Raindrop-Formed ‘Sandballs’ That Erode Hillsides Tenfold

In a late summer morning in Switzerland in 2021, Hugo Ulloa, a geophysicist at the University of Pennsylvania, was on a hike with colleague Bertil Trottet when they noticed something strange about how the rain droplets were hitting the dry ground along the hillside.

“When they landed, they splattered,” Ulloa says. The smaller rain droplets then started to roll and change shape, forming structures resembling snowballs rather than simply dissipating or sliding. They called the unusual structures “sandballs.”

These sandballs got the researchers thinking: “What conditions allow them to roll, and how do they aggregate soil and change shape? And has anyone else studied this behavior?”

Upon further investigation, Ulloa and Trottet were surprised to find that much of the literature on how droplets interact with granular landscapes focused on horizontal not sloping surfaces. That lack of literature, they knew, also limited the information used in erosion models.

This prompted a four-year-long, multi-lab study. They found that the process of these raindrops hitting a dry sloping landscape and forming sandballs greatly amplified the amount of soil a single drop could move by, in some instances, as much as 10 times. The work, now published in the Proceedings of the National Academy of the Sciences, provides fundamental insights into how rain-related erosion happens, helping to improve models of landscape change, soil loss, and agriculture.

“Current geological and agricultural models primarily estimate erosion based on the initial impact and splash of raindrops,” says co-first author Daisuke Noto, a former postdoctoral researcher in Ulloa’s GEFLOW Lab at Penn’s School of Arts & Sciences. But, by ignoring what happens after the initial splash, they revealed that sandballs are a powerful and previously overlooked erosion mechanism that can move as much as two orders of magnitude more mass than splash erosion alone.

Partnering with Penn geophysicist Douglas Jerolmack, Noto devised a 1.2-meter-long chute filled with silicate sand, tilted to exactly 30 degrees, just shy of the angle where the sand would naturally avalanche. When they tested raindrops made from mixtures of water and a viscosity-control substance (glycerol), they found that rebounding droplets pick up grains along the way down the chute, allowing the droplets to form two kinds of sandballs: peanut-shaped aggregates with liquid cores and donut-shaped, or toroidal, aggregates, centered with a little air sac that behaves like a solid, a never-before-seen phenomena.

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Pictured above: High-speed laboratory images capture two distinct “sandball” shapes formed when raindrops strike dry, sloped sand and roll downhill. (Top) Peanut-shaped sandballs, where grains coat the surface of a liquid core. (Bottom) Donut-shaped sandballs, which densify into rigid, wheel-like structures with a hollow center, enabling far more efficient sediment transport than splash erosion alone. (Credit: Daisuke Noto)