Increased mobility of a surfactant-retarded bubble at high bulk concentrations

We study theoretically the adsorption of surfactant onto the interface of g as bubbles in creeping flow rising steadily in an infinite liquid phase con taining surface-active agents. When a bubble rises in the fluid, surfactant adsorbs onto the surface at the leading edge, is convected by the surface flow to the trailing edge and accumulates and desorbs off the back end. Thi s transport creates a surfactant concentration gradient on the surface that causes the surface tension at the back end to be lower than that at the fr ont end, thus retarding the bubble velocity by the creation of a Marangoni force. In this paper, we demonstrate numerically that the mobility of the s urfactant-retarded bubble interface can be increased by raising the bulk co ncentration of surfactant. At high bulk concentrations, the interface satur ates with surfactant, and this saturation acts against the convective parti tioning to decrease the surface surfactant gradient. We show that as the Pe clet number (which scales the convective effect) increases, larger concentr ations are necessary to remobilize the surface completely. These results le ad to a technologically useful paradigm where the drag and interfacial mobi lity of a bubble can be controlled by the level of the bulk concentration o f surfactant.