Spontaneous velocity alignment of Brownian particles with feedback-induced propulsion

Based on Brownian dynamics simulations we study the collective behavior of a two-dimensional system of repulsively interacting colloidal particles, where each particle is propelled by a repulsive feedback force with time delay $\tau$. Although the pair interactions are purely isotropic we observe a spontaneous, large-scale alignment of the velocity vectors. This phenomenon persists for long times and occurs, at high densities, in the absence of steady-state clustering. We explain our observations by a combination of the effect of steric interactions yielding local velocity ordering, and the effect of time delay, that generates velocity persistence and cluster dissolution. Overall, the behavior reveals intriguing similarities, but also subtle differences, to systems of active particles.