Hysteresis, reentrance, and glassy dynamics in systems of self-propelled rods

TL;DR

This study explores the complex behaviors of self-propelled rods, revealing distinct flocking and nematic-laning states with unique mechanical properties, and proposes glass transition-like phenomena in active matter systems.

Contribution

It provides a comprehensive state diagram of self-propelled rods, identifying novel glassy and fluid-like phases and transitions driven by density and activity levels.

Findings

Flocking states exhibit high pressure and glassy dynamics.

Nematic-laning states behave fluid-like with ballistic transport.

Transitions between states resemble glass transitions with fluidization at higher densities.

Abstract

Non-equilibrium active matter made up of self-driven particles with short-range repulsive interactions is a useful minimal system to study active matter as the system exhibits collective motion and nonequilibrium order-disorder transitions. We studied high-aspect-ratio self-propelled rods over a wide range of packing fraction and driving to determine the nonequilibrium state diagram and dynamic properties. Flocking and nematic-laning states occupy much of the parameter space. In the flocking state the average internal pressure is high and structural and mechanical relaxation times are long, suggesting that rods in flocks are in a translating glassy state despite overall flock motion. In contrast, the nematic-laning state shows fluid-like behavior. The flocking state occupies regions of the state diagram at both low and high packing fraction separated by nematic-laning at low driving and a history-dependent region at higher driving; the nematic-laning state transitions to the flocking state for both compression and expansion. We propose that the laning-flocking transitions are a type of glass transition which, in contrast to other glass-forming systems, can show fluidization as density increases. The fluid internal dynamics and ballistic transport of the nematic-laning state may promote collective dynamics of rod-shaped microorganisms.