Influence of Surface-Active Solutes on the Coalescence, Clustering, and Fragmentation of Acoustic Bubbles Confined in a Microspace

Abstract

High-speed imaging was used to capture the effect of surface-active solutes on the behavior of acoustic bubbles, generated in a microspace using low-frequency ultrasound (60 kHz). By confining cavitation within a microspace, the dynamic behavior of bubbles, such as bubble coalescence, clustering, and fragmentation, could be observed directly. It was observed that bubbles coalesced instantly in water; however, in the presence of surface-active solutes (n-propanol and sodium dodecyl sulfate, SDS) the coalescence was hindered. Low-density bubble clusters were observed in the presence of 1 mM SDS and 0.1 M n-propanol. When 0.1 M sodium chloride was added to 1 mM SDS solution, the extent of clustering and the density of the clusters enhanced significantly; a similar observation was made at a higher SDS concentration (10 mM). The importance of these results in understanding multibubble sonoluminescence data published previously has been addressed. The collective oscillation of a bubble cluster consisting of different sized bubbles and images of bubbles emitting a fountain of microbubbles have also been presented.