THE INFLUENCE OF SURFACTANTS ON THE HYDRODYNAMICS OF SURFACE WETTING - I - THE NONDIFFUSING LIMIT

The hydrodynamic model of steady wetting developed by Boender et al. i s extended to include the effect of a (nonionic) surfactant in the lim iting case of negligible diffusion and low concentrations, confining a ttention to steady wetting between parallel plates. The approximation that the meniscus inclination becomes equal to the static contact angl e at a distance from the solid of the order of a molecular dimension i s extended to take account of the local surfactant concentration, maki ng use of Young's law. A second inner boundary condition, provided by a surfactant balance at the contact line, places a restriction on the speed at which the interface is shed, leading to surfactant accumulati on and partial or almost total immobilization of the interface which r educes the wetting speed. Under certain conditions, this immobilizatio n is self-stabilizing, leading to hysteresis effects. Both these effec ts and the reduced wetting speed correspond with results reported in t he literature. (C) 1998 Academic Press.