Invasion-wave induced first-order phase transition in systems of active particles

TL;DR

This paper demonstrates that soliton-like waves in a kinetic model of self-propelled particles induce a first-order phase transition, challenging the traditional second-order transition understanding in collective motion systems.

Contribution

It introduces a kinetic approach revealing how invasion waves cause a first-order transition in active particle systems, which was previously thought to be second order.

Findings

Soliton-like waves are observed near the transition to collective motion.

These waves cause hysteresis and abrupt changes in the order parameter.

The transition character shifts from second to first order due to invasion waves.

Abstract

An Enskog-like kinetic equation for self-propelled particles is solved numerically. I study a density instability near the transition to collective motion and find that while hydrodynamics breaks down, the kinetic approach leads to soliton-like supersonic waves with steep leading kinks and Knudsen numbers of order one. These waves show hysteresis, modify the transition threshold and lead to an abrupt jump of the global order parameter if the noise level is changed. Thus they provide a mechanism to change the second-order character of the phase transition to first order.