Sinking of light intruders in a shaken granular bed.

We present an experimental study of the displacement of a light intruder immersed in a vibrated granular bed. Using high speed video we resolve the motion, during one cycle of oscillation, of a cylindrical object inside a Plexiglas box partially filled with grains. We report experimental evidence that, in the absence of convection, at least two forces are behind the intruder's motion: an air drag force--due to the airflow through the granular bed--and a buoyancy force produced by an air-mediated granular fluid.