  When submillimetric particles are confined in a fluid such that a compact
cluster of particles lie above the clear fluid, particles will detach from the
lower boundary of the cluster and form an unstable separation front giving rise
to growing fingers of falling particles. We study this problem using both
experiments and hybrid granular/fluid mechanics models. In the case of
particles from 50 to 500 microns in diameter falling in air, we study the
horizontal density fluctuations at early times: the amplitude of the density
difference between two points at a certain horizontal distance grows as a power
law of time. This happens up to a saturation corresponding to a power law of
the distance. The way in which the correlation length builds up to this
saturation also follows a power law of time. We show that these decompaction
fronts in sedimentation problems follow a Family-Vicsek scaling, characterize
the dynamic and Hurst exponent of the lateral density fluctuations,
respectively z \sim 1 and \zeta \sim 0.75, and show how the prefactors depend
on the grain diameter. We also show from similar simulations with a more
viscous and incompressible fluid, that this feature is independent of the fluid
compressibility or viscosity, ranging from air to water/glycerol mixtures.

Fluid Dynamics;Statistical Mechanics