Consider the molecules (e.g., water H2O) or atoms (e.g., mercury, Hg) on the surface and in the interior of a liquid (Figure 2.3.2, see below). In the bulk of the liquid atoms are acted upon by attraction forces amongst themselves (van der Waals-London bonds), that are equal in all directions. The molecules that are on the liquid-gas surface only have attractive forces on the liquid side. This is why the molecules leave the surface and migrate to the liquid interior. The result is a spontaneous contraction of the surface. Macroscopically this manifests on the surface curves shown by liquids. This is why liquid drops and gas bubbles are spherical. This sphere is solid with the lowest (surface area/volume) ratio. The units for surface tension are N (Newtons)/m (meter).
