The explanation: each droplet, although small and soft, acts like a tiny bomb, releasing its impact energy explosively and thus creating punctual erosion
There are similar sayings in many cultures that “Dripping water hollows out stone“. In physics, the question is: how can something so soft and small like droplets hitting something so hard like rocks erode the stone’s surfaces over time? In a study led by scientists from the University of Minnesota Twin Cities, new technology was used to measure the forces involved in the impact. Their answer to the question: the destruction is possible because the impact energy is released explosively.
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 quantitative way to study the 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.
The research could help engineers design more erosion-resistant surfaces for applications that must weather the outdoor elements. Xiang Cheng, senior author on the paper and an associate professor in the University of Minnesota Department of Chemical Engineering and Materials Science, already plans to study how different textures and materials may change the force created by 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 damage.”
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 Álvarez-Novoa and Klebbert Andrade, and O’Higgins University, Chile Assistant Professor Pablo Gutiérrez.
The research was funded by the National Science Foundation.
Nature Communications: “Stress distribution and surface shock wave of drop impact”
(07.04.2022, USA: 04.07.2022)