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. Here, we experimentally investigate the formation of oil-coated bubbles at submerged coaxial orifices in quiescent liquids. A force balance model is developed to accurately predict the bubble size, which is determined by the revised Bond number, size ratio of the coaxial orifices, and the interfacial tension ratio of oil-water and oil-air interfaces. The results may be useful in estimating the size distribution of oil-coated bubbles as an input to characterize the transport and dynamics of bubbly flows where compound interfaces are present.