Mean rise rate of oil droplets under breaking waves

Entrainment of oil slicks by breaking waves generates a spectrum of droplets. Knowledge of the droplet rise rate under wave motion and turbulence is essential for predicting the fate of oil spills. This experimental work investigates the trajectories of 20 µm to 1 mm droplets with a specific gravity of 0.86 under lab breaking waves. The background turbulence energy dissipation rate is ~1 m²s^-3 shortly after wave breaking and 10^-3 m²s^-3 60 s later. The droplet trajectories are recorded using cinematic digital inline holography. Data analysis follows the temporal evolution of the droplet rise rate and its impact on the size distribution of the remaining droplets as a function of the initial droplet statistics and evolution of the turbulence dissipation rate. Results show that the ratio of mean rise rate over the quiescent rise rate (V_s/V_q) increases with decreasing diameter consistent with Friedman and Katz (Phy. Fluids, 14, 2002). For droplets larger than 0.3 mm V_s/V_q <1, but for smaller droplets V_s/V_q >1. The strong correlation between V_s and the turbulent level suggests that preferential sweeping plays a role in enhancing V_s of small droplets. Incorporating these modifications in the rise rate due to the wave motion and turbulence is necessary to improve the oil spill models.