Remotely sensing bound and free small-scale waves at the edges of monomolecular surface films in a wind-wave tank

We have carried out wind-wave tank measurements to investigate the small-scale roughness of the water surface at the edges of monomolecular surface films. Small amounts of oleyl alcohol, triolein, and palmitic acid methyl ester were carefully deployed on the water surface at wind speeds between 3 m/s and 9 m/s, thus allowing for wave measurements at either end (i.e., downwind and upwind) of the surface films. Data were acquired using a 10 GHz (X-band) scatterometer, and a two-dimensional laser slope gauge, and a video camera. We found that the effect of slick-edge asymmetry (i.e., a sharp downwind edge and a diffuse upwind edge) depends on both the deployed substance and the wind speed. Due to different states of film compression at the downwind or upwind edges of the surface film, the damping capability of the slick material can vary in these areas. This in turn may cause a varying spectral density of small-scale waves, which are bound to the dominant wind waves, while the slick is drifting by. This effect is shown and interpreted qualitatively using the video data, and quantitatively using the scatterometer and laser gauge data.