Enhanced water repellent surfaces discovered in nature
Researchers have theorized a coating that mimics the unique nanostructure could improve virus repellent face masks

Date:
July 17, 2020
Source:
Penn State
Summary:
Through the investigation of insect surfaces, researchers have
detailed a previously unidentified nanostructure that can be used
to engineer stronger, more resilient water repellent coatings.



FULL STORY ========================================================================== Through the investigation of insect surfaces, Penn State researchers
have detailed a previously unidentified nanostructure that can be used
to engineer stronger, more resilient water repellent coatings.


==========================================================================
The results of this research were published today (July 17) in Science Advances.

With an enhanced ability to repel droplets, this design could be applied
to personal protective equipment (PPE) to better resist virus-laden
particles, such as COVID-19, among other applications.

"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 used this approach to create superhydrophobic, or water repellent, surfaces. Traditionally, they
are 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, the Wormley
Early Career Professor of Engineering, associate professor of mechanical
and biomedical engineering and Wang's adviser.



========================================================================== Since it works effectively, human-made coatings tend to mimic the low
density of these nanostructures.

However, this paper details an entirely different approach. When examining surfaces like the eye of a mosquito, 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, such as raindrops.

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." With this knowledge gleaned from
nature, 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 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.

However, 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." As seen in this work, the Wong Laboratory
for Nature Inspired Engineering draws insights from biological phenomena
to make humanity's innovations better and more effective.

"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 developing a large scale, cost effective method
that can manufacture 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 research was funded by the National Science Foundation,
the PPG Foundation, the Wormley Family Early Career Professorship,
the Humanitarian Materials Initiative Award sponsored by Covestro and
the Materials Research Institute at Penn State. Additional contributors
include Ruoxi Wang, an undergraduate alumna, and Jing Wang, a doctoral graduate, both in the Department of Mechanical Engineering.


========================================================================== Story Source: Materials provided by Penn_State. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Lin Wang, Ruoxi Wang, Jing Wang, and Tak-Sing Wong. Compact
nanoscale
textures reduce contact time of bouncing droplets. Science Advances,
2020 DOI: 10.1126/sciadv.abb2307 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200717140740.htm

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