Bubble collapse modality and sono-chemical activity

SUMMARY Microscopic high-speed observations of acoustic cavitation bubbles are linked to their chemical activity in two set-ups: (i) alkali-metal line emission is investigated in a multi-bubble system below a sonotrode driven at 20 kHz in concentrated sulfuric acid, and (ii) the sono-chemical isomerization of fumaric from maleic acid in aqueous solution, mediated by bromine radicals, is observed in a single-bubble levitation cell at 18 kHz driving frequency. In both cases, non-spherical bubble dynamics and collapse is found to possibly play a key role for bubble activity. In the acid, surface unstable as well as fast translating and jetting bubbles seem to be the sources of pronounced Na* line emission. In the levitation cell, the transition of a single bubble to a cluster of few bubbles appears to mark the start of chemical activity in form of generation of a white streak of solid product. Bubble dynamics in the cluster is characterized by strong interaction with frequent splitting and merging of mainly one larger bubble with smaller fragments. We conclude that processes of microscopic mixing of liquid into the gas phase of the bubble interior are promoting the indicated activity, and we speculate on the exact mechanisms of such mixing by jetting collapse and bubble collision.