Rayleigh-Taylor instability of a particle packed viscous fluid (Michioka and Sumita, 2005, Geophys. Res. Lett. , 32, doi:10.1029/2004GL021827)
This is an example from a series of experiments with varying particle diameter and liquid viscosity. The upper layer of a thickness of 5mm contains glass beads with a diameter of 50 microns with a packing fraction of 0.6. The lower liquid layer is glycerine solution with a viscosity of 0.775 Pas. The video is 10 times faster than the actual experiment (in QuickTime). The onset of instability can be explained by applying the linear stability analysis of R-T instability of superposed viscous fluids by taking effective values for the layer thickness and viscosity of the particle bed. The later stage of instability shows number of features not seen for layered viscous fluids.