Fast and slow self-propelled particles interacting with asymmetric obstacles: Wetting, segregation, rectification, and vorticity

We study a mixture of "fast" and "slow" self-propelled particles in the presence of a regular array of large asymmetric obstacles. For this purpose, simulations of active Brownian particles interacting with a half-disk obstacle are performed in 2D with periodic boundary conditions. The system has two particle types, each of them characterized by its own self-propulsion speed. To isolate the effects of such "speed diversity", the system-average self-propulsion speed is kept unvaried as the degree of speed diversity is tuned. Because of their persistent motion, particles accumulate around the obstacle in a wetting phenomenon. Stationary segregation arises since faster particles are more likely to occupy new available spaces. For degrees of speed diversity $\geq40$\%, we observe a transition where the self-propulsion of the slower particles becomes too weak and thus these particles start to accumulate more easily over a "layer" of faster particles rather than near the wall. Also, particles traveling from the curved to the flat side of the obstacle spend less time trapped than in the opposite direction. As a result, directed motion emerges spontaneously. We find that the corresponding rectification current is amplified when the degree of speed diversity is increased. In the passive-active limit, the passive particles still undergo directed motion dragged by the active ones. Due to rectification, segregation profiles are different between the curved and flat sides. Near the obstacle corners, pairs of vortices that further contribute to rectification are observed. Their vorticities also increase with speed diversity. Our results provide useful insights into the behavior of active matter in complex environments.