Codina, JoanMahault, BenoitChate, HuguesDobnikar, JurePagonabarraga, IgnacioShi, Xia-qing2022-07-042022-07-042022-07-042022-05-2310.1103/PhysRevLett.128.218001https://infoscience.epfl.ch/handle/20.500.14299/189041WOS:000808312500002We show that arbitrarily large polar flocks are susceptible to the presence of a single small obstacle. In a wide region of parameter space, the obstacle triggers counterpropagating dense bands leading to reversals of the flow. In very large systems, these bands interact, yielding a never-ending chaotic dynamics that constitutes a new disordered phase of the system. While most of these results were obtained using simulations of aligning self-propelled particles, we find similar phenomena at the continuous level, not when considering the basic Toner-Tu hydrodynamic theory, but in simulations of truncations of the relevant Boltzmann equation.Physics, MultidisciplinaryPhysicstext::journal::journal article::research article