Flowing crystal of self-propelled particles

TL;DR

This paper investigates the formation and dynamics of a flowing crystalline phase in self-propelled particles, revealing a novel 'rheo-crystal' state characterized by shear-induced order and flow, supported by experiments and simulations.

Contribution

It introduces the concept of a 'rheo-crystal' in self-propelled particles, demonstrating shear-induced flow and order without frustration or noise, supported by combined experimental and numerical analysis.

Findings

Formation of a flowing crystalline 'rheo-crystal' state.

Flow speed increases with system size while maintaining order.

Flow is driven by shear along stacking faults, not frustration or noise.

Abstract

We experimentally and numerically study the structure and dynamics of a mono-disperse packing of spontaneously aligning self-propelled hard disks. The packings are such that their equilibrium counterparts form perfectly ordered hexagonal structures. Experimentally, we first form a perfect crystal in an hexagonal arena which respects the same crystalline symmetry. Frustration of the hexagonal order, obtained by removing a few particles, leads to the formation of a rapidly diffusing "droplet". Removing more particles, the whole system spontaneously forms a macroscopic sheared flow, while conserving an overall crystalline structure. This flowing crystalline structure, which we call a "rheo-crystal" is made possible by the condensation of shear along localized stacking faults. Numerical simulations very well reproduce the experimental observations and allow us to explore the parameter space. They demonstrate that the rheo-crystal is induced neither by frustration nor by noise. They further show that larger systems flow faster while still remaining ordered.