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We calculate the pair diffusion coefficient D(r) as a function of thedistance r between two hard-sphere particles in a dense monodispersesuspension. The distance-dependent pair diffusion coefficient describes thehydrodynamic interactions between particles in a fluid that are central totheories of polymer and colloid dynamics. We determine D(r) from thepropagators (Green's functions) of particle pairs obtained from discontinuousmolecular dynamics simulations. At distances exceeding 3 molecular diameters,the calculated pair diffusion coefficients are in excellent agreement withpredictions from exact macroscopic hydrodynamic theory for large Brownianparticles suspended in a solvent bath, as well as the Oseen approximation.However, the asymptotic 1/r distance dependence of D(r) associated withhydrodynamic effects emerges only after the pair distance dynamics has beenfollowed for relatively long times, indicating non-negligible memory effects inthe pair diffusion at short times. Deviations of the calculated D(r) from thehydrodynamic models at short distances r reflect the underlying many-body fluidstructure, and are found to be correlated to differences in the local availablevolume. The procedure used here to determine the pair diffusion coefficientscan also be used for single-particle diffusion in confinement with sphericalsymmetry.

Pair diffusion, hydrodynamic interactions, and available volume in dense fluids