Influence of soluble surfactant on wave evolution on falling liquid films

We have investigated the effect of soluble surfactant Triton X-100 on the initial development of waves on vertically falling liquid films. Field method of laser induced fluorescence was used to obtain instantaneous wave patterns forming at different concentrations of the surfactant. These concentrations were chosen on the basis of simple consideration that most pronounced influence of the surfactant on the wave characteristics should be observed when relaxation time of surface tension (dependent on surfactant concentration) is comparable with characteristic time of wavy motion. At low and moderate surfactant concentrations damping of waves is observed, which coincides with the results of other authors. At large surfactant concentration the waves start to grow again, but without initial stage of two dimensional wave development typical of pure liquids. The absence of this initial stage is due to the formation of rivulets on the free surface of films in the presence of the surfactant. The influence of the surfactant concentration on characteristics of rivulets and waves is described. (paper)