It flows through your fingers, but is one of the most destructive agents on Earth – what gives water its erosive power?

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

What gives this seemingly benign substance so much erosive power?

In a first-of-its-kind study led by researchers at University of Minnesota Twin Cities, US, the hidden strength that allows liquid droplets to erode hard surfaces has been revealed.

The study of droplets isn’t new – researchers have long been fascinated by them, from the way raindrops hit the ground to the transmission of pathogens in aerosols. But until now, their study has been limited to visual analysis using high-speed cameras.

Now, publishing in Nature Communications, researchers describe a newly developed technique that allows direct measurement of previously hidden quantities that endow droplets with their magnificent force. The new technique, called high-speed stress microscopy, provides a more quantitative way to study the liquid erosion 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 centre 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.

“There are similar sayings in both eastern and western cultures that ‘dripping water hollows out stone,’” explains 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.”

Having drawn back the veil on the mysterious power of the droplet, the researchers believe this new insight could be put to work in helping to design more erosion-resistant surfaces for applications that must weather the outdoor elements.

Cheng and his team are already hard at work studying how different textures and materials change the amount of force created by the impact of droplets.

“For example, we paint the surface of a building or coat wind turbine blades to protect the surfaces,” Cheng says. “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.”

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