Drop impact dynamics on granular beds and thin porous membranes: A review

The impact of liquid droplets on porous substrates governs a wide range of natural and industrial processes, from soil erosion and additive manufacturing to agricultural spraying and pathogen transmission. This review focuses on recent progress into two distinct types of porous substrates: deformable granular beds and thin porous membranes. On granular beds, interaction of liquid drop and particle gives rise to such rich phenomena as crater formation, liquid marble formation, and granule aggregation, as shaped by interplay of inertia, capillarity, viscous forces and substrate deformability. Thin porous membranes permit penetration of high-inertia liquid drop, thus exhibiting unconventional drop spreading and rebound behavior, and triggering post-penetration dynamics such as jet formation, microdroplet ejection, and spray generation. We examine the effects of viscosity, wettability, and non-Newtonian properties on these dynamics. By comparing these two classes of porous substrates, we highlight both common physical principles and system-specific behaviors. This unified perspective aims to guide future research and application of droplet-porous surface interactions in areas including inkjet printing, environmental remediation, and infection control.