Frequency effects during acoustic cavitation in surfactant solutions.

The acoustic cavitation-induced events, multibubble sonoluminescence (MBSL) and initial growth of MBSL have been studied in surfactant solutions and correlated with bubble coalescence data at three different ultrasound frequencies. For an ionic surfactant, both the number of ultrasonic pulses required to reach a steady state MBSL intensity (N(crit)) and the magnitude of this intensity increases to a maximum as the surfactant concentration increases and then falls again. The total bubble volume generated for a fixed sonication time, which is indirectly related to bubble coalescence, similarly falls as surfactant concentration increases and then rises again. These effects are caused by a combination of electrostatic and coalescence factors at relatively low surfactant concentrations and the screening of the electrostatic factor as surfactant concentration increases further. The peak in coalescence inhibition occurs almost at the same surfactant concentrations as the acoustic frequency is increased; however, the concentrations at which peaks in MBSL and N(crit) occur vary at different frequencies. These results have been discussed in terms of coalescence, electrostatic interactions, rectified diffusion growth, and the adsorption kinetics of the surfactants.