Spreading describes the expansion of a liquid phase on a second liquid or solid phase (complete wetting). In the theoretical case of an interface of any size, the appropriate phase spreads until a monomolecular layer is formed. The contact angle between the two phases is 0°.

Liquids with low surface tension generally spread well. In return, spreading frequently occurs on liquids with high surface tension (e.g. water) and on solids with high surface free energy (e.g. metals).

The tendency to spread is expressed by the spreading pressure S; This is the difference between the work of adhesion W1,2 between the phases and the work of cohesion W1,1 of the phase under consideration:

S = W1,2 - W1,1

If the spreading pressure is positive, the phase spreads; if it is negative, wetting is not complete.