Forces are acting between molecules which are resposible for the coherence of solids and liquids. The physical factor representing this interaction is the surface tension with the unit Newtons per square meter [N/m²] respectively [mN/m²]. The tendency of liquid drops to maintain a spherical shape is the result of the surface tension because the sphere is the shape of miniumum surface and minimum distance of molecules.
High surface tension results in a high tendency to maintain the spherical shape. Therefore oil drops with a low surface tension easier start spreading on surfaces than water with a high surface tension.
Generally liquid drops spread on a surface if they are stonger attracted by the surface molecules than by the internal attracting forces of the liquid. In this case the surface tension of the substrate is higher than the surface tension of the liquid.
In case of solid surfaces the physical unit of the surface energy with the dimension [J/m²] or [mJ/m²] respectively is often applied expressing the energy to be invested to break the attraction between surface molecules. The Value of the surface energy is identical with the value of the surface tension (1J = 1Nm).
Typical values of the surface tension of liquids are 20 mN/m up to 100 mN/m, of plastics and ceramics 25 mN/m up to 70 mN/m and metals up to more than 1000 mN/m.
Even if the surface tension of metals is very high liquids may unsufficiently wet metals because of oxidized surface layers or because of hydrocarbon surface deposites.