Diffuse Interface in a Critical Fluid Mixture

Abstract

The interface between the two fluids in equilibrium at temperature T below the critical (consolute) temperature Tc of a binary mixture in a gravitational field has been observed to expand into a transition layer that thickens, while the coexisting phases become more similar and the interfacial energy vanishes as T→Tc. The effective thickness L of the transition layer has been deduced from reflectivity measurements as a function of light wavelength λ and temperature. The shape of the interface concentration distribution function has been found from the λ dependence of the reflectivity to be close to an error functon and to a function recently proposed by Fisk and Widom and distinguishable from the classical hyperbolic tangent distribution. In the range 0.02 < Δ T < 3.0°C, L∝(ΔT)−ν where ν = 0.67 ± 0.02 and ξ the appropriately defined thickness L′ = 2(Δ T / Tc)−0.67 is comparable with the correlation length of the bulk fluids for corresponding ΔT. The critical index ν, together with the critical index β for the coexistence curve, and μ for the surface tension, can be combined to test various theoretical relations amongst the critical exponents and theories of the critical interface.