Magnetized granular particles running and tumbling on $S^{1}$

TL;DR

This paper models a magnetized granular particle in a circular channel, revealing a phase transition from erratic to persistent motion driven by magnetic energy, with theoretical predictions confirmed experimentally.

Contribution

It introduces a run and tumble model for a single magnetized particle on a circle, predicting a phase transition between disordered and ordered motion.

Findings

Existence of a dynamical phase transition at $ ext{l}_c = R/2$

Disordered phase resembles Brownian motion on a circle

Ordered phase corresponds to uniform circular motion

Abstract

It has been shown that a nonvibrated magnetic granular system, when it is feeded by means an altenating magnetic field, behaves with most of the distinctive physical features of active matter systems. In this work we focus our attention on the simplest granular system composed by a single magnetized spherical particle allocated in a quasi one-dimensional circular channel that receives energy from a magnetic field reservoir and transduces it into a running and tumbling motion. The theoretical analysis based on the run and tumble model on a circle of radius R forecasts the existence of a dynamical phase transition between an erratic motion (disordered phase) when the characteristic persistence length of the run and tumble motion, $\ell_{c} < R/2$, to a persistent motion (ordered phase) when $\ell_{c}> R/2$. It is found that the limiting behaviours of these phases correspond to a Brownian motion on the circle and a simple uniform circular motion, respectively. It is qualitatively shown that the lower magnetization of a particle, the larger persistence lenght is. It is so at least within the experimental limit of validity of our experiments. Our results show a very good agreement between theory and experiment.