Wetting and Random-Field Transition of Binary Liquids in a Porous Medium

Mixtures of lutidine and water imbibed in porous Vycor have been studied using Raman, elastic, and quasielastic light scattering. At low temperatures, adsorption of lutidine on the internal glass surface gives rise to local composition gradients. At higher temperatures, a transition from complete to partial wetting occurs. Composition fluctuations extend over several pores, and the local chemical and geometrical randomness sets the stage for a random-field transition. The critical exponent $\beta$ associated with this latter transition is smaller than in the pure system.