Insects inspire coating for repelling speedy water droplets

Through their investigation of insect surfaces, researchers at Penn State have detailed a previously unidentified nanostructure that can be used to engineer stronger, more resilient water-repellent coatings. With an enhanced ability to repel droplets, this new design could be applied to personal protective equipment (PPE) to better resist virus-laden particles, such as covid-19, among other applications. The researchers report their work in a paper in Science Advances.

"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 and the lead author of the paper.

Classical engineering theories have employed this approach to create superhydrophobic, or water-repellent, surfaces. Traditionally, these surfaces are manufactured using so-called low solid fraction textures that 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, professor of engineering, associate professor of mechanical and biomedical engineering and Wang's adviser. Since this approach works effectively in nature, man-made coatings tend to mimic the low density of the nanostructures.

But this new paper details an entirely different approach. When examining surfaces like 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. In engineering, such densely packed surfaces are termed high solid fraction textures.

Upon further exploration, the researchers found that this significant departure from a 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." More densely packed structures may also be able to repel liquid that is moving at a higher speed, such as raindrops.

While the design concept is new to humans, the researchers theorize that 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."

With this knowledge gleaned from nature, the researchers hope to apply the design principle to create next generation coatings. A water-repellent surface that can withstand faster moving and higher impact droplets could have a wide range of applications. From small, flying robotic vehicles, such as the drones that Amazon hopes to deliver packages with, to commercial airliners, a coating that can emulate these insect surfaces could provide increased efficiency and safety.

In light of the covid-19 pandemic, researchers have since realized this knowledge could have an additional 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."

"While we didn't imagine that application at the beginning of this project, covid-19 made us think about how we can use this design principle to benefit more people," Wong said. "It's up to us as engineers to take these discoveries and apply them in a meaningful way."

The next step for this work will be to develop a large-scale, cost-effective method for manufacturing a coating to mimic these properties. "In the past, we didn't have an effective surface that could repel high-speed water droplets," Wong said. "But the insects told us how. There are so many examples like this in nature; we just need to be learning from them."

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.