Dynamics of Gas-Fluidized Granular Rods

TL;DR

This study investigates the complex dynamics of granular rods in a fluidized monolayer, revealing anisotropic superdiffusive behavior, non-exponential collision statistics, and rotational-translational coupling modeled via a modified Langevin approach.

Contribution

It introduces a detailed experimental analysis of granular rod dynamics and proposes a modified Langevin model to describe their coupled rotational and translational motion.

Findings

Superdiffusive motion along the rod's axis

Non-exponential free path distributions

Rotational-translational coupling similar to anisotropic Brownian particles

Abstract

We study a quasi-two-dimensional monolayer of granular rods fluidized by a spatially and temporally homogeneous upflow of air. By tracking the position and orientation of the particles, we characterize the dynamics of the system with sufficient resolution to observe ballistic motion at the shortest time scales. Particle anisotropy gives rise to dynamical anisotropy and superdiffusive dynamics parallel to the rod's long axis, causing the parallel and perpendicular mean squared displacements to become diffusive on different timescales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the non-thermal character of the particle motion. The dynamics show evidence of rotational-translational coupling similar to that of an anisotropic Brownian particle. We model rotational-translation coupling in the single-particle dynamics with a modified Langevin model using non-thermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects.