Formation of oil-particle aggregates: Particle penetration and impact of particle properties and particle-to-oil concentration ratios

Oil droplets in marine environment interact with particles to form oil particle aggregates (OPA), and alters the transport and fate of oil. We investigated the impact of particles properties on the formation of OPAs. It was found that the distribution of 9 μm spherical silica (sand) particles on the oil droplet was more uniform than the 3 μm silica particles, and it is likely due to the inertia of the larger particles causing them to lodge into the droplet. Also, the OPAs of the 3 μm silica particles were much smaller than those of the 9 μm particles. For kaolinite particles that are rod-like of length around 10 μm, it was found that increasing the hydrophobicity of the particles from a contact angle (CA) of ~ 29^o to 38^o, increases the penetration of the particles in the oil through a projectile penetration mechanism, whereby the particle possesses sufficient inertia to penetrate into the oil. However, a further increase in hydrophbocitiy (CA ~ 57^o) caused the particles to agglomerate together and avoid the oil droplets. The oil droplets got smaller with time probably due to the penetration of the particles in them. For an oil concentration of 500 mg/L, a particle concentration of 100 mg/L was incapable of fragmenting the oil droplets, but particle concentration of 500 mg/L fragmented the droplets similarly to a concentration of 1500 mg/L. This is due to the larger coverage of the droplet surface area by the particles and the subsequent weakening of its structural rigidity through the reduction of the oil-water interfacial tension. The study shows that the fate (e.g., after 24 h) of OPAs greatly depends on the type of sediments where the oil spilled (sand versus clay) and their concentration.