Simultaneous Observation of Bubble Clouds and Microhollows Produced by Bubble Cloud Cavitation

Abstract

Sonoporation, which forms small pores through the cell membrane, is a useful method in ultrasonic-wave-mediated drug delivery systems. However, if microbubbles aggregate by acoustic radiation force and form bubble clouds, the mechanism of sonoporation becomes complicated. In this paper, bubble cloud cavitation is evaluated by simultaneous observation of bubble cloud motion and microhollows, which are produced on a flow channel wall. To observe the bubble cloud motion, three optical methods are adopted. Three-dimensional position measurement of bubble clouds using two cameras with different focal lengths, which is newly developed in this study, shows the three-dimensional motion of bubble clouds during cavitation. Microhollows on the flow channel wall are evaluated by confocal laser microscopy. It is found that bubble cloud cavitation can be classified into three stages. Among them, the first and second stages, which are characterized by both bubble cloud movement in the vicinity of the wall and the formation of large bubble clouds, play important roles in microhollow production.