Bubbly cavitating flow in a 200-kHz ultrasonic cleaning bath

Recent studies on ultrasonic cleaning suggest that acoustic cavitation bubbles play a dominant role in physical cleaning. Inside cleaning baths, standing-wave-like acoustic fields are formed due to repeated reflections of underwater ultrasound at the bath walls and the free surface. In these acoustic fields, however, acoustic intensity gradients can appear due to dissipative effects of the fluid and cavitation bubbles; in this case, cavitation bubbles can translate by the acoustic radiation force that appears under such acoustic intensity gradients, thereby entraining the surrounding liquid. Here, we experimentally examine bubbly cavitating flow that appears in an ultrasonic cleaning bath. We perform PIV measurement of streaming flow of both cavitation bubbles and water in a 200-kHz ultrasonic cleaning bath in order to quantify the liquid flow entrainment by the translating cavitation bubbles. We also perform a cleaning test using a glass slides on which small silica particles are spin-coated. SEM images of the cleaned samples show that the cleaned patterns agree with the direction of the bubbles' translation, supporting the fact that cavitation bubbles play a dominant contribution to particle removal.