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. I f the contact angle is between zero and 180 degrees, the droplet does not s pread, a situation called partial wetting. A wetting transition is a surfac e phase transition from partial to complete wetting. The wetting transition is generally first-order (discontinuous), implying a discontinuity in the first derivative of the surface free energy. As a consequence, at the trans ition a discontinuous jump in film thickness occurs from a molecularly thin to a thick film. We show here that the first-order nature of the transitio n can lead to the observation of metastable surface states and an accompany ing hysteresis. The second part of this review deals with the exceptions to the first-order nature of the wetting transition. Two different types of c ontinuous or critical wetting transitions have been reported, for which a d iscontinuity in a higher derivative of the surface free energy occurs. This consequently leads to a continuous divergence of the film thickness. The f irst type is long-range critical wetting, due to the long-range van der Waa ls forces. We show that this transition is preceded by the usual first-orde r wetting transition, which, however, is not achieved completely. This lead s to the existence of a new intermediate wetting state, in which droplets c oexist with a mesoscopic film: frustrated complete wetting. The film thickn ess diverges continuously from this mesoscopic film to a thick film. The se cond type of continuous transition is short-range critical wetting, for whi ch the layer thickness diverges continuously all the way from a microscopic to a macroscopically thick film. This transition is interesting, as renorm alization-group studies predict non-universal behaviour for the critical ex ponents characterizing the wetting transition. The experimental results, ho wever, show mean field behaviour, the reason for which remains unclear. (C) 2001 Elsevier Science Ltd. All rights reserved.