Spreading of surfactant solutions over thin aqueous layers: influence of solubility and micelles disintegration

A moving circular wave front forms after a small droplet of aqueous surfactant solution is deposited on a thin aqueous layer. The time evolution of the radius of the moving front was monitored. Surfactants of different solubility were used at concentrations above CMC. It is shown that the time evolution of the moving front proceeds in two stages: a rapid first stage, which is followed by a slower second stage. It is shown that the time evolution of the moving front substantially depends on the surfactant solubility. An exact solution for the evolution of the moving front was deduced for the case of insoluble surfactants. A qualitative theory was developed to account for the influence of the solubility on the front motion. Our experimental observations are in a good agreement with the theory predictions.