Oil-coated bubble formation from submerged coaxial orifices

Dispersions of bubbles with a compound interface in liquids are ubiquitous in nature and various industrial processes, such as marine oil transport and froth flotation, hence understanding surface phenomena effect on bubble formation in liquids remains as a canonical topic in fundamental multiphase flows. Here, we experimentally investigate the formation of an oil-coated bubble at submerged coaxial orifices in quiescent liquids. The air bubble is initially injected inside a confined oil domain and then exits into the surrounding water under buoyancy, forming an oil-coated bubble. Compared with the bubble formation in a single-phase liquid, the bubble size is significantly modified due to the presence of the oil phase. We develop a force balance model to predict the oil-coated bubble size, considering the orifice geometry as well as the compound air/oil/water interface, which agrees well with the experimental results. We further discuss the dependencies of the bubble size on the Bond number, size ratio of the coaxial orifices, and the interfacial tension ratio of oil-water and oil-air interfaces. Our study could contribute to further understandings on the dynamics of bubble formation with a compound interface.