Wetting Transitions at Fluid Interfaces and Related Topics

Abstract

Wetting transitions, in which a fluid wets the interface between two fluid phases, are defined. There may be first, second, and higher order transitions. The line tension, an excess free energy per unit length of a three-phase contact line, and the boundary tension, another excess free energy associated with the boundary of two surface phases at the prewetting transition, are also defined. Mean-field density functional models of the fluid interfaces are introduced and their results on the wetting transitions, the line and boundary tensions, and the fluid structures in the three-phase contact region are presented. Keywords: Wetting transition, Fluid interfaces, Line tension Wetting transitionWhen three fluid phases coexist, they do so either in the two possible modes: (i) they meet at a common line of contact and the three interfaces make dihedral angles or (ii) one of them wets the interface of the other two, i.e., it occupies between the two phases as an infinitely thick layer. The transition between the two states is the wetting transition [1, 2]. There must be at least two components in a fluid system if the system exhibits a wetting transition, because otherwise the thermodynamic state of three-phase coexistence cannot be varied. The two modes are illustrated in Fig. 1, where α, β, γ are the three coexisting phases and the αγ interface is either not wet or wet by β. In the non-wet states, the surface tensions of the αβ, βγ, and αγ interfaces satisfy the inequality:and the contact angle β is given by(2) In the wet states, the αγ interface may be considered as a composite of the αβ and βγ interfaces. Thenand the contact angle β = 0. The wetting transition may be viewed as a transition in the structure of the αγ interface, between one in which that interface does not, and one in which it does, consist of a layer of a macroscopic β phase.