In 1983 Pandit and
Fisher [5]
predicted that a number of interesting wetting phenomena can occur in the
vicinity of a bulk triple point. In particular, the presence of a triple
point can induce non-wetting of a substrate when it would normally be wet.
Consider for example a simple, single component system with the usual solid-liquid-vapor
bulk triple point as shown in the phase diagram of Figure 3.
If both solid and liquid phases wet the interface between a substrate
and the vapor, then at the triple point, where both liquid and solid compete
to wet the substrate, one phase will usually wet more strongly than the
other (unless things are perfectly balanced so that the wetting strengths
of both solid and liquid are equal). This situation is shown in the phase
diagram of Figure 4, for the case that the solid wets the substrate more
strongly than the liquid. The four boxes on either side of the phase diagram
illustrate the adsorption states for the case that the substrate forms
the walls of a container. At point A, on the solid-vapor coexistence
line, far from the triple point, the solid completely wets the substrate
forming thick films on the walls of the container.
Similarly, at point B, on the liquid-vapor coexistence line, the
liquid completely wets the substrate. At point T, the bulk triple
point, both liquid and solid would wet the substrate, but the solid wets
more strongly. If, starting at the triple point, the temperature is raised
slightly to point D, a solid film will still cover the substrate;
however, because the solid phase is no longer stable in the bulk, the film
is confined to a microscopic thickness and therefore the substrate is non-wet.
Upon raising the temperature further along the liquid-vapor coexistence
line, a point will eventually be reached, somewhere between points D
and B, at which the surface free energy of the thick, wetting liquid
films will be equal to that of the non-wetting solid films. This point
marks a first order triple point induced dewetting transition.
This description holds only for systems in which the liquid does not wet the interface between the solid and the vapor. If, on the other hand, the liquid were to wet the solid, then at point D, there would still be a microscopic solid film on the substrate, but there would be a thick film of liquid on top of that so that the substrate would be wet. In this case, instead of a dewetting transition, there would be a transition between thick liquid films at point B and thick liquid films with a layer of solid at the substrate at point D. Or in other words, a prewetting transition of the solid phase between the liquid and the substrate.