SELF-DIFFUSION IN THE NEMATIC-A-PHASE AND SMECTIC-A-PHASE OF AN ALIGNED FLUID OF HARD SPHEROCYLINDERS

The short to moderate time behaviour of the velocity-time correlation function (TCF) has been examined by molecular dynamics and by kinetic theory for a nematic and a smectic A liquid crystalline phase of perfe ctly aligned spherocylinders in three dimensions. The kinetic theory i s a generalized Langevin formulation in which the uncorrelated collisi ons are accommodated by means of an Enskog friction and the correlated caging recollisions via the memory function. In the nematic phase, th e TCFs both parallel and perpendicular to the director are well descri bed by the Enskog theory alone. In the smectic A phase, a second order parameter arises and the dynamics along the director are strongly cag ed (i.e., indicating the importance of correlated backscattering), whe reas the perpendicular motions still adhere to an uncorrelated collisi on scenario. The interesting questions of the existence and the nature of long-time tails in the TCFs are left for a future study.