Dispersion of Oil Spills by Breaking Waves

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Johns Hopkins University
Oil spilled at sea forms oil slick, which are subsequently broken up into droplets ranging from submicron to several millimeters by breaking waves. Application of dispersants to enhance the break-up process by drastically reducing the interfacial tension between oil and water has become increasingly popular in recent years. Knowledge of the resulting oil droplet size distribution is essential for determining and modeling their transport by oceanic currents and turbulence as well as their interaction with the chemical and biological environment. A major part of this experimental study involves comprehensive measurements up to five hours of the temporal evolution of the subsurface oil droplet size distributions generated by a single breaker, and their dependence on interfacial tension, oil viscosity and density, as well as breaking wave parameters. The measurements have been performed in a specially designed, 6 m long, 0.3 m wide, 0.6 m deep, totally transparent acrylic wave tank. The general entrainment processes have been visualized by high speed imaging and the temporal evolution of turbulence have been quantified using Particle Image Velocimetry (PIV). The measurements of the droplet size measurements have been performed in situ using digital inline holography at two magnifications. All early (2-10s) droplet size distributions display two distinct size ranges with different slopes. For low dispersant to oil ratios (DOR), the transition between them could be predicted based on a turbulent Weber number (We) in the 2-4 range, suggesting that turbulence plays an important role. For smaller droplets, all the number size distributions have power of about -2.1, and for larger droplets, the power decreases well below -3. The measured steepening of the size distribution over time is predicted by a simple model involving buoyant rise and turbulence dispersion. Conversely, for DOR 1:100 and 1:25 oils, the diameter of slope transition decreases from ~1mm to 46µm and 14µm respectively, much faster than the We-based prediction, and the size distribution steepens with increasing DOR. Furthermore, the concentration of micron-sized droplets of DOR 1:25 oil increase for the first ten minutes after entrainment. These phenomena are presumably caused by the observed formation and breakup of oil micro-threads associated with tip streaming whose length scale are well below the measured turbulence length scale. In addition to subsurface oil droplet characterization, preliminary efforts have been made to elucidate the health implications of oily marine aerosol generation by breaking waves. The investigated parameters include wave energy and oil/water interfacial tension. The waves are generated every 10 s for 15 min in an attempt to mimic characteristic coastal wave periods. Results show small difference in concentration in micron range, as confirmed by two independent measurements, using an Aerodynamic Particle Sizer and digital inline holography.
Breaking waves, dispersant, oil spill, dispersion