Spontaneously Flowing Crystal of Self-Propelled Particles
Abstract
We experimentally and numerically study the structure and dynamics of a monodisperse 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 a 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 “rheocrystal,” 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
rheocrystal is induced neither by frustration nor by noise. They further show that larger systems flow faster
while still remaining ordered.
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