Wetting transitions

Abstract

When a liquid droplet is put onto a surface, two situationsdistinguishable by the contact angle may result. If the contactangle is zero, the droplet spreads across the surface, asituation referred to as complete wetting. On the other hand,if the contact angle is between 0° and 180°, the dropletdoes not spread, a situation called partial wetting. A wetting transition is a surface phase transition from partial wetting tocomplete wetting. We review the key experimental findings onthis transition, together with simple theoretical models thataccount for the experiments.The wetting transition is generally first order (discontinuous),implying a discontinuity in the first derivative of the surfacefree energy. In this case, if one measures the thickness of theadsorbed film beside the droplet, at the wetting transition adiscontinuous jump in film thickness occurs from amicroscopically thin to a thick film. We show that this can lead to the observation of metastable surface states and anaccompanying hysteresis. The observed hysteresis poses, inturn, a number of questions concerning the nucleation ofwetting films that we also consider here. In addition, weconsider the equilibrium wetting film thickness that results from a competition between the long-range van der Waals forcesand gravity.Finally, the first-order character of the wetting transition canlead to a similar transition even when the phase that does thewetting is not (yet) stable in the bulk. For such prewettingtransitions, a discontinuous thin-to-thick film transitionoccurs off bulk coexistence. We show that, for the largevariety of systems for which prewetting transitions have been observed, the behaviour is surprisingly uniform, and can bemapped onto a simple generic phase diagram.The second part of the review deals with the exceptions to thefirst-order nature of the wetting transition. Two differenttypes of continuous or critical wetting transition have beenreported, for which a discontinuity in a higher derivative ofthe surface free energy occurs. This consequently leads to acontinuous divergence of the film thickness. The first type is the so-called long-range critical wetting transition, which isdue to the long-range van der Waals forces. We show under whatcircumstances such a transition can occur, and that it isusually preceded by a first-order wetting transition, whichhowever is not achieved completely. This leads to the existenceof an intermediate wetting state, in which droplets coexist with a relatively - but not macroscopically - thick film. Thesecond type of transition is the short-range critical wettingtransition, for which the layer thickness diverges continuouslyfrom a microscopic to a macroscopically thick film. Thistransition is interesting, as a number of renormalization-groupstudies predict non-universal behaviour for the criticalexponents. The experimental results indicate, however, mean-field behaviour, the reason for which is discussed in detail.