Molecular explanation of surface tension
On page 140 we explained that the molecules of a liquid are constantly oscillating to and fro alternately exerting repulsive and attractive forces on one another, yet free to move in the body of the liquid, exchanging partners as they go. Application of the kinetic theory, together with knowledge of how the force between the molecules varies with their distance apart, leads to the conclusion that molecules which have moved into the surface of the liquid become more widely paced than those inside it. Consequently, for most of the time they are exerting attractive forces on one another. The surface of a liquid is therefore under tension, which explains why liquids will form spherical drops, soap films spherical bubbles, and so on.
Thus, however much the surface of a liquid may expand or contract it behaves as though covered by an elastic skin under a constant tension, the value of which depends on the temperature.