Shear stimulated adsorption of surfactants from micellar solutions

The minimum entropy production for flow of a liquid in a very small pore may involve slip effects at the pore walls. With micellar surfactant solutions we measured the flow resistivity of filter pores. Surfactant molecules adsorb from the micellar solutions and cover the pore walls as continuous bi-layers, called admicelles. The material in these admicelles will be dragged along by viscous flow through the pores. This is seen when pore resistivities with and without adsorbed surfactant are compared. To explain the measured resistivities there must be a considerable mobility of surfactant in the adsorbed layers. A shear stimulated surfactant transport from the solution to the pore wall must be maintained for sustained flow of surfactant in the admicelle. This mechanism involves lower energy dissipation than the flow of the solution, which contains the surfactant as micelles. The entropy production turns out to be smaller than that for the pore walls with adsorbed surfactant bi-layers, as can be deduced from the observations when there is a gradient of the surfactant concentration over the length of the pores that leads to a Marangoni-effect. The observations point to shear stimulated adsorption of surfactant as a consequence of the thermodynamic principle of minimum entropy production during steady state flow.