What gives water its erosive power?

The erosive power of water is paradoxical. How can something that slips through your fingers also be one of the world’s most destructive agents? How can a raindrop gentle enough to be caught on the tip of the tongue carve through stone?

What gives this seemingly harmless substance so much erosive power?

A unique study led by researchers at the University of Minnesota Twin Cities, USA, has uncovered the hidden power that allows liquid droplets to erode hard surfaces.

The study of droplets is not new – researchers have long been fascinated by them, from the way raindrops hit the ground to the transmission of pathogens in aerosols. So far, however, their investigations have been limited to visual analysis using high-speed cameras.

Publish now in nature communicationresearchers describe a newly developed technique that allows direct measurement of previously hidden quantities that give droplets their magnificent power. The new technique, called high-speed stress microscopy, offers a more quantitative way to study the phenomenon of liquid erosion by directly measuring the force, stress and pressure under liquid droplets as they hit surfaces.

The researchers found that the force exerted by a droplet actually propagates with the impacting droplet – rather than being concentrated in the center of the droplet – and the speed at which the droplet propagates exceeds the speed of sound for brief periods and a shock creates wave across the surface. Each droplet behaves like a small bomb, explosively releasing its impact energy, giving it the force needed to erode surfaces over time.

“There are similar sayings in Eastern and Western cultures that ‘dripping water hollows out stones,'” explains Xiang Cheng, the article’s senior author and an associate professor in the University of Minnesota’s Department of Chemical Engineering and Materials Science.

“Sayings like this are meant to teach a moral lesson: ‘Be persistent. Even if you are weak, if you keep doing something, you will make a difference.’ But when something as soft as droplets meets something as hard as rocks, one can’t help but wonder, “Why does the fall impact cause damage in the first place?” This question has motivated our research.”

After lifting the veil on the mysterious power of the droplet, the researchers believe this new finding could help design more erosion-resistant surfaces for applications that need to withstand the elements outdoors.

Cheng and his team are already working hard to study how different textures and materials change the force generated by droplet impact.

“For example, we paint the surface of a building or coat wind turbine blades to protect the surfaces,” says Cheng. “But over time, raindrops could still cause impact damage. So, our research following this paper is to see if we can reduce the shear stress of droplets, which would allow us to design special surfaces that can mitigate the stress.”

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