Enhanced displacement of phase separating liquid mixtures in 2D confined spaces

Displacing liquids in a confined space is important for technological processes ranging from porous membrane separation to CO₂ sequestration. The liquid to be displaced usually consists of multiple components with different solubilities in the displacing liquid. Phase separation and chemical composition gradients in the liquids can influence the displacement rate. In this work, we investigate the effects of liquid composition on the displacement process of ternary liquid mixtures in a quasi-2D microchannel where liquid−liquid phase separation occurs concurrently. We focused on model ternary mixtures containing 1-octanol (a model oil), ethanol (a good solvent), and water (a poor solvent). These mixtures are displaced with water or with an ethanol aqueous solution. As a comparison, for some experiments, water was displaced by ternary mixtures. The bright-field and fluorescence imaging measurements reveal distinct phase separation behaviors. The spatial distribution of subphases arising from phase separation and the displacement rates of the solution are impacted by the initial ternary solution composition. The boundary between the solution and displacing liquid changes from a defined interface to a diffusive interface as the initial 1-octanol composition in the solution is reduced. The displacement rate also varies non-linearly with the initial 1-octanol composition. The slowest displacement rate arises at intermediate 1-octanol concentration, where a stable three-zone configuration forms at the boundary. At very low 1-octanol concentration, the displacement rate is fast, associated with droplet formation and motion driven by the chemical concentration gradients formed during phase separation. The excess energy provided from phase separation may contribute to the enhanced displacement at intermediate to high 1-octanol concentrations but not at low 1-octanol concentration with enhancement from induced flow in confinement. The knowledge gained from this study highlights the importance of manipulating phase separation to enhance mass transport in confinement for a wide range of separation processes.