Anomalous Diffusion in a Monolayer of Lightweight Spheres Fluidized in Airflow

Abstract

This paper presents statistical analyses of random motions in a single layer of fluidized lightweight spherical particles. Foam polystyrene spheres were driven by an upward airflow through the sieve mesh, and their two-dimensional motion was acquired using image analysis. In the bulk region, the particle velocity distributions changed from Gaussian to heavy-tailed distribution as the bulk packing fraction $\phi_b$ was increased. The mean square displacement of the particles exhibited transition to subdiffusion at much lower $\phi_b$ than observed in previous studies using similar setup but with heavier particles. A slight superdiffusion and significant growth of the correlation length in the two-body velocity correlation was observed at further large $\phi_b$. The effect of the wall on the dynamics of the particles was also investigated and the anisotropy of the granular temperature was found to be a useful index to discriminate between the wall region and the bulk. The turbulence statistics in the wake of a particle indicated a strong wall-normal asymmetry of aerodynamic forcing as the ``thermal'' agitation in the wall region.