Bubble Breathing during Dissolution in Confined Geometries under Partial Wetting

One expects that, during the dissolution of a gas bubble into surrounding liquid, the bubble volume will decrease monotonically as a result of continuous mass loss. This intuitive picture, however, changes in a surprising way when the dissolving bubble is confined in a geometry under partial wetting to the gas and the liquid. We show that the bubble then experiences cycles of volume expansion (``inhaling'') and shrinkage (``exhaling''), until it fully dissolves. This bubble ``breathing'' is the result of condensation, growth and merging of liquid droplets on the gas-solid surface, and their eventual expulsion from the gas bubble. Theoretical analysis shows that these counter-intuitive dissolution dynamics are driven by a reduction in the system's free energy. We provide a scaling argument that identifies the transition from this intermittent expansion-shrinkage ``breathing'' regime and the classical shrinkage-dominated regime. This behavior could play a major role in determining the macroscopic mass-transfer dynamics in partially wetting systems under confinement, such as oxygen transfer in low-temperature fuel cells and enhanced hydrocarbon recovery in porous rocks.