The Rheo-crystal : a flowing crystal of self-propelled particles*

We experimentally and numerically study the dynamics of a mono-disperse packing of self- propelled hard disks inside a hexagonal arena [1]. The packings are dense, with area fractions close to the maximal geometrically allowed value φhcp in two dimension. 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 rotating flow while conserving an overall crystalline structure. We shall call this state a “rheo-crystal”. Numerical simulations very well reproduce the experimental observations. They further demonstrate that when packing fraction is simply lowered without introducing frustration, the rheo-crystal phase subsists, while the droplet regime disappears. Finally, the larger the system is, the faster the ”rheo-crystal” rotates and the more ordered its structure.

[1] arXiv:1709.03844