Anomalous Diffusion in a Monolayer of Lightweight Spheres Fluidized in Airflow

TL;DR

This study investigates the motion of lightweight spherical particles in a fluidized bed, revealing transitions in diffusion behavior, velocity distribution changes, and wall effects through detailed statistical analysis.

Contribution

It provides new insights into the anomalous diffusion and velocity correlations of lightweight particles in fluidized systems, highlighting the influence of packing fraction and wall proximity.

Findings

Velocity distributions shift from Gaussian to heavy-tailed with increasing packing fraction

Mean square displacement shows transition to subdiffusion at lower packing fractions

Wall effects induce anisotropy and asymmetry in particle dynamics

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.