Kulim Kht Surface Tension & Contact angle

How come contact angle and surface tension are so important ? Molecules inside (bulk) a liquid/solid are in every direction affected by equal attraction forces, whereas the molecules at the surface lack a neighbor towards the air phase and therefore they have larger attraction forces towards the liquid/solid than air (see figure below). This leads to a situation where the interface has excess free energy. This excess free energy is characteristic for any liquid or solid. For liquids a spontaneous contraction of the surface will take place due to this free energy and the Surface Tension of a liquid is a direct measure of it. In the case of solids a contraction is hardly ever seen, but still this free energy is present at the interface of a solid. However, now it is called Surface Free Energy (instead of Surface Tension as for liquids) and can be accessed by measuring the contact angle of a series known liquids placed on the solid surface. The dimension of Surface Tension and Surface Free Energy is mN/m.