DIFFUSE INTERFACE IN A CRITICAL FLUID MIXTURE

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 X and temperature. The shape of the interface concentration distribution function has been found from the X 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 < ΔT < 3.0°C, L ∝(ΔT)-v where v=0.67±0.02 and ζ the appropriately defined thickness L′ = 2(ΔT/Tc)-0.67 is comparable with the correlation length of the bulk fluids for corresponding AT. The critical index v, 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.