FIG. 5 Situation corresponding to the complete wetting regime in which the liquid phase intrudes between the substrate and vapor phase.

We have seen that a three-phase system is in a state of either partial wetting or complete wetting. It is then not difficult to imagine the situation in which a thermodynamic variable of the system is changed, say the temperature, so that the system undergoes a transition from the partially wet state to the completely wet state. Such a transition is termed the wetting transition. One convenient way to monitor the wetting transition experimentally is to measure the contact angle as a function of temperature, for instance. On approach to the wetting transition, the contact angle decreases until at the wetting transition it is zero and a thick liquid layer spreads at the solid-vapor interface.