Molecular Dynamics Modeling of a Partially Saturated Clay-Water System

本研究的目的是通过大量的计算实验，在分子水平上推进对非饱和粘土-水体系界面物理性质(即接触角和毛细管应力)的基本认识。非饱和粘土-水系统在垃圾填埋场滑坡、干燥开裂、地下污染物迁移以及能量收集和储存等地质灾害中发挥着重要作用。非饱和粘土-水体系是由板状粘土颗粒、水和空气组成的三相多孔介质。在这三个阶段中，界面物理性质(接触角和水的弯月面曲率)对非饱和土水热力学行为有重要影响。虽然过去的研究已经在连续介质尺度上加深了对非饱和粘土热-水-力学行为的理解，但在原子尺度上的界面物理性质及其对热-水-力学行为的潜在影响还没有得到深入的研究。本项目旨在通过全尺度分子动力学模型研究非饱和粘土的界面物理性质及其与温度的关系。该项目将提供建立基于物理的多尺度计算框架所需的基本理解，用于对三相多孔材料中的多物理过程进行建模，这些材料在岩土工程和地球环境工程、石油工程、环境工程和科学、地球物理、地质科学、化学工程和制药工业中具有广泛的工程应用。该项目将通过分子动力学建模的新课程模块的设计，以及计算机集群上计算非饱和土力学的学生参与，在招聘科学和工程方面的人才方面迈出重要的一步。这项研究工作集中在通过分子动力学建模在零到99摄氏度的温度下对非饱和粘土(即高岭土)-水系统的界面物理性质的计算研究。研究工作包括：(1)在美国佛罗里达大学的超级计算机HiPerGator 2.0上进行全尺度分子动力学模拟，模拟不同温度下的非饱和粘土-水体系；(2)量化温度对接触角和水的半月面曲率的影响；(3)比较经典宏观理论和分子动力学模拟得到的粘土颗粒上的毛细应力。本研究旨在回答以下几个基本问题：(1)温度如何影响粘土颗粒上的毛细管应力？(2)如何准确确定高温下的接触角？(3)对于一般的三维情况，经典的宏观理论(即包含线张力的Young-Laplace方程)是否准确地量化了广泛温度范围内粘土颗粒上的毛细应力？这些科学发现还可以用来开发和验证中尺度(即从纳米到微米)的非饱和粘土的粗粒分子动力学模型。这项研究的长期目标是通过创新的计算实验(即分子动力学模拟)直接获得粘土颗粒上的毛细管应力等界面物理信息，建立自下而上的多尺度多物理计算技术，用于研究非饱和土壤中的局部不稳定性和扩散不稳定性以及多物理过程。

The goal of this research is to advance the fundamental understanding of interfacial physical properties (i.e., the contact angle and capillary stress) of unsaturated clay-water systems at the molecular scale via extensive computational experiments. Unsaturated clay-water systems play a substantial role in geohazards such as landfill slope failures, desiccation cracking, and contaminant transport in the subsurface, and energy harvesting and storage. Unsaturated clay-water systems are three-phase porous media comprised of plate-like clay particles, water, and air. Among those three phases, the interfacial physical properties (i.e., the contact angle and water meniscus curvature) have a significant impact on the hydro-thermal-mechanical behavior of unsaturated soils. While past research has advanced the understanding of the thermo-hydro-mechanical behavior of unsaturated clays at the continuum scale, the interfacial physical properties at the atomistic scale and their potential impact on the thermo-hydro-mechanical behavior have not yet been thoroughly investigated. This project aims to study the interfacial physical properties of unsaturated clays and their temperature dependence via a full-scale molecular dynamics modeling. The project will provide a fundamental understanding needed to build a physics-based multi-scale computational framework for modeling multi-physical processes in three-phase porous materials that have a broad spectrum of engineering applications in geotechnical and geoenvironmental engineering, petroleum engineering, environmental engineering and science, geophysics, geologic sciences, chemical engineering, and the pharmaceutical industry. The project will allow significant steps toward the recruitment of a talented workforce in science and engineering through the design of new course modules in molecular dynamics modeling, and the involvement of students in computational unsaturated soil mechanics on computer clusters.This research work is focused on the computational investigation of interfacial physical properties of unsaturated clay (i.e., Kaolinite)-water systems via molecular dynamics modeling at temperatures between zero and 99 degrees Celsius. The research tasks involve: (1) modeling an unsaturated clay-water system at different temperatures via the full-scale molecular dynamics modeling on the HiPerGator 2.0, a supercomputer at the University of Florida; (2) quantifying the impact of temperature on the contact angle and water meniscus curvature; and (3) comparing the capillary stress on clay particles obtained by classic macroscopic theory, and molecular dynamics modeling respectively. The research aims to answer the following fundamental questions: (1) how does temperature impact the capillary stress on clay particles? (2) how can the contact angle at the elevated temperature be accurately determined? and (3) does classic macroscopic theory (i.e., the Young-Laplace equation including the line tension) accurately quantify capillary stress on clay particles over a wide temperature range for a general three-dimensional case? The scientific findings can also be utilized to develop and validate the coarse-grained molecular dynamics model for unsaturated clays at a mesoscale (i.e., from nanometer to micrometer). The long-term goal of this line of research is to formulate the bottom-up multi-scale multi-physical computational technique for studying localized and diffusive instabilities and multi-physical processes in unsaturated soils through utilizing the interfacial physical information such as the capillary stress on clay particles obtained directly from the innovative computational experiment (i.e., molecular dynamics simulations).