SELF-DIFFUSION OF RODLIKE MOLECULES IN STRONG SHEAR FIELDS

We apply recently derived Green-Kubo relations for the various element s of the self-diffusion tensor (SDT) of shearing fluids to three diffe rent model systems of rodlike molecules: the Tildesley-Madden model fo r carbon disulfide, the Gay-Berne fluid, and the Ryckaert-Bellemans mo del for decane. We study how the various elements of the SDT depend on the shear rate. This dependence is explained in greater detail by exa mining the various velocity autocorrelation functions, the order param eter, and the shear induced rotation rate. We find that two different mechanisms are responsible for the behavior of the diagonal elements o f the SDT. The first is the elliptical distortion of the nearest-neigh bor shell and enhances the diagonal elements of the SDT. The second is the shear alignment of the molecules. This facilitates diffusion in t he alignment direction and suppresses diffusion in the perpendicular d irection. We also suggest ''collision'' sequences that are responsible for the sign of the off-diagonal elements of the SDT.