What is superhydrophobic? The principle and application of superhydrophobicity on material surfaces, superhydrophobic preparation technology

Due to the superhydrophobicity and self-cleaning properties exhibited by the “lotus leaf effect” of lotus leaves and water striders, people have launched in-depth research on superhydrophobic materials. The discovery of superhydrophobic materials has created favorable conditions for some underwater operations and the use of electronic products in humid environments. Common superhydrophobic materials on the market are often made of fluoropolymers such as PTFE and fluorinated polyethylene. Since fluoropolymers have excellent weather resistance, durability and chemical resistance, superhydrophobic materials made of fluoropolymers are easier to resist corrosion and the coating material has a relatively longer life.
However, due to the high price of fluoropolymers in the market, the cost of producing superhydrophobic materials is high and it is difficult to be widely circulated and used in the market. Therefore, we are looking for materials with more suitable prices and better material properties to prepare superhydrophobic materials. It is one of the current development directions of superhydrophobic materials.
1 Superhydrophobic material

1.1 Concept of superhydrophobic materials
We define hydrophobicity as the phenomenon that the liquid shows non-wetting to the solid surface when the material surface contact angle θ>90°, and the stable contact angle θ>150° of the material surface and the rolling contact angle α<10° Such materials are superhydrophobic materials. Based on the different adhesion between droplets and surfaces in the superhydrophobic state, the forms of superhydrophobic materials are divided into five states: Wenzel state, Cassie state, “lotus leaf state”, Wenzel-Cassie state and “gecko state”. Among these five forms, the adhesion between water droplets and the surface is extremely small, and the “lotus leaf state” with a rolling angle α <10° is currently the most widely studied state of superhydrophobic materials [1]. 1.2 Principle of superhydrophobicity The main factors affecting the wettability of solid surfaces are the chemical composition and surface microstructure of the surface. We generally use the contact angle θ to express the wettability of the liquid phase and the solid phase: if θ<90°, the material is said to be non-hydrophobic and wettable; if θ>90°, the material is said to be hydrophobic and non-wettable; θ=90° is the critical condition for the material to be wettable or non-wettable. The contact angle θ can also be described by Young [2] equation (Figure 1).

Figure 1 Young model

Fig.1 Young model

In the figure, YSV, YLV, and YSL represent the interfacial tensions of the solid-gas interface, solid-liquid interface, and liquid-gas interface respectively. The interaction of the three interfacial tensions is in equilibrium at this time.
Since Young’s equation is based on an ideal solid surface, in fact, all solid surfaces will have a certain degree of roughness, which makes the Young’s equation inconsistent with the actual situation. Rough surfaces will have a certain impact on superhydrophobic performance, causing the calculated value of Young’s equation to be very different from the actual value.

At present, only the Wenzel model, in which the liquid is in direct contact with the concave and convex surfaces of the solid surface microstructure, and the Cassie model, in which the liquid is only in contact with the convex surfaces of the solid surface microstructure, consider the influence of roughness and are relatively mature theories today [3 ] Figure 2.

Figure 2 Wenzel model and Cassie model[2]

Fig.2 Wenzel Model and Cassie Model

1.2.1 Wenzel model
In the figure, the liquid completely occupies the grooves of the rough surface, and the liquid contacts the concave and convex surfaces of the solid microstructure, making the actual contact area between the liquid and the solid much larger than the apparent geometric contact area, enhancing the hydrophobicity.
In the equation, r represents the material surface roughness factor, which is the ratio of the actual contact area to the apparent area, r≥1; θw is the apparent contact angle of the rough surface.
A solid surface with a certain roughness will enhance the hydrophobicity of the hydrophobic surface of the material, and will also enhance the hydrophilicity of the hydrophilic surface of the material.
1.2.2 Cassie model
The liquid is suspended on the grooves of the rough surface, but cannot fill the grooves of the rough surface and can only come into contact with the convex surfaces of the rough surface.
From f1+f2=1, θ2=180°, the above formula can be transformed into:
θ’ is the apparent contact angle in the model; f1 is the contact ratio between the liquid surface and the air, f2 is the contact ratio between the solid surface and the air; relative θ1 is the contact angle between the liquid and the air, and θ2 is the contact angle between the solid surface and the air. Contact angle of air.
Combining the two models, it can be seen that the larger the contact angle and the smaller the rolling angle, the better the superhydrophobicity of the material surface will be [4].
2 Preparation method of superhydrophobic materials

The two factors that determine the superhydrophobic properties of materials are the roughness of the solid surface of the material and the chemical composition of the material. Therefore, it can be seen that there are two ways to prepare superhydrophobic materials: (1) modifying low surface energy substances on the surface of rough structures; (2) forming an appropriate rough structure on the surface of low surface energy materials.
2.1 Phase separation method
Disperse a certain solid in a liquid or another solid to obtain a stable mixture system, and then change the experimental conditions to make the stable mixture system form two or more phases. After phase separation, materials with superhydrophobic surfaces can be prepared [5- 6].
Liu Hailu [7] and others prepared a superhydrophobic acrylic polyurethane hydrophobic film using a phase separation method. After heat treatment, the surface of the material can be converted from superhydrophobic to hydrophilic through certain adjustments. The phase change causes changes in the optical diffraction characteristics of the material, which has certain application prospects in the field of photonic crystal patterned anti-counterfeiting.
This method has lower experimental requirements, easy to control experimental conditions, simple operation, and can prepare large-area superhydrophobic surface materials. Therefore, it is more common to use this method to prepare superhydrophobic surfaces.
2.2 Template method
A material with a rough surface or a hole structure is used as a template, and the film-forming liquid is formed on the template by coating, pouring, etc., and then the template is removed to obtain a film with a superhydrophobic surface.
Huang Junjie et al. [8] used the water droplet template method to successfully prepare polymer porous membranes with viscous superhydrophobic properties, regular structure, and controllable pore size. Such materials can be used in fields such as non-destructive liquid transmission and biochemical separation. This method is simple to operate, low in cost, and highly effective, so it has relatively good application prospects.
2.3 Electrochemical deposition method
Placing a solid material in an electrolytic cell loop causes the material to undergo an oxidation reaction, causing ions in the solution to deposit on the surface of the solid material, forming a superhydrophobic surface.
Liu Chunyue et al. [9] prepared superhydrophobic cotton fabric materials through electrochemical deposition technology, which allowed the textile to form a superhydrophobic surface and have special performance properties, expanding the application range of superhydrophobic materials. This method has low experimental conditions and low cost, so it is widely used.
2.4 Sol-gel method
The precursor containing highly chemically active components is hydrolyzed into a sol, and a gel is prepared after a condensation reaction. The gel is then formed into a film on the surface of the substrate through certain treatments to obtain a surface with a superhydrophobic structure.

He Meiying et al. [10] used tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) as precursors to prepare a sol through the sol-gel method. After modification, they obtained ultra-low refractive index hydrophobic Film layer, this type of material has certain applications in the field of template agents due to its unique high cavitation efficiency and low refractive index properties.
The superhydrophobic surface prepared by this method has good comprehensive mechanical properties and has a variety of substrates, but the experimental conditions are relatively high, the operation is complicated, and the adhesion is relatively poor.
2.5 Spraying method
A method of spraying the previously prepared coating onto the surface of a solid material to form a superhydrophobic surface. Xu Lijie et al. [11] prepared a composite superhydrophobic coating by spraying the prepared mixture of SiO2 and silicone glue on the surface of the glass substrate.
2.6 Other methods
In addition to the above methods, hydrothermal methods [12], etching methods [13], layer-by-layer self-assembly methods [14], and one-step scalable plasma arc oxidation can also be used to prepare superhydrophobic coatings based on θ-Al2O3 [ 15], preparation of durable superhydrophobic coatings using sulfonated plasmonic photocatalysts [16] and other methods to prepare superhydrophobic materials.
3 Applications of superhydrophobic materials

Current superhydrophobic materials are mostly prepared with fluoropolymers, and the price of fluoropolymers is relatively high, making superhydrophobic materials prepared from fluoropolymers more expensive, and most of them are only conducted in the laboratory. preparation, but difficult to circulate in the market. Therefore, it is hoped that superhydrophobic materials prepared from relatively cheap silicone glue and other materials can be widely prepared and used [11].
Due to the poor surface mechanical stability of superhydrophobic materials, research and development of multifunctional superhydrophobic materials that can be reused is a major problem today. And it is hoped that more environmentally friendly superhydrophobic materials and superhydrophobic materials that can be used in different environments will be developed [38].
Entering a new era, materials are required to have better performance and wider applications, and preparation methods are gradually becoming simpler, easier to operate and easier to control. In recent years, some methods of synthesizing superhydrophobic materials proposed at home and abroad also consider relatively simple operations and the use of relatively cheap raw materials. However, some of these methods include a series of continuous multi-stage procedures, which require specific technical conditions and will greatly affect the quality of the materials. limiting their practical implementation on an industrial scale [39]. Therefore, future research on superhydrophobic materials needs to pay more attention to methods that use lower-priced and simple-to-operate materials while preparing methods with excellent performance and easy-to-control process conditions for practical applications.
Based on current research results, superhydrophobic materials can generally be used in the fields of oil-water separation, self-cleaning, medicine, anti-icing, textiles, anti-corrosion, cultural relic protection, prevention of pollutants in water, anti-reflection, etc. . However, the current research on superhydrophobic materials has not reached its ultimate and mature level. Therefore, developing superhydrophobic materials that are more suitable for the development of the new era and broadening the application fields of superhydrophobic materials are also the focus of current research on superhydrophobic materials [39].
And most of the methods for preparing superhydrophobic materials today are limited to laboratory and model theoretical processes. Therefore, how to truly put these methods into practice, successfully achieve commercialization, and make superhydrophobic materials truly circulate in the market is one of the next directions for the development of superhydrophobic materials [40].

3.1 Application in oil-water separation
The special hydrophobic and lipophilic properties of superhydrophobic materials cause the water in the oil-water mixture in the water body to be discharged when an external magnetic field is applied to such materials, and the oil is adsorbed on the surface of the material to achieve the purpose of oil-water separation [17-19].
3.2 Applications in medicine
The non-wetting properties of superhydrophobic materials make this type of material unique in antibacterial and self-cleaning properties. Therefore, the application of superhydrophobic materials in medicine to prevent bacterial infections has great potential.
Academician Jiang Lei is based on the phenomenon that air remains in the gap between superhydrophobic rough surfaces, minimizing the contact area between peptidoglycan and the substrate. These air layers are used to inhibit and reduce the adhesion of bacteria, thereby inhibiting the formation of biofilm and reducing the possibility of infection [20].
Superhydrophobic materials can also be used as matrices to control protein adsorption and maintain bacterial growth in the biomedical field. They can also become platforms for drug delivery devices and diagnostic tools.
Eric J. Falde[21] and others have discussed the application of superhydrophobic materials in the biomedical field, expanding the application scope of superhydrophobic materials.
3.3 Application in self-cleaning
Due to the phenomenon that water droplets on the lotus leaf gather and take away dust, making the surface of the lotus leaf relatively clean, people have learned about the self-cleaning properties of the surface of superhydrophobic materials.
Yu Wang[22] and others prepared a superhydrophobic material grafted onto wood with good wear resistance, self-cleaning ability and anti-mildew properties.
Ye Xiangdong et al. [23] applied superhydrophobic coatings to the protection of building walls. Particles and liquid pollutants attached to walls with superhydrophobic surfaces are easily cleaned by water flow, demonstrating the self-cleaning properties of superhydrophobic materials. .
At present, the self-cleaning properties of superhydrophobic materials are mainly reflected in the applications of outdoor glass [24], solar panels [25], etc.
3.4 Application in anti-icing
Jiang Guo and Chen Liang [26] invented a super-hydrophobic anti-icing composite material that can be bent and actively de-iced. It uses a vulcanized liquid silicone rubber matrix and micro-nano materials as materials at room temperature, using spray coating. This method has prepared a bendable superhydrophobic material that can actively photothermal de-icing and is combined with superhydrophobic anti-icing.
Guoyong Liu[27] and others used radio frequency magnetron sputtering to prepare nano-ZnO superhydrophobic surfaces that have good anti-frost and anti-icing properties when applied to aluminum alloys.
3.5 Applications in textiles
Applying the non-adhesion and self-cleaning properties of superhydrophobic materials to textiles allows textiles to have better properties to meet the needs of people’s daily lives [28].
Liyun Xu[29] and others used chemical vapor deposition method to prepare superhydrophobic cotton fabric, which has good adsorption capacity for different oils and organic solvents.
3.6 Application in corrosion protection
A large number of studies have shown that for the Cassie model, the superhydrophobic surface in the wet state has the following characteristics: the air prevents the intrusion of some corrosive ions or substances and the low superhydrophobic surface energy causes the corrosive liquid to be squeezed out of the surface voids. Better corrosion resistance [30].
3.7 Application in cultural relics protection
Also due to the self-cleaning and anti-wetting properties of superhydrophobic materials, superhydrophobic surfaces can prevent the adhesion and penetration of liquids and spontaneously clean attachments on the surface of cultural relics, thus making the superhydrophobic surface play a certain role in protecting cultural relics [ 31].
Yijian Cao [32] et al. took advantage of the good hydrophobicity, oleophobicity and self-cleaning properties of superhydrophobic materials and proposed the use of superhydrophobic materials to protect stone artworks. The use of superhydrophobic materials to protect stone artworks can protect the physical and chemical integrity of the artworks while retaining the artistic and aesthetic characteristics of these artworks, so that such artworks can be preserved for a longer period of time without being damaged. Leave information for future historical research.
3.8 Application in removing pollutants from water
Since traditional sewage treatment methods adsorb water while adsorbing pollutants, thereby reducing the material’s adsorption effect on pollutants, superhydrophobic materials can maintain hydrophobicity while adsorbing oily pollutants, reducing the adsorption effect on water. Adsorption increases the adsorption effect on pollutants [33].
Yuanfei Lv[34] prepared an antibacterial superhydrophobic membrane separation material with a layered lotus leaf micronipple structure to solve the problem of effective separation and treatment of industrial oily wastewater and oil spill accidents.
3.9 Other applications
Superhydrophobic materials can also be used for anti-seepage of earth-rock dams in cold areas [35], electrical sensing [36], and as fluid reinforcement materials in engineering applications [37].
4 Outlook


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