Flocking at a distance in active granular matter
Nature Communications 5:4688 doi: 10.1038/ncomms5688 (Sept. 3, 2014)
Flocking, the self-organised motion of vast numbers of living creatures in a single direction, relies on organisms sensing each other's presence, orientation and direction of movement. Physical scientists interested in this spectacular example of spontaneous symmetry breaking in living systems have adopted a variety of approaches to the problem, including experiments in vivo and in vitro, computer simulations and theory, and artificial analogues made of energised non-living components. In most imitations of motility alignment came about through direct mechanical contact, possible only at high concentrations.
In our paper published in Nature Communications, we show that millimetre-sized asymmetrically tapered cylinders -- polar rods -- rendered motile by a vertically vibrated horizontal surface, communicate their orientation and directed motion over many rod diameters through a medium of non-motile spherical beads. This results, as the area fraction of beads is increased, in a phase transition of the polar rods from the isotropic state to an oriented and coherently moving flock, at fixed low rod concentrations. Our findings, reinforced by large-scale numerical simulations, include a phase diagram in the plane of rod and bead concentrations, power-law spatial correlations upon approaching the phase boundary, and insights into the underlying mechanisms. We account for our observations through an analytical theory with two simple ingredients: a moving rod drags beads; neighbouring rods reorient in the resulting flow like a weathercock in the wind.