Landau Theory of Wetting Transitions
An interface between two coexisting bulk phases costs free energy1. The macroscopic surface tension is nothing but this excess free energy per unit area. Suppose now that three bulk phases coexist. A natural question is then: Are all three interfaces stable against the intension of a macroscopic layer of the third phase ? If every interfacial tension is less than the sum of the other two, the answer is yes. One could express this by saying that none of the bulk phases wet the interface between the other two. (There might be a wetting layer of microscopic thickness. In that case one could reasonably call the interface partially wet.) On the other hand, if the triangular inequality does not hold for the three tensions, one of the interfaces will be completely wet by the third phase. In 1977 Cahn predicted2 that for suitably chosen systems there should be a (interfacial) phase transition between states of partial and complete wetting at a temperature below bulk criticality. This prediction has since been verified experimentally3.
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