Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA.
J Chem Phys. 2012 Jul 21;137(3):034110. doi: 10.1063/1.4732515.
We calculate the pair diffusion coefficient D(r) as a function of the distance r between two hard sphere particles in a dense monodisperse fluid. The distance-dependent pair diffusion coefficient describes the hydrodynamic interactions between particles in a fluid that are central to theories of polymer and colloid dynamics. We determine D(r) from the propagators (Green's functions) of particle pairs obtained from molecular dynamics simulations. At distances exceeding ~3 molecular diameters, the calculated pair diffusion coefficients are in excellent agreement with predictions from exact macroscopic hydrodynamic theory for large Brownian particles suspended in a solvent bath, as well as the Oseen approximation. However, the asymptotic 1/r distance dependence of D(r) associated with hydrodynamic effects emerges only after the pair distance dynamics has been followed for relatively long times, indicating non-negligible memory effects in the pair diffusion at short times. Deviations of the calculated D(r) from the hydrodynamic models at short distances r reflect the underlying many-body fluid structure, and are found to be correlated to differences in the local available volume. The procedure used here to determine the pair diffusion coefficients can also be used for single-particle diffusion in confinement with spherical symmetry.
我们计算了两个硬球粒子在密集单分散流体中距离 r 处的对扩散系数 D(r)。距离相关的对扩散系数描述了流体中粒子之间的流体动力学相互作用，这对于聚合物和胶体动力学理论至关重要。我们从分子动力学模拟中获得的粒子对传播器（格林函数）确定 D(r)。在距离超过~3 个分子直径的情况下，计算出的对扩散系数与悬浮在溶剂浴中的大布朗粒子的宏观流体动力学理论以及 Oseen 近似的预测非常吻合。然而，与流体动力学效应相关的 D(r)的渐近 1/r 距离依赖性仅在对距离动力学经过相对较长的时间后才出现，这表明在短时间内对扩散存在不可忽略的记忆效应。在短距离 r 处，计算出的 D(r)与流体力学模型的偏差反映了基础多体流体结构，并发现与局部可用体积的差异相关。此处用于确定对扩散系数的方法也可用于具有球对称性的受限单粒子扩散。
