The effect of surfactants on the surface profiles and droplet generation in a plunging breaker

An experimental study is conducted to compare the free surface profile evolution and droplet generation in a deep-water plunging breaker in the presence of three bulk concentrations of soluble surfactant (zero, below the critical micelle concentration (CMC) and above the CMC). The breakers are generated by a programmable wave maker that is set with a single motion profile that produces a dispersively focused wave packet. The wave profiles are measured with a cinematic LIF technique. It is found that a plunging breaker is formed at all surfactant conditions. In clean water and at the surfactant concentration above the CMC, the plunging jet has very smooth surfaces up to jet impact. However, at the intermediate surfactant concentration the jet becomes convoluted before impact and the water surface under the jet appears to fall on the forward wave face before impact. Droplets are measured with a cinematic inline holography technique. A preliminary experiment showed that large changes in surface dynamic properties between surfactant conditions have a large effect on the distribution of droplet diameters and velocities. Comprehensive droplet measurements are currently underway.