New developments in the study of binary fluids under shear flow

Abstract

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 $\overrightarrow{\mathrm{q}}$. 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 ($\beta =\frac{1}{2}$). 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 $\overrightarrow{\mathrm{q}}$ is well represented by the Onuki-Kawasaki formulation, in particular the exponent $\gamma$ shows the classical value ($\gamma =1\mathrm{}$). (ii) At equilibrium. The susceptibility and the correlation length have been measured on the critical isochore above $T_c$, on the coexistance curve, and on the critical isotherm. The universal amplitude ratios $\frac{{\xi }_0^{+}}{{\xi }_0^{-}}$, $\frac{C^{+}}{C^{-}}$, $R_{\chi }^{+}$, and $Q_2$ 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.