Morphology scaling of drop impact onto a granular layer.

We investigate the impact of a free-falling water drop onto a granular layer. First, we constructed a phase diagram of crater shapes with two control parameters, impact speed and grain size. A low-speed impact makes a deeper cylindrical crater in a fluffy granular target. After high-speed impacts, we observed a convex bump higher than the initial surface level instead of a crater. The inner ring can be also observed in a medium impact speed regime. Quantitatively, we found a scaling law for a crater radius with a dimensionless number consisting of impact speed and density ratio between the bulk granular layer and water drop. This scaling demonstrates that the water drop deformation is crucial to understanding the crater morphology.