Study Shows Bursting Bubbles Can Redistribute Oil Droplets into a Water Column
Dr. Howard Stone (right), the Donald R. Dixon ’69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering at Princeton University, and gradute student Jie Feng (left) observe bubbles in a tank. (Photo credit: Frank Wojciechowski, Princeton University)
Scientists using a high-speed camera to observe bubbles bursting have gained new insight into the hydrodynamics of complex fluids.
They found that collapsing bubbles emit a downward spray into the water and that the presence of a surfactant can facilitate the transport of tiny, submicron-diameter oil droplets into the underlying water column. While this phenomenon could increase pollutant distribution, it could also be a useful and cost-efficient dispersal method with industrial and oil biodegradation applications. The team published their findings in the August 2014 edition of Nature Physics: Nanoemulsions obtained via bubble-bursting at a compound interface.
The authors reported that bubbles in an oil-water interface are covered by two film layers – a water film that forms the bubble cap covered by an oil film – and burst in two steps. First, the top oil film breaks and retracts. A hole then forms in the water film, and the bubble cap ruptures and retracts. The bubble walls collapse inward and the surrounding fluids converge, pushing the flow down and ejecting a spray from the bottom of the bubble, projecting the oil droplets into the water below.
Video above: A side-view of the bubble-bursting process at a compound interface made of air, oil, and water. The formation of a spray emitting from the bottom of the bubble can be observed. (Provided by Jie Feng)
Diagram of a bubble bursting at an oil-water interface. a) The bubble rests near the oil-water interface, with oil and water films on top of the bubble. b) The oil film ruptures and retracts first. c) A hole opens in the water film of the bubble cap, and the water film retracts. d) The bubble collapses and the fluids converge, creating a spray into the water column. The flow direction is indicated with the arrows in the black oval. (Courtesy of Howard Stone and Jie Feng)
Researchers replicated the bubble-bursting process by releasing air from the bottom of a tank of water with a thin top layer of either oil or surfactant-oil mixture. A camera operating at 6,800 – 32,000 frames per second captured each stage of the bursting bubbles. They identified and measured small droplets that traveled down into the water column using dynamic light scattering – a process that analyzes fluctuations in the intensity of scattered laser light to determine particles’ size.
The team also examined the behavior and fate of the newly formed droplets. They observed that larger oil droplets rose back to the surface, while smaller droplets remained deep in the water column. The small droplets’ size remained constant for at least a week, indicating that they were stable. The smaller droplets appeared only when surfactant was present in the water phase. They concluded that hydrodynamics did not affect the size of these droplets, because heightened energy in and resulting from the collapse increased droplet size rather than decreasing it. Therefore, the physical chemistry of interactions between oil and surfactant molecules determined droplet size.
The researchers suggested that increasing bubble frequency could easily scale this dispersal method, potentially offering a low-cost, sustainable method of dispersing tiny particles under various interfacial tensions. “This system offers an energy-efficient route to produce nanoparticles […] for applications in a variety of fields such as drug delivery, food production, and materials science,” explained researcher Jie Feng.
The team also acknowledged the complexities of this top-down method. Bubble-bursting can continue to disperse oil into the water column when other mechanisms, such as wave breaking, are limited. This redistribution of oil particles could make toxins more available for ingestion by sea creatures. But, the droplets’ small size could also speed up pollutant biodegradation by hydrocarbon-eating bacteria.
The study’s authors are Jie Feng, Matthieu Roché, Daniele Vigolo, Luben N. Arnaudov, Simeon D. Stoyanov, Theodor D. Gurkov, Gichka G. Tsutsumanova, and Howard A. Stone.
The 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/.