New developments in the study of binary fluids under shear flow

Three binary fluids, aniline-cyclohexane, nitrobenzene-n-hexane, and isobutyric acid---water, have been studied by light scattering and turbidity techniques near their critical points, both at the thermodynamic equilibrium and under a shear flow, as a function of the variables temperature T, relative concentration M, shear rate S, and wave vector q. The following results have been obtained: (i) Out of equilibrium. The region where a shear affects the critical behavior has been determined in the plane (M, T) ; the crossover temperature varies as M^2, and the coexistence curve exhibits the classical exponent (=12). A small temperature change due to the shear was detected; its value is about four times lower than that calculated by the Onuki-Kawasaki theory. The susceptibility versus T, M, S, and q is well represented by the Onuki-Kawasaki formulation, in particular the exponent shows the classical value (=1). (ii) At equilibrium. The susceptibility and the correlation length have been measured on the critical isochore above T₂, on the coexistance curve, and on the critical isotherm. The universal amplitude ratios {₀^+}{₀^-}, C^{+}{C^-}, R_^+, and Q₂ have been obtained. The typical time taken by the system to return to equilibrium after having been perturbed by shear has been analyzed in terms of mass diffusion.