Spontaneous gas film expansion at an oil-water interface.

Spontaneous interfacial separation of oil and water, by inserting a gas film between the two liquids, is thermodynamically unfavorable because the oil-water interface is replaced by oil-gas and water-gas interfaces with a higher combined interfacial energy. Here, we demonstrate that microstructural confinement enables this transition after bubble introduction through the coupling of wetting, capillarity, and gas transport. Spontaneous oil imbibition into a water-submerged micropillar array, together with local contact-line pinning, deforms the oil-water interface into a metastable meniscus and generates a local pressure deficit in the confined oil. This deficit, combined with the bubble's interfacial configuration, lowers its air partial pressure and promotes dissolved-air uptake from the surrounding water. Geometric confinement, reduced oil pressure, and low oil-gas interfacial tension favor bubble lateral invasion of adjacent oil-filled cells over vertical extrusion. As the gas front advances, meniscus relaxation and oil redistribution promote further wetting, renewing the capillary conditions that support gas film propagation. We develop a coupled transport-capillarity framework that captures these morphological transitions and predicts a characteristic timescale for gas film evolution, determined by the interaction among geometry, capillarity, and gas-phase transport. Spatially varying the microstructure further controls the propagation pathway and morphology of the gas film. These findings show how favorable wetting can support an energetically costly interfacial transformation through a confinement-induced capillary pressure field, providing a general mechanism for gas-mediated separation in confined multiphase systems.