Drop impact on wet granular beds: effects of water-content on cratering

Drop impact events on a wet granular bed show a rich variety by changing the substrate composition. We observe the drop impact onto dry/wet granular substrates with different grain sizes (50-400 mu m) and water contents (0-22 vol%). Despite the fixed impactor conditions (impact velocity: 4.0 m s-1, water drop radius: 1.8 mm), the experiment reveals that the post-impact behaviors of both the impactor and target are strongly influenced by the substrate composition. We categorize these behaviors into several phases concerning liquid splashing and crater shapes left after the event. As these phases are relevant to each other, we measure the mechanical characteristics of the substrates and find that the onset of splashing and particle ejection is explained via the fracture of the substrate. Furthermore, we discuss several timescales of the event to understand the phase separations in more detail. Consequently, we find that the splashing phase and the crater shape are determined by competition among the timescales of impact, penetration, and contact. Drop impact experiments on wet/dry granular beds revealed that liquid splashing and crater morphology vary widely under identical impactor conditions. These differences are explained via substrate fracture and fluid motion timescales.