Wetting transitions

When a liquid droplet is put onto a surface, two situations distinguishable by the contact angle may result. If the contact angle is zero, the droplet spreads across the surface, a situation referred to as complete wetting. On the other hand, if the contact angle is between 0 degrees and 180 degrees, the droplet does not spread, a situation called partial wetting. A wetting transition is a surface phase transition from partial wetting to complete wetting. We review the key experimental findings on this transition, together with simple theoretical models that account for the experiments. The wetting transition is generally first order (discontinuous), implying a discontinuity in the first derivative of the surface free energy. In this case, if one measures the thickness of the adsorbed film beside the droplet, at the wetting transition a discontinuous jump in film thickness occurs from a microscopically thin to a thick film. We show that this can lead to the observation of metastable surface states and an accompanying hysteresis. The observed hysteresis poses, in turn, a number of questions concerning the nucleation of wetting films that we also consider here. In addition, we consider the equilibrium wetting film thickness that results from a competition between the long-range van der Waals forces and gravity. Finally, the first-order character of the wetting transition can lead to a similar transition even when the phase that does the wetting is not (yet) stable in the bulk. For such prewetting transitions, a discontinuous thin-to-thick film transition occurs off bulk coexistence. We show that, for the large variety of systems for which prewetting transitions have been observed, the behaviour is surprisingly uniform, and can be mapped onto a simple generic phase diagram. The second part of the review deals with the exceptions to the first-order nature of the wetting transition. Two different types of continuous or critical wetting transition have been reported, for which a discontinuity in a higher derivative of the surface free energy occurs. This consequently leads to a continuous divergence of the film thickness. The first type is the so-called long-range critical wetting transition, which is due to the long-range van der Waals forces. We show under what circumstances such a transition can occur, and that it is usually preceded by a first-order wetting transition, which however is not achieved completely. This leads to the existence of an intermediate wetting state, in which droplets coexist with a relatively-but not macroscopically-thick film. The second type of transition is the short-range critical wetting transition, for which the layer thickness diverges continuously from a microscopic to a macroscopically thick film. This transition is interesting, as a number of renormalization-group studies predict non-universal behaviour for the critical exponents. The experimental results indicate, however, mean-field behaviour, the reason for which is discussed in detail.