The work required to expand the surface of a liquid by unit area.
The surface behaves as if it were covered with an elastic stretched skin. The effect is due to the fact that at the surface the attraction between the molecules of the liquid is unbalanced, surface molecules being attracted towards the body of the liquid, whereas in the body of the liquid a molecule is attracted equally in all directions.
Small droplets or bubbles of one fluid immersed in another fluid have a spherical shape because of the interfacial tension, or surface tension, between the two fluids. Whenever the fluids have different densities (which is almost always the case) the droplets or bubbles move downward or upward because the buoyant force is not in balance with the gravity force. However, droplets or bubbles can remain stationary in a gravitational field when they come into contact with a solid surface provided the surface tension force exceeds the buoyant force. Since the former is proportional to the drop radius while the latter is proportional to the cube of the drop radius, a sufficiently small drop or bubble can defy gravity and remain attached to a solid surface. The bubble attached to the side of a glass of carbonated beverage and the rain drop sticking to the window pane are two common examples of this phenomenon.