Sandballs

Raindrops do more than spatter sand—they can build it. We report a robust pathway in which an impacting drop on an inclined granular surface rebounds, rolls downslope, and continually entrains grains to self-assemble a mobile aggregate—a "sandball." Controlled laboratory experiments reveal two reproducible morphologies: (i) two-lobed, peanut-like bodies form when the drop remains fluid and acquires a granular skin while rolling; and (ii) toroidal, doughnut-like bodies emerge when grains are swept into the drop interior, thicken the suspension, and densify into a jammed annulus. High-speed imaging and post-impact mass measurements show that hydrodynamic forcing acting on grains sets the entrainment rate and selects the morphology. Complementary field observations document naturally occurring sandballs, establishing environmental relevance. We identify a transition criterion between coated-drop and toroidal regimes and develop scaling relations for the entrained solid mass. The results quantify how a single drop can act as a vector that gathers, transports, and deposits sediment over long distances, providing a mechanism for rainfall-driven microscale sculpting on hillslopes. Our findings connect impact hydrodynamics, granular physics, and geomorphology in a single, testable framework with predictive power.