FREE AND PENDULAR-LIKE ROTATION - ORIENTATIONAL DYNAMICS IN HARD ELLIPSOID FLUIDS

Rotational motions in molecular fluids may exhibit a coherence arising from collision-free propagation or from highly correlated collisions. Information on both of these behaviors are derived from the analysis of orientational and angular momentum relaxation. In the present study , the rotating body is taken to be a hard ellipsoid, immersed in a har d sphere fluid, and its dynamics are analyzed within a framework of ki netic theory. When the fluid bath is dilute, collisions are regarded a s uncorrelated events and orientational relaxation has elements of the Fokker-Planck and Gordon diffusion models. When the correlated collis ions are dominant, the rotational dynamics are frustrated. In this lim it the rotational diffusion coefficient vanishes, the hard sphere shea r viscosity diverges and their product is well behaved and yields a re lationship in close accord to the Stokes-Einstein-Debye relation. (C) 1995 American Institute of Physics.