Effect of viscosity ratio on structure evolution during mixing/demixing of regular binary mixtures.

We simulate the mixing (demixing) process of a quiescent binary mixture with a composition-dependent viscosity which is instantaneously brought from the two-phase (resp. one-phase) to the one-phase (resp. two- phase) region of its phase diagram. Our theoretical approach follows a standard diffuse-interface model of partially miscible regular binary mixtures wherein convection and diffusion are coupled via a nonequilibrium capillary force, expressing the tendency of the phase-separating system to minimize its free energy. Based on 2D simulation results, we discuss the influence of viscosity ratio on basic statistics of the mixing (segregation) process triggered by a rapid heating (quench), assuming that the ratio of capillary to viscous forces (a.k.a. the fluidity coefficient) is large. We show that, for a phase-separating system, at a fixed value of the fluidity coefficient (with the continuous phase viscosity taken as a reference), the separation depth and the characteristic length scale of single-phase microdomains increase monotonically for increasing values of viscosity ratio; however, for a mixing system the attainment of a single-phase equilibrium state by coalescence and diffusion is retarded by an increase in viscosity ratio at a fixed fluidity for the dispersed phase.