spaceSpace and Physics

Slow Motion Video Reveals How Raindrops Fall on Sand Like Asteroids

445 Slow Motion Video Reveals How Raindrops Fall on Sand Like Asteroids
R. Zhao et al., PNAS 2014

What does a rainy day on the beach and catastrophic asteroid strikes have in common? Despite the massive difference in scale, a whole lot it turns out, according to new work published in Proceedings of the National Academy of Sciences this week. Using high-speed imaging, researchers have captured the impact of raindrops falling on sandy surfaces: the surface deforms like a liquid while preserving a circular crater like a solid -- just like when an asteroid strikes Earth. 

We know quite a bit about the mechanism behind impact cratering by solid spheres, but what we know about the same phenomenon caused by liquid spheres is pretty limited. So, by combining photography with lasers, a University of Minnesota team led by Xiang Cheng studied the dynamics of these liquid-drop impacts on a granular surface made of tiny glass beads, as well as the impact craters that resulted. In addition to deionized water, they also experimented with liquids like methanol, ethylene glycol, and mineral oil, dropping the droplets from as high as 11.5 meters. 


Surprisingly, despite the enormous energy and length difference, granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters, the team writes. Here are snapshots of a 3.1-millimeter water drop impacting at different energy levels (they increase from the top row to the third): 


As the droplet splashes slowly, Popular Science describes, it spreads out horizontally, creating a small crater before retracting, pulling in some particles and bouncing back up. At higher impact velocities, the drop spreads out more and picks up more sand, increasing its weight and reducing the height of its bounce. Increasing the energy of the impact creates projections (like the limbs of a cartoon amoeba) that pick up so many beads that the droplet stops being a perfect sphere, and with even more energy, they eventually stop bouncing back up. 

Here’s a complication of some very cool slow-motion movies, via American Physical Society’s Division of Fluid Dynamics




This newfound similarity allows the team to apply planetary science models to rain-drop-sized scales that are relevant to natural, agricultural, and industrial processes -- from soil erosion and irrigation to the dispersal of microbes and the spray-coating of powders. Also, the vestige of raindrop imprints in fossilized granular surfaces, they write, can be used to calculate the air density on Earth 2.7 billion years ago. 

Images: R. Zhao et al., PNAS 2014
Video: APS Physics Gallery of Fluid Motion




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