Publications

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In this work we investigate the Orowan hypothesis, that decreases in surface energy due to surface adsorbates lead directly to lowered fracture toughness, at an atomic/molecular level. We employ a Lennard-Jones system with a slit crack and an infiltrating fluid, nominally with gold-water properties, and explore steric effects by varying the soft radius of fluid particles and the influence of surface energy/hydrophobicity via the solid–fluid binding energy. Using previously developed methods, we employ the J-integral to quantify the sensitivity of fracture toughness to the influence of the fluid on the crack tip, and exploit dimensionless scaling to discover universal trends in behavior.

Observation of vibrational properties of phyllosilicate edges via a combined molecular modeling and experimental approach was performed. Deuterium exchange was utilized to isolate edge vibrational modes from their internal counterparts. The appearance of a specific peak within the broader D2O band indicates the presence of deuteration on the edge surface, and this peak is confirmed with the simulated spectra. These results are the first to unambiguously identify spectroscopic features of phyllosilicate edge sites.

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Fracture toughness of silicates is reduced in aqueous environments due to water-silica interactions at the crack tip. To investigate this effect, classical molecular dynamics simulations using the bond-order-based reactive force field (ReaxFF) were used to simulate silica fracture. The chemical and mechanical aspects were separated by simulating fracture in (a) a vacuum with dynamic loading, (b) an aqueous environment with dynamic loading, and (c) an aqueous environment with static subcritical mechanical loading to track silica dissolution. The addition of water to silica fracture reduced the silica fracture toughness by ~25%, a trend consistent with experimentally reported results. Analysis of Si─O bonds in the process zone and calculations of dissipation energy associated with fracture indicated that water relaxes the entire process zone and not just the surface. Additionally, the crack tip sharpens during fracture in water and an increased number of microscopic propagation events occur. This results in earlier fracture in systems with increasing mechanical loading in aqueous conditions, despite the lack of significant silica dissolution. Therefore, the threshold for Si─O bond breakage has been lowered in the presence of water and the reduction in fracture toughness is due to structural and energetic changes in the silica, rather than specific dissolution events.

We report a fluid flow in a nanochannel highly depends on the wettability of the channel surface to the fluid. The permeability of the nanochannel is usually very low, largely due to the adhesion of fluid at the solid interfaces. Using molecular dynamics (MD) simulations, we demonstrate that the flow of water in a nanochannel with rough hydrophilic surfaces can be significantly enhanced by the presence of a thin layer of supercritical carbon dioxide (scCO₂) at the water–solid interfaces. The thin scCO₂ layer acts like an atomistic lubricant that transforms a hydrophilic interface into a super-hydrophobic one and triggers a transition from a stick- to- a slip boundary condition for a nanoscale flow. Here, this work provides an atomistic insight into multicomponent interactions in nanochannels and illustrates that such interactions can be manipulated, if needed, to increase the throughput and energy efficiency of nanofluidic systems.

Predicting chemical-mechanical fracture initiation and propagation in materials is a critical problem, with broad relevance to a host of geoscience applications including subsurface storage and waste disposal, geothermal energy development, and oil and gas extraction. In this project, we have developed molecular simulation and coarse- graining techniques to obtain an atomistic-level understanding of the chemical- mechanical mechanisms that control subcritical crack propagation in materials under tension and impact the fracture toughness. We have applied these techniques to the fracture of fused quartz in vacuum, in distilled water, and in two salt solutions - 1M NaC1, 1M NaOH - that form relatively acidic and basic solutions respectively. We have also established the capability to conduct double-compression double-cleavage experiments in an environmental chamber to observe material fracture in aqueous solution. Both simulations and experiments indicate that fractures propagate fastest in NaC1 solutions, slower in distilled water, and even slower in air.

The adsorption equilibrium constants of monovalent and divalent cations to material surfaces in aqueous media are central to many technological, natural, and geochemical processes. Cation adsorption-desorption is often proposed to occur in concert with proton transfer on hydroxyl-covered mineral surfaces, but to date this cooperative effect has been inferred indirectly. This work applies density functional theory-based molecular dynamics simulations of explicit liquid water/mineral interfaces to calculate metal ion desorption free energies. Monodentate adsorption of Na+, Mg2+, and Cu2+ on partially deprotonated silica surfaces are considered. Na+ is predicted to be unbound, while Cu2+ exhibits binding free energies to surface SiO- groups that are larger than those of Mg2+. The predicted trends agree with competitive adsorption measurements on fumed silica surfaces. As desorption proceeds, Cu2+ dissociates one of the H2O molecules in its first solvation shell, turning into Cu2+(OH-)(H2O)3, while Mg remains Mg2+(H2O)6. The protonation state of the SiO- group at the initial binding site does not vary monotonically with cation desorption.

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Using an atomistic technique consistent with continuum balance laws and drawing on classical fracture mechanics theory, we estimate the resistance to fracture propagation of amorphous silica. We discuss correspondence and deviations from classical linear elastic fracture mechanics theory including size dependence, rigid/floppy modes of deformation, and the effects of surface energy and stress.

Classical molecular dynamics simulation was used to study the adsorption of Na+, Ca2+, Ba2+, and Cl- ions on gibbsite edge (1 0 0), basal (0 0 1), and nanoparticle (NP) surfaces. The gibbsite NP consists of both basal and edge surfaces. Simulation results indicate that Na+ and Cl- ions adsorb on both (1 0 0) and (0 0 1) surfaces as inner-sphere species (i.e., no water molecules between an ion and the surface). Outer-sphere Cl- ions (i.e., one water molecule between an ion and the surface) were also found on these surfaces. On the (1 0 0) edge, Ca2+ ions adsorb as inner-sphere and outer-sphere complexes, whereas on the (0 0 1) surface, outer-sphere Ca2+ ions are the dominant species. Ba2+ ions were found as inner-sphere and outer-sphere complexes on both surfaces. Calculated ion surface coverages indicate that, for all ions, surface coverages are always higher on the basal surface compared to those on the edge surface. More importantly, surface coverages for cations on the gibbsite NP are always higher than those calculated for the (1 0 0) and (0 0 1) surfaces. This enhanced ion adsorption behavior for the NP is due to the significant number of inner-sphere cations found at NP corners. Outer-sphere cations do not contribute to the enhanced surface coverage. In addition, there is no ion adsorption enhancement observed for the Cl- ion. Our work provides a molecular-scale understanding of the relative significance of ion adsorption onto gibbsite basal versus edge surfaces and demonstrates the corner effect on ion adsorption on NPs.

Methane (CH4) and carbon dioxide (CO2), the two major components generated from kerogen maturation, are stored dominantly in nanometer-sized pores in shale matrix as (1) a compressed gas, (2) an adsorbed surface species and/or (3) a species dissolved in pore water (H2O). In addition, supercritical CO2 has been proposed as a fracturing fluid for simultaneous enhanced oil/gas recovery (EOR) and carbon sequestration. A mechanistic understanding of CH4-CO2-H2O interactions in shale nanopores is critical for designing effective operational processes. Using molecular simulations, we show that kerogen preferentially retains CO2 over CH4 and that the majority of CO2 either generated during kerogen maturation or injected in EOR will remain trapped in the kerogen matrix. The trapped CO2 may be released only if the reservoir pressure drops below the supercritical CO2 pressure. When water is present in the kerogen matrix, it may block CH4 release. However, the addition of CO2 may enhance CH4 release because CO2 can diffuse through water and exchange for adsorbed methane in the kerogen nanopores.