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Superhydrophobic property

Superhydrophobic theoretical model

Superhydrophobic properties of animals and plants in nature

Artificial preparation of superhydrophobic surface

Practical Application of Super hydrophobic Surface

01. Super hydrophobic characteristics when enjoying the haughty and beautiful black swans, we will find out why the black swans' feathers are not wet. To investigate the reason, this is inseparable from a knowledge point, super-hydrophobic characteristics.

Super hydrophobic property is a high-sounding word that can actually be seen everywhere in people's lives. If you have ever observed a lotus leaf, you will often notice that there is a perfect drop of water on the surface of the leaf. Not only does the surface of the leaves stay dry, but the water droplets also take away dust and debris, keeping the leaves clean.

This is because the nanoscale small "bumps" make the contact surface between the water droplets and the leaves actively small. This minimizes the attraction between water molecules and surface atoms and allows water to "bead" and roll off. This is the phenomenon of super hydrophobicity. So the superhydrophobic property is sometimes called the lotus effect [1].

02. The secret of superhydrophobic phenomenon in superhydrophobic theoretical model lies in low surface tension. In 1804, the British scientist T. Young put forward the concept of surface contact angle. The surface contact angle, as shown in Figure 1, is determined by air, water, and the surface tension of the contact surface. The better the surface wettability is, the smaller the surface contact angle is.

Figure-1 Young's equation and contact angle diagram Young's equation only describe the change of contact angle in an ideal case, but it does not exist in practice. Wenzel theory and Cassie theory further describe the influence of different surface roughness on the contact angle. The Wenzel equation reveals the relationship between the apparent contact angle and the intrinsic contact angle of the homogeneous rough surface, as shown by Equation 1.

Here is the roughness factor, defined as the ratio of the actual solid-liquid contact area to the apparent solid-liquid area. By introducing roughness factor, Wenzel equation explains a natural phenomenon, that is, surface roughness can enhance hydrophilic and hydrophobic phenomena. Rough surfaces tend to be more hydrophilic and hydrophobic than smooth surfaces.

Equation 1 Wenzel equation Cassie and Baxter further extend Young's equation. They believe that the rough surface will retain some of the air, and this thin layer of air will also have an effect on the antennae. The Cassie equation is defined as follows

Equation 2 Cassie equation when the contact surface of the droplet is composed of two substances with different chemical properties (air and solid surface), the intrinsic contact angle is expressed as sum, and the contact area is expressed by F1 and f2, respectively. In this way, the apparent contact angle can be calculated from the Cassie equation.

Cassie equation and Wenzel equation are not contradictory, they are the theoretical summary of docking antennae under different surface conditions respectively. Wang & Jiang [2] classified the state of droplets on the superhydrophobic surface into five types: Wenzel state, Cassie state, lotus leaf state (a special case of Cassie state), transition state (transition between Wenzel and Cassie state) and gecko state.

03. The super-hydrophobic characteristics of animals and plants in nature in 1964, Dettre and Johnson first studied the lotus effect. With the introduction of SEM (scanning electron microscope), Wilhelm Barthlott, a botanist at the University of Bonn in Germany, identified the mechanism behind the super hydrophobicity of lotus leaves. Surface hydrophobic chemical properties (waxy) and surface roughness (micro-texture) are two key factors for superhydrophobicity of materials [3].

In nature, many animals and plants show this super-hydrophobic property. For example, the feathers of plants such as rice, taro, canna, kale, red roses and waterfowl, butterflies and waterbirds.

Bird feathers belong to a kind of keratin, which is similar to human hair and has warm and hydrophobic properties. The feathers of most waterfowl have good super-hydrophobic properties, and scientists have done in-depth research in this area [4]. It is considered that the texture structure of the surface and the oily surface coating are the main reasons for the superhydrophobicity.

Kennedy [5] discovered in 1970 that the feather surface of birds is porous, and the water droplets always fall along the direction of the feather branchlets. Bormashenko et al. [6] found that although the pigeon feather itself is composed of hydrophilic keratin, the surface micropore structure makes the pigeon feather hydrophobic on the whole, which satisfies the assumption of Cassie equation. Through in-depth research, Sullivan et al. [7] further found that the micropores on the feather surface constitute a kind of one-way valve structure with grooves. This structure helps to reduce the air resistance of birds flying in the air. Zhang et al. [8] found that this structure gives the feather the ability of self-repair, so that the super-hydrophobic structure can be maintained for a long time. As shown in figure 2, the duck feather consists of a fiber rod in the middle and branchlets extending to both sides. The feather branchlets near the middle of the fiber rod form a fine structure with a groove structure and become a superhydrophobic interface with Cassie state [9].

Zhou Yongwu et al. [10] used scanning electron microscope to study the feather structure of many kinds of birds, including mallard duck, little swan, black swan, small mackerel, black-tailed gull and cormorant. They found that the belly feathers of all kinds of swimming birds were similar and were made up of feather branchlets. However, the specific shape is obviously different, which can be distinguished by scanning electron microscope. And the diameter and Internode distance of down-feathered branchlets are slightly different.

In the literature search, it is found that there are few studies on the superhydrophobic mechanism of swan feathers at home and abroad, and more superhydrophobic studies are focused on common domestic birds. Many studies have emphasized that the grooved three-dimensional structure on the feather surface is the main reason for the formation of feather superhydrophobicity, and more bionic studies have focused on how to use engineering means to form micro-nanostructures on the surface to improve the surface superhydrophobicity. At the same time, the bird feather surface has the ability of self-repair, which is not the property of artificial superhydrophobic surface at present. How to improve long-term stability should be an interesting research direction. At present, there is little reference to swan feathers in the literature at home and abroad, but there is still no research on this aspect.

04. Artificial preparation of super-hydrophobic surfaces through the bionic study of the super-hydrophobic characteristics of animals and plants in nature, people can use a variety of engineering and technical means to make all kinds of surfaces have super-hydrophobic properties. in order to solve the surface self-cleaning, friction and drag reduction, prevent road icing and other practical problems. In the past decade, people have been able to reduce the surface free energy and nanotechnology to make the surface with a variety of textures to increase its roughness, developed a variety of artificial self-cleaning surfaces [11].

The preparation of superhydrophobic surface should have two basic conditions: 1) the substrate with micro-nanostructure and 2) the chemical modification of the substrate. The properties (such as roughness) and chemical composition of the substrate directly determine its superhydrophobic properties.

At present, metals and polymers are common base materials. Metal-based superhydrophobic materials take metal (metal-like, metal oxide) as the main composition of the substrate. It has been reported in the literature that the metals used for substrate preparation include a small number of alloys such as copper, titanium, aluminum, zinc and aluminum-lithium alloys [12]. On the whole, the preparation of metal-based superhydrophobic materials is limited by the availability of the substrate, the duration of superhydrophobicity is limited, and the target pollutants are single. Polymer-based superhydrophobic materials are based on polymers (or the main components of substrates). Common polymer-based materials include polyurethane sponges [13] and polydopamine [14]. In contrast, polymer-based superhydrophobic materials have the advantages of simple preparation and low cost, and have a better application prospect in oil-water separation. Generally speaking, polyurethane sponge material can quickly remove oil from water (contact angle is 0 °) and other incidental organic pollutants, and the adsorption capacity of diesel oil and gasoline is 19.6 g / g and 18.4 g / g, respectively.

However, it should be noted that most of the researches on oil-water separation of metal-based superhydrophobic materials and polymer-based superhydrophobic materials are still in the laboratory stage due to various factors such as cost, treatment effect, R & D process and so on. In addition, in addition to environmental protection performance and oil-water separation performance, durability is another important factor affecting the application prospect of superhydrophobic materials.

05. The practical application of super-hydrophobic surface makes it widely used in engineering and technology. Due to the rapid development of photovoltaic technology, surface cleaning technology has obvious practical significance. This is because the dirt on the surface of solar cells will hinder the improvement of cell power generation efficiency, and the self-cleaning effect brought by super-hydrophobic technology has been successfully applied in photovoltaic technology [15].

In chemical engineering technology, oil-water separation usually requires expensive professional equipment, and super-hydrophobic technology provides a simple solution. This is because super hydrophobic surfaces are usually made up of good lipophilic properties. When the oil-water mixture comes into contact with the super-hydrophobic surface, the oil phase passes through the surface, while the water stays on the surface. In this way, the separation of oil and water is realized.

Superhydrophobic materials are also used in the field of medicine. A superhydrophobic protective layer was prepared by electrospinning outside the water-soluble drug. The superhydrophobic grid will form a thin layer of air on the drug surface, thus increasing the drug dissolution time to achieve the purpose of sustained release [17].

In 2008, the ice and snow disaster in China caused a large area of circuit line damage and railway traffic interruption, which attracted people's attention to the anti-freezing protection technology of power lines. Superhydrophobic treatment on the surface of power lines will reduce the possibility of freezing rain and ice on the surface of power lines, which is more energy-saving and environmentally friendly than simply using electric heating and other means [18]. The use of super-hydrophobic technology to treat the surface of textile fabric, so that the textile has the characteristics of pollution resistance and waterproof has always been the focus of scientific and technological research. Ordinary waterproof fittings are made of plastic, rubber and other materials. Because it is airtight, it is not comfortable to wear on the body, so it can only be used on specific occasions. Can you, like birds, have a fabric that walks in the mud without getting wet? Superhydrophobic technology has made this ideal a reality in the laboratory [19].

Due to air pollution, not long after the building is completed, the dust accumulated on the surface will make the new building look as if it has been used for a long time. Building surface cleaning is technically challenging, and now it is often maintained by means of surface painting. Superhydrophobic coating technology can reduce the deposition of dust and other pollutants on the building surface, and simple water erosion can change the old appearance of the building, which has a bright application prospect in the construction field.

In recent years, the frequent oil spill accidents and the extensive use of oil substances have caused serious ecological pollution. Researchers have thought of using super-hydrophobic materials to efficiently separate the oil-water mixture to solve this pollution problem. The key to solve the pollution problem is to separate the oil-water mixture efficiently. Superhydrophobic materials have different wettability between oil and water, so the application prospect of superhydrophobic materials in the process of oil-water separation has attracted wide attention. However, traditional superhydrophobic materials have some shortcomings, such as toxic raw materials, secondary environmental pollution, poor reuse and so on, so they can not be used on a large scale. Based on this, the preparation of super hydrophobic oil-water separation materials should take into account both environmental protection performance and oil-water separation performance.

However, there are still many uncertainties in the practical application of superhydrophobic technology, and many technical details need to be solved to realize large-scale commercial applications. For example, the preparation technology is complex and tedious, and the cost of large-scale commercial application is high. And the stability of super-hydrophobic surface has always been a problem, because the super-hydrophobic fine three-dimensional structure will be destroyed due to external forces such as impact and friction in the process of processing, thus reducing the super-hydrophobicity. In addition, it is difficult to clean the super-hydrophobic surface, when all kinds of dirt in the air pollute the super-hydrophobic surface, and the cleaning process will bring mechanical damage. In this regard, so far there is no better solution [1].

reference

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This article comes from the official account of Wechat: Hao Zhen popular Science talk (ID:sigsxskxjsxh). Picture and text: Zhang Mingrui, an Xuyao, du Binghang, Liu Ziqi, Wang Yuyang, Huang Yiping, typesetting: Zhang Mingrui, Review: Hao Zhihan, an Xuyao, Zhao Runze, du Binghang, Zhang Mingrui

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