Shear stimulated adsorption of surfactants from micellar solutions

The minimum entropy production for flow of a liquid in a very small pore ma y involve slip effects at the pore walls. With micellar surfactant solution s we measured the flow resistivity of filter pores. Surfactant molecules ad sorb 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 resistivit ies with and without adsorbed surfactant are compared. To explain the measu red resistivities there must be a considerable mobility of surfactant in th e adsorbed layers. A shear stimulated surfactant transport from the solutio n to the pore wall must be maintained for sustained flow of surfactant in t he admicelle. This mechanism involves lower energy dissipation than the flo w of the solution, which contains the surfactant as micelles. The entropy p roduction turns out to be smaller than that for the pore walls with adsorbe d 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 stimulate d adsorption of surfactant as a consequence of the thermodynamic principle of minimum entropy production during steady state flow. (C) 2001 Published by Elsevier Science B.V. All rights reserved.