Vibration induced airflow through granular beds and density-dependent segregation

We describe a quantitative model for air-driven fluidization in a vertically vibrated granular system and use experimental results and simulations to evaluate it. The model involves a mechanism for vibrationally induced interstitial gas flow [L.I. Reyes, I. Sanchez, G. Gutierrez, Physica A, cond-mat/0502376, in press] that can generate fluid-like states that produce density-dependent segregation of large objects, through a buoyancy force. A criterion for the onset of fluidization analogous to that of a gas-fluidized static bed is used. We calculate the vertical displacement of a large sphere occurring within one period of oscillation, in specific parts of the cycle where the granular system behaves like a regular fluid.