Non-reciprocal reorientation mechanism exhibits flocking*

A non-reciprocal alignment interaction [1] that reorient the self-propulsion direction of circular disks along the inter-particle separation and away from the neighboring disks is found to exhibit flocking in a system of extremely small area fraction of 15 percent. The non-reciprocal alignment interaction is considered to be extremely short-ranged, and it effects the neighboring particles that comes within a small fraction of their diameter. Interestingly, the flocking phase is observed in a specific range of the strength of reorientational interaction [1], and the system reenters the homogeneous phase without any global dynamical ordering in the limit of large strength of alignment. Flocking bands are found to move with a speed [1], depending on the self-propulsion speed of the particles, without forming any shock front. The system is further investigated to explore the spatio-temporal correlation of various dynamical parameters. On the other hand, reversing the reorientational interaction [2] of the particles results in completely new mechanism of motility-induced phase separation.