Rectified diffusion in the presence of surfactants

Abstract

Measurements are reported of the growth of air bubbles by rectified diffusion in water at 22.1 kHz. Values were obtained of both the threshold and the rate of growth of bubbles by rectified diffusion as a function of bubble radius, acoustic pressure amplitude, liquid surface tension, and equilibrium gas concentration. Good agreement with theory was obtained for normal values of the surface tension of water for both the rectified diffusion threshold and the growth rate. With addition of a surfactant, measured values of the threshold and growth rate deviate from the calculated values, the disagreement becoming considerable at large surfactant concentrations (low surface tension). Surface‐wave activity of the bubble that would increase the diffusion rate by acoustic streaming was not observed at low bubble radii and is not thought to be the cause of the disagreement. A possible explanation is given in terms of the retardation of gas diffusion by the surfactant monolayer on the surface of the bubble. [Work su...