Molecular Dynamics Modeling of a Partially Saturated Clay-Water System

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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).