New study solves mystery of how soft liquid droplets erode hard surfaces
Date:
March 31, 2022
Source:
University of Minnesota
Summary:
A new study shows why liquid droplets have the ability to erode
hard surfaces, a discovery that could help engineers design more
erosion- resistant materials.
FULL STORY ==========================================================================
A first-of-its-kind study led by University of Minnesota Twin Cities researchers reveals why liquid droplets have the ability to erode hard surfaces. The discovery could help engineers design better, more erosion- resistant materials.
========================================================================== Using a newly developed technique, the researchers were able to measure
hidden quantities such as the shear stress and pressure created by the
impact of liquid droplets on surfaces, a phenomenon that has only ever
been studied visually.
The paper is published in Nature Communications,a peer-reviewed, open
access, scientific journal published by Nature Research.
Researchers have been studying the impact of droplets for years, from
the way raindrops hit the ground to the transmission of pathogens such
as COVID-19 in aerosols. It's common knowledge that slow-dripping
water droplets can erode surfaces over time. But why can something
seemingly soft and fluid make such a huge impact on hard surfaces?
"There are similar sayings in both eastern and western cultures that
'Dripping water hollows out stone,'" explained Xiang Cheng, senior author
on the paper and an associate professor in the University of Minnesota Department of Chemical Engineering and Materials Science. "Such sayings
intend to teach a moral lesson: 'Be persistent. Even if you're weak, when
you keep doing something continuously, you will make an impact.' But,
when you have something so soft like droplets hitting something so hard
like rocks, you can't help wondering, 'Why does the drop impact cause any damage at all?' That question is what motivated our research." In the
past, droplet impact has only been analyzed visually using high-speed
cameras. The University of Minnesota researchers' new technique, called
high- speed stress microscopy, provides a more quantitative way to study
this phenomenon by directly measuring the force, stress, and pressure underneath liquid drops as they hit surfaces.
The researchers found that the force exerted by a droplet actually
spreads out with the impacting drop -- instead of being concentrated in
the center of the droplet -- and the speed at which the droplet spreads
out exceeds the speed of sound at short times, creating a shock wave
across the surface. Each droplet behaves like a small bomb, releasing
its impact energy explosively and giving it the force necessary to erode surfaces over time.
Besides paving a new way to study droplet impact, this research could help engineers design more erosion-resistant surfaces for applications that
must weather the outdoor elements. Cheng and his lab at the University
of Minnesota Twin Cities already plan to expand this research to study
how different textures and materials change the amount of force created
by liquid droplets.
"For example, we paint the surface of a building or coat wind turbine
blades to protect the surfaces," Cheng said. "But over time, rain droplets could still cause damage via impact. So, our research after this paper
is to see if we can reduce the amount of shear stress of droplets, which
would allow us to design special surfaces that can mitigate the stress."
In addition to Cheng, the research team included University of Minnesota chemical engineering Ph.D. student Ting-Pi Sun, University of Santiago,
Chile Assistant Professor Leonardo Gordillo and undergraduate students
Franco A'lvarez-Novoa and Klebbert Andrade, and O'Higgins University,
Chile Assistant Professor Pablo Gutie'rrez.
The research was funded by the National Science Foundation.
Slow-motion video of water droplet impacting sandy surface:
https://youtu.be/ 6n4lsx5aXEQ
========================================================================== Story Source: Materials provided by University_of_Minnesota. Note:
Content may be edited for style and length.
========================================================================== Related Multimedia:
* Droplet_impacts ========================================================================== Journal Reference:
1. Ting-Pi Sun, Franco A'lvarez-Novoa, Klebbert Andrade, Pablo
Gutie'rrez,
Leonardo Gordillo, Xiang Cheng. Stress distribution and surface
shock wave of drop impact. Nature Communications, 2022; 13 (1)
DOI: 10.1038/ s41467-022-29345-x ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/03/220331101521.htm
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