  We study the motility-induced aggregation of active Brownian particles (ABPs)
on a porous, circular wall. We observe that the morphology of aggregated
dense-phase on a static wall depends on the wall porosity, particle motility,
and the radius of the circular wall. Our analysis reveals two morphologically
distinct, dense aggregates; a connected dense cluster that spreads uniformly on
the circular wall and a localized cluster that breaks the rotational symmetry
of the system. These distinct morphological states are similar to the
macroscopic structures observed in aggregates on planar, porous walls. We
systematically analyze the parameter regimes where the different morphological
states are observed. We further extend our analysis to motile circular rings.
We show that the motile ring propels almost ballistically due to the force
applied by the active particles when they form a localized cluster, whereas it
moves diffusively when the active particles form a continuous cluster. This
property demonstrates the possibility of extracting useful work from a system
of ABPs, even without artificially breaking the rotational symmetry.
