Mechanisms of oil droplet detachment by cavitation bubbles in ultrasonic cleaning

Efficient removal of oily contaminants from solid surfaces remains a significant challenge in various engineering applications. While ultrasonic cleaning mechanisms for solid particles are well established, the detachment of oil droplets is less understood due to the distinct interfacial and viscous characteristics of oils. Here, we experimentally investigate oil droplet removal under ultrasonic excitation, focusing on the role of cavitation bubble dynamics. High-speed visualizations reveal that cleaning is initiated by bubble penetration into the droplet, followed by successive coalescence and growth of bubbles. This process progressively reduces the droplet-substrate contact area, ultimately resulting in detachment. We identify a critical acoustic pressure threshold above which bubble penetration becomes frequent and cleaning efficiency increases sharply. The results indicate that oil detachment is governed by microjets generated during bubble collapse, which must overcome interfacial resistance for successful penetration. These findings provide mechanistic insight into the ultrasonic removal of oil-based residues and offer practical guidance for enhancing cleaning performance through control of acoustic parameters.