Revealing the Nonequilibrium Nature of a Granular Intruder: The Crucial Role of Non-Gaussian Behavior

TL;DR

This paper investigates the nonequilibrium behavior of a granular intruder by analyzing non-Gaussian velocity fluctuations and time-reversal asymmetry, introducing a new coupled-variable model that captures experimental observations across different densities.

Contribution

It introduces a novel model with coupled variables and mixed noise types to describe the nonequilibrium dynamics of a granular intruder, emphasizing non-Gaussian effects.

Findings

The angular velocity shows significant deviations from equilibrium behavior.

The model accurately reproduces experimental results across various densities.

Non-Gaussian velocity increments are crucial for understanding the system's irreversibility.

Abstract

The characterization of the distance from equilibrium is a debated problem in particular in the treatment of experimental signals. If the signal is a 1-dimensional time-series, such a goal becomes challenging. A paradigmatic example is the angular diffusion of a rotator immersed in a vibro-fluidized granular gas. Here, we experimentally observe that the rotator's angular velocity exhibits significative differences with respect to an equilibrium process. Exploiting the presence of two relevant time-scales and non-Gaussian velocity increments, we quantify the breakdown of time-reversal asymmetry, which would vanish in the case of a 1D Gaussian process. We deduce a new model for the massive probe, with two linearly coupled variables, incorporating both Gaussian and Poissonian noise, the latter motivated by the rarefied collisions with the granular bath particles. Our model reproduces the experiment in a range of densities, from dilute to moderately dense, with a meaningful dependence of the parameters on the density. We believe the framework proposed here opens the way to a more consistent and meaningful treatment of out-of-equilibrium and dissipative systems.