CRITICAL-POINT RHEOLOGY OF A SHEARED PHASE-SEPARATING MICELLAR SOLUTION

We have investigated the rheological behavior of a phase-separating critical mixture of a nonionic micellar solution with a lower consolute point. For this purpose we have observed forward light scattering and measured the viscosity during the phase-separation process that occurs after quenches into the unstable region. The light-scattering experiments show that at a finite shear rate S the formation of concentration domains is suppressed until a characteristic temperature TS larger than the critical temperature Tc is reached. After quenches to temperatures T>TS, concentration domains strongly elongated in the direction of the flow are formed. We observe an effective viscosity enhancement Δη(t) as a function of the time t after the quench which depends on the shear rate S and which decays to zero as t→. The approach to a nearly stationary state after quenching, where Δη0, is accompanied with the formation of a stringlike morphology of concentration domains which does not resist shearing in contrast to recent theoretical predictions which assume that the decay of the viscosity enhancement is due to coalescence of the domains. © 1995 The American Physical Society.