Critical Phenomena in Thick Films of a Binary Liquid Mixture

摘要

The first experimental data on the behavior of a critical system as it approaches two dimensionality are presented. Measurements of the bulk coexistence curve by use of refractive index techniques were done on the binary fluid mixture methanol--cyclohexane. These measurements gave the critical exponent beta = 0.326 +- 0.003, which agrees with recent Ising-model calculations. This same binary fluid mixture was then constrained between two highly reflective, optically flat pieces of fused silica in an interferometer. The critical temperature and coexistence curve were determined as the spacing between the flats was varied from 1 mu m to 60 mu m. The critical temperature was directly measured for spacings between 3 and 60 mu m. It was found that, if the walls were close enough together (less than or equal to 6 mu m), then the drops that form on phase separation would span the intervening space. The coexistence curves of these thick films (less than or equal to 6 mu m) were determined from measurements of the difference in refractive index between the two phases that appeared as drops. It was found that the shift in the critical temperature as the spacing was varied followed a logarithmic dependence. Such a dependence is not expected from Scaling Theory for an Ising model, but is to be expected of systems with effectively infinite-range interactions. The coexistence curves for each spacing of the thick film indicated that the critical exponent beta was close to 0.5, which is the mean-field (infinite-range-interaction) value and not the two-dimensional Ising-model value of 0.125. The amplitude of the coexistence curves was found to vary with spacing as L/sup z/ with z in the range 0.6 implies 0.8. This was a much larger dependence than expected from the theory. 29 figures, 5 tables. (ERA citation 03:010711)