Escape of an air bubble from a droplet under power ultrasound

Escape of air bubbles from a droplet driven by ultrasound is of great relevance to many industrial processes, featuring the passage of air bubbles through a moving interface between two immiscible liquids. In this study, we experimentally investigated the interactions between an air bubble and a moving liquid–liquid interface, including the identification of different interaction outcomes, bubble velocity change, and the role of bubble-to-droplet size ratio. Five bubble-interface interaction outcomes were observed, namely, upward and downward penetration, semi-penetration, inward bouncing and interface locking. A theoretical model was developed for predicting the interaction outcome between a bubble and a moving interface. The results revealed that the driving pressure of power ultrasound that yields an acoustic radiation force acted as the main mechanism driving the bubble to penetrate the fluid interface. On the contrary, the interfacial tension strengthened bubble-droplet attachment and suppressed the bubble’s passage. The effect of the confined liquid size was found to be marginal. The study presents the knowledge that could be used to inform the design and operation of relevant technologies where ultrasound is applied to control bubble dynamics in the confined liquid.