Incollection,
Long-lived Giant Number Fluctuations in a Granular Nematic
Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)
Abstract
Coherently moving flocks of birds, beasts or bacteria are phases of active
matter with spontaneous orientational order -- living liquid crystals. Their
nonequilibrium nature is manifested most strikingly in the giant density
fluctuations predicted by theories of flocking. We present the experimental
observation of these fluctuations, in the nematic phase of an agitated
monolayer of granular rods. We find number fluctuations consistent with
standard deviation proportional to the mean, in stark contrast to any thermal
equilibrium system away from a critical point or, indeed, to any situation
where the Central Limit Theorem applies. These large-scale variations are
imprinted even on the small-scale dynamics, with local density fluctuations
decaying only logarithmically in time. We compare these results in apolar
active particles with those in bacteria, which are polar -- each bacterium has
a distinct head and tail. Among the lessons of our findings is that the
mechanisms behind anomalous number fluctuations in active orientable
media are quite generic, be the system composed of inanimate but agitated rods
or living bacteria.
