Nature Commun. 5, 4688 (2014)

Credit: NPG

Analogies between birds moving in the same direction and aligned phases in condensed matter (such as iron's ferromagnetic phase, where all spins are on average aligned) have been made for about two decades. Now, Nitin Kumar et al. show another example of collective self-organization in inanimate matter: the spontaneous alignment of macroscopic tapered metal rods lying on a mechanically vibrated bed of smaller metal beads. The researchers find that on increasing the surface coverage of either the rods or the underlying beads, the system undergoes a 'flocking' transition from a disordered to an aligned configuration (even at low concentration of rods). They also show with computer simulation and hydrodynamic theory that the rods transmit information about their orientation by dragging nearby beads, whose flow causes the reorientation of neighbouring rods. Such 'hydrodynamic coupling' has been shown to lead to oriented motion in self-propelled rolling colloidal particles, and has analogies to the coupling of local velocity fluctuations in neighbouring birds within a flock.