Experimental studies of the rheology of a simple liquid mixture during phase separation.

Abstract

We report measurements of the effective viscosity and elastic shear modulus of the critical and two off-critical mixtures of isobutyric acid and water in the process of phase separation under shear. All three mixtures exhibit enhancements of the viscosity caused by the presence of concentration domains, as has been suggested by Onuki [Phys. Rev. A 35, 5149 (1987)]. The critical mixture and an off-critical mixture rich in isobutyric acid also possess significant elasticity during phase separation. Elasticity is absent in the case of a water-rich mixture. During phase separation of the critical mixture, we observe rapid decreases of the viscosity enhancement and the elastic shear modulus. This behavior is due to rapid capillary-driven coarsening of the concentration domains, as we show in a detailed application of a theory recently proposed by Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)]. The off-critical mixtures exhibit constant viscosity enhancements and, in the case that it is present, an elasticity that increases with time. Some of these features can be understood by regarding the phase-separating off-critical mixtures as dilute emulsions.