Small but dangerous: A new study shows that aerosols from exploding polluting bubbles are much smaller than expected

Cold sparkling water. Waves crashing on the beach. crackle of fire Steam from a boiler.

These are not only weekend getaways, but also aerosol sources in our environment. Although some of these sources of aerosols are not a major concern, aerosols originating from industrial sources, such as wastewater treatment plants, and even natural sources, such as sea spray and dust, have the potential to have a greater impact on the environment. environment and even public health.

An aerosol is a suspension of liquid droplets or fine solid particles suspended in the air. Although aerosols are generated in a variety of ways, one of the most important sources is the bursting of bubbles at the liquid-air interface. Most of the previous research has focused on “clean” bubbles, although contaminated interfaces are common.

New research from University of Illinois Urbana-Champaign assistant professor of mechanical sciences and engineering Ji Feng and graduate student Zhenjiu Yang shows that explosive bubbles coated in a thin oil film produce droplets with smaller sizes and a greater total number of droplets, and are ejected at a single point. Higher speed of bubbles generated in clean water. This research was recently published in nature physics.

Aerosols are ubiquitous in our environment, and they can be natural or anthropogenic. Bubble-bursting aerosols play a major role in mass transfer across liquid interfaces. Sea spray mist, for example, is primarily generated by bubbles bursting at the ocean’s surface. Droplets from bursting bubbles can affect air pollution, the global climate, and even the transmission of infectious diseases. An important factor for these droplets is their size as this is indicative of their residence and transport time in the atmosphere – small droplets are more easily lifted by wind and can travel much farther.

Feng says, “We’ve polluted the water everywhere. When a bubble rises from deep water to the surface, it will collect pollutants and form an organic layer around it. We call this a pollutant bubble. When it reaches the surface and bursts, it can, in effect, transport these pollutants into tiny droplets.” ”

Feng and Yang investigated the effect of a thin film of oil on bubble bursting, as a model system for contaminated bubbles. The bursting of a millimeter-sized bare bubble on a water surface produces droplets of a typical size of about 100 micrometers (a typical human hair is about 100-200 micrometers). In this work, they found that droplets can be as small as a few micrometers when the bursting bubble is covered with a thin layer of oil. In addition, the bursting of a naked bubble produces droplets with a typical ejection velocity of 1 meter per second (m/s), while the bursting of an oil-covered bubble produces droplets with an ejection velocity of up to 10 m/s.

Feng sums up, “The main conclusion of our work is that we found that, for these pollutant bubbles, they can effectively convert pollutants into micron-sized droplets.”

In an industrial setting such as a wastewater treatment plant, smaller contaminated droplets can pose a significant danger to those working in the plant. Bubbles bursting in these locations may generate acid mist and biological spray. Understanding the effect of contaminant bubbles on size distribution and ejection velocities is critical to designing effective personal protective equipment and implementing additional air and water quality guidance near these facilities.

On a larger scale, aerosols have an impact on weather, climate, and even human health.

“These droplets can transmit pathogens, bacteria and viruses,” Yang said. “When you have these small aerosols, and they can be ejected larger, the smaller size and higher ejection velocity can help them stay in the atmosphere for longer.” Understanding aerosol size and composition is critical to improving global modeling efforts. Moreover, these contaminated droplets can pose a greater risk of dissemination of pollutants as well as infection because smaller aerosols are able to penetrate the respiratory tract more than larger aerosols.

Other contributors to this work include Bingqiang Ji (co-author, postdoctoral researcher, MechSE, UIUC) and Jesse T. Ault (Assistant Professor, School of Engineering, Brown University).