Anomalous Damping of Capillary Waves With Surfactant Solutions

We have studied the dilational viscoelasticity of the surface of various surfactant solutions by using an excited capillary waves technique. We find that the dilational viscosity is negative in some cases, depending on the surfactant chain length, ionic strength, pH, bulk concentration and frequency of the waves. We have investigated the linearity of the response of the monolayer, and the possible influence of surface modes coupling. A correlation with phase behavior in the monolayer is discussed.