Dynamic behavior of natural sea surfactant films

Abstract

The dynamic behavior of sea surfactants is studied at timescales from 0.04 to 2 seconds by generating waves on the water containing its natural surfactants in the laboratory and comparing dynamical measurements with theoretical predictions for prescribed surfactant properties. The properties considered are film pressure, elasticity and surface viscosity. For longitudinal Marangoni waves in the frequency range of 0.5 to 4.0 Hz, time‐varying film pressures are measured. For transverse waves in the frequency range of 3 to 25 Hz, spatial decay rates are measured. Prior to conducting experiments with sea water containing soluble natural surfactants, the procedures and methods of analysis are validated by experiments with clean fresh water and with an insoluble oleyl alcohol film. A notable finding is that the static film elasticity accurately predicts the dynamic behavior of both the insoluble oleyl alcohol film and the soluble natural sea surfactant films. To better understand the reasons for this finding, sea surfactant adsorption and desorption time histories were measured. The adsorption/desorption time scales ranged from 46 to 196 min. One reason for the accurate prediction of surfactant dynamic behavior by the static elasticity is that the timescales of the waves are much shorter than the adsorption/desorption timescales. The conclusion is that the static elasticity controls the interactions of surfactants with most hydrodynamic disturbances having timescales up to several min.