Capillary waves of fluid interfaces near a critical point

A fluid interface near a critical point is commonly pictured as an intrinsic interface broadened by thermally excited capillary waves. A comparison of the capillary-wave theory with experiments is hampered by the presence of two short-wavelength cutoff parameters in the theory. We review a procedure introduced earlier for fixing these cutoff parameters, so that a definite comparison can be made with experimental reflectivity data reported by Huang and Webb for a binary liquid (methanol + cyclohexane) near the consolute temperature, and by Wu and Webb for a one-component fluid (sulfurhexafluoride) near the critical temperature. We show that the temperature dependence of the reflectivity data for the binary liquid is consistent with a nonuniversal temperature dependence predicted by capillary-wave theory. The reflectivity data of the vapor-liquid interface of SF6 do not appear to be consistent with the binary-liquid reflectivities and do not show the predicted temperature dependence.