Brownian motion of ellipsoidal particles on a granular magnetic bath

Abstract

We study the Brownian motion of ellipsoidal particles lying on an agitated granular bath composed of magnetic particles. As we decrease the magnetic forcing, the bath takes characteristics of a molecular system that is cooled towards the glass transition. We quantify the mobility of different floating ellipsoidal particles using the mean square displacement and the mean square angular displacement, and relate the diffusion coefficients to the bath particle motion. The ratio of translational and rotational diffusion constants for the floating particles is forcing-independent, although the particle shape matters (with longer floating particles rotating slower than shorter). Unusual aspects of the floating particle motion include non-Gaussian statistics for their displacements, and a shape-dependent and forcing-dependent anisotropy of translational diffusion coefficients parallel and perpendicular to the ellipsoid long axis.