Kelvin-Helmholtz instability in binary fluids with miscibility gap

Abstract

The isothermal spatio-temporal evolution of an interface between binary fluids, with temperature sensitive miscibility gap, subjected to shear flow is investigated using direct numerical simulations. The thermophysical properties and the interfacial tension in such fluids exhibit a dynamic temperature dependence based on the degree of miscibility. These fluid pairs can therefore be deployed in crucial microfluidic applications involving enhancement of liquid-liquid mass transfer. In this study, a modified phase-field approach that allows modeling of partially miscible fluids and continuous transition from an initially immiscible state to a completely miscible state is employed to investigate the Kelvin-Helmholtz (KH) instability. The analysis entails two different configurations based on the thermodynamic equilibrium of the system. In the first configuration, the fluid pair is considered in equilibrium at different degrees of partially miscibility. The second configuration deals with fluids out of equilibrium thereby allowing simultaneous mass transport across the interface. The results reveal the effect of apparent surface tension in miscible fluids, shear flow profile, the degree of stratification and the effect of mass diffusion across the interface. The shear instability in a fluid pair close to the consolute point is found to be independent of the extent of stratification. Finally, the competition between the time scales of the instability and the diffusion process reveals the importance of the surface tension of binary fluids in the immiscible limit.