The lotus leaf is a typical self-cleaning plant in nature. Water droplets falling on the lotus leaf will form water droplets that roll freely on the surface and take away dust and other dirt. Through in-depth analysis, it was found that the multi-scale structure of the lotus leaf surface and the existence of epidermal biowax are responsible for the “self-cleaning” of the lotus leaf surface. This property of the lotus leaf is called “superhydrophobicity”.
Even if you pour sewage on the lotus leaves, there will be no stains on the leaves. This kind of “self-cleaning” surface can be realized through the application of nanotechnology after people understand the reason why it “comes out of mud without staining”.
Why are the water droplets on some leaves spherical and able to roll around, while others are flat and stay in one place?
A drop of water is on a solid surface and there are three interfaces in the entire diagram. The red one is the interface between solid and water, the blue one is the interface between solid and air, and the black one is the interface between water and air. The black interface is curved. If we draw a line from the intersection of red, black and blue along the direction of the black surface, it is called the tangent line of the curve at that point. In the picture, it is the green line. There is an angle between the red line and the green line, which we call the “contact angle”.
What does it look like if the contact angle is very large?
When the contact angle is large, the water droplets will take on a spherical shape. The place where the water contacts the leaves (equivalent to the red line in the picture above) is very small. The water will not stay in one place, and the entire water droplet can roll around. go.
What would it look like if the contact angle was small?
This is the shape of water droplets on a typical leaf. It is flat, with a large boundary between the water and the leaf surface (that is, the red line is very long). The contact angle is very small, and the water droplets cannot move freely. Thinking further, what would happen if the contact angle was very small, say zero? That’s right, there’s no blue line anymore, all the solids are taken up by water. In daily life, if our bowl or glass is not very clean, for example, there is oil, then the tentacles will be larger and we can see water droplets. If you wash them very clean with detergent and put dripping water on them, the water will spread immediately and no water droplets will be seen.
The physical reasons for contact angle are somewhat abstract. We need to start from the concept of surface energy to understand: increasing the interface between any two substances requires a certain amount of energy, which is numerically equal to the interfacial tension of the interface composed of the two substances. The surface tension we are more familiar with is the interfacial tension between air and water. In fact, there is interfacial tension not only between air and liquid, but also between air and solid, liquid and solid. Look at the picture above again. A drop of water placed on a solid surface creates three interfaces: black air and water surface, red water and solid interface, and blue air and solid interface. Multiply the respective interfacial tensions by the interface area and add them up to get the interfacial energy of the entire system.
The specific mathematical derivation will not be made. Let us consider two extreme situations. If the gas-solid interfacial tension is large and the liquid-solid interfacial tension is small, nature obviously tends to spread the water droplets completely (everyone likes to do labor-saving work). This is the case of cleaned ordinary glass. On the contrary, if the liquid-solid interfacial tension is large and the gas-solid interfacial tension is small, nature is inclined to let the air come into contact with the solid and leave the liquid alone. This is the case with lotus leaves or feathers. In the middle situation, no one can dominate the two interfacial tensions of gas-solid and liquid-solid. The contact angle is the result of the compromise between the two parties to divide the sphere of influence. The decisive factor behind this is that nature likes to save effort, that is, the surface energy of the entire system is low. In the specific division, the surface tension between air and liquid will also jump out and insert a bar, so the contact angle is determined by the interfacial tension between solid, liquid and gas.
If we don’t want water to stay on a solid surface, we need to increase the contact angle. For example, the contact angle of water on ordinary cloth is very small,
When the water reached the top, it wetted the cloth. But when we use cloth to make umbrellas, we apply some special substances on the cloth so that the contact angle of the cloth becomes large and it will not get wet by rain.
Extended reading: https://www. alltextile.cn/product/product-4-124.html
Extended reading: https://www.brandfabric.net/full-dull-nylon-dobby-2-5-layer-with-silver-coated-fabric/
Extended reading:https://www.yingjietex.com/product/coating-Oxford-Fabric.html
Extended reading:https://www.yingjietex.com/product /Polyester-dobby-3-Laminated-Fabric.html
Extended reading:https: //www.china-fire-retardant.com/post/9403.htm
