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arXiv 1902.01043cond-mat.soft
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纳米尺度布朗运动中的分子动力学与流体动力学之间的桥梁

Bridging the gap between molecular dynamics and hydrodynamics in nanoscale Brownian motions

Keisuke Mizuta, Yoshiki Ishii, Kang Kim, Nobuyuki Matubayasi

摘要:

通过分子动力学模拟,我们研究了富勒烯颗粒基于分子间相互作用的布朗运动的流体力学行为。溶剂化自由能和速度自相关函数(VACF)的计算通过使用Lennard-Jones(LJ)和Weeks-Chandler-Andersen(WCA)势的溶质-溶剂和溶剂-溶剂的相互作用,并通过改变富勒烯颗粒的尺寸。我们还测量了富勒烯颗粒的扩散常数和宿主流体的剪切粘度,然后从Stokes-Einstein关系量化的流体动力学半径$a_\mathrm{HD}$。当使用LJ电势时溶剂化自由能表现出很大的负值时,$a_\mathrm{HD}$值超过富勒烯的回转半径。相比之下,当使用WCA势时溶剂化自由能为正时,$a_\mathrm{HD}$变得与裸富勒烯的尺寸相当。此外,富勒烯颗粒的VACF与利用不可压缩和可压缩形式的纳维-斯托克斯方程的解析表达式直接可比。流体动力学的长时间尾巴$t^{-3/2}$的时间尺度比运动学时间的粒子的大小扩散证明。然而,VACF在较短的时间尺度偏离流体动力学描述,特别是对于较小的富勒烯颗粒和LJ电位。即使在表征VACF在颗粒尺寸上的声音传播时间尺度周围的衰减时考虑了可压缩效应,也会发生这种情况。这些结果表明,纳米级的布朗运动的溶质粒子周围的溶剂化结构的分子相互作用的影响。

英文摘要:

Through molecular dynamics simulations, we examined hydrodynamic behavior of the Brownian motion of fullerene particles based on molecular interactions. The solvation free energy and the velocity autocorrelation function (VACF) were calculated by using the Lennard-Jones (LJ) and Weeks-Chandler-Andersen (WCA) potentials for the solute-solvent and solvent-solvent interactions and by changing the size of the fullerene particles. We also measured the diffusion constant of the fullerene particles and the shear viscosity of the host fluid, and then the hydrodynamic radius $a_\mathrm{HD}$ was quantified from the Stokes-Einstein relation. The $a_\mathrm{HD}$ value exceeds that of the gyration radius of the fullerene when the solvation free energy exhibits largely negative values using the LJ potential. In contrast, $a_\mathrm{HD}$ becomes comparable to the size of bare fullerene, when the solvation free energy is positive using the WCA potential. Furthermore, the VACF of the fullerene particles is directly comparable with the analytical expressions utilizing the Navier-Stokes equations both in incompressible and compressible forms. Hydrodynamic long-time tail $t^{-3/2}$ is demonstrated for timescales longer than the kinematic time of the momentum diffusion over the particles' size. However, the VACF in shorter timescales deviates from the hydrodynamic description, particularly for smaller fullerene particles and for the LJ potential. This occurs even though the compressible effect is considered when characterizing the decay of VACF around the sound propagation time scale over the particles' size. These results indicate that the nanoscale Brownian motion is influenced by the solvation structure around the solute particles originating from the molecular interaction.

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