Oil column pinch-off controls the oil fraction of the oil-coated bubble

We study experimentally the formation of millimeter-sized oil-coated bubbles at a customized co-axial orifice system and the pinch-off dynamics of the oil column attached below the rising gas bubble. 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, with the oil fraction set by the pinch-off location. We show that this pinch-off location is dominated by the size ratio of the gas bubble/oil tail to the outer orifice, and a theoretical model is proposed to predict the oil fraction, describing the experimental results well. Our findings provide potential guidelines for the controllable generation of compound multiphase bubbles using co-axial orifices.