STABILITY OF THIN EVAPORATING CONDENSING FILMS IN THE PRESENCE OF SURFACTANTS

A new equation is presented for the evolution of film thickness which, in addition to van der Waals dispersive forces and surface tension, includes flow effects arising from surface tension gradients. The effect of surfactant is quantified in terms of its adsorption at the liquid-gas interface. A linear stability analysis shows that the dynamics of evaporating/condensing thin films in the presence of surfactant are entirely different from isothermal films with surfactant and non-isothermal films without surfactant. Thus, flows driven by surface tension gradients originating from surfactant concentration variations, i.e. Marangoni flows, are in a direction opposite to that of similar flows originating from surface temperature variations, i.e. thermocapillary flows. The former usually dominate; they are destabilizing for condensing films and stabilizing for evaporating films..