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Theory for aerosol droplets from contaminated bubbles may shed light on spread of pollution, microplastics, and more

Bubbles burst when their caps rupture. Children discover this phenomenon every summer day, but it also underpins key mechanisms for the spread of pollutants, contaminants, and even infectious disease through the generation of aerosol droplets. While bubble bursting has been extensively studied in pure substances, the impact of contaminants on bursting dynamics has not received widespread attention.

Researchers in The Grainger College of Engineering at the University of Illinois Urbana-Champaign have conducted a systematic study to investigate bubble-bursting jets—aerosol particles sprayed when bubble surfaces rupture—when surface contaminants are present. The laboratory of mechanical science and engineering professor Jie Feng developed a model predicting the influence of contaminants on jet size and experimentally confirmed it.

The study is in the journal Â鶹ÒùÔºical Review Letters, where it was selected as an Editors' Suggestion.

"Bubbles are commonly formed or intentionally used in many natural and engineered settings," Feng said. "As they rise due to buoyancy, they become coated with surrounding chemicals, such as micro- and nano-plastics, bio-surfactants, and even bacteria and viruses, and they spread through the fluid jets and the resulting aerosol droplets created when the bubbles burst at an air-liquid interface.

"This mechanism is crucial in understanding phenomena like airborne contaminants from oil spills and the spread of respiratory diseases, but there have been no systematic studies to date for size quantification of the contaminant-laden droplets. We aim to perform controlled experiments and develop a theoretical framework to predict their size."

When the cap of a bubble ruptures, a mechanism is triggered by which the focusing of surface waves causes fluid to be ejected—so-called Worthington jets. This phenomenon has been studied for decades, but a significant knowledge gap remains. Most research focuses on the case when the bubble surface consists of a pure substance, but many of the most important occurrences involve bubbles with impurities or contaminants.

"For instance, in wastewater treatment ponds contaminated with viruses or bacteria, bubbles formed by mechanical agitation will rise and collect microorganisms on their surfaces," Feng said. "When they escape into the atmosphere and burst, they spray fine droplets that contain these pathogens. Bubble-bursting jets pose significant environmental and safety concerns in the area surrounding contaminated aquatic environments, and cleanup efforts need to understand and account for these."

Finding that there was limited data available and no theoretical work, Feng's laboratory turned to its capabilities in multiphase flows to study the phenomenon. To mimic a contaminated bubble, the researchers used a specialized coaxial orifice system that injects gas into water to form bubbles and then coats them with silicone oil. By varying the properties of the oil, the impact of its characteristics on jet features could be studied with a high-speed camera.

"My research has been focusing on fluid mechanics and environmental impacts related to bubble bursting," said Zhengyu Yang, a graduate student in Feng's laboratory and the study's lead author. "We had already established the experimental capabilities required to generate high-quality data on this problem. From there, it was a question of building the right theory to explain what we found."

The results showed that the size of the ejected aerosol droplets depends on the thickness of the oil layer, the oil's viscosity and surface tension. To develop a mathematical model explaining this behavior, the researchers introduced a new parameter accounting for the presence of the oil layer. They call this parameter the "revised Ohnesorge number" after the classical Ohnesorge number governing the dynamics of pure bubble-bursting jets.

"Our results are valuable for understanding and mitigating airborne transmission of contaminants mediated by bubbles and drops," Feng said. "To realize their full power, our next step is to consider collective bubble bursting—studies of many bubbles producing aerosol jets at once, which is a more practical scenario in the real world. We hope the tools and knowledge obtained in this research will be broadly applicable to elucidate the mechanistic influence of a variety of contaminated interfaces on multiphase flows."

Yang Liu of Tsinghua University also contributed to this work.