Engineers uncover why tiny particles form clusters in turbulent air

(Press-News.org) BUFFALO, N.Y. — Tiny solid particles – like pollutants, cloud droplets and medicine powders – form highly concentrated clusters in turbulent environments like smokestacks, clouds and pharmaceutical mixers.

What causes these extreme clusters – which make it more difficult to predict everything from the spread of wildfire smoke to finding the right combination of ingredients for more effective drugs – has puzzled scientists.

A new University at Buffalo study, published Sept. 19 in Proceedings of the National Academy of Sciences, suggests the answer lies within the electric forces between particles.

“Small, uneven electric charges between particles in turbulent airflows play a much more important role than we previously thought,” says corresponding author Hui Meng, PhD, UB Distinguished Professor in the Department of Mechanical and Aerospace Engineering. “Uncovering this hidden mechanism could lead to better predictions and controls in climate research, medicine, engineering and science.”

The team began its work with the idea that particles exchange small portions of electric charge when they collide in turbulent air. But instead of spreading out evenly, the charges form irregular patches across the surface of each particle – what the team refers to as a “mosaic charge.”

These patchy charges create electrical dipoles that attract one another, which leads to more collisions and more charges.

“Ultimately, this action strengthens the attraction between the particles, creating a positive feedback loop that we named IMPACT, which is short for Inhomogeneous Mosaic Potential Amplified Collisions in Turbulence,” says first author Danielle R. Johnson, who recently earned her PhD from UB.

To test this hypothesis, the team placed hollow glass spheres – a stand-in for solid particles – into a chamber where they controlled turbulent airflows. Researchers then used a high-resolution, high-speed, 3D particle tracking system, as well as atomic force microscopic tools, to measure nanoscale charge patterns on the particles.

They found that the glass spheres acted as hypothesized, with movements matching those of the dipoles. While performed in controlled setting, the team says the results can be applied to a bevy of real-world scenarios where particle interactions are key. For example:

Drug development – In pharmaceutical manufacturing, powders mix and behave in a variety of ways. If they’re forming extreme clusters, it could prompt drugmakers to change how the drugs are made, and ultimately make them more effective in fighting disease. Extreme rainfall – Cloud droplets and ice crystals collide to create rainstorms. Clustering may change these interactions, leading to less predictable or stronger storms. Better understanding such behavior could improve predictions, and help save lives and property. Air pollution – Smog particles may clump differently than models predict, changing the smog’s intensity and how long it stays in the air. Fuel combustion – Tiny particle interactions in engines and other sources of combustion have great impacts. Better understanding clustering could lead to more efficient energy use. “What’s really exciting about this finding is that it sheds light on a previously overlooked phenomenon in particulate turbulence, and it has broad environmental, industrial and societal implications,” says co-author James Chen, PhD, associate professor in the Department of Mechanical and Aerospace Engineering.

Additional co-authors include Adam Bocanski, a UB PhD candidate in the Department of Mechanical and Aerospace Engineering, and Emily M. Diorio, a UB senior majoring in electrical engineering.

The research was supported by the National Science Foundation and the University at Buffalo Experiential Learning Network Funding.

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