In our daily life, we may be used to what we see and think it should be like this, but we don't think too much about why. For example, after the rain, we can see that the small water droplets on the leaves and grass are close to spherical; Accidentally broke the thermometer, the mercury inside fell to the ground, and the small mercury drops were spherical. In addition, we can perform a little magic trick. In a glass of water, carefully place a needle horizontally on the water. The needle floats on the water without sinking into the cup, and a concave surface is formed on the water below the needle. If you are skilled, you can even use buttons, small flat metal or coins instead of needles. All these phenomena are related to surface tension.

So, what is surface tension? It turns out that liquid contacts gas to form a surface layer, and the mutual attraction in this surface layer is surface tension, which can automatically shrink the liquid level. Surface tension is caused by great cohesion between liquid molecules. The molecules in the liquid surface layer are less than those in the liquid, so the force introduced into the liquid makes the liquid surface layer shrink like a taut rubber film, thus reducing the surface area of the liquid as much as possible. As we know, a sphere is a geometric shape with the smallest surface area under a certain volume. Therefore, under the action of surface tension, water droplets always try to keep spherical, which is also the reason why the water droplets on our common leaves are nearly spherical.

The direction of surface tension is tangent to the liquid surface and perpendicular to the dividing line between any two parts of the liquid surface. The surface tension is only related to the properties and temperature of the liquid. Generally speaking, the higher the temperature, the smaller the surface tension. In addition, impurities will obviously change the surface tension of liquid. For example, the surface tension of clean water is very large, while the surface tension of water stained with soap solution is relatively small, which means that the surface of clean water has a greater contraction trend.

There is a "surface layer" not only between liquid and gas, but also between liquid and solid wall. This thin liquid layer, which is often called an adhesive layer, also has surface tension. This surface tension determines that when liquid and solid contact, there will be two phenomena: non-wetting and wetting. When mercury falls on glass, it is spherical, that is to say, the contact surface between mercury and glass tends to shrink, which is impermeability. When water drops on the glass, they slowly spread along the glass, and the contact surface tends to expand. This phenomenon is infiltration. Although mercury can't penetrate into the glass, after washing the zinc plate with dilute sulfuric acid and dropping mercury on the plate, we will see that mercury slowly diffuses along the zinc plate instead of being spherical. Therefore, the same liquid can soak some solids, but not other solids. Mercury can penetrate zinc, but not glass; Water can soak glass, but not paraffin.

The two phenomena of wetting and non-wetting determine that there are two different shapes at the contact between liquid and solid wall: concave and convex.

Now we understand the reason why coins don't sink in the little magic trick introduced earlier. It actually takes advantage of the huge surface tension and impermeability of water. If we coat the surface of the coin with oil in advance, the coin can be easily placed on the water without sinking. In engineering technology and daily life, people often make use of the property that water is insoluble in oil. Like drawing a paper umbrella to make an umbrella; Apply engine oil to metal equipment to prevent it from rusting due to water; Even in the mineral processing method, the property that water does not soak the oiled object is used. Flotation beneficiation is to put the crushed ore into a pool, put water and oil that only soak the useful minerals in the pool, coat a thin layer of oil on it, and then send air to the pool to make bubbles adhere to the useful mineral particles and bring them to the water surface, so as to separate them from impurities such as rocks.

An important phenomenon of surface tension is capillary phenomenon. That is to say, the immersion liquid rises in the thin tube and the non-immersion liquid falls in the tube. We can easily do a small experiment to observe this phenomenon. Put the thin glass tube into the water tank and the water will rise rapidly from the thin glass tube. The water level in the pipe is higher than that in the water tank. The thinner the pipe, the higher it rises, and the water level in the pipe is concave. If mercury is put into the sink, the situation is just the opposite, and the liquid level in the test tube is lower than that in the sink.

Why can wetting liquid rise in capillary? It turns out that when the immersion liquid contacts the inner wall of the capillary, the liquid surface is concave, and the surface tension acts along the tangent direction of the liquid surface, so the surface tension acting along the tube wall forms an upward resultant force, which makes the liquid in the tube rise until the upward pulling force of the surface tension is equal to the weight of the rising liquid column in the tube. Similarly, for non-wetting liquid, the resultant force of capillary wall surface tension is downward, which makes the liquid in the tube drop.

We usually see the phenomenon of wiping sweat with towel and absorbing ink with chalk on paper, which can be explained by capillary phenomenon. Towels, cotton, chalk, mud and other objects have many small holes inside, which play a capillary role. In alcohol, using cotton thread as wick can make alcohol rise along the wick; If silk thread is used as wick, the alcohol lamp may not light. This is because alcohol can't penetrate into the silk thread, and alcohol drops in the wick of the silk thread.

Capillary phenomenon is also of great significance to plant growth. The nutrients and water they need are absorbed from the soil through the small tubes in roots, leaves and stems and transported to the green leaves. This is like a never-ending water pump, which tirelessly sends water and nutrients to every cell of the plant. In addition, there are many capillaries in the soil, along which groundwater rises to the ground and evaporates. If we want to save groundwater for plants to absorb, we should hoe the soil on the surface and cut off these capillaries to reduce the evaporation of water. Therefore, farmers often loosen the soil for crops after rain to keep water.

Using capillary phenomenon, people also produce various pens, signature pens and colored pens. When they are used to write on paper, the paper immediately shows handwriting, which is what we are used to on weekdays, but few people think about why ink keeps coming out when writing, but it doesn't come out when not writing. Now we know that it transports the ink in the pen container to the pen tip by a series of capillary grooves on the pen body and the fine cracks on the pen tip; The pen tips of the signature pen and the colored pen are connected by a slender tube, and the inner wall of the tube is covered with a cotton roll filled with ink. Some colored pen tips are also made of materials containing multiple capillary holes. When writing, as soon as the pen tip touches the paper, the ink sticks to the paper and leaves handwriting on it.

Why doesn't the ink come out when you don't write? We can do another experiment to explain it. Cover the glass filled with water with a piece of cardboard (the glass doesn't need to be filled with water), hold down the cardboard, quickly turn the glass upside down and take your hands off the cardboard. At this time, a strange phenomenon happened: the cardboard stopped at its original place, and the water stayed in the cup without flowing out. Can't the weight of a glass of water push a piece of paper? That's not true. This is due to the interaction between atmospheric pressure and surface tension of water. When the glass is inverted, the water column drops a little, which reduces the air pressure in the glass. The pressure difference between the top and bottom of the water column overcomes the weight of the water column itself, so that the water in the glass can't come out. The surface tension between water and paper and between water and glass also keeps the cardboard in its original position. When you don't write, it's the same if the ink in the pen doesn't flow out.

The use of surface tension is far more than that, it is also widely used in biology, medicine and microcirculation system. Toy manufacturers often use it to produce all kinds of interesting toys.