Velocity correlations of two-dimensional hard needles from molecular dynamics.

We present velocity correlations of a two-dimensional system of perfectly smooth hard needles from molecular dynamics. In a nematic phase the autocorrelation velocity function (ACF) clearly separates into two domains with, first, a very quick decay and, then, a long-lasting exponential-like decay that pertains to several characteristic times of the fast decay. The latter one is strongly subjected to the order of the system. We demonstrate that the existence of two time scales corresponds to different relaxations of the transverse and longitudinal components of the ACF.