  Displacing liquid in a confined space is important for technological
processes, ranging from porous membrane separation to CO$_{2}$ 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, ethanol, and water. These mixtures are displaced
with water or with ethanol aqueous solution. 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 in the 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 excessive energy
provided from phase separation may contribute to the enhanced displacement at
intermediate to high 1-octanol concentrations, but not at the 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.
