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

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Abstract
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 τt, the average lifetime of a cage. As one would expect from the Doi–Edwards theory, we find that τt 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.