1 Center for Natural Resources, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, 323 MLK Blvd., Newark, NJ 07102, USA.
2 Center for Natural Resources, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, 323 MLK Blvd., Newark, NJ 07102, USA. Electronic address: boufadel@gmail.com.
3 ExxonMobil Upstream Research Company, 22777 Springwoods Village Pkwy, Houston, TX 77389, USA.
4 Department of Fisheries and Oceans, Dartmouth, NS B2Y 4A2, Canada.
5 Department of Building, Civil, and Environmental Engineering, Concordia University, 1455 Boulevard de Maisonneuve O, Montréal, QC H3G 1M8, Canada.
6 Department of Civil Engineering, Memorial University of Newfoundland, St. John's, NL A1C 5S7, Canada.

Spilled oil slicks are likely to break into droplets offshore due to wave energy. The fate and transport of such droplets are affected by suspended particles in local marine environment, through forming oil particle aggregates (OPAs). OPA formation is affected by various factors, including the mixing energy and duration. To evaluate these two factors, lab experiments of OPA formation were conducted using kaolinite at two hydrophobicities in baffled flasks, as represented by the contact angle of 28.8° and 37.7° (original and modified kaolinite). Two mixing energies (energy dissipation rates of 0.05 and 0.5 W/kg) and four durations (10 min, 30 min, 3 h, and 24 h) were considered. Penetration to the oil droplets was observed at 3-5 µm and 5-7 µm for the original and modified kaolinite by confocal microscopy, respectively. At lower mixing energy, volume median diameter d50 of oil droplets increased from 45 µm to 60 µm after 24 h mixing by original kaolinite; for modified kaolinite, d50 decreased from 40 µm to 25 µm after 24 h mixing. The trapped oil amount in negatively buoyant OPAs decreased from 35% (3 h mixing) to 17% (24 h mixing) by original kaolinite; and from 18% to 12% after 24 h mixing by modified kaolinite. Results indicated that the negatively buoyant OPAs formed with original kaolinite at low mixing energy reaggregated after 24 h. At higher mixing energy, d50 decreased from 45 µm to 17 µm after 24 h mixing for both kaolinites. And the trapped oil amount in negatively buoyant OPAs increased to 72% and 49% after 24 h mixing for original and modified kaolinite, respectively. At higher mixing energy, the OPAs formed within 10 min and reached equilibrium at 3 h by original kaolinite. For modified kaolinite, the OPAs continued to form through 24 h.