ESPCI web site
Flocking through disorder
How do flocks, herds and swarms proceed through disordered environments ? This question is not only crucial to the displacement of animal groups in the wild, but also to virtually all applications of collective robotics, and active materials composed of motile units. In stark contrast, appart from very rare exceptions, our physical understanding of flocking motion has been hitherto restrained to homogeneous media. I will attempt to elucidate how collective motion survives to geometrical disorder. I will report both measurements on colloidal bots flocking through random repelling obstacles and analytical theory. Firstly, we explain how the effective bending elasticity of these broken-symmetry fluids, restrains their spontaneous flows to sparse channel networks akin those found beyond plastic depinning in driven condensed matter. Secondly, we demonstrate how further increasing disorder, collective motion is suppressed in the form of a first-order phase transition generic to all polar active materials.