Dissolution of a freely rising bubble in aqueous surfactant solutions

A numerical simulation is used to determine the dissolution rate of a freel y rising bubble in both pure water and aqueous surfactant solutions. Result s are presented for three different gases representing a broad range of sol ubilities. In pure water, highly soluble gases dissolve more slowly than on e would expect from a quasi-steady boundary layer theory. It is argued that this is a consequence of the effect of the moving boundary. While surfacta nts strongly affect the dissolution rate for gases with small solubilities, they have relatively little effect on gases with large solubilities. The e ffects of surfactant sorption kinetics are investigated using the Langmuir model. Over a broad range of sorption parameters, a surfactant cap forms an d grows as the bubble accelerates. For highly soluble gases, the surfactant cap is too small to significantly affect the dissolution process until mos t of the bubble's mass is gone. A criterion for a critical surfactant conce ntration below which the surfactant has relatively little effect on the bub ble is developed and tested with the simulations. For a range of surfactant concentrations, a bubble can achieve a maximum speed that is larger than t he steady-state speed of a non-dissolving bubble. A criterion for the criti cal surfactant concentration below which this will occur is developed and t ested with the simulations. (C) 2000 Elsevier Science Ltd. All rights reser ved.