This dissertation is about fabrication and functionalization of metal nanoparticles for use in plasmonic and nonlinear optical (NLO) applications. In the first two chapters, I describe a series of experiments, where I combined silver nanoparticles fabricated by nanosphere lithography with ionic self-assembled multilayer (ISAM) films, tuning the geometry of the particles to make their plasmonic resonances overlap with the frequency of optical excitation. The designed hybrid metallic/organic nanostructures exhibited large enhancements of the efficiency of second harmonic generation (SHG) compared to conventional ISAM films, causing a modified film with just 3 bilayers to be optically equivalent to a conventional 700-1000 bilayer film. SHG responses from Ag nanoparticle-decorated hybrid-covalent ISAM (HCISAM) films were investigated as the next logical step towards high-Ï ²⁺ ISAM films. I found that the plasmonic enhancement primarily stems from interface SHG. Interface effects were characterized by direct comparison of SHG signals from PAH/PCBS ISAM films and PAH/PB HCISAM films. Though interface &chi²⁺ is substantially smaller in PAH/PCBS than in PAH/PB, plasmonically enhanced PAH/PCBS films exhibit stronger NLO response. I propose that the structure of PAH/PB film makes its interface more susceptible to disruptions in the nanoparticle deposition process, which explains our observations. During the fabrication of monolayer crystals for nanosphere lithography, I developed a variation of the technique of convective self-assembly, where the drying meniscus is restricted by a straight-edge located approximately 100 μM above the substrate adjacent to the drying zone. This technique can yield colloidal crystals at roughly twice the growth rate compared to the standard technique. I attribute this to different evaporation rates in the thin wet films in the two cases. I also found that the crystal growth rate depends strongly on the ambient relative humidity. Finally, dithiocarbamate (DTC)-grafted polymers were synthesized and employed to functionalize surfaces of Au nanopartciles. PAH-DTC shows greater stability in different environments than PEI-DTC. I also investigated the stability of PAH-DTC coated particles in suspensions with UV-Vis spectroscopy and autotitration. The covalently bonded PAH-DTC enhances the colloidal stability of the Au nanoparticles and enables subsequent ISAM film deposition onto the particles. / Ph. D.

The Federal Aviation Administration (FAA) has been modernizing the United States' air transportation system within a series of initiatives called the Next Generation Air Transportation System (NextGen). The goal of NextGen is to increase the safety, efficiency, capacity, predictability, and resiliency of American Air Traffic Control (ATC) by implementing satellite-based communication, and navigation systems. Because of the vast oceanic areas controlled by Oakland, New York, and Anchorage air traffic control centers, improving oceanic operations is significant for the United States. According to the FAA, oceanic flights generate 31% of passenger revenue and 40% of cargo revenue in U.S.-controlled airspace. New NextGen procedures offer the opportunity for aircraft to save fuel consumption by allowing oceanic flights to fly at more efficient routes and flight levels. This dissertation investigates three areas to improve flight operations over oceanic airspace. The first area studies the operational benefits of providing satellite-based meteorological information to aircraft operating in remote and oceanic airspace. This research effort uses two approaches as follows: 1) statistical flight analysis, and 2) simulation-based analysis. The second area provides an optimization technique to improve the current procedures for assigning flights to the Organized Track System (OTS) in the Atlantic Ocean based on the Collaborative Decision Making (CDM) concept. The third area investigates the potential savings of "In-Trail Procedure" (ITP) as one of the advanced surveillance operations in the Pacific and Atlantic oceanic airspace. To quantify the operational benefits of the proposed procedures, a fast-time simulation tool, the Global Oceanic (GO) model, is developed and employed. The GO model is a microscopic flight simulation tool that has been developing by the Air Transportation Systems Laboratory at Virginia Tech offering realistic and inexpensive evaluations of novel technologies and procedures to improve flight operations over global oceanic airspace. the results of these studies are analyzed in terms of fuel consumption, travel distance, travel time, level of service, and potential air traffic controllers' workload. / Doctor of Philosophy / The economic growth and social connectivity of nations are highly correlated to effective and efficient air transportation systems. The Federal Aviation Administration (FAA) has initiated a program to modernize America's air transportation system and make flight operations safer, and more efficient. This program is called the Next Generation Air Transportation System (NextGen) and its goal is transforming the communication and navigation technologies to satellite-based systems. Improving oceanic flights is one of the main concerns of the NextGen program since the United States controls massive oceanic areas in the Atlantic and the Pacific Ocean. The FAA needs to evaluate the benefits and costs of advanced technologies and procedures to justify the NextGen initiatives. The FAA has employed computer simulation tools to support decisions for future infrastructure investments and encourage airlines to equip their aircraft with more advanced avionics. The Global Oceanic (GO) model is a microscopic flight simulation tool developed jointly by the Air Transportation Systems Laboratory at Virginia Tech and the FAA providing quick, realistic, and inexpensive evaluations of advanced procedures to improve flight operations over oceans. This dissertation investigates the operational benefit of three advanced procedures using the GO model. The areas to improve flight operations over oceanic airspace are as follows: 1) operational benefits of providing satellite-based meteorological information to aircraft operating in remote and oceanic airspace, 2) operational benefits of an optimization technique for flight assignments to the Organized Track System (OTS) in the Atlantic Ocean, 3) operational benefits of "In-Trail Procedure" (ITP) as one of the advanced surveillance operations in the Pacific and Atlantic oceanic airspace. These studies quantify the potential savings of these procedures in terms of reducing fuel consumption, travel distance, travel time, greenhouse gas emissions, and potential air traffic controllers' workload.

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, and the height increased by 6-12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10-550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of un-aggregated AgNP dissolution. / Master of Science

Metal-based nanoparticles (MNPs) are becoming increasingly common in commercial products. Release of these materials into the environment raises concerns about the potential risks they pose to aquatic life. Predicting these risks requires an understanding of MNPs' chemical transformations. In this study, arrays of immobilized MNPs fabricated by nanosphere lithography (NSL) were used to investigate environmental transformations of MNPs. Specifically, sulfidation of silver nanoparticles (Ag NPs) and dissolution of copper-based nanoparticles (Cu NPs) were investigated. Atomic force microscopy (AFM) and transmission electron microscopy were the primary analytical techniques for these investigations. Because the MNPs were immobilized on a solid surface, the samples were field deployable, environmentally relevant metal concentrations were maintained, and the confounding influence of MNP aggregation was eliminated. Ag NP samples were deployed in a full-scale wastewater treatment plant. Sulfidation occurred almost exclusively in anaerobic zones of the WWTP, where the initial sulfidation rate was 11-14 nm of Ag converted to Ag2S per day. Conversion to Ag2S was complete within 7-10 d. Dissolution rates of Cu-based NPs were measured in situ over a range of pH by flow-cell AFM. Based on the measured rates, CuO/Cu(OH)2 NPs dissolve completely within a matter of hours at any pH, metallic Cu NPs persist for a few hours to days, and CuxS NPs do not dissolve significantly over the time scales studied. Field deployment of samples in a freshwater stream confirmed these conclusions for a natural aquatic system. This research demonstrates that environmental transformations of MNPs will be a key factor in determining the ultimate form and concentration of NPs that aquatic organisms will be exposed to. / Ph. D.

Supersonic hot jet noise causes significant hearing impairment to the military workforce and results in substantial cost for medical care and treatment. Detailed insight into the turbulence structure of high-speed jets is central to understanding and controlling jet noise. For this purpose a new instrument based on the Doppler global velocimetry technique has been developed. This instrument is capable of measuring three-component velocity vectors over ex-tended periods of time at mean data-rates of 100 kHz. As a demonstration of the applicability of the time-resolved Doppler global velocimetry (TR-DGV) measurement technique, statistics of three-component velocity measurements, full Reynolds stress tensors and spectra along the stream-wise direction in a cold, supersonic jet at exit Mach number Mj = 1.4 (design Mach number Md = 1.65) are presented. In pursuance of extending the instrument to planar op- eration, a rapid response photomultiplier tube, 64-channel camera is developed. Integrating field programmable gate array-based data acquisition with two-stage amplifiers enables high-speed flow velocimetry at up to 10 MHz. Incor- porating this camera technology into the TR-DGV instrument, an investigation of the perfectly expanded supersonic jet at two total temperature ratios (TTR = 1.6 and TTR = 2.0) was conducted. Fourth-order correlations which have direct impact on the intensity of the acoustic far-field noise as well as convective velocities on the lip line at several stream-wise locations were obtained. Comprehensive maps of the convective velocity and the acoustic Mach number were determined. The spatial and frequency scaling of the eddy convective velocities within the developing shear layer were also investigated. It was found that differences in the radial diffusion of the mean velocity field and the integral eddy convective velocity creates regions of locally high convective Mach numbers after the potential core. This, according to acoustic analogies, leads to high noise radiation efficiency. The spectral scaling of the eddy convec- tive velocity indicates intermittent presence of large-scale turbulence structures, which, coupled with the emergence of Mach wave radiation, may be one of the main driving factors of noise emission observed in heated supersonic jets. / Ph. D.

Two series of experiments were performed to determine the flow characteristics and to quantify the heat transfer components from a propane diffusion flame impinging onto a ceiling. A 0.3 m square sand burner with propane as the fuel type provided a steady-state fire. In the first series of experiments, measurements of gas temperature and velocity were made at 76 mm vertical intervals above the burner up to the ceiling. Fire heat release rates (HRRs) of 50 kW and 90 kW with free flame length to ceiling height ratios, Lf/H, of 2, 1.5, 1, 0.8, 0.85 were used to determine their effects on the measured parameters. Gas temperatures within the continuous flaming region were relatively constant, and measured to be independent of ceiling height and HRR, while velocities increased with elevation and were independent of ceiling height yet weakly dependent on HRR. Within the intermittent region, gas temperature was weakly affected by the presence of the ceiling at various heights, while the effect on velocity was more pronounced. HRR had an effect on both temperature and velocity within the intermittent region of the fire plume. Comparisons with existing fire plume correlations showed that the unbounded correlations can be used to provide a good approximation of the gas temperature for the ceiling bounded case; while the correlations for the velocity can only be used for elevations up to approximately 60% of the ceiling height. Elevations above this cutoff were significantly affected by the presence of the ceiling. The second series of experiments investigated HRRs of 50 kW and 90 kW with free flame length to ceiling height ratios, Lf/H, of 2, 1.5, and 1. Heat flux and gas temperature at the stagnation point of the ceiling were measured using hybrid heat flux gauges and an aspirated Type K thermocouple. Four methods of calculating the convective heat transfer coefficient, h, were developed and adapted; two reference methods and two slope methods. The components of heat transfer at the impingement point were separated using these calculated h values. The reference method 2, and both slope methods only required the use of the non-cooled hybrid gauge measurements and were in overall good agreement with one another. The reference method 1 differed significantly, being up to 15.8 times lower than the others. The trends in the two groups were contradictory, with the h calculated using the reference method 1 increasing with ceiling height while the others showed no strong trend with ceiling height. The disagreements between the methods greatly affected the components of heat transfer, particularly at the lowest ceiling heights. Convection calculated using the h from reference method 1 contributed only 2-5% of the total exposure heat flux at the lowest ceiling heights, whereas with the other methods convection contributed 20-50% of the total exposure heat flux. The limitations of each method are discussed. Further investigation is required for all methods to determine their applicability within the flaming region of a fire. / Master of Science

Yes / The molecular stress function model with convective constraint release (MSF with CCR) constitutive model [J. Rheol. 45 (2001), 1387] is capable of fitting all viscometric data for IUPAC LDPE, with only two adjustable parameters (with difference found only on reported ¿steady-state¿ elongational viscosities). The full MSF with CCR model is implemented in a backwards particle-tracking implementation, using an adaptive method for the computation of relative stretch that reduces simulation time many-fold, with insignificant loss of accuracy. The model is shown to give improved results over earlier versions of the MSF (without CCR) when compared to well-known experimental data from White and Kondo [J. non-Newt. Fluid Mech., 3 (1977), 41]; but still to under-predict contraction flow opening angles. The discrepancy is traced to the interaction between the rotational dissipative function and the large stretch levels caused by the contraction flow. A modified combination of dissipative functions in the constraint release mechanism is proposed, which aims to reduce this interaction to allow greater strain hardening in a mixed flow. The modified constraint release mechanism is shown to fit viscometric rheological data equally well, but to give opening angles in the complex contraction flow that are much closer to the experimental data from White and Kondo. It is shown (we believe for the first time) that a constitutive model demonstrates an accurate fit to all planar elongational, uniaxial elongational and shear viscometric data, with a simultaneous agreement with this well-known experimental opening angle data. The sensitivity of results to inaccuracies caused by representing the components of the deformation gradient tensor to finite precision is examined; results are found to be insensitive to even large reductions in the precision used for the representation of components. It is shown that two models that give identical response in elongational flow, and a very similar fit to available shear data, give significantly different results in flows containing a mix of deformation modes. The implication for constitutive models is that evaluation against mixed deformation mode flow data is desirable in addition to evaluation against viscometric measurements.

Les échangeurs de chaleur constitués de microcanaux parallèles sont considérés une bonne solution technologique pour dissiper de grands flux de chaleur dans les composants et les systèmes miniaturisés. D’une manière générale cette réduction de taille permet une diminution des coûts des matériaux et l'utilisation de plus faible quantité de fluides frigorigènes pour les systèmes de refroidissement. Cette étude est divisée en deux parties complémentaires A et B. Elles visent à étudier le comportement thermo-hydraulique dans les échangeurs de chaleur constitués de microcanaux pour une meilleure compréhension des transferts de chaleur et des écoulements diphasiques dans les évaporateurs miniatures. Dans la partie A, nous étudions l'ébullition convective du réfrigérant R134a dans un mini échangeur composé de neuf microcanaux parallèles de section transversale circulaire, placés horizontalement, avec un diamètre interne de 0,77 mm et longueur et 150 mm. Les résultats expérimentaux montrent que la configuration d'écoulement a une forte influence sur le coefficient de transfert de chaleur, et que différents mécanismes de transfert de chaleur ont lieu dans chacune de ces configurations d'écoulement. En revanche la perte de pression est une fonction directe de la vitesse massique, du titre de vapeur et de la pression du système. La partie B, porte sur l’analyse de la distribution de l'écoulement diphasique en l’absence de transferts de chaleur et de changement de phase liquide-vapeur. A partir de ces résultats nous montrons que les effets du titre de gaz sur la répartition du liquide change considérablement selon la position de l'ensemble (tube d'alimentation, distributeur-canaux). / Heat exchangers consisting of parallel micro-channels are considered a good technological solution in response to the increasing demand for compact systems, which require high heat flux dissipation, ensuring a decrease in the material costs and the use of a lower quantity of refrigerants. The aim of this study was to investigate the thermo-hydraulic behavior inside these components provided by microchannels. This study is divided into two experimental studies (A and B) which are complementary. In part A, the convective boiling of the refrigerant R134a is analyzed within nine parallel microchannels of circular cross section, positioned horizontally, with internal diameter and length of 0.77 mm and 150 mm, respectively. The experimental results show that the flow pattern has a strong influence on the heat transfer coefficient, and that different heat transfer mechanisms are associated with each of the flow patterns observed, whereas the frictional pressure drop is a direct function of the mass velocity, vapor quality and pressure of the system. In part B, the two-phase flow distribution, using as the working fluid a mixture of air and water, is analyzed inside a circular header coupled to nine branched parallel microchannels of circular cross-section with internal diameter and length of 0.8 mm and 150 mm, respectively. The results show that the effect of the gas quality on the liquid distribution changes considerably depending on the configuration (feeder tube-header-channels).

The majority of this work is concerned with the local-linear convective instability analysis of the incompressible boundary-layer flows over prolate spheroids and oblate spheroids rotating in otherwise still fluid. The laminar boundary layer and the perturbation equations have been formulated by introducing two distinct orthogonal coordinate systems. A cross-sectional eccentricity parameter e is introduced to identify each spheroid within its family. Both systems of equations reduce exactly to those already established for the rotating sphere boundary layer. The effects of viscosity and streamline-curvature are included in each analysis. We predict that for prolate spheroids at low to moderate latitudes, increasing eccentricity has a strong stabilizing effect. However, at high latitudes of ϴ ≥ 60, increasing eccentricity is seen to have a destabilizing effect. For oblate spheroids, increasing eccentricity has a stabilizing effect at all latitudes. Near the pole of both types of spheroids, the critical Reynolds numbers approach that for the rotating disk boundary layer. However, in prolate spheroid case near the pole for very large values of e, the critical Reynolds numbers exceed that for the rotating disk. We show that high curvature near the pole of prolate spheroids is responsible for the increase in critical Reynolds number with increasing eccentricity. For both types of spheroids at moderate eccentricity, we predict that the most amplified modes travel at approximately 76% of the surface speed at all latitudes. This is consistent with the existing studies of boundary-layer flows over the related rotating-disk, -sphere and -cone geometries. However, for large values of eccentricity, the traveling speed of the most amplified modes increases up to approximately 90% of the surface speed of oblate spheroids and up to 100% in the prolate spheroid case.

Ce travail avait pour objectif l'étude de l'impact des deux procédés : la déshydratation imprégnation par immersion (DII) et le séchage convectif menés séparément ou combinés sur les transferts couplés d'eau et de saccharose et les principaux critères de qualité de fruit (écart de couleur, teneur en phénols totaux (PT), teneur en acide ascorbique (AA), retrait volumique, activité de l'eau) en prenant la poire Conférences comme modèle. L'effet des procédés sur les morceaux de poire a été appréhendé selon deux approches : (i) une étude globale de l'impact des variables de commandes sur la qualité du produit fini et (ii) une étude cinétique des composés d'intérêt nutritionnel et de la qualité (couleur, retrait, activité de l'eau).Un plan d'expériences composite centré à quatre facteurs et cinq niveaux a été établi (concentration en saccharose de la solution osmotique : 25-65%, température de DII : 20-60 °C, durée de la DII : 0,5-6,5 h, température du séchage convectif : 30-70 °C) pour étudier l'effet global des procédés sur le produit fini. En DII, les paramètres « °Brix, « durée » et « la température de la solution » ont un effet significatif (p < 0,01) sur les pertes en eau, le gain en soluté et les attributs de qualité de morceaux de poire. Les pertes en AA sont plus élevées que les pertes en PT et sont essentiellement dues à l'oxydation et à l'entraînement par l'eau. Durant le séchage convectif, les pertes en PT et en AA dépendent plus de la durée du procédé que de la température d'exposition. Elles atteignent respectivement 80% et 34% après 10 h de séchage à 30 °C. Le changement de couleur de morceaux de poire dépend de la durée et de la température de traitement et est plus prononcé en fin de séchage. Des modèles quadratiques prédictifs ont été proposés pour relier les variables de réponse (aw, différence de couleur, perte en PT et durée totale de traitement) aux paramètres opératoires des deux procédés étudiés. Cependant, la perte en AA décroit linéairement avec la durée de DII. Une optimisation multicritères en utilisant la méthodologie des surfaces de réponse (MSR) a été proposée (pertes minimales en PT et en AA, durée totale du traitement et différence de couleur réduites et une aw inférieure à 0,6) pour la combinaison DII/séchage. Les conditions optimales sont : solution osmotique à 28°C et 25°Brix, une durée de DII de 30 min et un séchage convectif à 60 °C. Par ailleurs, les effets de la température (30, 45 et 60 °C) et de l'imprégnation en saccharose par DII (10 et 65 min, 70°Brix, 30 °C) sur les isothermes de désorption et sur la température de transition vitreuse (Tg) de morceaux de poires, de pommes et d'abricots ont été déterminés. L'imprégnation en saccharose du tissu végétal atténue l'influence de la température sur sa capacité de sorption. Elle engendre un effet dépresseur de l'activité de l'eau et une augmentation de l'hygroscopicité de la poire, la pomme et l'abricot à température élevée. Les isothermes de désorption des abricots frais et enrichis en saccharose présentent un croisement des courbes marquant l'inversement de l'effet de la température sur les isothermes. La Tg dépend de la température, de la teneur en saccharose et de la teneur en eau des fruits. Pour une teneur en eau constante, l'augmentation de la température et la durée de DII se traduisent par l'augmentation de la Tg. Les digrammes de phase (Tg=f(X) et Tg=f(aw)) des fruits ont été établis à 30, 45 et 60°C et les teneurs en eau critiques ainsi que les activités d'eau critiques ont été déterminées. Une teneur en eau en fin de séchage inférieure ou égale à 0,02 g/g M.S. est recommandée pour l'obtention de morceaux de fruit stables aux niveaux microbiologique, physicochimique et rhéologique. / The aim of this work was the study of the impact of both drying processes: dehydration-impregnation by soaking (DIS) and convective drying performed separately or in combination on mass transfers of water and sucrose and on the main quality characteristics of the fruit (total difference of colour, total phenols (TP) and ascorbic acid (AA) contents, shrinking, water activity) by taking the pear Conference as a model. The processes effect on the pieces of pear was apprehended according two approaches: (i) a global study of the impact of the variables of commands on the quality of finished product and (ii) a kinetic study of nutritional compounds and the quality (colour, shrinkage, and water activity). Central Composite Design with four factors (sucrose concentration: 25-65 %, temperature: 20-60 °C, immersion time: 0.5-6.5 hr and drying temperature: 30-70 °C) at five levels each was used for pears processing. During DIS, the variables: immersion time, temperature and concentration of sucrose solution had a significant effect (p<0.01) in the loss of water, the solid gain and the quality attributes of pears. The losses in AA are higher than those in TP and are essentially due to the oxidation and the leaching with water. During the convective drying, the losses in TP and in AA depend more on the time of the process rather than the temperature of exposition. They reach respectively 80% and 34% after 10 h of drying at 30 °C. The color change of pear depends on the time and temperature of treatment and is more pronounced at the end of drying. Predictive and quadratic models were suggested to link the responses variables (aw, total colour difference, TP loss and total time treatment) to operational parameters of both studied processes. The losses in AA decrease linearly with the duration of DIS. A multicriteria optimization using response surface methodology (RSM) was proposed (minimal losses in TP and in AA, reduced total processing time and total difference in colour and an aw inferior to 0.6) for the combination DIS/convective drying. The optimal conditions are: osmotic solution at 28°C and 25°Brix, a DIS lasting 30 min and a convective drying at 60 °C. Otherwise, the temperature effects (30, 45 and 60 °C) and the sucrose impregnation by DIS (10 and 65 min, 70°Brix, 30 °C) upon the desorption isotherms and the transition temperature (Tg) of pears, apples and apricots were determined. The sucrose impregnation diminishes the influence of temperature on its sorption ability. It generates a depressing effect on water activity and a rise of pear hygroscopicity, apple and apricot at a high temperature. The desorption isotherms of fresh and sucrose impregnated apricot present a crossing of curves which mark the inverse of the effect of temperature on isotherms. The Tg depends on the temperature, the sucrose content and the fruit water content (X). For constant water content, the rise of temperature and of the duration de DIS result in the increase of Tg. The phase diagrams (Tg=f(X) et Tg=f(aw)) of fruits were established at 30, 45 and 60°C and the critical water content as well as the critical water activities were determined. Water content at the end of drying inferior or equal to 0.02 g/g d.b. is recommended to obtain stable fruit pieces at microbiological, physiochemical and rheological levels.