Pinch-off dynamics of oil column determines the oil fraction of compound bubble *

Compound multiphase bubbles are widely present in nature and industrial processes, such as gas releases from natural seeps and oily bubbles used in froth flotation. In these scenarios, the effect of the outer coating phase on bubble formation and the coating fraction remains unknown. Here, we use a customized co-axial orifice system to investigate the formation mechanism of millimeter-sized compound multiphase bubbles. We experimentally study how the orifice geometry and liquid properties control the oil fraction of the formed oil-coated bubbles. After the gas bubble detaches from the inner orifice, it rises under buoyancy, and stretches the oil column to cause pinch-off, forming an oil-coated bubble. The pinch-off location of the oil column sets the oil fraction of the oil-coated bubble. We show that the oil column pinch-off location is mainly determined by the size ratio of the gas bubble/oil tail to the outer orifice. A theoretical model is proposed to predict the oil fraction, which agrees well with the experimental results. Our findings provide potential guidelines for the controllable generation of compound bubbles using co-axial orifices.