Small Obstacle in a Large Polar Flock

TL;DR

This paper demonstrates that a small obstacle can induce complex, chaotic behavior in large polar flocks, revealing a new disordered phase through simulations and Boltzmann equation analysis.

Contribution

It introduces the discovery that small obstacles can cause chaotic dynamics in large flocks, supported by particle simulations and Boltzmann equation truncations.

Findings

Small obstacles trigger dense band formations in flocks.

Large systems exhibit chaotic, disordered behavior.

Similar phenomena occur in Boltzmann equation simulations.

Abstract

We show that arbitrarily large polar flocks are susceptible to the presence of a single small obstacle. In a wide region of parameter space, the obstacle triggers counter-propagating dense bands leading to reversals of the flow. In very large systems, these bands interact yielding a never-ending chaotic dynamics that constitutes a new disordered phase of the system. While most of these results were obtained using simulations of aligning self-propelled particles, we find similar phenomena at the continuous level, not when considering the basic Toner-Tu hydrodynamic theory, but in simulations of truncations of the relevant Boltzmann equation.