High-speed visualization of ultrasonic emulsification in microgravity: interactions between cavitation dynamics and liquid-liquid interface topology.

Ultrasonic emulsification is a widely used technique for generating fine dispersions of immiscible liquids, yet its underlying mechanisms remain only partially understood, particularly under reduced-gravity conditions. In the absence of gravity, buoyancy-driven phase separation vanishes, fundamentally altering the organization of liquid-liquid interfaces and their interaction with cavitation structures. This study investigates the dynamics of ultrasonic emulsification in microgravity using high-speed visualization during parabolic flight experiments, with systematic variation of oil-water volume ratios. The results reveal that microgravity leads to capillarity-dominated interface configurations, where the position and stability of the oil-water interface become less predictable compared to normal gravity conditions. As a consequence, sustained interaction between the cavitation zone and the interface is more difficult to achieve. Since such interaction was previously identified as the key mechanism driving droplet breakup, its reduction leads to slower emulsification rates and lower final emulsion homogeneity. Image analysis shows that emulsification under normal gravity proceeds faster and produces more homogeneous dispersions, while microgravity conditions lead to delayed evolution and lower overall homogeneity. The findings demonstrate that ultrasonic emulsification in microgravity is governed by an interplay between interface topology and cavitation dynamics and highlight the importance of controlled interface positioning for enabling efficient multiphase fluid processing in space environments.