Waves at liquid surfaces II. Surfactant action and coupled oscillators

The effects of surfactant action upon the dispersion of capillary and dilational modes on liquid surfaces have been considered. The recently predicted reduction in stability of the dilational waves is shown to have profound consequences. In particular, under conditions of strong coupling between the two surface modes, this reduced stability can induce the appearance of mixed excitations, neither capillary nor dilational in nature. The predicted observable effects are discussed and shown to differ significantly from those characteristic of the only other known route to surface wave mixing, involving the transverse shear viscosity. Some of the present predictions have been verified by data from an experimental study of dilute aqueous solutions of n -heptanol.