Because many natural and industrial processes rely on the transport of bubbles and droplets through fluids, the behavior of ordinary air bubbles and water droplets has been well documented.
What's less understood is how they react to turbulence when coated with another liquid, such as oil. New research from Binghamton University and Georgia Tech, supported by a $590,000 grant from the National Science Foundation (NSF), seeks better modeling for newly-formed bubbles, smaller droplets, and oil-coated bubbles that could guide diverse applications, from environmental cleanups to drug delivery.
Assistant Professor Cosan Daskiran - a faculty member at the Thomas J. Watson College of Engineering and Applied Science's Department of Mechanical Engineering - points to the 2010 Deepwater Horizon oil spill in the Gulf of Mexico and its aftermath as a prime case study for why this research is important.
"When an oil-coated bubble bursts at the water's surface, it can eject a very tiny oil droplet into the atmosphere," he said. "This is crude oil, and it is toxic. The size of the injected droplets depends on several factors, including the size of the oil-coated bubble and its oil content. If people are fishing nearby or involved in cleanup activities, they may inhale these oil droplets, which poses health risks."
Bubbles or droplets with an outer coating act differently than "bare" bubbles, creating unique structures that break up into "daughter" bubbles, droplets, and oil-coated bubbles in complex ways. While the overall mass stays the same, the surface area for each daughter droplet becomes different based on size distribution.
If scientists and engineers could more accurately model the size distribution of daughter droplets, they could predict their mass, momentum, and heat transfer with surrounding fluid more accurately.
Daskiran and collaborator Chungkei "Chris" Lai, an assistant professor at Georgia Tech, will take a two-pronged approach to this NSF project. Daskiran will develop computer models using multiphase direct numerical simulations, a method of computational fluid dynamics that replicates real turbulence with dispersed bubbles and droplets. Lai simultaneously will perform experiments in Georgia Tech's unique experimental tank that creates and tracks oil-coated bubbles and their breakup in turbulence.
As the experiments validate the simulations, the researchers will cover a large matrix of nondimensional numbers governing the breakup, allowing them to extrapolate from a smaller scale to full size.
"The tank is equipped for high-speed imaging, so we can see how fast the bubbles break up," Daskiran said. "After the breakup, we can determine the probability of forming bare bubbles, oil-only droplets, and oil-coated bubbles, as well as their size distributions."
Forecasting how oil-coated bubbles react to ocean conditions is just one application for Daskiran and Lai's research. Specially engineered bubbles could gather water pollutants and bring them to the surface, where they could be skimmed off the top and disposed of. The research might also lead to new ways for targeted drug delivery.
"You could engineer droplets or bubbles with medication inside," Daskiran said. "Once the medication reaches a specific location in the body, technicians could use ultrasonic waves to rupture the bubbles and release the drug. Across all of these applications, breakup kernels and daughter-droplet/bubble size distributions are crucial for predicting the behavior of compound droplets, and these will be addressed in this collaborative project."
In 2024, Daskiran received funding from the U.S. Department of Energy to develop a more energy-efficient method to turn seawater into fresh water for human consumption or agricultural use. The integrated tidal desalination system will create drinkable water through renewable energy using the rotational power of hydrokinetic turbines, rather than electrical energy.
