Casimir effect in critical films of binary liquid mixtures

We present experimental evidence for the Casimir effect within critical films of binary liquid mixtures possessing opposite boundary conditions (+ -) by studying the thickness of these vapor-adsorbed films on a silicon wafer as a function of temperature near the critical temperature. Our results fur two different critical mixtures demonstrate that the critical Casimir pressure scaling function theta (+ -)(y) scales with y = L/xi, where L is the equilibrium film thickness and xi is the bulk correlation length. Additionally, on approaching the critical temperature T-c an increase in the film thickness L is observed, implying that the sign of the universal Casimir amplitude Delta (+ -) = theta (+ -)(0)/2 at T-c is positive, consistent with theoretical predictions. However, the magnitude of the Casimir amplitude that we measure is approximately two orders of magnitude smaller than that given by prevailing, theories. In the two-phase region of the liquid mixture, preliminary evidence suggests that the adsorbed him undergoes a surface phase transition from a film near the critical composition at T less than or similar to T-c to a film near one of the bulk phases at T much less than T-c. This low temperature film composition most likely corresponds to the bulk phase rich in the component that preferentially adsorbs at the silicon surface.