50. Surface energy

The work performed when new areas of the rarefied surface layer appear increases the potential energy, which is called surface energy. Surface energy is the excess potential energy of surface layer molecules compared to the energy of molecules inside a liquid.

If two environment (two liquids or a liquid and hard body) border, both layers close to the border have special properties. The surface energy is equal to the difference between the energy of the molecules of the surface layers of both bodies environment and the energy that the same molecules would possess if they were inside the respective bodies.

With each new surface area, the surface energy increases. The surface energy \(U_a ~\) is proportional to the area \(A\) of the interface surface

\( U_a ~= ~\sigma A \)

The proportionality coefficient \(\sigma\) depends on the nature of the boundary bodies environment and their condition, mainly temperature. It is called surface tension coefficient and can be measured in joules per square meter (in SI).

When the surface area is reduced, the surface energy is decreased, and the liquid performs positive work. In a state of equilibrium, the surface energy is of minimal value. Liquid takes the form of a ball if there are no other forces distorting its natural spherical shape.

As the temperature rises, the difference between the liquid and the saturated vapour of the liquid gradually disappears and at critical temperatures disappears completely. Therefore, the surface tension coefficient for the boundary between liquid and saturated vapour decreases as the temperature rises and becomes zero at critical temperature.