Optical visualization of ultrasonic cavitation bubbles near an oil-deposited glass surface

Low-frequency underwater ultrasound is conventionally used to degrease metallic parts. It is important to reveal its cleaning mechanism in order to achieve more efficient cleaning for practical purposes. In this study, we set up a simple ultrasonic cleaning test and optically visualize ultrasonic cavitation bubbles. Glass plates on which an oil drop is deposited are used as a cleaning sample. The sample is soaked in a water container and subjected to 28-kHz underwater ultrasound. The ultrasound-induced cavitation bubbles near the glass surface are recorded by a high-speed camera. It is observed that cavitation bubbles nucleate preferentially in the water (not in the oil) and subsequently penetrate into the oil, perhaps, due to Bjerknes force. In the presentation, we will report on the complex dynamics of bubbles in the oil and their role in degreasing.