Collective Order Decouples Boundary Selection from Macroscopic Chirality in Confined Active Matter

Abstract: Can a boundary determine the direction of circulation in a collectively moving active fluid? We study chiral self-propelled particles with nematic bulk alignment, confined by walls whose orientational coupling varies continuously from nematic to polar. For an isolated particle at a wall, we derive the exact parameter condition under which a stable orientation is tangent to the wall. With predominantly nematic wall coupling, however, the many-body circulation changes sign at a chirality that is nearly independent of the wall symmetry. This sign change follows a pronounced decrease in nematic order and disappears when bulk alignment is removed. As the polar wall coupling becomes dominant, opposite-handed circulation is suppressed and same-handed polar locking emerges. Thus local orientation at a wall and the direction of macroscopic circulation are selected by distinct physical processes: the wall determines the possible local orientations, whereas collective order determines whether those orientations control the global flow.