Confinement-induced collective motion in suspensions of run-and-tumble particles

Collective motion is ubiquitous in active systems at all length and time scales. The mechanisms behind such collective motion are usually alignment interactions between active particles, effective alignment after collisions between agents, or symmetry-breaking fluctuations induced by passive species in active suspensions. In this article, we introduce a new type of collective motion in the shape of a traveling band induced purely by confinement, where no explicit or effective alignment is prescribed among the active agents. We study a suspension of run-and-tumble particles confined in microchannels comprising asymmetric boundaries: one flat wall and one array of funnel-like obstacles. We study the phase behavior of the confined active suspension upon changes in the packing fraction and persistence length to define the stability region of the traveling band. We characterize the traveling band structurally and dynamically and study its stability with respect to the geometry of the microchannel. Finally, we describe the mechanism of motion of the band, which resembles the tracked locomotion of some heavy vehicles such as tractors, finding that a counter-flux of active particles in the lower part of the band, explained in terms of source-sink and vacancy diffusion mechanisms, is the facilitator of the traveling band and sustains its motion. We name this new collective phenomenon confinement-induced tracked locomotion.