Experimental study of liquid drop impact onto a powder surface

The initial dynamics of liquid drop impact onto powder surfaces is studied experimentally using high-speed photography. For a range of bed packing fractions, ϕ, liquid physical properties and impact velocities, ui, we observe a variety of phenomena that can be representative of a hydrophobic surface, a rough surface or a porous medium. The solids packing fraction in the bed, 0.38 ≤ ϕ ≤ 0.65, and the impact Weber number, 3.5 ≤ We = ρDui2/σ ≤ 750, (where ρ, D and σ are the drop density, diameter and surface tension respectively) are shown to be the critical parameters governing the outcome of an impact. For high packing fractions, ϕ ≳ 0.5, we show that the observed spreading, rebound and splashing can be broadly characterised in terms of the Weber number while for looser packing fractions, ϕ ≲ 0.5, we observe powder ejectas and provide a qualitative description of the granule nucleation at the centre of the impact sites.