Title: From Drops to Sandballs

Author (Invited): Hugo Ulloa, University of Pennsylvania

Abstract:

Raindrops striking soil are usually viewed as agents of impact, splash, and erosion. Yet recent experiments reveal a more surprising pathway: under the right conditions, a drop can capture grains, detach, roll downslope, grow by entrainment, and transform into a dense sediment aggregatea sandball. This keynote uses raindrop-induced aggregates as the motivation for exploring a broader problem in fluidgranular physics: how liquid drops create, mobilize, reshape, and jam granular matter. By connecting impact, wetting, capillary cohesion, rolling friction, morphology selection, and arrest, the talk frames sandballs as transient material bodies whose short lives reveal rich physics and open questions now beginning to be explored.