TRANSPORT OF A PASSIVE SOLUTE BY SURFACTANT-DRIVEN FLOWS

A localized insoluble monolayer of nonuniform concentration on the fre e surface of a thin viscous fluid layer will, through the generation o f surface-tension gradients, drive a shear flow causing the monolayer to spread and inducing large deformations of the free surface of the l iquid layer. An existing model of surfactant-driven flows, based on lu brication theory, is extended here to incorporate the transport of a p assive solute and absorption of the solute at the wall beneath the liq uid layer. A combination of numerical and averaging techniques are emp loyed, appropriate to a wide range of solute diffusivities, time scale s and absorption rates. It is shown that diffusion of the solute acros s the liquid layer causes the solute to be advected on average more sl owly than the surfactant, because the solute is carried into regions m oving more slowly than the fluid at the free surface; diffusion along the layer may overwhelm advection in certain circumstances, however. B ecause of the flow-induced deformation of the liquid layer depth, abso rption at the lower boundary, if it occurs, does so in a spatially non uniform manner. The results have implications for techniques of pulmon ary drug delivery and flow visualization.