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

Abstract

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 ≥ 40%, 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.

Rectification in a mixture of active and passive particles subject to a ratchet potential

“…Previous work on 2D systems also suggest that currents may still flow in either direction. For example, current reversal is observed [21] for active particles alone in a 2D potential, and passive currents were also seen in an AP mixture, in presence of disk-shaped obstacles [32]. Of course, 2D systems also support more complex behaviour including dynamical clustering and phase separation [11,45], which would also affect the ratchet currents.…”

“…Use of external, asymmetric potentials is also promising to sort mixtures; indeed, in recent experiments bacteria were shown to assist the transport of passive colloidal beads towards asymmetric micrometric obstacles [31]. Less sharp but asymmetric obstacles, such as half-disks, can also rectify passive particles in a mixture [32]. A better understanding of the parameters influencing the passive particle flux induced by active motion is then necessary to control more accurately the currents in the system.…”