Unsteady mixing of binary liquid mixtures with composition-dependent viscosity

The laminar mixing of two miscible liquids in a T junction is investigated through Direct Numerical Simulations. In particular the liquids have the same density and the same viscosity, but the viscosity of the mixture strongly depends on composition and can be up to three times larger or smaller than that of the pure liquids; this is aimed at mimicking the behavior of many aqueous and organic mixtures of interest in the pharmaceuticals and fine chemistry industry. The mixing process is found to be largely affected by the dependence of viscosity on composition. When the viscosity of the mixture is larger than that of the pure components, a viscous layer forms at the confluence of the inlet flows, which tends to keep the two streams separated, resulting in a shift of the engulfment regime towards larger Reynolds numbers. On the contrary, when the viscosity of the mixture is lower than that of the pure components, an acceleration of the fluid at the center of the mixing channel takes place, leading to an enhancement of the degree of mixing. At larger Reynolds number, unsteady, time periodic flow regimes, able to strongly increase the degree of mixing, are established for all types of mixtures. Although fluid oscillations are all characterized by typical Strouhal numbers lying in the range 0.1-0.3, the onset of the unsteady regimes, together with the corresponding vortical structures, strongly depends on the fluid rheology.