Domain-wall networks rule the ordering dynamics of flocking matter.

We address the ordering dynamics of flocking matter.
When motile units self-assemble into flocks where all particles propel along the same direction, they realize one of the most stable phase observed in Nature. Unlike in active nematics or passive systems such as ferromagnets or liquid crystals, the long range orientational ordered active fluids formed by flocking units are robust to defect proliferation even in two dimensions.
Building on model experiments based on Quincke rollers, I will show how the velocity field of a colloidal flock initially marred by a number of topological defects, heals and reaches pristine orientational order. Combining experiments, simulations and theory I will present how to elucidate the elementary excitations of 2D polar active matter and explain their phase ordering dynamics. I will explain how self-similar dynamics emerges from the annihilation of +/-1 vortices along a filamentous network of domain walls with no counterparts in passive systems. Remarkably, the structure of this fully connected network is mainly determined by extended singularity lines growing from −1 vortices. The two body interactions between the defects correctly account for the self-similar coarsening of the density and flow excitations of flocking liquids.