By investigating insect surfaces, Penn State researchers have detailed an unidentified nanostructure that can be used to engineer stronger, more resilient water-repellent coatings. With an enhanced ability to repel droplets, it could be applied to personal protective equipment (PPE) to better resist virus-laden particles such as COVID-19.

“For the past few decades, conventionally designed water-repellent surfaces have usually been based on plants, like lotus leaves,” said Lin Wang, a doctoral student in the Department of Materials Science and Engineering at Penn State. Classical engineering theories have used this approach to create superhydrophobic, or water-repellent, surfaces. Traditionally, they’re manufactured with low solid fraction textures, which maintain an extremely thin layer of air above a low density of microscopic, hair-like nanostructures, which the researchers liken to an air hockey table.

“The reasoning is if the droplet or object is floating on top of that air, it won’t become stuck to the surface,” said Tak-Sing Wong, associate professor of mechanical and biomedical engineering and Wang’s adviser. Since it works effectively, man-made coatings tend to mimic the low density of these nanostructures.

However, the new research focuses on a different approach. When examining surfaces such as the eye of a mosquito, the body of a springtail, or the wing of a cicada under high-resolution electron microscopes, Wang found that the nanoscopic hairs on those surfaces are more densely packed, referred to in engineering as high solid fraction textures. Upon further exploration, this significant departure from plants’ structure may imbue additional water-repelling benefits.

“Imagine if you had a high density of these nanostructures on a surface,” Wang said. “It could be possible to maintain the stability of the air layer from higher impact forces.” This could also mean the more densely packed structures may be able to repel liquid that is moving at a higher speed.

While the design concept is new to humans, the researchers theorize this nanostructure boosts the insect’s resiliency in its natural environment. “For these insect surfaces, repelling water droplets is a matter of life and death. The impact force of raindrops is enough to carry them to the ground and kill them,” Wang said. “So, it is really important for them to stay dry, and we figured out how.”

The researchers hope to apply this design principle to create next-generation coatings. By developing a water-repellent surface that can withstand faster moving and higher impact droplets, the applications are abundant. From drones that deliver packages to commercial airliners, a coating that can emulate these insect surfaces could provide increased efficiency and safety. However, in light of the COVID-19 pandemic, researchers have realized this knowledge could have an impact on human health. “We hope, when developed, this coating could be used for PPE. For example, if someone sneezes around a face shield, those are high-velocity droplets. With a traditional coating, those particles could stick to the surface of the PPE,” Wong said. “However, if the design principles detailed in this paper were adopted successfully, it would have the ability to repel those droplets much better and potentially keep the surface germ-free.”

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