Amélie Chardac
Post-doctorante au Laboratoire de Physique, ENS de Lyon

“Life and death of topological defects in polar active matter”

Active-matter physics describes the mesmerizing dynamics of interacting motile bodies: from bird flocks and cell colonies, to collections of synthetic units independently driven far from equilibrium. When motile units self-assemble into flocks where all particles propel along the same direction, they realize one of the most robust ordered phase observed in Nature.
But, after twenty five years of intense research the very mechanism controlling the ordering dynamics of both living and artificial flocks has remained unsettled.

In this talk, building on model experiments based on Quincke rollers, I will first explain how a flock suppresses its singularities to form an ordered spontaneous flow. Combining experiments, simulations and theory I will show how to elucidate the elementary excitations of 2D polar active matter and explain their phase ordering dynamics as a self-similar process emerging from the annihilation of $\pm 1$ defects along a filamentous network of domain walls with no counterparts in passive systems (

In a second part, I will address the robustness of long range order and discuss the stabilization of topological defects in a polar active fluid through disordered media. Combining experiments and theory, I will show that colloidal flocks collectively cruise through disorder without relaxing the topological singularities of their flows, unlike in pure systems. % where they reach a macroscopic global order.

Introducing colloidal flocks in micro patterned circular chambers, we reveal a state of strongly disordered active matter with no counterparts in equilibrium: a dynamical vortex glass. The resulting state is highly dynamical but the flow patterns, shaped by a finite density of frozen vortices, are stationary and exponentially degenerated (

1. Topology-driven ordering of flocking matter. Chardac A., Hoffmann L. , Poupart Y.,
Giomi L., Bartolo D. Phys. Rev. X 11, 031069 (2021).
2. Emergence of dynamic vortex glasses in disordered polar active fluids. Chardac A., Shan-
kar S., Marchetti M. C., Bartolo D. Proc. Natl. Acad. Sci. USA, 118(10) (2021).

Key words
Active matter, Quincke rollers, collective motion, topological defects, phase ordering dynamics,
disordered media.