Evaporative phase separation in polymer microdroplets

Evaporation of sessile drops has been the subject of extensive research due to its underlying physics and consequential impact on various applied processes. While there is an extensive understanding of the evaporation of pure and colloidal drops, especially in the context of the well-known phenomenon known as the "coffee-stain effect", less research has been dedicated to exploring the evaporation of multicomponent drops. The evaporation of such drops can initiate a process called liquid-liquid phase separation (LLPS), near the regions where significant evaporative fluxes occur.

Evaporation can drive initially homogeneous multiphase liquid systems out of equilibrium to induce liquid-liquid phase separation (LLPS). Here, we demonstrate evaporative LLPS in microfluidic-generated emulsion microdroplets of polymer mixtures. The evaporation produces distinct polymer phases within the microdroplets. Phase separation occurs even with polymer combinations that do not form distinct phases in sessile droplet evaporation. We attribute this aspect to evaporation-driven solutal Marangoni flows and the interface capture accumulating the nuclei at the apex where the evaporation rate is the maximum. A fast coalescence and growth of the accumulated polymer nuclei occurs inside the droplets, unlike the capillary-flow-induced spread-out of the nuclei along the contact line in sessile drops. Our method of evaporation of the droplet cluster may facilitate studying LLPS in volume-limited environments and have implications for understanding LLPS in biological systems.