  We perform experiments on an active chiral fluid system of self-spinning
rotors in confining boundary. Along the boundary, actively rotating rotors
collectively drives a unidirectional material flow. We systematically vary
rotor density and boundary shape; boundary flow robustly emerges under all
conditions. Flow strength initially increases then decreases with rotor density
(quantified by area fraction $\phi$); peak strength appears around a density
$\phi=0.65$. Boundary curvature plays an important role: flow near a concave
boundary is stronger than that near a flat or convex boundary in the same
confinements. Our experimental results in all cases can be reproduced by a
continuum theory with single free fitting parameter, which describes the
frictional property of the boundary. Our results support the idea that boundary
flow in active chiral fluid is topologically protected; such robust flow can be
used to develop materials with novel functions.
