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.