Abstract
How do flocks, herds and swarms move through disordered environments? The answer to this question is crucial not only to animal groups in the wild, but also to effectively all applications of collective robotics and active materials composed of synthetic motile units1. In stark contrast, aside from rare exceptions, our physical understanding of flocking has so far been limited to homogeneous media.
Here we explain how collective motion survives in geometrical disorder. To do so, we combine experiments and analytical theory to examine motile colloids cruising between randomly positioned microfabricated obstacles. We elucidate how disorder and bending elasticity compete to channel the flow of polar flocks along sparse river networks akin those found beyond plastic depinning in driven condensed matter21. Further increasing the disorder, we demonstrate that collective motion is suppressed in the form of a first-order phase transition generic to all polar active materials.

References: Distortion and destruction of colloidal flocks in disordered environments,  A. Morin, N. Desreumaux, J.B. Caussin and D. Bartolo,  Nature Physics (2016), doi:10.1038/nphys3903