Propagating Waves in a Monolayer of Gas-Fluidized Rods

TL;DR

This study observes propagating compression waves in a monolayer of gas-fluidized rods, revealing their speed, fluctuation behavior, and phase conditions, with implications for understanding collective dynamics in granular systems.

Contribution

It reports the first observation of propagating waves in fluidized granular rods and characterizes their properties and phase behavior, highlighting the relationship between wave propagation and self-propulsion.

Findings

Waves propagate faster than individual particles by an order of magnitude.

Number fluctuations are anomalously large, scaling as N^{0.72}.

Wavespeed increases linearly with packing fraction, independent of airflow.

Abstract

We report on an observation of propagating compression waves in a quasi-two-dimensional monolayer of apolar granular rods fluidized by an upflow of air. The collective wave speed is an order of magnitude faster than the speed of the particles. This gives rise to anomalously large number fluctuations dN ~ $N^{0.72 \pm 0.04}$, which are greater than ordinary number fluctuations of N^{1/2}. We characterize the waves by calculating the spatiotemporal power spectrum of the density. The position of observed peaks, as a function of frequency w and wavevector k, yields a linear dispersion relationship in the long-time, long-wavelength limit and a wavespeed c = w/k. Repeating this analysis for systems at different densities and air speeds, we observe a linear increase in the wavespeed with increasing packing fraction with no dependence on the airflow. Although air-fluidized rods self-propel individually or in dilute collections, the parallel and perpendicular root-mean-square speeds of the rods indicate that they no longer self-propel when propagating waves are present. Based on this mutual exclusivity, we map out the phase behavior for the existence of waves vs self-propulsion as a function of density and fluidizing airflow.