Study Reveals New Mechanism for Particle Attachment to Oil Droplets

Dr. Michel Boufadel and Dr. Lin Zhao study the 3D structure of oil-particle aggregates using confocal microscope in the lab of New Jersey Institute of Technology. Photo provided by L. Zhao.

Dr. Michel Boufadel and Dr. Lin Zhao study the 3D structure of oil-particle aggregates using confocal microscope in the lab of New Jersey Institute of Technology. Photo provided by L. Zhao.

Using confocal microscope imaging techniques, researchers discovered a new mechanism of sediment attachment to oil in turbulent flows. Photo provided by L. Zhao.

Using confocal microscope imaging techniques, researchers discovered a new mechanism of sediment attachment to oil in turbulent flows. Photo provided by L. Zhao.

Researchers analyzed 3D renderings of oil-particle aggregates to better understand their interactions in turbulent environments. The team found that hydrodynamic forces can cause sediment particles to act as projectiles that penetrate oil droplets. Some particles only protruded the droplets’ surface, and the longer that aggregates were in turbulent conditions, a greater number of these protruding particles were torn from the droplets. This action enhanced smaller oil-particle aggregate formation and potentially enhanced dissolution and biodegradation. Since more particles may enter an oil droplet than attach to its surface and increase its weight, this mechanism suggests a pathway other than adsorption for oil sedimentation. The researchers published their findings in Environmental Science & Technology: A new mechanism of sediment attachment to oil in turbulent flows: Projectile particles.

Oil droplets from natural seeps or spills often interact with particles in the ocean environment, particularly sediment, and form oil-particle aggregates (OPAs) that can affect the trajectory and fate of oil. These particles stabilize oil droplets, prevent coalescence, and cause the resulting OPAs to have a lower buoyancy than oil droplets alone, which facilitates their transport by deeper currents. This study’s  researchers generated OPAs in turbulent conditions to reveal the mechanisms of their interactions in such environments, such as when oil approaches shallow water where waves stir up sediment.

The team added oil to a seawater-filled baffled flask, generated droplets using an orbital shaker, added particles to create 1:3 and 5:1 oil-to-particle ratios (ranges observed in the natural environment), and left the flasks to settle overnight. Depending on how many particles attached to a droplet, OPAs were either negatively buoyant and settled at the bottom or positively buoyant and floated to the surface. The team sampled and processed the negatively buoyant OPAs using a confocal laser-scanning microscope, which scans thin cross-sections and builds a 3D reconstruction similar to a CT Scan.

Most particles penetrated the oil droplet ~2 – 3 µm deep with smaller particles typically penetrating deeper than larger particles. Most particle penetration occurred within the first minute of mixing and showed no further deepening of penetration, suggesting that no permanent forces drove the particles deeper into the oil droplet. As shaking time progressed, oil droplet sizes decreased and OPAs began to form clusters.

The team’s findings suggest that the presence of particles caused the fragmentation of droplets in turbulent flows. Previously, the assumption was that particles stuck flat to the droplet’s surface. “Over a duration of 6 to 24 hours, the particles can act as dispersant causing the oil droplets to get smaller,” explained study author Lin Zhao. “In essence, particles stuck in droplets get uprooted and take with them some of the oil from the oil droplets. The droplet size was reduced 5-fold from a median diameter of 22 µm to a median diameter of 3.5 µm. Concurrently, the OPAs that tended to form massive clusters were ‘glued’ together by the oil that gets torn out of droplets by the particles.”

Study co-author Michel Boufadel explained that the findings from this study could aid predicting the fate of oil in shorelines and the development of oil spill countermeasures, especially in areas with significant particle concentrations.

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7RF5SGD.

The study’s authors are Lin Zhao, Michel C. Boufadel, Joseph Katz, Gal Haspel, Kenneth Lee, Thomas King, and Brian Robinson.

See related research: Study Develops Predictive Model for Oil-Particle Aggregate Formation.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Dispersion Research on Oil: Physics and Plankton Studies II (DROPPS II) consortium. Other funding sources include the Department of Fisheries and Oceans Canada.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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