  An active colloidal fluid comprised of self-propelled spinning particles
injecting energy and angular momentum at the microscale demonstrates
spontaneous collective states that range from flocks to coherent vortices.
Despite their seeming simplicity, the emergent far-from-equilibrium behavior of
these fluids remains poorly understood, presenting a challenge to the design
and control of next-generation active materials. When confined in a ring, such
so-called polar active fluids acquire chirality once the spontaneous flow
chooses a direction. In a perfect ring, this chirality is indefinitely
long-lived. Here, we combine experiments on self-propelled colloidal Quincke
rollers and mesoscopic simulations of continuum Toner-Tu equations to explore
how such chiral states can be controlled and manipulated by obstacles. For
different obstacle geometries three dynamic steady states have been realized:
long-lived chiral flow, an apolar state in which the flow breaks up into
counter-rotating vortices and an unconventional collective state with flow
having an oscillating chirality. The chirality reversal proceeds through the
formation of intermittent vortex chains in the vicinity of an obstacle. We
demonstrate that the frequency of collective states with oscillating chirality
can be tuned by obstacle parameters. We vary obstacle shapes to design chiral
states that are independent of initial conditions. Building on our findings, we
realize a system with two triangular obstacles that force the active fluid
towards a state with a density imbalance of active particles across the ring.
Our results demonstrate how spontaneous polar active flows in combination with
size and geometry of scatterers can be used to control dynamic patterns of
polar active liquids for materials design.
