Complex Rheology in a Model of a Phase-Separating Fluid

The time-dependent effective shear viscosity of a two-dimensional phase-separating fluid is measured by numerical simulation of a momentum-conserving lattice-gas model of an immiscible binary mixture. Simulations of phase separation in a simple shear flow reveal that the effective viscosity can be greater than 5 times the viscosity of the constituent fluids prior to mixing. The effective viscosity is strongly time dependent, with a behavior characterized by a rapid increase (strain thickening) followed by a gradual drop-off (strain thinning) to a steady state. Explicit measurements of the time-dependent excess stress are shown to be well correlated with analogous measurements made only from knowledge of the orientation of interfaces.