Due to the cohesive force between molecules, the free surfaces of liquids exhibit a tendency to contract. Due to the fact that cohesion can only be manifested within a very small radius (about 1/1000000 cm), molecules located at a large distance from the interface are mutually counteracted by the gravitational forces of surrounding molecules, and therefore have no effect on the interface; But if the distance from the molecule to the interface is too small The liquid molecules below exert a downward pulling force on the molecules above, and the molecules at the liquid surface are subjected to this force There is also a tendency to shrink towards the interior of the liquid. Therefore, it can be imagined that the liquid interface is a layer of elastic film, which is in a tense state due to the force of internal tension acting on the interface. This tension is called surface tension.

The molecules near the interface between gas and liquid, or between immiscible liquids, are subjected to the splitting force of the two adjacent media. The characteristics of these two media determine the magnitude of the interfacial tension and the different shapes of the interface. Like dewdrops in the air, bubbles in water, and a film of mercury on the surface.

The phenomenon in which a liquid rises or falls along a solid wall when it comes into contact with the wall is called capillary action. Liquid can rise in a thin tube because the cohesion between liquid molecules is less than their adhesion to the wall tube. For example, water, oil, etc. can wet the tube wall, causing the liquid level to bend upwards Surface tension causes the liquid to rise; If the cohesion between liquids is greater than their adhesion to the wall tube, such as mercury, it cannot wet the tube wall, causing the liquid surface to bend downwards and the surface tension to lower the liquid.

As a flow verification device. When reading the liquid level with a level gauge glass tube, regardless of whether the liquid level is convex or concave, in order to reduce reading errors, it is necessary to read the height of the horizontal section of the liquid level inside the tube, and not the height of the liquid bend at the tube wall.