility of the molecular chains in the shape memory polymer increases after heating at 85 ℃ for 2 min, and the damaged molecular structure returns to its original state (Figure 7). AFM results show that the surface morphology and roughness are basically the same before and after recovery, and the surface shows superhydrophobicity again macroscopically. Cao et al. Prepared superhydrophobic coatings by blending supramolecular silicone polymers with nano-SiO2 particles and spraying them into films[57]. A simulated defect with a width of 10 μm and a depth of 60 μm was made on the surface of the coating with a graver, and the scratched area was completely healed under the condition of heating at 120 ℃ for 3 min. The superhydrophobicity of the coating was lost after 100 times of sanding, but the worn surface was restored to the original micro-nano rough structure and superhydrophobicity after heating at 100 ℃ for 30 min. This is because when the temperature is higher than the glass transition temperature of the polymer, a large number of hydrogen bonds and coordination bonds are activated to promote the phase transition of the polymer and restore the original morphology and integrity of the coating.
Only from the low surface energy or micro-nano structure to carry out unilateral self-repair, often attend to one thing and lose another, the development of double self-repair superhydrophobic surface has become a new development direction[58]. For the first time, Lv et al prepared a super-hydrophobic surface with shape memory polymer (SMP), which can simultaneously repair damaged micro-nano structures and low surface energy substances[59]. The method comprises the following steps of: firstly, constructing a micro-nano columnar structure surface by using a template method, and modifying by using fluorosilane to obtain an SMP super-hydrophobic layer. Then, under the dual destruction of external pressure and O3 plasma, the surface micro-nano pillars are flattened into scale-like plates, the low surface energy substances are oxidized, hydrophilic OH groups appear on the surface, and the contact angle is reduced to less than 60 degrees. When heated above the glass transition temperature of the polymer, the molecular chain migration of SMP is activated, and the molecular conformation tends to return to the original stable structure thermodynamically. In addition, the surface energy difference between the polymer surface and the contact medium leads to the diffusion of hydrophobic groups such as methyl, methylene and phenyl in the film to the surface layer, and the molecular structure of the surface is reorganized, thus restoring the low surface energy characteristics of the film. Therefore, above the glass transition temperature, the micro-nano structure of the SMP surface and the low surface energy substances recover synchronously, and finally the film returns to the initial superhydrophobic state from the hydrophilic state. Pan et al. Also constructed the micro-nano pillar structure on the surface of SMP by template method, and then filled the pH-responsive capsule encapsulated with fluorosilane into the micro-nano pillar on the surface of SMP[46]. When the superhydrophobic state of the surface is destroyed, the micro-nano columnar structure can be reconstructed and the low surface energy can be restored by heating. Fluorosilanes in pH-responsive capsules can be released under acidic conditions and also play a role in the repair of low surface energy materials (Fig. 8).
Different from the previous self-repair strategies, the three-dimensional superhydrophobic surface developed in recent years can restore superhydrophobic properties after bulk self-repair. The idea is to endow the coating or block with superhydrophobicity as a whole, and when the superhydrophobic layer on the surface is damaged, the new surface exposed by mechanical friction still has superhydrophobicity, so as to achieve self-repair effect[60⇓⇓⇓~64]. Qing et al. used three-dimensional porous iron as a carrier, blended fluorinated epoxy resin and hydrophobic nano-SiO2 and filled them into the porous iron, and added KH-550 to improve the bonding strength between the organic coating and the inorganic filler to prepare a three-dimensional super-hydrophobic block (Figure 9)[63]. After 500 m friction travel, the thickness of the block is reduced by 1. 1 mm.Because the superhydrophobic material is a blocky whole, the exposed fresh surface after wear still maintains the physical and chemical properties of the original surface, and still shows good Cassie state. Peng et al. Blended fluorinated epoxy resin with perfluoropolyether as the resin substrate, and used polytetrafluoroethylene nanoparticles as the filler to build a micro-nano rough structure to prepare perfluorinated superhydrophobic coatings[64]. Due to the excellent chemical and mechanical stability of the perfluorinated organic coating, the surface is still in Cassie state after immersion in aqua regia for 60 min. In that continuous wear process, the coat is continuously thinned, the exposed new surface can still maintain the micro-nano structure, and the element components are basically unchanged, so that the self-supplement effect of the body is achieve, and the service life of the super-hydrophobic surface layer is remarkably prolonged.
Self-repair strategy can respond to sound, light, electricity, heat and other stimuli to repair the damaged superhydrophobic surface[65]. However, the current self-repair methods for low surface energy and micro-nano structures are complex, expensive and difficult to apply in engineering. The bulk self-replenishing and self-repairing strategy is relatively simple and inexpensive, but it requires a large number of fillers to build a three-dimensional superhydrophobic surface, which increases the defects in the coating and reduces the interfacial bonding strength and mechanical strength.
3.5 Air film supplement or replacement
According to Cassie theory, the fundamental reason for the superhydrophobicity of the surface is the air layer stored in the micro-nano structure, and the failure of the superhydrophobicity is essentially the escape of the air layer, and the surface changes from Cassie state to Wenzel state. Therefore, the damaged air layer is replenished, and the superhydrophobicity of the surface can be restored[66⇓~68]. Lee et al. First constructed a micro-nano structure to obtain hydrophobic properties, then placed the gold wire film on the surface as a cathode, used the copper wire as an anode, and electrolyzed in water with an external power supply[66]. The wetted state of the surface is equivalent to a switch. When the surface is wetted by water, the external circuit is turned on, the gas generated by electrolysis restores the air layer between the micro-nano structures, and the Cassie state reappears on the surface. The transition process is shown in Figure 10. The gas produced by electrolysis can automatically adjust its pressure to adapt to the pressure of the liquid, and can maintain the stability of the gas film even under 7 standard atmospheres, which is expected to be applied in the field of deep water drag reduction and anti-fouling.
According to the metastable wetting theory, the Wenzel state is thermodynamically more stable and requires additional energy to overcome the energy barrier to recover to the Cassie state. The film replenishment strategy consumes a lot of energy and has limited effect, so it gradually fades out of people's vision. Then researchers took a different approach, replacing the original air layer with a lubricating liquid, and developed a smooth porous liquid injection surface (SLIPS)[69⇓~71]. When Aizenberg et al observed pitcher plants, they found that insects could slide along the edge to the digestive juice at the bottom. The reason was that the surface of pitcher plants was covered with a layer of oil film. According to this phenomenon, the concept of SLIPS was first proposed[69]. Unlike the lotus leaf effect superhydrophobic surface, SLIPS is to inject lubricant into the micro-nano structure to replace the original air layer. Compared with the traditional super-hydrophobic surface, SLIPS can not only be hydrophobic, but also repel a variety of oily liquids, and repair mechanical damage through rapid leveling, so the lubricating liquid shows more stable characteristics than the original air layer under high pressure and external impact, but it also has the problem that the lubricating layer can not be effectively replenished after natural volatilization. On the basis of SLIPS, Jiang Lei and others used self-replenishing organic gel (OG) as a reservoir to replace the micro-nano structure substrate in SLIPS, which can be used for the adhesion of waxy oil on the inner wall of oil pipelines[72]. The three-dimensional network structure in the organic gel can absorb oily substances in the surrounding environment, form a surface lubricating layer in a self-replenishing manner, and significantly reduce the adhesion of waxy oil stains, thereby achieving a self-cleaning effect. Jiang et al. First constructed a micro-arc oxidation layer (PEO) on the surface of magnesium alloy as the bottom layer, and then grew a layered double hydroxide film (LDH) on the surface.And then that perfluoropolyether is inject into the LDH to serve as a lubricating lay, so that the double effects of encapsulating PEO holes on the bottom lay and surface super-lubrication are achieved, and the corrosion resistance of the magnesium alloy can be comprehensively improved by combining with a corrosion inhibitor in the LDH[2,73,74][2]. After a decade of rapid development, SLIPS technology has been widely used in the fields of anti-icing, decontamination and anti-corrosion, and has become an important branch of the development of superhydrophobic surfaces[75][76][77].
4 Evaluation Method of Durable Superhydrophobic Surface
The previous paper mainly describes how to improve the durability of superhydrophobic surfaces, but the difference of preparation strategies also leads to the difference of superhydrophobic properties. For the same super-hydrophobic surface, its durability is different under different external damage stimuli, so it is necessary to establish a unified evaluation method of super-hydrophobic durability[78]. At present, there are various methods to evaluate the durability of super-hydrophobic surface, which can be summarized into two aspects: one is mechanical durability evaluation, which mainly includes friction and wear test, tape peeling, impact test and cycle washing; The second is chemical durability evaluation, which mainly includes UV irradiation, temperature test, acid-base test and solution immersion[79]. In order to comprehensively evaluate the mechanical wear resistance and chemical stability of superhydrophobic surfaces, it is often necessary to combine the above evaluation methods[80,81]. The durability of different superhydrophobic surfaces will be evaluated from these two aspects to provide a basis for establishing a unified evaluation standard.
4.1 Mechanical Durability
According to Cassie's theory, one of the keys to achieve superhydrophobic durability is to improve the mechanical stability of the surface micro-nano hierarchical structure[82]. There are three main ways to cause mechanical damage to micro-nano structures, namely, transverse friction shear failure, adhesion weakening and vertical dynamic impact, which correspond to friction wear test, tape peeling test and impact test respectively. The schematic diagram of mechanical damage test is shown in Figure 11. For fabric superhydrophobic surfaces, a cycle wash test is also required[83].
Friction and wear tests include linear wear and rotational wear. Linear wear is to simulate mechanical damage by reciprocating motion on the same contact line, and the durability failure process of super-hydrophobic surface under different friction conditions is simulated by adjusting the load, abrasive particle size and friction distance, which has become a common means to quantitatively evaluate the wear resistance of super-hydrophobic surface[84,85]. The common way to adjust the load is to change the mass of the weight, and the change of the abrasive can be realized by choosing sandpaper with different particle sizes. Li et al. Studied the effects of superhydrophobic coatings prepared with different filler particle sizes on surface morphology and hydrophobic properties under the same load, different abrasive particle sizes and friction distances[86]. The results show that the larger the filler particle size is, the better the wear resistance is. When the roughness of the abrasive is greater than that of the superhydrophobic coating, the new surface exposed after friction still has superhydrophobicity, otherwise the coating will lose superhydrophobicity. Rotational wear testing is similar to linear wear, but is not commonly used because of the difference in friction speed due to the different radii of rotation of the friction area during rotation[64]. Adhesive tape peeling can be used to measure the adhesion of micro-nano particles on the substrate, and the relationship between the number of adhesive tape peeling and the surface contact angle can be used to evaluate the anti-peeling performance of the surface[87]. Zhao et al. Used SEM to characterize the micro-morphology of the surface of the super-hydrophobic coating before and after tape stripping, compared the number of nanoparticles on the surface per unit area before and after stripping, qualitatively evaluated the bonding strength between particles with different particle sizes and epoxy resin substrates, and optimized the durability of the super-hydrophobic surface by adjusting the ratio of fillers with different particle sizes[88].
Under external mechanical impact, the air layer in the microstructure of the superhydrophobic surface can cross the energy barrier and escape from the superhydrophobic surface, resulting in the transition from Cassie state to Wenzel state. The stability of the air layer on the super-hydrophobic surface was evaluated by comparing the relationship between the height, speed, number of impacts and the total mass of alluvium and the surface contact angle[89,90]. Liu et al comprehensively evaluated the impact resistance of superhydrophobic surfaces by recording the change of surface contact angle with the impact time of water column and the number of sand impact cycles[91]. The results show that the Cassie state is still maintained after 12 H of water column impact, but the Cassie state is unstable after 2 sand impact cycles, and the Wenzel state is completely transformed after 10 sand impact cycles, which indicates that the damage of sand impact to superhydrophobic properties is much greater than that of water column impact. Fabric based superhydrophobic surfaces are mostly used in the textile industry, and cyclic washing is the main reason for the failure of superhydrophobic properties[92,93]. Xue et al. Used stearylamine low surface energy compound to modify polyester fiber and PDMS as adhesive to evaluate the difference of wear resistance of superhydrophobic surface by cyclic washing method[94]. The results showed that the superhydrophobic fabric adhered by PDMS still maintained good Cassie state after 120 cycles of washing, which significantly improved the mechanical stability of the superhydrophobic fabric surface.
4.2 Chemical Durability
The stability of superhydrophobic surface depends not only on the micro-nano hierarchical structure, but also on the modification of low surface energy materials, so whether the low surface energy state can be maintained also determines the durability of superhydrophobic surface. There are three main mechanisms for the destruction of the low surface energy state: first, the low surface energy material is destroyed and lost along with the micro-nano structure; Second, the low surface energy substance fails after reacting with the corrosive medium; The third is the natural aging of low surface energy materials.
The first failure mechanism is related to the mechanical stability of the micro-nano structure, which has been described above and will not be described again. The second damage mechanism can be used to evaluate the chemical stability of the surface with different pH solutions and soaking time[95,96]. Saddiqi et al. prepared superhydrophobic surfaces with silicone nanofibers and immersed them in strong acid and alkali solutions at pH = 2 and pH = 14, respectively[97]. After soaking in strong acid solution for 5 days, the surface contact angle decreased from 170 ° to 127 °, and the hydrophobicity remained good. In the same immersion time, the contact angle of the surface in strong alkali environment decreased to less than 90 °, showing hydrophilicity. Combined with SEM, it can be seen that the silicone nanofiber-based superhydrophobic surface can maintain good chemical stability in acidic conditions, and the surface morphology is relatively complete. However, in alkaline environment, the surface morphology is severely etched. Yin et al. Used ZIF-8 as filler to prepare superhydrophobic coatings to improve the corrosion resistance of magnesium alloys, and immersed the coating samples in 3. 5 wt% NaCl solution to investigate the durability of superhydrophobic coatings in simulated seawater[98]. The coating can maintain the superhydrophobic property within 3 d immersion time, and the surface contact angle of the coating is still above 130 ° after 7 d. Combined with electrochemical impedance spectroscopy (Eis) test, it was found that good hydrophobicity was the key to improve the corrosion resistance of magnesium alloy.
The third damage mechanism can be tested by UV irradiation and temperature and humidity chamber to simulate the aging environment. Huang et al. Used polytetrafluoroethylene (PTFE) particles to prepare powder coatings without solvent and low surface energy modification[99]. Due to the high bond energy and chemical inertness of PTFE, the surface contact angle of the coating remained above 160 ° after UV irradiation for 84 H, showing excellent chemical durability. Lin et al used dopamine to combine CNT and MgO into composite nano-fillers, formed aluminum phosphate network through crosslinking reaction and hydrogen bonding, and dispersed the fillers in waterborne PTFE coating to prepare superhydrophobic surface[100]. The coating surface can maintain a stable Cassie state in the high temperature range of 350 ℃, which is attributed to the excellent chemical inertness and high temperature resistance of both the inorganic composite nano-filler and the organic coating.
In order to compare the durability of superhydrophobic surfaces under different preparation strategies more intuitively, the mechanical stability is characterized by wear resistance, and the chemical stability is characterized by acid and alkali resistance, as shown in Table 1. It should be noted that the durability of superhydrophobic surfaces prepared by gas film supplementation or replacement strategies can not be characterized by wear and acid and alkali immersion tests.
5 Existing problems
Durable superhydrophobic surfaces have great commercial value in many fields, but there are still some problems to be solved. For example, the explanation of superhydrophobic state transition mostly relies on the classical Cassie and Wenzel theoretical model, and the dynamic mechanism under the stimulation of environmental damage needs to be further improved. In addition, the existing durable superhydrophobic surface preparation process is complex, costly, and the raw materials are not environmentally friendly, which hinders its large-scale application. Superhydrophobic durability can be evaluated from two aspects of mechanical stability and chemical stability, but the specific parameters of evaluation methods are quite different, and it is difficult to form a unified evaluation standard. At present, the indoor accelerated test is often used to evaluate the durability of super-hydrophobic surface, but the durability data of super-hydrophobic surface under actual working conditions are lacking for verification.The mathematical model between accelerated test and field test has not yet been established, and the aging factor acceleration ratio in accelerated test is not clear, which weakens the advantage of rapid evaluation of indoor accelerated test.
6 Conclusion and prospect
The commercial application of superhydrophobic surface depends on its durability under service conditions. In this paper, three wetting models including classical wetting, metastable wetting and contact line theory are introduced, and the preparation strategies of durable superhydrophobic surface are introduced, and the advantages and disadvantages of each preparation strategy are compared and analyzed. Then, several commonly used superhydrophobic durability evaluation methods are explained in detail from two aspects of mechanical stability and chemical stability. Finally, by summarizing the current problems of durable superhydrophobic surface, the future research of durable superhydrophobic surface is prospected as follows.
(1) By studying the dynamic process of superhydrophobic state transition under different external stimuli, the failure mechanism model of superhydrophobic surface is constructed, which provides theoretical guidance for the development of new preparation strategies.
And (2) the preparation process is further simplified, the production cost is reduced, and environment-friendly raw materials are used, for example, toxic fluorine-containing materials are replaced by siloxane, and water-based resin is used as a substrate, so that the preparation process of the super-hydrophobic material is more environment-friendly.
(3) By comparing the failure process of superhydrophobic materials in indoor accelerated test and outdoor actual working conditions, the acceleration ratio of aging factors is defined, and a unified superhydrophobic durability evaluation standard is established, which provides a basis for the rapid evaluation of superhydrophobic performance.
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