Phase-field modeling of colloid-polymer mixtures in microgravity.

Colloid-polymer mixtures are an archetype for modeling phase transition processes, as they exhibit a low-density gas phase, high-density crystalline phase and an intervening liquid phase. While their equilibrium behavior has been studied extensively, the role of hydrodynamics in driving their phase separation is not yet understood. We present a theoretical model that describes hydrodynamic interactions in colloid-polymer mixtures in a microgravity environment. Our phase-field model consists of the Cahn-Hilliard equation, which describes phase separation processes in multicomponent mixtures, coupled with the Stokes equation for viscous fluid flow. We account for the dependence of the suspension viscosity on the colloid concentration, and the so-called Korteweg stresses that arise at the interfaces of colloidal phases. We process video microscopy images from NASA's Binary Colloid Alloy Test (BCAT) experiments, which were performed on the International Space Station. While terrestrial experiments would be dominated by gravitational forces and buoyancy-driven flows, the microgravity environment of the BCAT experiments allows for the visualization of phase separation by low interfacial tension, and thus enables a quantitative comparison between experiment and our model predictions.