Molecular force mechanism of hydrodynamics in clay nanopores

Abstract

Nanopores are prevalent within various clay morphologies, and water flow in clay nanopores is significant for various engineering applications. In this study, we performed non-equilibrium molecular dynamics (NEMD) simulations to reveal the molecular force mechanisms of water flow in clay nanopores. The water dynamic viscosity, slip length, and average flow velocity were obtained to verify the NEMD models. Since the water confined in the nanopores maintained a dynamic mechanical equilibrium state, each water lamina can be regarded as a simply supported beam. The applied driving force, the force from clay crystal layers, the force from compensating sodium ions, and the force from other water laminae were further calculated to investigate the force mechanisms. The van der Waals barrier above the surface and hydraulic gradient lead to distribution differences in water oxygen atoms, which contribute to a net van der Waals resistance component of the force from clay crystal layers. Meanwhile, the water molecules tend to rotate to generate the electrostatic resistance component of the force from clay crystal layers and balance the increasing hydraulic gradient. Due to the velocity difference, the water molecules in the slower lamina have a higher tendency to lag and generate a net electrostatic resistance force as well as a net van der Waals driving force on the water molecules in the faster lamina, which together make up the viscous force.

摘要

目的

连续介质方程在描述流体在黏土纳米孔内流动时具有较大偏差。本文旨在通过非**衡动力学模拟揭示纳米尺度 下黏土边界效应以及流体内部粘滞作用的力学机制,为更好地控制黏土纳米孔内流动提供新的分析思路。

创新点

1. 通过非**衡动力学模拟,计算了黏土纳米孔内流动过程中黏土与阳离子对孔间受限水的作用力大小,并揭示 其空间分布规律;2. 建立了水分子取向分布、空间形态与其所受作用力间的微细观联系,成功揭示黏土边界效 应和流体粘滞作用的力学机制。

方法

1. 通过非**衡动力学方法,模拟黏土纳米孔内的流动过程(图1和2),并通过流速、粘滞系数和渗透系数的 计算验证模型的**确性(表1);2. 通过对孔间受限水的受力分析,计算各项分子间作用力,得到作用力 大小和空间分布(图3和4);3. 计算水分子**面分布和取向角,建立与所受作用力间的微细观联系,揭 示黏土边界效应和流体粘滞作用的力学机制(图5~7)。

结论

1. 黏土和阳离子对孔内受限水的合力随时间波动满足高斯分布,且其时均值与施加的驱动力基本相等,以维持 流动过程中纯水的动态力学**衡。2. 黏土对孔内水分子的作用力类似于简支梁上的支座反力,集中作用在 Stern层的水分子上;其中,范德华作用项与水氧原子**面分布相关;由于范德华排斥势垒与定向流动, 所以水分子更倾向于在阻力区滞留,而随着水力梯度的增大,阻力区与动力区分布概率差增大,进而产生 更大的范德华阻力;库仑作用项则与水分子取向矢量的转动相关;水力梯度的增大诱导水分子取向转动, 进而导致承受库仑阻力的水分子数量增多。3. 由于相邻水层间存在速度差,所以慢层中的水分子有更高概 率停留在中心水分子的上游,进而对中心水分子产生净库仑阻力和净范德华动力,而随着速度梯度的增大, 分布概率差异进一步扩大,进而产生了更**的粘滞阻力。