Microscopic Exploration of Liquid-liquid Phase Separation Mechanism in LCST Solutions

Abstract

A partially miscible solution with a lower critical solution temperature (LCST) is a potential coolant for microchannel heat sinks to meet the multi-goals of high cooling performance, low-pressure drop and stable operation. However, it is a great challenge to fundamentally understand the liquid-liquid phase separation mechanism, and to harvest benefits from it for evaluating the cooling performance and regulating the flow pattern of the LCST solutions in microchannels. Here, we observed and explained the liquid-liquid phase separation behaviors of 2-Butoxyethanol/water solutions with various mass fractions (ωp) and heating rates, consisting of spinodal decomposition or nucleation, droplet growth (diffusion-driven), and droplet coalescence (convectiondriven, stage Ⅰ and stage Ⅱ). The results proved that the separated molecular cluster is covered with a layer of water molecules, causing the mass fraction of 2-Butoxyethanol in the separated liquid to be 65.4% instead of 100%. To keep a mist or droplet flow pattern in microchannels, some efforts must be made to reduce the droplet diameter and suppress the droplet coalescence in stage Ⅱ. Although the solutions with better cooling performance have been determined at ωp=27.4%~35%, future works should focus on whether the liquid-liquid phase separation ends at the droplet growth stage or coalescence stage.