On the Morphology and Viscosity of Water-in-Crude Oil Emulsions Generated by Impingement of Breaking Waves on Oil Slicks*

This study examines the time evolution and effect of photo-oxidation on the microscopic morphology and bulk viscosity of salt water-in-crude oil emulsions generated by wave impingement on oil slicks. Slicks of Alaskan North Slope (ANS) and Hibernia crude oils with an initial thickness of 2 mm are emulsified by 40 s periodic plunging breaking waves over a 12-day period. Rheology is used to determine the viscosity, and fluorescence microscopy, followed by a machine-learning-based analysis, to measure the droplet size and spatial distributions of emulsion samples collected daily. Results show that ANS emulsifies water faster than Hibernia oil, resulting in a greater concentration of water droplets and a more interconnected droplet structure. Consequently, the (non-Newtonian) viscosity of ANS-based emulsion is at least three times higher than that of Hibernia. Additionally, photo-oxidation results in faster emulsification, accelerated formation of droplet clusters, larger characteristic droplet size, and higher water volume fraction. Hence, photo-oxidation increases the viscosity of Hibernia oil-based emulsion. These trends are consistent with our previously determined Ohnesorge number-based empirical relationships between the emulsion morphology and its viscosity.