Rayleigh-Taylor instabilities in thin films of tapped powder.

We observe powder "droplets" forming when tapping repeatedly a horizontal flat plate initially covered with a monolayer of fine powder particles. Starting from a simple model involving both the air flow through the porous cake and avalanche properties, we set up an analytical model which satisfactorily fits the experimental results. We observe a close analogy between the governing equations of the phenomenon and the basic physics of wetting liquids, including the equivalent of the Laplace law and the surface tension parameter leading to the well known Rayleigh-Taylor instability.