Shulou(Shulou.com)11/24 Report--

In the past two days, the topic of home-made stormsuit has been hit by a hot search. I wonder if you have thought that there is still a big relationship between the stormsuit and the lotus leaf when using it.

Fig. 1 the dewdrop on the surface of the lotus leaf did you ever pick a lotus leaf as an umbrella in the lotus pond when you were young? When the water droplets roll back and forth on the lotus leaves, I wonder if you realize that the phenomenon that the water droplets do not wet the lotus leaves is not simple. This "lotus leaf effect" was later widely used in waterproof materials, super-hydrophobic materials and self-cleaning materials.

What does waterproof and hydrophobic mean respectively? In order to facilitate the discussion, let's take a moment to distinguish the concepts: waterproof and hydrophobic. What do the twins mean?

Waterproofing is the ability of a material to resist the "invasion" of water. The resistance of a material (in this case, fabric) to water is measured by the grade of hydrostatic head. Imagine a piece of fabric being pressed under a tall pipe filled with water. The height of the water in the pipe (in mm units) when the water begins to permeate the fabric is called the hydrostatic head grade. Materials with hydrostatic head grades greater than or equal to 1000mm are usually regarded as waterproof materials.

Fig. 2 the surface hydrophobicity of charge clothing fabric refers to the ability of the material to resist being soaked by water, which can generally be described by the contact angle theta. When the droplet is in equilibrium, the angle from the "liquid-solid" interface to the "gas-liquid" interface is called the contact angle. The contact angle less than 90 degrees is considered as a hydrophilic surface, and the contact angle greater than 90 degrees is considered as a hydrophobic surface.

Figure 3 contact angle diagram, so the smart friends have thought: only children make choices, we all want waterproofing and drainage! If you want to get a material that is very waterproof, you should use a waterproof matrix material plus a hydrophobic coating.

Where does the waterproof of the suit come from? The suit is waterproof, of course, because water droplets cannot "enter" into the inside of the suit fabric. However, in fact, all fabrics have holes, but they cannot penetrate because the size of the water droplets is larger than the pores of the fabric. it is obvious that something full of holes can not cover the rain. (if there is a friend to get to the bottom of how the water is kept out of the material, the most fundamental reason is from the electromagnetic force. We all know that matter is made up of nuclei and electrons, and this "resistance" is caused by the repulsion between electrons. So the denser the fabric is, the better it will be waterproof (but for the suit, it will also make the air permeability worse, so it is not as dense as possible).

Fig. 4 Water droplets on the fabric but the waterproof ability of the fabric alone is not ideal. The most common way to produce waterproof fabric is to apply a hydrophobic coating on the fabric.

"Lotus leaf effect"-when it comes to hydrophobicity, we have to mention the "lotus leaf effect". In 1997, W.Barthlott and C.Neinhuis published the original article "Lotus Leaf effect". It can be said that this article started an upsurge of surface science research on superhydrophobicity. By studying the microstructure of the lotus leaf surface, Barthlott and Neinhuis found that the lotus leaf surface is not smooth, but a rough surface with small papillae. This surface is the reason why the water droplets roll freely on the surface of the lotus leaves without getting wet. This phenomenon is called the "lotus leaf effect" (by the way, in fact, the more accurate translation of the "lotus leaf effect" in the original articles of Barthlott and Neinhuis should be called the "holy lotus effect".

Let's explain how it works.

Generally speaking, the lotus leaf effect is the result of low energy hydrophobic surface and rough surface.

First of all, let's explain why the surface energy of hydrophobic surface is relatively low. Students who are not interested in the formula look directly at the following conclusion.

Students who have come into contact with surface physical chemistry should know the famous Young's equation in 1805. The expression is as follows:

Where, is the contact angle, is the surface energy.

After its transformation form, the following equation is obtained:

This equation points out that wetting depends on the amount of energy needed by the droplet to enlarge the surface and the energy obtained by covering the substrate material, and the latter can compensate for the infiltration of the former, thus minimizing the energy of the system; on the contrary, when the surface energy of the substrate material is lower than the surface energy of the droplet, the droplet will not spread. So if you want to get a hydrophobic surface, it must be a low-energy surface.

The second factor affecting hydrophobic performance is surface roughness. The rough surface reduces the contact area between the droplet and the surface, the droplet can only stay on the tip of the mastoid (figure 6), the air is sealed in the middle of the mastoid, and the droplet is supported.

Fig. 6 the state of droplets on the surface of lotus leaves (Barthlott W, Neinhuis C 1997) it should be noted that roughness is not the fundamental condition leading to hydrophobicity. The Wenzel model (a classical model, shown in figure 7) holds that the existence of rough surfaces makes the actual "solid-liquid" contact area larger than the apparent geometric contact area, geometrically enhancing hydrophobicity (or hydrophilicity). That is, the roughness will make the hydrophobic surface more hydrophobic and the hydrophilic surface more hydrophilic.

Fig. 7 Wenzel model combines the above two items: first, if the energy on the surface is very low and the energy obtained by water through adsorption is very small, then the water will not expand, that is, the water will remain spherical. Surfaces with little or no polar groups (such as organic solids, polymer solids) have very low surface energy. This is why the waterproof coating of the suit generally chooses polyurethane (PU), polyether polyurethane (PE) and silicone and their combination.

Second, in the case of a rough surface, air is wrapped between the surface and water droplets to form a composite surface (this composite surface model is called the Cassie-Baxter model (figure 8), which is another classical model. The real world situation is usually a combination of the Wenzel model and the Cassie model. The composite surface expands the water / air interface and reduces the water / solid interface. This surface can be regarded as a composite surface of air and solid, and the surface energy of the composite surface of air and solid is lower than that of the solid itself. this makes the droplet expand and absorb less energy and less inclined to expand or infiltrate. This is the principle of the "lotus leaf effect". Apply it to a stormsuit and you will get a waterproof suit.

Fig. 8 the hydrophobic surface of the Cassie-Baxter model is easily destroyed but the hydrophobic surface is unstable. Such as mechanical wear, or chemical reactions such as hydrolysis can destroy the surface and affect its performance. This is a huge challenge in biomimetic materials. Organisms can regenerate this papillary structure because of their biological activity, but how to prepare self-healing hydrophobic materials is a difficult problem. Therefore, this is a challenge to the durability and life of the suit. At the same time, the coating may affect the tear strength of the fabric. One disadvantage of PU and PE, for example, is that it reduces the strength of the fabric. The coating affects the sliding mode of the fabric, and the pressure is more concentrated, making the fabric easier to tear. However, silicone is not easy to hydrolyze and increases the strength of the fabrics they use, so it is widely used in high-end products (of course, it is more expensive).

It is obvious that the quality of a piece of clothing cannot be judged solely by its waterproofing (if so, you might as well go out in a raincoat). For the suit, it is necessary to take into account a series of functions, such as waterproof, air permeability, weather resistance to complex weather conditions, warm performance, windproof performance, quick-drying performance and so on. These functions may contradict each other and require the perspective of system engineering. This may also be one reason why high-end stormtrousers get stuck in their necks.

References:

Purity of the sacred lotus,or escape from contamination in biological surfaces

Https://baijiahao.baidu.com

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). By opzk and Landuo.

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