  We investigate the response of colloidal flocks to external fields. We first
show that individual colloidal rollers align with external flows as would a
classical spin with magnetic fields. Assembling polar active liquids from
colloidal rollers, we experimentally demonstrate their hysteretic response:
confined colloidal flocks can proceed against external flows. We theoretically
explain this collective robustness, using an active hydrodynamic description,
and show how orientational elasticity and confinement protect the direction of
collective motion. Finally, we exploit the intrinsic bistability of confined
active flows to devise self-sustained microfluidic oscillators.
