Controlling Vortex Rotation with Obstacles in Nonaligning Dry Active Matter.

In many systems with chiral symmetry, including dry active matter around circular obstacles, vortices exhibit no intrinsic preference for clockwise or counterclockwise rotation. Here, we investigate the rotation of an active vortex around a circular obstacle in a nonaligning dry active-matter system with M half-circles distributed around the central disk. We define a dimensionless control parameter as the ratio between the mean angular velocity of the controlled vortex (M>0) and the root-mean-square angular velocity of the isolated vortex (M=0), when there is no half-circular obstacle distributed around the circular obstacle. Two rotational regimes emerge from the obstacle geometry: clockwise rotation when the flat sides of the half-circles face the vortex and counterclockwise rotation when the curved sides face the vortex. We further show that this geometric control induces a nonmonotonic dependence of vortex stability (maintenance and rotation direction) on the distance between the half-circles from the circular obstacle, demonstrating how geometric constraints can be exploited to control spontaneous collective motion in nonaligning dry active-matter systems.