Abstract
The initial dynamics of liquid drop impact onto powder surfaces is studied experimentally using high-speed photography. For a range of bed packing fractions, h, liquid physical properties and impact velocities, u sub(i) , 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 , h , 0.65, and the impact Weber number, 3.5 , We = h rDu sub(i) super(2)/s , 750, (where r, D and s 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, h > 30.5, we show that the observed spreading, rebound and splashing can be broadly characterised in terms of the Weber number while for looser packing fractions, h < 0.5, we observe powder ejectas and provide a qualitative description of the granule nucleation at the centre of the impact sites.