Flocking through a sea of rods

TL;DR

This study uses simulations to explore how motile rods form flocks in a vibrating monolayer, revealing segregation, flock structure dependence on rod shape, and a disorder mechanism.

Contribution

It uncovers a novel antidiffusive instability leading to flock formation and analyzes how rod anisotropy influences flock structure and order.

Findings

Motile rods segregate from apolar medium forming larger flocks at higher concentrations.

Flock shape depends on rod aspect ratio, elongating perpendicular or parallel to motion.

Segregation reduces global polar order, indicating disorder mechanisms.

Abstract

We investigate the collective behavior of motile rods immersed in a monolayer of apolar rods confined between vertically vibrating plates using numerical simulations. We uncover an antidiffusive instability whereby motile rods segregate from the apolar medium and form flocks whose size increases with the medium concentration. Remarkably, enhanced segregation leads to a reduction of the global polar order. The flock structure is strongly influenced by the anisotropy of the medium rods. For small aspect ratios, the flocks are elongated perpendicular to the mean direction of motion, whereas for larger aspect ratios, they elongate along the direction of motility. We rationalize the emergence of segregation-induced disorder using a minimal mean-field model.