Molecules exercise forces of attraction on each other which are responsible for the bonding between the surfaces of solid bodies and liquids. The physical variable for this force is the surface tension with the dimension force per unit length, and the unit [N/m] or else [mN/m]. By reason of surface tension, drops of liquid tend to assume a spherical shape to have the smallest possible surface. The higher the surface tension, the harder it is to break up this spherical shape. For this reason, oils with a low surface tension tend much more to roll off surfaces than water with a high surface tension. Generally, drops of liquid dissolve on solid surfaces when the attraction of the liquid molecules by the solid molecules outweighs the internal surface tension. In particular with solid bodies, the term surface energy in the dimension [J/m²] or [mJ/m²] is used to describe the energy required to break up the attraction between surface molecules. The values for surface tension and for surface energy have the same meaning. Typical values for the surface tension of liquids are 20 mN/m to 100 mN/m, of plastics and ceramics 25 - 70 mN/m, of metals up to more than 1000 mN/m. Nevertheless, it happens that metal surfaces are not adequately wetted by liquids. This is because the surface of almost all metals, when exposed to the atmosphere, is covered by an oxide layer and often also by hydrocarbon residues (greases, oils). (also see ⇒ Surface oxide, ⇒ Surface cleaning).