Making dew droplets so small, they're invisible

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

In Boreyko's lab, then-undergraduate Megan Mulroe 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.

"These results, correlated with a theoretical model, revealed that the bottleneck for jumping is how the droplets inflate inside of the surface after they first form," Boreyko said.

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, who was first author on the paper, 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.