Rayleigh-Taylor Instabilities in Thin Films of Tapped Powder

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We observe powder ''droplets'' forming when tapping repeatidly an 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 setup 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-Young law and the surface tension parameter leading to the familiar Rayleigh Taylor instability.