Anyone who has watched raindrops falling on a beach is likely familiar with the craters the raindrops create in the sand. But impact cratering in granular materials by liquid drops is distinctly different from the well-studied granular cratering by solid spheres and is much less understood. This snapshot from a high-speed video taken by researchers in Xiang Cheng’s soft-matter physics group at the University of Minnesota captures a particularly dramatic moment after a falling water drop, 3.9 mm in diameter, violently strikes a bed of 90-micron glass beads. As the drop initially spreads out and then retracts and jumps off the granular bed, it entrains a layer of the fine particles on its surface, which gives rise to an exquisite “liquid marble.” A permanent crater is left underneath.
Investigating granular impact dynamics and crater formation for drop energies spanning four orders of magnitude, Cheng and company found a most surprising...
