Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item ATOMISTIC AND THEORETICAL DESCRIPTION OF LIQUID FLOWS IN POLYELECTROLYTE-BRUSH-GRAFTED NANOCHANNELS(2021) Sachar, Harnoor Singh; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyelectrolyte (PE) chains grafted in close proximity stretch out to form a “brush”-like configuration. Such PE brushes can represent a special class of nanomaterials that are capable of exhibiting stimuli-responsive behavior. They can be manipulated as needed by changing the environmental conditions like pH, solvent quality, salt concentration, temperature, etc. This responsiveness renders them very useful for a plethora of applications such as lubrication, emulsion stabilization, current rectification, nanofluidic energy conversion, drug delivery, oil recovery, etc. Therefore, gaining fundamental insights into PE brush systems is of utmost importance for both industrial as well as academic research. In this dissertation, we make use of theoretical and computational tools to improve our understanding of planar PE brushes and then use this understanding to probe flows in PE brush-grafted nanochannels. We begin our quest by conducting all-atom Molecular Dynamics (MD) simulations to probe the microstructure of planar PE brushes with an unprecedented atomistic resolution. This allows us to not only investigate the properties of the PE chains but also the local structure and arrangement of the counterions and water molecules trapped within the brushes. Next, we use our atomistic model to probe the effects of variation in charge density on the microstructure of weak polyionic brushes. Such a variation in the charge density is typically enforced by a change in the surrounding pH and is a characteristic behavior of pH-responsive (annealed) PE brushes. Furthermore, we go on to develop the most exhaustive theoretical model for pH-responsive PE brushes known as the augmented Strong Stretching Theory (SST). Our model is an improvement over the existing state-of-the-art as it considers the effects of the excluded volume interactions and an expanded form of the mass action law. We further improve this model by including several non-Poisson Boltzmann effects, especially relevant at high salt concentrations. This improved model is in excellent agreement with the results of our all-atom MD simulations. Next, we use our augmented SST to model pressure-driven transport in backbone-charged PE brush-grafted nanochannels. Our results are an improvement over previous electrokinetic studies that did not consider a thermodynamically self-consistent description of the brushes. Finally, we conduct all-atom MD simulations to probe the pressure-driven transport of water in PE brush-grafted nanochannels using an all-atom framework. The nanoscale energy conversion characteristics obtained from our simulations are in reasonable agreement with the predictions of our continuum framework and lie within the range of values reported by a prior experimental study.Item FRACTURE BEHAVIOR AND THERMAL CONDUCTIVITY OF POLYCRYSTALLINE GRAPHENE(2014) Fox, Andrew Oliver; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the effect of grain boundaries (GBs) in polycrystalline graphene on the tensile fracture behavior and thermal conductivity of the graphene sheets. Current techniques to fabricate large-scale graphene intrinsically introduce defects, e.g., GBs, resulting in polycrystalline graphene sheets. Though GBs in graphene are expected to affect the mechanical properties of graphene, mechanistic understanding and quantitative determination of such effects are far from mature. For example, existing studies on the effect of GBs on the tensile behavior of graphene only focus on a twin GB perpendicular to the tensile loading direction. However, GBs in a polycrystalline graphene sheet under uniaxial tension could be subject to tension in any arbitrary directions, depending on the GB and grain orientation in the graphene sheet. In this dissertation, we focus on the effect of GBs on the tensile and thermal response of polycrystalline graphene. The fracture process of polycrystalline graphene sheets under uniaxial tension was studied using molecular dynamics (MD) simulations to determine how GBs affects the ultimate strength and critical failure strain of the graphene. We also study the flow of heat through polycrystalline graphene to determine the effect of GBs on the thermal conductivity of graphene. A comprehensive study including 24 GB misorientation angles ranging from 2.1° to 54.3° and the whole range of loading angle (i.e., that between a GB and in-plane tensile loading direction, ranging from 0° to 90°) was carried out to quantitatively determine the effect of GBs. Stress-strain data were generated from the MD simulations and the failure strength and critical strain were analyzed. A theoretical model combining continuum mechanics theory and disclination dipole theory was introduced to predict the failure strength of the polycrystalline graphene sheets, which was shown to be in good agreement with the MD simulation results. Various failure modes of polycrystalline graphene under tension were also analyzed. The thermal conductivity of polycrystalline graphene as a function of GB misorientation angle and thermal loading angle was also quantitatively determined through systematic simulations. The quantitative findings from this dissertation could potentially bridge the knowledge gap toward a better understanding of defects and their effects on two-dimensional materials, and also shed light on possible defect control and engineering to achieve desirable properties of graphene in applications.Item A Proposed Mechanical-Metabolic Model of the Human Red Blood Cell(2014) Oursler, Stephen Mark; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The theoretical modeling and computational simulation of human red blood cells is of interest to researchers for both academic and practical reasons. The red blood cell is one of the simplest in the body, yet its complex behaviors are not fully understood. The ability to perform accurate simulations of the cell will assist efforts to treat disorders of the cell. In this thesis, a computational model of a human red blood cell that combines preexisting mechanical and metabolic models is proposed. The mechanical model is a coarse-grained molecular dynamics model, while the metabolic model considers the set of chemical reactions as a system of first-order ordinary differential equations. The models are coupled via the connectivity of the cytoskeleton with a novel method. A simulation environment is developed in MATLAB® to evaluate the combined model. The combined model and the simulation environment are described in detail and illustrated in this thesis.Item MOLECULAR DYNAMICS STUDIES OF METALLIC NANOPARTICLES(2009) Henz, Brian John; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal nanoparticles have many desirable electrical, magnetic, optical, chemi-cal, and physical properties. In order to utilize these properties effectively it is neces-sary to be able to accurately predict their size-dependent properties. One common method used to predict these properties is with numerical simulation. The numerical simulation technique used throughout this effort is the molecular dynamics (MD) si-mulation method. Using MD simulations I have investigated various metallic nano-particle systems including gold nanoparticles coated with an organic self-assembled monolayer (SAM), the self-propagating high-temperature synthesis (SHS) reaction of nickel and aluminum nanoparticles, and the mechano-chemical behavior of oxide coated aluminum nanoparticles. The model definition, boundary conditions, and re-sults of these simulations are presented in the following dissertation. In the first material system investigated MD simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. Numerous results and observations from this parametric study are pre-sented here. By analyzing the mechanical and chemical properties of gold nanopar-ticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we have determined that the material system is metastable. The model and computational results that provide support for this hy-pothesis are presented. The second material system investigated, namely sintering of aluminum and nickel, is explored in chapter 4. In this chapter MD simulations are used to simulate the kinetic reaction of Ni and Al particles at the nanometer scale. The affect of par-ticle size on reaction time and temperature for separate nanoparticles has been consi-dered as a model system for a powder metallurgy process. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles of one material embedded within a matrix of the second. Simulation results show that the sintering time for separate and coated nanoparticles is dependent upon the number of atoms or volume of the sintering nanoparticles and their surface area. We have also found that nanoparticle size and surface energy is an important factor in determining the adiabatic reaction temperature for both systems, coated and separate, at nanoparticle sizes of less than 10nm in diameter. The final material system investigated in chapters 5 and 6 is the oxide coated aluminum nanoparticle. This material system is simulated using the reactive force field (ReaxFF) potential which is capable of considering the charge transfer that occurs during oxidation. The oxidation process of oxide coated aluminum nanoparticles has been observed to occur at a lower temperature and a faster rate than micron sized nanoparticles, suggesting a different oxidation mechanism. From this effort we have discovered that the oxidation process for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. In contrast to the currently assumed pressure driven diffusion process the results presented here demonstrate that the high temperature oxidation process is driven by the electric field present in the oxide layer. This electric field ac-counts for over 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experi-mental results. The final chapter includes some important conclusions from this work and highlights some future work in these areas. Future work that is outlined includes ef-forts that are currently underway to analyze the interactions of multiple alkanethiolate coated gold nanoparticles in vacuum and in solvent. Other future efforts are farther out over the horizon and include using advanced computing techniques such as gen-eral purpose graphical processing units (GPGPU) to expand simulation sizes and physical details over what it is currently possible to simulate.