Molecular kinetic modelling of nanoscale slip flow using a continuum approach
Molecular kinetic modelling of nanoscale slip flow using a continuum approach
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DOI：
10.1017/jfm.2022.186
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发表时间：
2022-03
影响因子：
3.7
通讯作者：
Baochao Shan;Peng Wang;Runxi Wang;Yonghao Zhang;Zhaoli Guo
中科院分区：
文献类型：
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作者：
Baochao Shan;Peng Wang;Runxi Wang;Yonghao Zhang;Zhaoli Guo
摘要描述纳米尺度受限流体流动的连续介质模型的一个主要挑战是缺乏能够捕捉分子尺度滑移行为的边界条件。在这项工作中，我们提出了一个分子动力学边界条件来模拟使用Lennard-Jones型势的流体-表面和流体-流体分子相互作用，并添加一个平均场力的动量方程。然后，使用由Guo等人开发的广义流体动力学模型（Phys. Fluids，第18卷，第6期，2006 a，067107），将该新的边界条件应用于研究纳米级Couette和Poiffille流。我们的模型的准确性进行了验证，分子动力学模拟和其他模型的广泛的参数，包括密度，剪切速率，润湿性和通道宽度。我们的模拟结果揭示了一些意想不到的和不直观的滑移行为在纳米尺度上，包括外延分层结构的流体和滑移长度的最小值。滑移长度的最小值，这是类似于努森最小值，可以解释为竞争的流体-固体和流体-流体的分子相互作用的密度变化。提出了一种新的滑移长度标度律，不仅考虑了流固、流液分子间相互作用的竞争效应，还考虑了流体内部势能和动能的竞争以及禁闭效应等物理机制.在低剪切速率下，滑移长度几乎不变，而在高剪切速率下，由于摩擦减小，滑移长度迅速增加。这些分子尺度的滑移行为是由流体-固体界面处的能量波纹引起的，在该界面处，强烈的流体-固体和流体-流体分子相互作用相互作用。
Abstract One major challenge for a continuum model to describe nanoscale confined fluid flows is the lack of a boundary condition that can capture molecular-scale slip behaviours. In this work, we propose a molecular-kinetic boundary condition to model the fluid–surface and fluid–fluid molecular interactions using the Lennard–Jones type potentials, and add a mean-field force to the momentum equation. This new boundary condition is then applied to investigate the nanoscale Couette and Poiseuille flows using the generalised hydrodynamic model developed by Guo et al. (Phys. Fluids, volume 18, issue 6, 2006a, 067107). The accuracy of our model is validated by molecular dynamics simulations and other models for a broad range of parameters including density, shear rate, wettability and channel width. Our simulation results reveal some unexpected and unintuitive slip behaviours at the nanoscale, including the epitaxial layering structure of fluids and the slip length minimum. The slip length minimum, which is analogous to the Knudsen minimum, can be explained by competing fluid–solid and fluid–fluid molecular interactions as density varies. A new scaling law is proposed for the slip length to account for not only the competing effect between the fluid–solid and fluid–fluid molecular interactions, but also many other physical mechanisms including the competition between the fluid internal potential energy and kinetic energy, and the confinement effect. While the slip length is nearly constant at the low shear rates, it increases rapidly at the high shear rates due to friction reduction. These molecular-scale slip behaviours are caused by energy corrugations at the fluid–solid interface where strong fluid–solid and fluid–fluid molecular interactions interplay.