Laboratory study of the damping of parametric ripples due to surfactant films

Laboratory studies of damping of capillary-gravity waves (CGW) due to organic films were carried out in a wide range of surfactant concentrations using a method of parametrically excited surface waves (Faraday ripples) at wave frequencies from about 15 Hz to 30 Hz. The threshold acceleration for CGW excitation and CGW wavelengths were measured and the wave damping and the surface tension coefficients were retrieved for a clean water surface and for water covered with organic films. It is obtained that the damping coefficient for ordinary surface-active substances (e.g., oleyl alcohol and oleic acid) exhibits a maximum at small surfactant concentrations, comparable with the concentrations needed for monomolecular coverage and remains practically constant at large concentrations. The damping coefficient for polymer films (polyoxyalkylene glycol - “Emkarox”) shows two maxima, one of which is a narrow peak at small concentrations similar to the case of ordinary surfactants, and a “plateau”-like maximum at large concentrations. The dynamic film elasticities are retrieved from the measured damping coefficient using a theory of wave damping for purely elastic films. The dynamic elasticities are shown to differ strongly from the static elasticities, especially at large surfactant concentrations.