Traveling Fronts and the Smoothing of the Collective-Motion Phase Transition in Vibrated Polar Disks

TL;DR

This study demonstrates that the phase transition to collective motion in vibrated polar disks appears continuous in experiments, contrasting with the discontinuous transition observed in larger systems, highlighting the importance of finite size effects.

Contribution

It reveals that finite size effects can change the nature of the phase transition in active matter systems, challenging previous assumptions based on larger or infinite systems.

Findings

Transition appears continuous in experiments with hundreds of disks

Finite size effects influence the observed phase transition nature

Comparison between experimental fluctuations and traveling bands

Abstract

One of the most well known features of active matter is the tendencey of self-propelled particles to undergo system-wide collective motion. With low enough rotational noise or high enough global density, these systems spontaneously break symmetry and transition to a state with nonzero net momentum. The transition is currently understood as discontinuous, with phase coexistence manifesting in terms of dense travelling bands propagating through a dilute background. Here we show that the phase transition appears continuous in experiments with a system of hundreds of polar vibrated disks, and compare the fluctuations in this system to the traveling bands that are present in larger systems. We argue that this difference is due to finite size effects, which are often considered an artifact of simulations in studies with equilibrium systems, but that are of fundamental importance in many systems of self-propelled particles such as those composed of living individuals.