Spreading coefficient, spreading parameter

The spreading coefficient or parameter is a measure of the tendency of a liquid phase to spread (complete wetting) on a second, liquid or solid phase. The spreading coefficient S 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

Equally, the spreading coefficient can be expressed as the difference between the surface tensions σ1 and σ2 and the interfacial tension σ1,2:

S = σ2 - σ1 - σ1,2

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