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New research could make dew droplets so small, they’re invisible

August 18, 2017

An image of water droplets jumping off of a surface.
Jonathan Boreyko has been studying the phenomena of 'jumping' dew droplets since graduate school.

By better understanding the behavior of water in its smallest form, a Virginia Tech professor and his undergraduate student could be improving the efficiency of removing condensation in a major way.

Jonathan Boreyko, an assistant professor in the Department of Biomedical Engineering and Mechanics in the Virginia Tech College of Engineering, has been studying "jumping" dew droplets since he discovered the phenomenon in graduate school.

According to Boreyko, dew droplets jump from water-repellent surfaces only 
when they reach a large enough size, about 10 micrometers, but it was unclear why until Boreyko and his students made a breakthrough discovery, published in the high-impact journal ACS Nano.

In Boreyko’s lab, then-undergraduate Megan Mulroe, who is first author on the published paper and studied engineering science and mechanics, experimented with the surface of silicon chips to see how the nanoscopic topography of the surface might impact the jumping ability of condensation.

By creating and testing six different types of surfaces covered with so-called nanopillars — reminiscent of stalagmites on a cave floor — Mulroe found that the critical size of the jumping droplet can be fine-tuned based on the height, diameter, and pitch of the nanopillars.

“I didn't expect a minor alteration to change so much about how condensation behaves on a surface,” Mulroe said. “Only a few hundred nanometers difference in the surface's topography can stop the phenomenon [of jumping] altogether.”

Essentially, when the nanopillars are tall and slender, the droplets formed inside and on the crevices can jump off the surface at a much smaller size, down to two micrometers. Likewise, short and stout pillars increase the size of the droplet required to jump — up to 20 micrometers in the case of Mulroe’s experiment.

While the jumping-droplets phenomena has been found to be the most-efficient form of condensation removal, the ability to tweak the size of the droplets can allow for improved efficiency in removing condensation from surfaces.

“We expect that these findings will allow for maximizing the efficiency of jumping-droplet condensers, which could make power plants more efficient and enable robust anti-fogging and self-cleaning surfaces,” Boreyko said. “The ultimate goal is for all dew droplets forming on a surface to jump off before they are even visible to the eye.”

Mulroe conducted all of the experiments, while graduate student Farzad Ahmadi, who is pursuing a Ph.D. in engineering mechanics, backed up the findings with a theoretical model.

For Mulroe, the unique opportunity to publish research in a high-impact journal as an undergraduate was eye-opening: she had previously never written at such a high technical level or using a research-paper-writing program called LaTeX. She said she is thankful for the support of Boreyko and her labmates throughout the intensive process.

“I had a lot of help and it was a very difficult task for me while still in classes, but I wanted the experience and refused to give up,” Mulroe said. “I recommend anyone who is able to get this experience to take it.”

Since graduating, Mulroe landed a job as a nuclear engineer at the Norfolk Naval Shipyard. She starts in September, but until then plans to travel and spend some time relaxing.

“I don't know what to expect, but I'll take it in stride and make sure to continue to learn and grow,” Mulroe said.

Written by Erica Corder

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