The non-uniform
potential near a surface can have different effects on a bulk phase transition.
If the potential is effectively attractive, the surface can serve to nucleate
the transition as with dew on a glass. Whereas, at surfaces with an effectively
repulsive potential, the more dense bulk phase will be repelled like water
beading on a newly waxed car. The study of wetting phenomena was greatly
enriched in 1977 with the work of Cahn [1]
and Ebner and Saam [2].
They demonstrated that as a consequence of thermodynamics, there should
be phase transitions on the surface between effectively repulsive and effectively
attractive behavior. In other words, they predicted the existence of wetting
transitions.
The simplest example of a wetting transition involves the interaction between the bulk liquid-vapor phase transition and an inert solid surface or a wall. When the solid surface is exposed to the environment of the coexisting liquid and vapor, two different things can happen. If the molecules of the liquid interact strongly with the surface, the liquid will spread across the surface in a thick film and the surface is said to be wet. If however, the surface is weakly binding, so that the interactions between the liquid molecules are stronger than the interaction with the surface, the liquid will clump around itself and will not wet the surface [1,2]. This is illustrated in Figure 1 for the case that the surfaces in question are the walls of a container. If a surface is non-wet at a given temperature, it will usually become wet at some higher temperature [7]. The temperature at which the surface becomes completely wet is the wetting temperature, Tw, and the transition from the non-wet surface to the wet surface as the temperature is raised is the wetting transition.