Dynamics of gas-fluidized granular rods

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-square displacements to become diffusive on different time scales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the nonthermal character of the particle motion. The dynamics show evidence of rotationaltranslational coupling similar to that of an anisotropic Brownian particle. We model rotationaltranslational coupling in the single-particle dynamics with a modified Langevin model using nonthermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects. 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-square displacements to become diffusive on different time scales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the nonthermal character of the particle motion. The dynamics show evidence of rotationaltranslational coupling similar to that of an anisotropic Brownian particle. We model rotational-translational coupling in the single-particle dynamics with a modified Langevin model using nonthermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects.

Propagating waves in a monolayer of gas-fluidized rods

“…For high air flows, out-of-plane motion dominates and in-plane motion slows down. For most densities, the speed parallel to the rod axis is slightly larger than the transverse speed, although the effect is smaller than that observed for dilute systems [16]. For all densities and airspeeds analyzed, the wave speed remains an order of magnitude larger than the particle speed and the components of the particle speed remain nearly equal.…”

“…Postprocessing of the video data is accomplished in LabVIEW, using the same tracking programs used in Ref. [16]. At high densities, it becomes impossible to track all the rods at all times because the bright regions of close neighbors overlap and cannot be resolved as two separate particles; nevertheless, it is still possible to track enough rods for long enough to obtain speed statistics.…”

“…Conversely, above some air flow, the rods gain enough energy to lose contact with the substrate and overlap one another out of plane. This [16]. The vertical dashed line in both plots corresponds to the density above which the rods are uniformly distributed about the system.…”

“…For this paper, we extend our earlier experiments on individual and dilute systems of air-fluidized rods [16] in order to search for giant number fluctuations and collective flocklike motion. While flocking was never observed, we do find giant number fluctuations.…”

“…Individual air-or vibro-fluidized granular rods have also been observed to self-propel, in the sense of developing a greater speed along their long axes than along their short axes [13][14][15][16]. The idea of active self-propulsion has garnered great theoretical interest [17,18] due to the potential of unifying a broad spectrum of systems over a huge range of length scales: from biological systems, like bacteria [19][20][21][22], fish [23], locusts [24], and birds [25,26], to physical systems, such as agitated granular materials [7,11] and human or animal traffic [27,28].…”