Observation of short-range critical wetting

Mean-field theory correctly predicts the critical behaviour of systems close to a phase transition, provided that fluctuations can be neglected. Fluctuations, however, become important if the dimensionality of the system is lower than a certain upper critical dimension. For such systems, it is necessary to use renormalization-group methods to describe the critical behaviour. Investigation of three-dimensional systems in which the upper critical dimension is also three can therefore provide a probe of the way in which mean-field theory breaks down when fluctuations become important1,2,3,4,5,6,7,8,9,10,11,12. An important example is the critical wetting transition that is predicted1,2 to occur in systems in which long-range forces are negligible, involving a continuous and reversible increase in the thickness of an adsorbed film. Here we present experimental observations of the short-range wetting transition close to the critical point in methanol–alkane binary liquid mixtures. We observe second-order, critical wetting for nonane (as characterized by the surface specific-heat exponent). The measured value is consistent with the predictions of mean-field theory, but disagrees strongly with renormalization-group calculations, which predict1,2,3,4 non-universal behaviour for this transition. The reasons for the apparent failure of the renormalization-group approach remain unclear; further experiments are needed to investigate the effects of fluctuations in more detail.