Experimental study of secondary oil droplet formation during coalescence process of oil Droplet-Gas bubble

•Full coalescence and partial coalescence are elucidated.•Interfacial force at the oil droplet bridge affects coalescence process.•Critical line of partial coalescence and full coalescence has been determined. Flotation is an efficient pretreatment technology for oily water. In this study, the coalescence processes of air bubbles and isooctane droplets with different size ratios, 1.0–2.2, were investigated using a high-speed imaging system. Two types of oil droplet-gas bubble interaction have been identified: full coalescence and partial coalescence. Through analysis of interfacial force at the liquid bridge, it has been observed that interfacial force at the oil droplet bridge affects the lateral contraction of the liquid bridge, which in turn affects the coalescence of oil droplets and gas bubbles. Through dimensional analysis, a phase diagram of dimensional parameter coalescence behavior has been constructed, and the critical line of partial coalescence and full coalescence has been determined. This work fills the void in the study of secondary oil droplet coalescence within the oil droplet-gas bubble coalescence in gas flotation, which is potentially applied for industrial air flotation system design and optimization.