Friday, October 7, 2022

Mystery solved: How do soft liquid droplets erode hard surfaces?

The discovery could help engineers design better, more erosion-resistant materials.

Sometimes soft liquid droplets can potentially erode hard surfaces. This phenomenon- drop impact- dictates many significant natural, environmental, and engineering processes and calls for substantial prevention and preservation efforts. However, despite several studies on the kinematic features of impacting drops over the last two decades, the dynamic process that leads to drop-impact erosion remains unclear.

In a first-of-its-kind study led by the University of Minnesota Twin Cities, scientists solved this mystery. They developed a new method of high-speed stress microscopy to measure hidden quantities such as the shear stress and pressure created by the impact of liquid droplets on surfaces.

By using this new method, scientists could directly measure the force, stress, and pressure underneath liquid drops as they hit surfaces. They found that the force exerted by a droplet spreads out with the impacting drop. Plus, the speed at which the droplet spreads out exceeds the speed of sound at short times, creating a shock wave across the surface.

When the droplet hits a hard surface, it acts as a small bomb. It releases its impact energy explosively and gives it the force necessary to erode surfaces over time.

Xiang Cheng, senior author of the paper and an associate professor in the University of Minnesota Department of Chemical Engineering and Materials Science, said, “There are similar sayings in eastern and western cultures that ‘Dripping water hollows out a stone.’ 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?’ That question is what motivated our research.”

In addition to offering a way to study droplet impact, this study could help engineers design more erosion-resistant surfaces for applications that must weather the outdoor elements. Scientists have already planned to expand this research to study how different textures and materials change the amount of force created by liquid droplets.

Journal Reference:

  1. Stress distribution and surface shock wave of drop impact, Nature Communications (2022). DOI: 10.1038/s41467-022-29345-x
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