Swirling in a Vibrated Monolayer of Rods

We report observations of the spatiotemporal behaviour of a vertically vibrated horizontal monolayer of copper rods (aspect ratio $\approx $ 5) etched to a rolling-pin-like shape. The spatial organization of the rods resembles a highly-defected nematic state with large, coherently moving swirls. We measure spatiotemporal correlations of the single-particle and collective velocities, and study the structure and dynamics of the system as a function of density and vibration amplitude. We analyze the observed patterns in the light of theories\footnote{ J. Toner, Y. Tu and S. Ramaswamy, Ann. Phys. 318 (2005) 170.} of orientational ordering, dynamics, and topological defects in systems of driven particles. We make comparisons to related but different experiments\footnote{ D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).}, as well as to our earlier measurements\footnote{ V. Narayan, N. Menon and S. Ramaswamy, J. Stat. Mech. (2005) in press; cond-mat/0510082.} on similar particles with higher aspect ratio.