Due to the cohesion between molecules, the free surface of the liquid shows a tendency to shrink. Since cohesion can only be manifested in the range of action with a small radius (approximately 10-7cm), molecules with a large distance from the interface will be offset by the gravitational forces of the surrounding molecules and will have no effect on the interface. However, if the distance between the molecules and the interface is too small, the liquid molecules below will exert a downward pulling force on the molecules above, and the molecules at the liquid surface will have a tendency to shrink inside the liquid due to this pulling force. Therefore, it is conceivable that the liquid interface is a layer of elastic film, and the film is in a state of tension due to the component force of the internal tension on the interface. This tension is called surface tension.
Between gas and liquid or between non-mixed liquids, the molecules near the interface are affected by the molecular forces of the two media. The characteristics of the two adjacent media determine the size of the interface tension and the different shapes of the interface. Such as dew in the air, bubbles in the water, mercury film on the surface of mercury.
When the liquid is in contact with the solid wall, the phenomenon that the liquid rises or falls along the wall is called the capillary phenomenon. The liquid can rise in the thin tube because the cohesive force between the liquid molecules is less than the adhesion between the liquid molecules and the wall. For example, water, oil, etc. can wet the tube wall, the liquid surface bends upwards, and the surface tension pulls the liquid up. The cohesive force is greater than the adhesion force between it and the wall pipe. For example, mercury cannot wet the pipe wall. The liquid surface is bent downward, and the surface tension pulls the liquid down.
As a flow calibration device, when reading the liquid level with a liquid level gauge glass pig tube, no matter whether the liquid level| is convex or concave, in order to reduce the reading error, the height of the horizontal section of the liquid level in the tube must be read, and it cannot be read. The height of the liquid bend on the pipe wall.