A laboratory study of spilling breakers in the presence of light‐wind and surfactants

Spilling breaking waves in the presence of light‐wind and surfactants are studied experimentally in a wind‐wave tank. The breaking waves are mechanically generated with a single wave maker motion that produces a weak spilling breaker in clean water without wind. Separate experiments are performed with the same wave maker motion at different low wind speeds in clean water and in water with various concentrations of Triton X‐100 (soluble surfactant). The crest profiles of the waves along the center plane of the tank are measured with a laser‐induced fluorescence (LIF) technique that utilizes a high‐speed camera. In clean water with wind speeds lower than 2.3 m/s (the minimum wind speed of wind‐generated waves for clean water in our tank), the breaking of the waves is initiated with a similar bulge‐capillary‐waves pattern on the forward face of the wave crest as reported in Duncan et al. (1999). When the wind speed is above 3 m/s, wind waves are generated. The wind waves strongly affect the breaking process of the mechanically generated waves. It is found that the bulge‐capillary‐waves pattern is independent of the wind, but is dramatically affected by surfactants. The slope of the back face of the wave crest decreases with increasing wind speed. At the moment of incipient breaking, the distances between the leading edge of the bulge (called the toe) and the highest point of the wave crest in all cases are linearly proportional to the surface wind drift. After the fluid in the bulge slides down the front face of the wave, the maximum horizontal distance of the toe away from the crest increases as the wind speed increases.