The transport properties of rod-like particles via molecular dynamics. I. Bulk fluid

Molecular dynamics studies of pure bulk fluids of hard, infinitely thin rods are reported. In the semidilute fluid above 70 rods/L³, the rotational diffusion coefficient scales with concentration, with an exponent approaching, but not reaching -2, the value predicted by the Doi-Edwards theory. We estimate the semidilute transition concentration to be at least 70 rods/L³, a concentration twice as large as previously reported from simulations of this rod fluid, but more consistent with recently published theoretical estimates. In the Doi-Edwards theory, a given rod executes a series of step rotations through a sequence of "cages" formed by neighboring rods. Using the simulation results, we estimate a value of τ_t, the average lifetime of a cage. As one would expect from the Doi-Edwards theory, we find that τ_t decreases sharply when we turn off the hard core repulsive interactions between the rods which comprise the cages. This result is in disagreement with a recently published Brownian dynamics study. In a previous molecular dynamics study of this model rod fluid, the translational self-diffusion coefficient was observed to increase with concentration above 30 rods/L³; an increase attributed in the theory of Frenkel and Maguire to the decreasing amount of rotational motion. Our results confirm the increase above 30 rods/L³. However, at higher concentrations the theory is inadequate; it underestimates the diffusivity and the lifetime of the rod velocity along its axis.