The Role of Surfactant Headgroup, Chain Length, and Cavitation Microstreaming on the Growth of Bubbles by Rectified Diffusion

The role of a surfactant on the rate of bubble growth via rectified diffusion during acoustic cavitation has been investigated. The charge of the headgroup of ionic surfactants did not affect the bubble growth rate at low surface loadings. However, at higher surface loading, the growth rate was significantly dependent upon the type and charge of the headgroup. Dodecyl trimethyl ammonium chloride (DTAC) caused a greater increase in growth rate than sodium dodecyl sulfate (SDS) or dodecyl dimethyl ammonium propane sulfonate (DDAPS). With charge suppressed by the addition of salt, DDAPS showed the highest growth rate. Particle-image velocimetry (PIV) has been used to characterize the microstreaming around a bubble in aqueous solutions of these surfactants. Results indicate an enhancement of microstreaming velocities in the vicinity of the bubble when surfactant of any form is present in the solution, with the greatest enhancement for the surfactant with the most bulky headgroup. A clear difference could also be observed when bubbles underwent surface mode oscillations that dramatically increased the streaming velocities and resulted in a chaotic flow. The enhancement in surface oscillations and microstreaming was greatest for DTAC reflecting the bulkiness and charge of the headgroup. The resistance to mass transfer also played a role in enhancing the rectification of gas into the bubble. Surfactants with a longer chain length provide greater mass transfer resistance, and this contributes to higher growth rates.