Bubble oscillation and motion in a filamentary streamer cavitation structure induced by power ultrasound

Acoustically induced cavitation bubbles are known for their feature of accumulating energy in ultrasound field by means of bubble oscillation. Accomp anying with bubble oscillation is the complex bubble motion, such as streaming, fragmentation, coalescence, and collapse, which play an important role in many practical applications, such as ultrasound cleaning, ultrasound homogenizing, ultrasound disintegration, and sonochemistry. The focus of the present study is to characterise bubble oscillation and motion in one of the typical cavitation structures, i.e. filamentary streamer structure, occurring under power ultrasound (Branson Sonifier 450) operated at a low energy intensity. High speed photographing analysis had revealed that the streamer structure formed under current experimental conditions had a various lengths varying from a few millimeters to centimeters. Regardless of the structure size, bubbles were clearly noticed to occur and move strongly through all filaments. After passing through the main streamer stem, all bubbles tended to join at a certain location and eventually annihilated. Upon the movements, coalescence and collapse phenomena were found to be very vigorous for both bubbles and bubble clusters. Some small streamers also united to yield a larger, more complexstreamer structure. Through coalescence, small bubbles gre w up quickly; meanwhile their oscillations altered from a stable status to a violent collapse. With a diameter less than ~120 um, most bubbles remained a relatively spherical shape although small bubbles were seen to suffer from surface instability. Bubble clusters were important participants in the discussed streamer structure. They were not only critical for the inception of streamer, but also vital for maintaining such a structure during bubble migration. It was observed that initial filaments were developed upon the bubble cluster, then extended to produce complex streamer structures. Besides, the end location for all the bubble campaign was found to have a noticeable trace of bubble cluster actions.