Transport properties of nano-confined fluids such as diffusivity can exhibit utterly distinctive characteristics compared to the transport properties in the bulk due to the interactions between atoms in the solid walls and fluid atoms as well as the confinements. In this paper, the diffusivity of water confined in the graphene nanochannels is calculated by molecular dynamics simulations through the Einstein equation, and the results show that the diffusivity of nano-confined water is obviously anisotropic, i.e., the perpendicular (vertical to the graphene walls) diffusivity is obviously lower than the diffusivity in the parallel plane. By studying the Lagrangian dynamics of molecules in the confined region, we realize that the anisotropy can be attributed to the trapping of water molecules in the potential wells near the graphene walls, resulting in the inhibition of the molecular mobility in the perpendicular direction. Meanwhile, the proportion of confined water molecules decreases with increasing channel height and the contributions of the trapped water molecules on the inhibited mobility in the perpendicular direction are weakened. As a result, the diffusivity in all directions approaches the bulk values at high channel heights. The obtained results are helpful in revealing the mechanisms of water diffusion in nanospaces from the molecular level.

中文翻译：

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石墨烯纳米通道中水扩散率的分子动力学研究

由于固体壁中的原子与流体原子之间的相互作用以及限制，纳米限制流体的传输特性，例如扩散率，与本体中的传输特性相比，可以表现出完全不同的特性。本文通过分子动力学模拟通过爱因斯坦方程计算了石墨烯纳米通道内水的扩散率，结果表明纳米级水的扩散率具有明显的各向异性，即垂直（垂直于石墨烯壁） ) 扩散率明显低于平行平面内的扩散率。通过研究受限区域中分子的拉格朗日动力学，我们意识到各向异性可归因于石墨烯壁附近势阱中水分子的捕获，导致分子在垂直方向上的运动受到抑制。同时，随着通道高度的增加，受限水分子的比例减小，并且被困水分子对垂直方向上的抑制迁移率的贡献减弱。结果，所有方向的扩散率在高通道高度处接近体值。所得结果有助于从分子水平揭示水在纳米空间中的扩散机制。在所有方向的扩散率接近高通道高度的体值。所得结果有助于从分子水平揭示水在纳米空间中的扩散机制。在所有方向的扩散率接近高通道高度的体值。所得结果有助于从分子水平揭示水在纳米空间中的扩散机制。