Estimating Bubble Plume Dynamics in Breaking Waves using the Thermal Signature of the Residual Foam

This study is motivated by the observation that after a wave breaking event in the ocean, the residual surface foam left in the wake of the breaker rapidly cools down. The relationship between the cooling foam and the characteristics of the breaking wave such as bubble plume dynamics, visible surface foam, and energy dissipation is investigated experimentally. Previous studies have suggested that the decay time of the visible foam can be used to determine the dynamics of the subsurface bubble plume, and to estimate the energy dissipation by the breaking process. But the foam decay process can be greatly affected by the surfactant concentration in the ocean and this effect need to be accounted for independently. We present a new approach that utilizes the thermal signature of the cooling foam to infer the breaking characteristics and is less sensitive to surfactant concentration. The experiments are conducted in a wave flume that is equipped with a piston-type wavemaker and is filled with salt water. In order to study the effects of surfactants on the cooling of the residual foam, two sets of experiments are carried out; In the first set clean salt water is used and in the second set, Triton X-100 at a concentration of approximately 200 mg/L is added to the water. Breaking waves are generated using the focusing wavepacket technique and are designed to cover a wide range of slopes and breaking intensities. The bubble plume and the surface foam are imaged using visible cameras and the surface temperature is captured using an IR camera with an overlapping field of view with the visible foam camera. It is observed that the average temperature of the foam initially increases after the passage of a breaking wave due to the disruption of the cool skin layer, but the foam starts to cool down soon after the bubble plume has subsided, and the foam regeneration is not sustained by the bubbles anymore. It is found that the time from the start of the breaking process to the onset of cooling of the foam (τ_cool) scales with the decay time of the bubble plume (τ_plume), energy dissipation, and the wavepacket slope of the breakers. It is also observed that the foam decay time is prolonged greatly by the presence of additional surfactants (consistent with the literature), but the bubble plume decay time and the onset of cooling of the foam are not significantly affected.