SURFACE ENERGY OF LIQUID

The molecules of a liquid are in a state of random motion inside the liquid. A molecule is attracted by other molecules lying within a range of  molecular force. A sphere of radius equal to the range of molecular force is called Sphere of molecular influence. The molecule at the centre of molecular influence is attracted equally in all the directions by all other molecules in the sphere.


For a molecule A, well inside the liquid, the forces of attraction on the molecule are equal in all directions. Hence net force on it is zero. consider a molecule B near the surface. In this case the sphere of molecular influence is partially outside the liquid. But the molecules within the liquid lying in the molecular influence attract the molecule B. So there is a net downward force.

Consider another molecule C on the surface of liquid. Half of the sphere of influence is outside the liquid. Hence the downward force on molecule is maximum. Thus when a molecule comes to the surface , it is doing work to over come this downward pull. The work done resides as potential energy in the molecule on the surface.

Thus  molecules on the surface of liquid have  potential energy. This energy is called the surface energy. To increase the area of a liquid, work has to be done. Thus energy is required to increase the surface area. The energy required to increase unit area of the surface is called the surface energy.