  The temporal evolution of a water-sand interface driven by gravity is
experimentally investigated. By means of a Fourier analysis of the evolving
interface the growth rates are determined for the different modes appearing in
the developing front. To model the observed behavior we apply the idea of the
Rayleigh-Taylor instability for two stratified fluids. Carrying out a linear
stability analysis we calculate the growth rates from the corresponding
dispersion relations for finite and infinite cell sizes. Based on the
theoretical results the viscosity of the suspension is estimated to be
approximately 100 times higher than that of pure water, in agreement with other
experimental findings.
