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How liquid transport occurs in confined spaces is relevant to many industrialand lab-scale processes, ranging from enhanced oil recovery to drug deliverysystems. In this work, we investigate propelling microdroplets that form fromliquid-liquid phase separation in a quasi-2D chamber, focusing on the directionand speed of microdroplets in response to local composition gradients. Theconfined ternary solution in our experiments comprises a model oil, a goodsolvent (ethanol) and a poor solvent (water). Depending on the initial solutioncomposition, water-rich or oil-rich microdroplets, form and move spontaneouslyas the ternary solution mixes with and is displaced by water diffusing from adeep side channel. Microdroplet movement is followed in situ using high-speedbright-field imaging or fluorescence imaging when the solution is doped with adye. Local ethanol composition gradients are estimated from the variation offluorescence intensity in the local continuous liquid surrounding the mobilemicrodroplets. From phase separation of the ternary solution with high oilconcentration, mobile oil-rich microdroplets form in a water-rich zone,accompanying the formation of water-rich microdroplets in an oil-rich zone. Thefast movement of oil-rich microdroplets induces directional flow transport thatmobilizes water-rich microdroplets close to the water-rich zone. The averagemicrodroplet speed increases with the initial oil concentration in the ternarysolution. The fastest speed of oil-rich microdroplets observed in ourexperiments is ~150 ${\mu}$m/s along the surface of a hydrophobic wall. Thepresence of a sharp ethanol composition gradient is the primary driving forcefor the fast movement of oil-rich microdroplets in confinement. Our resultsdemonstrate the potential of enhancing liquid transport in confinement throughcomposition gradients arising from phase separation.

How fast do microdroplets generated during liquid-liquid phase separation move in a confined 2D space?