  Using MRI and high-speed video we investigate the motion of a large intruder
particle inside a vertically shaken bed of smaller particles. We find a
pronounced, non-monotonic density dependence, with both light and heavy
intruders moving faster than those whose density is approximately that of the
granular bed. For light intruders, we furthermore observe either rising or
sinking behavior, depending on intruder starting height, boundary condition and
interstitial gas pressure. We map out the phase boundary delineating the rising
and sinking regimes. A simple model can account for much of the observed
behavior and show how the two regimes are connected by considering pressure
gradients across the granular bed during a shaking cycle.
