Linear and nonlinear gravity-capillary water waves with a soluble surfactant

Naturally occurring soluble-surfactant slicks influence the properties of w ater waves. This paper describes results from wave tank experiments involvi ng a soluble surfactant, and linear and nonlinear gravity-capillary waves. Instantaneous surface deflections were measured using optical techniques to determine the damping, phase speed, and the frequency content of the waves for wavemaker frequencies from 4 to 22 Hz. Measured linear-wave phase spee d and damping agree well with existing theory at surfactant concentrations away from that leading to maximum damping. Under conditions leading to nonl inear waves, an as-yet-unexplained subharmonic wave with one-sixth the wave maker frequency was found only with soluble surfactant present.