Diffusion-mediated Spinodal Decomposition in Ternary Mixtures

Ternary mixtures can undergo phase separation in response to concentration changes. It has been observed that a ternary mixture of oil-water-ethanol in a microchannel with water leads to the formation of a phase separating front, leaving oil- and water-rich stripes in its wake due to ethanol diffusion out of the mixture. We explore these dynamics via a system that is comprised of a stable ternary mixture and a stable single-component phase in contact. This interface allows for the preferential diffusion of component 3 out of the mixture (mimicking ethanol), causing the mixture to become unstable and undergo spinodal decomposition from the interface. By rewriting a Flory-Huggins free energy, assuming for simplicity that the interaction parameters involving component 3 are zero, we get an effective binary mixture description, parameterized by component 3. This allows the ternary mixture to potentially become unstable once component 3 concentration goes below a cut-off value. Using Cahn-Hilliard dynamics, we explore features of diffusion-mediated patterns such as length scale, phase composition, and front velocity. We can then extend this model to microfluidic systems for industrial use such as aqueous two-phase co-flows, where advection can affect the phase-separated patterns.