Modeling microdistillation within microfluidics devices*

Distillation is a well-known and widely-used process for the separation of binary fluids based on their differences in volatilities. The microscale counterpart, microdistillation, is a relatively new process, where surface tension becomes dominant over gravity, but has received little attention so far. Specifically, a temperature gradient is imposed across a microfluidic chip such that the temperature in the middle is above the boiling point of phase A and below that of phase B. A mixture introduced in the middle will undergo a phase separation, where one phase will evaporate while the other remains liquid. The distillate phase is then collected at the hot end of the chip, and the remaining phase at the cold output. Enhancement is achieved by guiding liquid phase using micropillars. From the modeling point of view this problem is challenging as it couples different physical processes of fluid mechanics and thermodynamics within a complex geometry. Here we present a diffuse-interface approach to model flow and phase change in confinement, by incorporating different levels of complexity progressively, ensuring that the system respects key equilibrium properties at each stage.