While it is well known that bursting bubbles can throw tiny particles into the air, the new research demonstrates that the physics behind a bursting bubble also push some particles down into the liquid as well.
"It is well known that bursting bubbles produce aerosol droplets, so we were surprised, and fascinated, to discover that when we covered the water with oil, the same process injected tiny oil droplets into the water," said project leader Professor Howard Stone of Princeton.
Such findings provide a new insight into the mixture of non-soluble liquids, and could have far reaching applications because the need to mix non-soluble liquids if at the centre of many areas of industry - including drug manufacturing, oil spill clean-ups, and the food and beverage industry.
Writing in Nature Physics the team describe how they reached their conclusions after examining bubbles in containers holding water covered by a layer of oil. Using several experimental approaches, they presented a detailed physical description of how the bubbles burst and how that affected the oil and water mix.
"If you look at this system, which has a thin layer of oil over water, the bursting bubbles were dispersing the oil phase in the form of nano-droplets into the water," explained Jie Feng, a graduate student in Stone's lab and the first author of the paper. "Essentially, it is an unrealized form of mass transport related to bubble bursting."
Indeed, in one observation, the team found that the water in one container changed from clear to translucent after bubbles ran through the mixture for some time. This change in appearance "suggested that small objects had been dispersed in the lower water phase," the researchers wrote.
Nano-emulsions
To get a better understanding of how bubble bursting was creating such nano-emulsions, the team used a high-speed camera to break down the steps involved in a bubble's final pop.
They found that a bubble's collapse caused a pressure wave just below the bubble; this wave pushed a small amount of liquid out and down, away from the collapsing void.
Stone and his team also reported that the addition of a surfactant, which decreases surface tension, was critical to the formation of the nano-droplets. In fact, they concluded that without a proper amount of surfactant, the droplets would not form.
"Bubbles are used to make foams and are part of common gas-liquid processes used in chemical processing," Stone said. "But bubbles also occur in lakes, rivers and oceans because of wave breaking and rain. As a consequence, bubbles can impact many systems."
Feng also said that applying this approach could play an important role in many industrial mixing systems. For one, this manner of bubbling to produce nano-emulsions uses much less energy than traditional mixers, so it is cheaper and more efficient.
It also does not require extremely low surface tensions, which some types of industrial processes require. And it provides a good method to mix typically insoluble liquids, such as oil and water.
"This system offers an energy-efficient route to produce nanoparticles, with the potential to increase in scale, for applications in a variety of fields such as drug delivery, food production and materials science," he said.
Source: Nature Physics
Volume 10, Pages 606–612, doi: doi:10.1038/nphys3003
"Nanoemulsions obtained via bubble-bursting at a compound interface"
Authors: Jie Feng, Matthieu Roché, et al