Antibubble formation and dynamics in detergent solution: Effects of droplet impact conditions and flow parameters

An antibubble is a liquid droplet encapsulated by a thin air shell and immersed in the same liquid phase, representing the inverse configuration of a conventional soap bubble. In this work, we present a systematic experimental investigation of antibubble formation and dynamics using a controlled drop-by-drop method. Antibubbles are generated by the impact of a liquid droplet onto a quiescent air–liquid interface, followed by air entrapment during cavity collapse. The influence of geometrical and flow parameters, including nozzle diameter, impact height, and volumetric flow rate, on antibubble formation is examined in detail. Key characteristics, including antibubble diameter, penetration depth, jet height evolution, primary droplet formation, and the probability of antibubble formation, are quantified. Our results show that antibubbles form only when a secondary droplet is generated during jet retraction, while also satisfying the additional requirements of air film entrapment and correctly timed cavity closure. The number of antibubbles generated exhibits a non-monotonic dependence on impact height, with a nozzle-specific optimum corresponding to a critical Weber number. At low-impact heights, insufficient kinetic energy prevents air entrapment, while at higher-impact heights, excessive inertia suppresses secondary droplet formation, thereby inhibiting antibubble formation. Higher flow rates promote increased antibubble formation. The penetration depth of antibubbles initially increases with Weber numbers due to enhanced momentum transfer but decreases with higher Weber numbers owing to increased interfacial disturbances and dissipation. In contrast, the mean antibubble diameter scales primarily with the impacting droplet diameter and shows only a weak dependence on the Weber number.