Emergent states in dense systems of active rods: from swarming to turbulence

TL;DR

This paper uses computer simulations to explore the diverse non-equilibrium behaviors of dense suspensions of self-propelled rods, revealing various collective states like swarming, turbulence, and jamming across different densities and aspect ratios.

Contribution

It provides a comprehensive phase diagram of emergent states in dense active rod systems using a minimal model and numerical simulations, highlighting the role of shape and density.

Findings

Disordered phase at low density and aspect ratio

Swarming state at large aspect ratio

Jamming transition at high densities

Abstract

Dense suspensions of self-propelled rod-like particles exhibit a fascinating variety of non-equilibrium phenomena. By means of computer simulations of a minimal model for rigid self-propelled colloidal rods with variable shape we explore the generic diagram of emerging states over a large range of rod densities and aspect ratios. The dynamics is studied using a simple numerical scheme for the overdamped noiseless frictional dynamics of a many-body system in which steric forces are dominant over hydrodynamic ones. The different emergent states are identified by various characteristic correlation functions and suitable order parameter fields. At low density and aspect ratio, a disordered phase with no coherent motion precedes a highly-cooperative swarming state at large aspect ratio. Conversely, at high densities weakly anisometric particles show a distinct jamming transition whereas slender particles form dynamic laning patterns. In between there is a large window corresponding to strongly vortical, turbulent flow. The different dynamical states should be verifiable in systems of swimming bacteria and artificial rod-like micro-swimmers.