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, phi, liquid physical properties and impact velocities, u(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 <=phi <= 0.65, and the impact Weber number, 3.5 <= We = rho Du(i)(2)/sigma <= 750, (where rho, D and sigma 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, phi greater than or similar to 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, phi less than or similar to 0.5, we observe powder ejectas and provide a qualitative description of the granule nucleation at the centre of the impact sites. (C) 2010 Elsevier B.V. All rights reserved.