Vibrated granular materials can appear very much like a fluid. Yet there are important differences. A completely counter-intuitive property is that if a sufficiently heavy large object is placed inside a vibrated container filled with granular particles, it will rise to the top. Even more confounding is that a very light intruder can either rise or sink. Using magnetic resonance imaging and high-speed video techniques, we have demonstrated that both the rising and the sinking behavior is determined by the interaction of the granular medium with the air in its interstices: when the system is evacuated, the intruder follows the motion of the background particles. These results suggest a new model for understanding the role of the interstitial air on the so-called "Brazil Nut Effect", by which larger objects typically rise to the top of a shaken granular medium.

Here's more, including movies demonstrating the effect of interstitial gas (air) on the rising or sinking of large granular particles in a vibrated bed of smaller particles

1. •Matthias E. Möbius, Benjamin E. Lauderdale, Sidney R. Nagel and Heinrich M. Jaeger, “Size Separation of Granular Particles”, Nature 414, 270 (2001). pdf
   

2. •Matthias E. Möbius, Xiang Cheng, Peter G. Eshuis, Gregory Karczmar, Sidney R. Nagel, and Heinrich Jaeger, “The Effect of Air on Granular Size Separation in a Vibrated Granular Bed”, Phys. Rev. E 72, 011304 (2005)