Subsurface Droplet Size Distribution generated as breaking waves entrain an oil slick

Breaking waves are a primary mechanism for entraining and dispersing oil spills. Knowledge of the resulting droplet size distribution is crucial for predicting the transport and fate of this oil. In this on-going experimental study, a controlled oil slick of varying viscosity ($\mu_{d})$, density ($\rho_{d})_{,\thinspace }$interfacial tension ($\sigma )$, and thickness $\delta =$0.5mm are entrained by waves of varying energy ($E_{w})$. The changes to droplet size over time, from seconds to hours, are measured at several locations using multi-resolution holography, which covers sizes ranging from $\mu $m to mm. Using dispersants to reduce $\sigma $, the Webber number, \textit{We}$=E_{w}\delta /\sigma $, and Ohnesorge number, \textit{Oh}$= \mu_{d} /(\rho_{d}\delta \sigma )^{0.5}$, are varied from 6 to 813 and from 0.09 to 0.95, respectively. Droplets smaller than the turbulence scale (2-30 $\mu $m -- diameter), are generated by ``micro-threading''. Their size distribution becomes steeper and their total number increase substantially with decreasing interfacial tension. For slopes smaller than -3, measured for $\sigma $ around 10$^{\mathrm{-1\thinspace }}$mN/m, the volumetric size distribution decreases with diameter, i.e. most of the oil breaks into micron-scale droplets. For high interfacial tension oil, the concentration of small droplets increases with wave energy, but this effect diminishes as $\sigma $ decreases. Droplets larger than 100 $\mu $m are generated by turbulent shear. Hence, their number is impacted by $\mu_{d}$ and $E_{w}$. Increasing \textit{We} from 6 to 15 (\textit{Oh} from 0.09 to 2.95) increases the initial number of droplets by up to 5 times, but the distribution slopes remain largely similar.