Motile dissenters disrupt the flocking of active granular matter

We report flocking in the dry active granular matter of millimeter-sized two-step-tapered rods without an intervening medium. The system undergoes the flocking phase transition at a threshold area fraction ~ 0.12 having high orientational correlations between the particles. However, the one-step-tapered rods do not flock and are used as the motile dissenters in the flock-forming granular matter. At the critical fraction of dissenters ~ 0.3, the flocking order of the system gets completely destroyed. The variance of the system's order parameter shows a maximum near the dissenter fraction f ~ 0.05, suggesting a finite-size crossover between the ordered and disordered phases.

Pradip K. Bera, A. K. Sood · 2020