[论文解读] 2D microscopic and macroscopic simulation of water and porous material interaction

E. G. Nikonov, Miron Pavluš+1 more|arXiv (Cornell University)|Sep 18, 2017

Gas Dynamics and Kinetic Theory参考文献 5被引用 3

一句话总结

本文提出了一种混合模拟方法，结合二维分子动力学（微观）与宏观扩散模型，研究水蒸气与狭缝状孔隙的相互作用。结果表明，两种模型在扩散系数和密度方面结果一致，验证了该混合方法在不同热力学条件下更精确模拟水-孔隙动力学的可行性。

ABSTRACT

In various areas of science, technology, environment protection, construction, it is very important to study processes of porous materials interaction with different substances in different aggregation states. From the point of view of ecology and environmental protection it is particularly actual to investigate processes of porous materials interaction with water in liquid and gaseous phases. Since one mole of water contains $6,022140857\cdot 10^{23}$ molecules of $\mathtt{H_2O}$, macroscopic approaches considering the water vapor as continuum media in the framework of classical aerodynamics are mainly used to describe properties, for example properties of water vapor in the pore. In this paper we construct and use for simulation the macroscopic two-dimensional diffusion model describing the behavior of water vapor inside the isolated pore. Together with the macroscopic model it is proposed microscopic model of the behavior of water vapor inside the isolated pores. This microscopic model is built within the molecular dynamics approach. In the microscopic model a description of each water molecule motion is based on Newton classical mechanics considering interactions with other molecules and pore walls. Time evolution of water vapor - pore system is explored. Depending on the external to the pore conditions the system evolves to various states of equilibrium, characterized by different values of the macroscopic characteristics such as temperature, density, pressure. Comparisons of results of molecular dynamic simulations with the results of calculations based on the macroscopic diffusion model and experimental data allow to conclude that the combination of macroscopic and microscopic approach could produce more adequate and more accurate description of processes of water vapor interaction with porous materials.

研究动机与目标

• 研究水蒸气在不同热力学条件下与孤立二维狭缝状孔隙的相互作用。
• 比较微观分子动力学模拟与宏观扩散建模在预测精度与一致性方面的表现。
• 评估混合微-宏方法在模拟多孔介质中水-孔相互作用的可行性。
• 将模拟结果与实验数据及理论扩散模型进行对比验证。
• 确定微观与宏观模型在预测关键输运性质方面实现收敛的条件。

提出的方法

• 采用经典分子动力学（MD）方法，使用Lennard-Jones势能（σ = 3.17 Å，ε = 6.74×10⁻³ eV）模拟二维孔隙中单个H₂O分子的运动。
• 基于Bitsadze-Kalinichenko框架构建宏观二维扩散模型，描述孔隙中水蒸气的传输行为。
• 通过MD模拟计算干燥与润湿过程中时间相关的扩散系数。
• 利用公式 D = (kT / (6πηr)) × (1 / (1 + 2r / (3λ)))，结合温度与压力输入，计算宏观模型中的恒定扩散系数。
• 模拟不同外部空间缩放因子（k = 1, 2, 3）下的系统，评估边界条件的影响。
• 追踪并比较MD模型与宏观模型中水蒸气浓度（通过空间平均）及分子密度的时间演化。

实验结果

研究问题

• RQ1 在预测二维狭缝状孔隙中水蒸气扩散方面，微观分子动力学与宏观扩散模型的表现如何比较？
• RQ2 MD模拟得到的扩散系数与宏观扩散模型推导出的系数在多大程度上一致？
• RQ3 外部压力与温度的变化如何影响孔隙系统中的平衡状态与输运性质？
• RQ4 外部空间缩放（k = 1, 2, 3）对扩散动力学与系统平衡化过程有何影响？
• RQ5 在何种条件下，微观与宏观模型在预测水蒸气密度与浓度方面实现收敛？

主要发现

• 在25 °C和k=2条件下，宏观扩散模型得到的恒定扩散系数为1121.58 m²/s，与MD模拟的时间平均结果高度一致。
• 在T₀ = 25 °C和k=2条件下，MD模拟得到的最终系统密度约为1.2×10²⁵ molecules/m³，与宏观预测结果一致。
• MD模拟中空间平均水蒸气浓度（w_sm）随时间推移与宏观模型解趋于一致，尤其在t₀ = 66532 psec时表现明显。
• 在T₀ = 25 °C和k=2条件下，宏观模型预测的恒定扩散系数为326.88 m²/s，与MD模拟在平衡阶段估算的值相符。
• 当MD模拟中分子数量足够时，混合方法表现出更高的精度，支持Norman & Stegailov（2012）的研究发现。
• 扩散系数随温度显著变化：D从k=1、T=15 °C时的1542.39 m²/s降至k=3、T=35 °C时的168.95 m²/s，反映出强烈的热依赖性。