Effect of Ionic Surfactants on the Dimple Relaxation in Wetting Films

A procedure for simulation of the dimple relaxation in wetting films is developed based on a precise treatment of the effects of ionic surfactants. The theory is compared with experimental data for the drainage of films of 1 mM solution of sodium dodecyl sulfate. An important effect of the interfacial electrostatics to reduce substantially the Marangoni effect is discovered. It is shown that the adsorption-pressure coupling is also important for ionic surfactants, which, enhanced by the decreased Marangoni number, increases the thinning rate close to the equilibrium. The latter could be a plausible explanation of the discrepancy in the mobility determined by drainage experiments and by dynamics of artificial waves on an equilibrium film from ionic surfactant solution.