Microfluidics of Temperature-Responsive Partially Miscible Binary Liquid Mixtures

Two-phase liquid-liquid microfluidics relies on the intricate control over the fluid properties of liquid mixtures. Herein, we report on the use of partially miscible binary liquid mixtures that lend their microfluidic properties from a highly temperature-sensitive mixing and phase separation behaviour. For a blend composed of the thermotropic liquid crystal 4-Cyano-4'-pentylbiphenyl (5CB) and methanol, mixing at temperatures above the upper critical solution temperature (UCST) leads to a uniform single phase while partial mixing can be achieved at temperatures below the UCST. Thermally-driven phase separation inside the microfluidic channels results in the formation of very regular phase arrangements, namely in droplets, plug, slug and annular flow. We map different flow regimes and relate findings to the role of viscous, interfacial and inertial forces. As the interfacial tension of the mixture and the dynamic viscosity of the separated phases are inversely proportional to temperature, different flow regimes can be achieved at constant channel architecture and flow rate. A consistent behaviour is observed for a binary liquid mixture with lower critical solution temperature, namely 2,6-lutidine and water. This temperature-responsive approach to microfluidics is an interesting candidate for multi-stage processes, selective extraction and sensing applications.