Oscillation and Migration of Bubbles within Ultrasonic Field**Supported by National Natural Science Foundation of China under Grant Nos 51327901, 51727803 and 51571164, the NPU Excellent Personnel Supporting Project, the Fundamental Research Fund of Northwestern Polytechnical University under No 3102018jcc039, and the Key Research Plan in Shanxi Province (2018GY-104).

Abstract

The oscillation and migration of bubbles within an intensive ultrasonic field are important issues concerning acoustic cavitation in liquids. We establish a selection map of bubble oscillation mode related to initial bubble radius and driving sound pressure under 20 kHz ultrasound and analyze the individual-bubble migration induced by the combined effects of pressure gradient and acoustic streaming. Our results indicate that the pressure threshold of stable and transient cavitation decreases with the increasing initial bubble radius. At the pressure antinode, the Bjerknes force dominates the bubble migration, resulting in the large bubbles gathering toward antinode center, whereas small bubbles escape from antinode. By contrast, at the pressure node, the bubble migration is primarily controlled by acoustic streaming, which effectively weakens the bubble adhesion on the container walls, thereby enhancing the cavitation effect in the whole liquid.