Coalescence of rising oil droplets – A combined experimental and modelling study to investigate the influence of surface active components on oil droplet coalescence

Understanding the stability of oil-in-water dispersions is critical for predicting the fate of oil released during subsea blowouts and for evaluating response strategies. This study investigates coalescence of rising oil droplets in large-scale laboratory experiments, comparing chemically dispersed oil (with surfactants) to mechanically dispersed oil (without surfactants). Eight experiments were conducted in a 42 m 3 seawater tank using a full factorial design, varying droplet size (20–30 μm vs. 250–300 μm) and oil concentration (50 ppm vs. 400 ppm). Droplet size distributions and concentrations were monitored over time using laser diffraction instruments, and results were interpreted with an Eulerian transport and fate model incorporating buoyant rise, diffusion, and Smoluchowski-based coalescence kinetics. Model simulations indicate that coalescence is strongly dependent on number concentration, which scales inversely with droplet size. Significant coalescence effects were observed only for small droplets at high concentration, where collision rates are highest. For these conditions, mechanical dispersions showed faster decay in suspended oil concentration and a shift towards larger droplet sizes compared to chemical dispersions, suggesting reduced stabilization in the absence of surfactants. However, under realistic field conditions, rapid dilution and larger droplet sizes are expected to minimize coalescence, regardless of dispersion method. These findings indicate that while coalescence may influence laboratory-scale observations, its operational impact on subsea mechanical dispersion (SSMD) is likely negligible. The study provides experimental evidence and modelling insights to support decision-making for subsea dispersant injection (SSDI) and SSMD technologies in oil spill response. • Study compares coalescence & stability of chemical- and mechanical oil dispersion. • Experiments and modelling include buoyant rise, diffusion& coalescence kinetics. • Coalescence observed for small droplets, high concentration & high collision rates • Under field conditions, rapid dilution and larger droplets minimize coalescence. • Provides evidence for decision-making using SSDI and SSMD in oil spill response