Flocking at a distance in
active granular matter
-Nitin Kumar, Harsh Soni, Sriram Ramaswamy and A. K. Sood
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.
Link: http://www.nature.com/ncomms/2014/140903/ncomms5688/full/ncomms5688.html