No abstract

We report an ellipsometry study of the wetting of hexane on water. By adding salt to the water, we are able to tune the Hamaker constant of this system. This allows us to demonstrate, for the first time, that two rather than one wetting transitions can exist in a single system. Upon increasing the temperature, a discontinuous (first-order) transition from a microscopic film to a mesoscopic film occurs, followed by a continuous (critical) wetting transition that leads to a thick adsorbed film. The latter is due to the Hamaker constant which changes sign with temperature. The first-order transition temperature changes by the same amount as the critical wetting temperature upon changing the Hamaker constant.[S0031-9007(98)06032-3] PACS numbers: 68.45.Gd If one considers a liquid droplet on a substrate, in general one distinguishes two possible situations. If the sum of the liquid-substrate and the liquid-vapor interfacial tension is larger than the substrate-vapor interfacial tension, the droplet will have a contact angle between 0 ± and 180 ± , a situation called partial wetting. On the other hand, the situation may arise that the sum of the liquid-substrate and the liquid-vapor surface tension equals the substratevapor surface tension. The contact angle will then be zero, and the droplet will form a uniform film that covers the whole substrate surface: The liquid completely wets the substrate. The transition between these states, say as a function of temperature, is believed to be a first-order (discontinuous) surface phase transition, as there is a discontinuity in the first derivative of the surface free energy with respect to the temperature [1].The last few years have seen tremendous progress in the study of these phenomena. For the first time, evidence for the existence of the prewetting line has been obtained in a few systems [2]. Several observations of metastable surface states, such as the discovery of hysteresis in wetting transitions, also underline the generic first-order (discontinuous) character of the wetting transition [2]. On the other hand, Ragil et al. recently demonstrated the existence of a counterexample [3]. Studying the wetting behavior of pentane on water, a transition from a mesoscopic ͑ഠ50 Å͒ to a very thick film was found that was completely continuous. The possible existence of such a continuous (or critical) wetting transition had been disputed for a long time [4]. The conclusion was that it might occur in real systems with long-range interactions, but that these interactions, quantified by the Hamaker constant, should change sign at the critical wetting temperature [4]. It was demonstrated that this was indeed the case for the pentanewater system, and, moreover, it was shown that this is a necessary but not a sufficient condition for critical wetting to occur. Additionally, the system has to be in a state that would show complete wetting in the absence of long-range forces [3].Ragil et al. thus proposed a new scenario for wetting transitions: Instead of two possible surface state...