The Effects of Ambient Surfactants on Plunging Breakers

An experimental study of the free surface profile evolution of a deep-water plunging breaker in the presence of five surface concentrations of ambient surfactants. 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. Surfactant conditions are created by starting with a tank full of very clean water and performing measurements after wait-times ranging from 15 minutes to 21 hours. For each wait time, the surface tension isotherm of the tank water is measured in a Langmuir trough. For a wait time of 15 minutes, the ambient surface tension is 72 mN/m, remains constant through a surface compression of 80% and then falls off at larger surface compressions. In this case, the plunging jet is smooth and a smooth cavity of air is entrapped at impact. As the wait time is increased, the ambient surface tension remains 72 mN/m, but the surface tension begins to fall off at progressively smaller compressions. In these cases, the plunging jet curls inward and breaks up before impact, and the entrained air cavity appears rough and broken as well. The distribution of surface compression up to jet impact is estimated using a DNS of a plunging breaker in water with constant surface tension. In light of the numerical and experimental results, it is hypothesized that the above-described breaker behavior is the result of Marangoni stresses.