Zhou Runfeng, Sun Chengzhen, Bai Bofeng
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
J Chem Phys. 2021 Feb 21;154(7):074709. doi: 10.1063/5.0039228.
The accurate determination of fluid viscosity based on the microscopic information of molecules is very crucial for the prediction of nanoscale flow. Despite the challenge of this problem, researchers have done a lot of meaningful work and developed several distinctive methods. However, one of the common approaches to calculate the fluid viscosity is using the Green-Kubo formula by considering all the fluid molecules in nanospace, inevitably causing the involvement of the frictional interaction between fluid and the wall into the fluid viscosity. This practice is certainly not appropriate because viscosity is essentially related only to the interactions among fluid molecules. Here, we clarify that the wall friction should be decoupled from fluid viscosity by distinguishing the frictional region and the viscous region for the accurate prediction of nanoscale flow. By comparing the fluid viscosities calculated from the Green-Kubo formula in the whole region and viscous region and the viscosity obtained from the velocity profile through the Hagen-Poiseuille equation, it is found that only the calculated viscosity in the viscous region agrees well with the viscosity from the velocity profile. To demonstrate the applicability of this clarification, the Lennard-Jones fluid and water confined between Lennard-Jones, graphene, and silica walls, even with different fluid-wall interactions, are extensively tested. This work clearly defines the viscosity of fluids at nanoscales from the inherent nature of physics, aiming at the accurate prediction of nanoscale flow from the classical continuum hydrodynamic theory.
基于分子微观信息准确测定流体粘度对于预测纳米尺度流动至关重要。尽管这个问题具有挑战性，但研究人员已经做了很多有意义的工作并开发了几种独特的方法。然而，计算流体粘度的常见方法之一是通过考虑纳米空间中的所有流体分子来使用格林 - 库博公式，这不可避免地导致流体与壁之间的摩擦相互作用被纳入流体粘度。这种做法肯定不合适，因为粘度本质上仅与流体分子之间的相互作用有关。在这里，我们阐明，为了准确预测纳米尺度流动，应通过区分摩擦区域和粘性区域将壁摩擦与流体粘度解耦。通过比较在整个区域和粘性区域中由格林 - 库博公式计算出的流体粘度以及通过哈根 - 泊肃叶方程从速度分布获得的粘度，发现只有在粘性区域计算出的粘度与从速度分布得到的粘度吻合良好。为了证明这种阐明的适用性，对限制在伦纳德 - 琼斯壁、石墨烯壁和二氧化硅壁之间的伦纳德 - 琼斯流体和水进行了广泛测试，即使流体与壁之间的相互作用不同。这项工作从物理的固有性质清晰地定义了纳米尺度下流体的粘度，旨在从经典连续介质流体动力学理论准确预测纳米尺度流动。
