Browsing by Subject "Surface"

Now showing 1 - 7 of 7

Constraining fracture permeability by characterizing fracture surface roughness
(2010-12) Al-Johar, Mishal Mansour; Sharp, John Malcolm, 1944-; Ketcham, Richard A.; Cardenas, Meinhard B.
Open and connected fractures, where present, control fluid flow and dominate solute transport. Flow through fractures has major implications for water resource management, underground waste repositories, contaminant remediation, and hydrocarbon exploitation. Complex fracture morphology makes it difficult to quantify and predict flow and transport accurately. The difficulty in usefully describing the complex morphology of a real fracture from a small 3-D volume or 2-D profile sample remains unresolved. Furthermore, even when complex fracture morphology is measured across three-dimensions, accurate prediction of discharge remains difficult. High resolution x-ray computed tomography (HXRCT) data collected for over 20 rock surfaces and fractures provide a useful dataset to study fracture morphology across scales of several orders of magnitude. Samples include fractured rock of varying lithology, including sandstone, volcanic tuffs and crystalline igneous and metamorphic rocks. Results suggest that the influence of grain size on surface roughness is not readily apparent due to other competing variables such as mechanics, skins and coatings, and weathering and erosion. Flow tests of HXRCT-scanned fractures provide real discharge data allowing the hydraulic aperture to be directly measured. Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures. Often, a distinction is not made between the apparent and true fracture apertures for rough fractures measured on a 2-D topographic grid. I compare a variety of local aperture measurements, including the apparent aperture, two-dimensional circular tangential aperture, and three-dimensional spherical tangential aperture. The mechanical aperture, the arithmetic mean of the apparent local aperture, is always the largest aperture. The other aperture metrics vary in their ranking, but remain similar. Results suggest that it may not be necessary to differentiate between the apparent and true apertures. Rock fracture aperture is the predominant control on permeability, and surface roughness controls fracture aperture. A variety of surface roughness characterizations using statistical and fractal methods are compared. A combination of the root-mean-square roughness and the surface-to-footprint ratio are found to be the most useful descriptors of rock fracture roughness. Mated fracture surfaces are observed to have nearly identical characterizations of fracture surface roughness, suggesting that rock fractures can be sampled by using only one surface, resulting in a significantly easier sampling requirement. For mated fractures that have at least one point in contact, a maximum potential aperture can be constrained by reflecting and translating a single surface. The maximized aperture has a nearly perfect correlation with the RMS roughness of the surface. These results may allow better predictions of fracture permeability thereby providing a better understanding of subsurface fracture flow for applications to contaminant remediation and water and hydrocarbon management. Further research must address upscaling fracture morphology from hand samples to outcrops and characterizing entire fracture networks from samples of single fractures.
The effects of walking speed and an uneven surface on dynamic stability margins in young adult subjects with and without traumatic unilateral trans-tibial amputations
(2009-12) Scott, Shawn James; Dingwell, Jonathan B.; Abraham, Larry; Jensen, Jody; Wilken, Jason M.; Childs, John
Dynamic stability is commonly defined as the ability to maintain balance through center of mass control during locomotion. Patients with locomotor impairments are especially challenged when walking over uneven surfaces (Richardson 2004). We studied dynamic stability margins in young healthy adults and in adults with unilateral traumatic trans-tibial amputations (TTA). To date, studies have not controlled for walking speed over an uneven surface in a patient population. We hypothesized that: 1) DSMs would increase over the uneven rocky surface (URS) for both groups, 2) DSMs would be greater on the involved side at faster walking speeds for subjects with TTA and, 3) DSMs would increase more for the involved limb when on the URS. 17 (4 females, 13 males) young healthy military service members (22.8 ± 6.4 years) and 12 (1 female, 11 males) service members (27.2 ± 4.7 years) with traumatic unilateral trans-tibial amputations participated in two study designs. A 15-segment model was used to estimate whole body COM motions. All subjects walked at 5 dimensionless speeds over a flat level surface (FLS) and an URS. Subjects completed 6-10 trials over each surface at each speed. Minimum frontal plane DSM values were extracted for each stride for statistical analyses. For young healthy subjects a two factor (speed x surface) ANOVA was used to test significance (p<.05). The DSMs were not greater over the URS (p=.307), but a main effect due to speed was found (p<.001) for young healthy subjects. In contrast, DSMs were significantly larger when subjects with TTA walked on the URS compared to the FLS (p = 0.011). For subjects with unilateral TTA, a three-factor ANCOVA ((amputation) side x speed x surface) with residual limb length (p=.029) and time in prosthesis (p=.741) as covariates was used for hypothesis testing. When limb length and time in prosthesis were accounted for there was no significant within subjects effect due to speed (p=.656). There were no significant differences between involved and uninvolved limbs (p = 0.211). There were no significant interaction effects. In conclusion, there was a difference in DSMs due to speed in unimpaired subjects and due to surface and residual limb length in subjects with unilateral TTAs. In subjects with unilateral TTA side-to-side symmetry was found for DSM measures, which was in contrast to an earlier study of subjects with unilateral trans-femoral amputations (Hof 2006). It appears that symmetry and dynamic stability are reasonable expectations for young adults with isolated TTAs.
Enzyme immobilization on gold surfaces : effects of surface chemistry and attachment strategies on binding and activity
(2023-04-20) Correira, Joshua Manuel; Webb, Lauren J.; Samanta, Devleena; Baiz, Carlos R; Gordon, Vernita; Shear, Jason B
Functional enzymes are the basis for many biotechnological systems, including biosensors, bio-fuel cells, and heterogeneous biocatalysts. In these systems, enzymes are often immobilized on a solid support or surface to capture their catalytic activity. Immobilization has the advantage of improving enzyme stability and reusability but often results in a significant loss of catalytic activity (1-2 orders of magnitude) when compared to the native enzyme. This inactivation is due to direct interactions between the enzyme and the solid support. Here, we developed methods to study enzyme immobilization and resulting inactivation. The aim of this work was to identify optimal surface chemistries and attachment strategies that promote high binding efficiencies while minimizing activity losses. Subsequently, we studied this using three enzymes (acetylcholinesterase (AChE), β-galactosidase (β-gal), horseradish peroxidase (HRP)) immobilized on gold surfaces by direct adsorption, covalent attachment, and DNA-directed attachment. First, AChE was directly adsorbed onto a variety of gold surfaces modified with self-assembled monolayers (SAMs) terminated with -COO⁻, -NH₃⁺, -OH, and -CH₃ functional groups at varying mole % to study the effect of surface hydrophobicity and charge on binding and activity. We found that binding was directly proportional to surface hydrophobicity (r = 0.75) and activity was inversely proportional to surface hydrophobicity (r = -0.62). The highest binding observed was ~40% of a monolayer on the most hydrophobic surfaces and the lowest binding observed was ~10% of a monolayer on the most hydrophilic surfaces. Conversely, on the most hydrophobic surfaces AChE retained <10% of its native activity, and on the most hydrophilic surfaces AChE retained ~40% of its native activity. This illuminated an inherent problem with direct adsorption: high binding and high activity are mutually exclusive. Due to these findings, we next immobilized β-gal and HRP on DNA-functionalized gold surfaces using DNA-DNA interactions, to avoid direct interactions between the enzyme and surface. We found that β-gal retained 62% of its native activity following immobilization, a significant improvement over previous direct adsorption strategies.
First principle study of transition metal oxide (catalytic) electrodes for electrochemical energy technologies
(2017-08-08) Tsai, Yu-Hao; Hwang, Gyeong S.; Manthiram, Arumugam; Yu, Guihua; Ferreira, Paulo
TTo fulfill the needs for developing the alternative energy technologies, searching for the adequate electrode materials which catalyze the electrochemical reactions utilized in devices such as fuel cell, Li-ion batteries, and related applications such as hydrogen generation and storage, has been a longstanding challenge. Among various catalysts, transition metal oxides (TMO) draw great attentions due to their low-cost, high stability, and, most importantly, a great variety of structures and electrical properties. Nonetheless, studying electrochemical reactions catalyzed by TMO is a challenging task due to the possible multivalent systems, flexible coordinations of lattice atoms, adjustable surface structures and diverse surface species. In the past decades, many innovative approaches have been explored with encouraging results; however, the mechanisms of incorporating the bulk/surface TMO structures in various chemical reactions still remain unclear. In this dissertation, using quantum mechanical calculations, we attempt to improve the fundamental understandings of how structures and electronic properties of TMO materials facilitate the electrochemical reactions of interest. To identify the possible causes for CuO and Cu structures having different selectivity in catalysis, in Chapter 3, we study the CO₂ reduction reaction (CO2RR) catalyzed by CuO (111) surface structure, and compare the results with the more widely studied Cu (100) surface. The roles played by the electronic properties of two materials in their selectivity are elucidated. In Chapter 4 and 5, we study the oxygen evolution reaction (OER) for LiCoO₂ surface structure. The structures and stabilities of Li-, O-, and H-terminated surface are investigated comprehensively. Based on the results, the formation of H-terminated surface results from Li/H exchange at the solid/liquid interface is proposed (Chapter 4). Along with the findings, we explore the possible mechanisms for the OER for non-metal terminated LiCoO₂ surface (Chapter 5). In Chapter 6, we study the oxygen reduction reaction (ORR) for Co₃O₄ (111) H-terminated surface structure. The possible reaction steps for both four-electron and two-electron pathway are investigated. In Chapter 7, the PO₄-decicient LiFePO₄/FePO₄ structures are investigated to understand how the presence of polyanion defects in the matrices could potentially improve the performance of the materials as electrodes in Li-ion batteries.
Insights into computational methods for surface science and catalysis
(2021-12-06) Ciufo, Ryan Anthony; Henkelman, Graeme; Humphrey, Simon M; Hwang, Gyeong S.; Webb, Lauren J.
The fundamental understanding of both the reactions at catalytic surfaces and the ways in which these surfaces change throughout a catalytic cycle and lifetime are important for both academic and industrial disciplines. To develop these understandings on complex catalytic systems, ultra-high vacuum techniques such as molecular beam studies, temperature programmed desorption, reflection-absorption infrared spectroscopy and Auger electron spectroscopy can be used to study the simplest interactions between gas molecules and surfaces. These interactions can be studied from a bottom-up approach to learn about the system in question, upon which additional complexities can be added. To parallel these experimental techniques, a number of computational methods can be used to support findings and guide new experiments. Ab-initio electronic structure calculations allow for a better understanding of adsorbate-surface interactions, while long timescale dynamic simulations provide insight into the time evolution and kinetics of catalysts and catalytic surfaces. Empirical and machine-learning guided potentials can be developed to lessen computational cost while retaining accuracies comparable to ab-initio calculations. Fitting such potentials ultimately allows for larger calculations to be performed and longer timescales to be simulated. The above methods will be applied to a number of industrially and academically relevant catalytic systems, including studying the interaction of H₂ and CO with Cobalt based Fischer-Tropsch catalysts and the interaction between hydrogen and palladium surfaces. Additionally, the development of a machine learning package to fit and use interatomic potentials will be discussed.
Novel tools for ultrafast spectroscopy
(2011-12) Jarvis, Thomas William; Li, Elaine; Fink, Manfred; Keto, John; Lim, Sang-Hyun; Shih, Chih-Kang; Sitz, Greg
Exciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.
Where we are not
(2019-05-06) Lai, Renee Hoi Yee; Canright, Sarah
This report outlines the primary concerns in my painting practice, that of material, surface, line weight, and barriers