Brownian motion of ellipsoidal particles on a granular magnetic bath

TL;DR

This study investigates the Brownian motion of ellipsoidal particles on a magnetic granular bath, revealing how diffusion scales with particle size and magnetic forcing, and noting non-Gaussian displacement statistics.

Contribution

It provides the first quantitative analysis of how magnetic bath agitation influences the translational and rotational diffusion of floating ellipsoids, including size and forcing dependencies.

Findings

Diffusion coefficients scale as 1/R^2 (translational) and 1/R^4 (rotational).

Magnetic forcing changes bath diffusivity by a factor of ten, affecting particle diffusion by fifty.

Displacement statistics of floating particles are non-Gaussian.

Abstract

We study the Brownian motion of ellipsoidal particles lying on an agitated granular bath composed of magnetic particles. 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. In terms of the particle major radius $R$, we find the translational diffusion coefficient scales roughly as $1/R^2$ and the rotational diffusion coefficient scales as roughly $1/R^4$; this is consistent with the assumption that diffusion arises from random kicks of the bath particles underneath the floating particle. By varying the magnetic forcing, the bath particles diffusivity changes by a factor of ten; over this range, the translational and rotational diffusion of the floating particles change by a factor of fifty. However, the ratio of the two diffusion constants for the floating particles is forcing-independent. Unusual aspects of the floating particle motion include non-Gaussian statistics for their displacements.