Free drainage in aqueous foams: Model and experimental study

Abstract

AbstractFree drainage in compressed‐air foams were studied experimentally and theoretically. The time evolution of liquid holdup profiles in a 0.2‐m‐high by 0.29‐m‐diameter foam column was determined at various heights by measuring sonic velocity. A new experimental technique was devised to measure the true drainage rate of surfactant solution leaving the foam column. A drainage model was outlined to predict the discharge rate and evolution of the liquid‐fraction profile in aqueous foams. The model led to the formulation of a nonlinear partial differential equation in which the liquid fraction was used explicitly as a dependent variable. The model was applied with one adjustable parameter to simulate drainage in foams made from fluorocarbon surfactants containing mobile plateau border and film walls. The liquid‐fraction profiles, drainage rates, and final equilibrium liquid profiles depended strongly on the surface mobility of plateau border and film walls; the bubble size; and therefore on the coarsening histoy in the foam under study. Over longer time periods this model needs to be coupled with interbubble gas diffusion to account for coarsening‐induced drainage.