UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Burning Emulations of Condensed Phase Fuels Aboard The International Space Station(2022) Dehghani, Parham; Sunderland, Peter B; Quintiere, James G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Little is known about the fire hazards of solids and liquids in microgravity. Ground-based tests are too short to overcome ignition transients and testing dozens of condensed fuels in orbit is prohibitively expensive. Burning rate emulation is one way to address this gap. It involves emulating condensed fuels with gases using a porous burner with embedded heat flux gages. This is a study of microgravity burning rate emulation aboard the International Space Station. The burner had porous round surfaces with a diameter of 25 mm. The fuel mixture was gaseous ethylene, and it was diluted with various amounts of nitrogen. The resulting heats of combustion were 15 – 47.2 kJ/g. The flow rate, oxygen concentration in the ambient, and pressure were varied. Heat flux to the burner was measured with two embedded heat flux gages and a slug calorimeter. The effective heat of gasification was determined from the heat flux divided by the fuel flow rate. Radiometers provided the radiative loss fractions. A dimensional analysis based on radiation theory yielded a relationship for radiative loss fraction. RADCAL, a narrow-band radiation model, yielded flame emissivities from the product concentrations, temperature, flame length, and pressure. Previously published analytical solutions to these flames allowed prediction of flame heights and radius, and when combined with the radiation empirical relationship led to corrections of total heat release rate from the flames due to radiative loss. Average convective and radiative heat flux were obtained from the analytical solution and a model based on the geometrical view factor of the burner surface with respect to the flame sheet, that was used to calculate the heat of gasification. All flames burning in 21% by volume oxygen self-extinguished within 40 s. However, steady flames were observed at 26.5, 34, and 40% oxygen. The analytical solution was used to quantify flame steadiness just before extinction. The steadiest flames reached more than 94% of their steady-state heat fluxes and heights. A flammability map as a plot of the heat of gasification versus heat of combustion was developed based on the measurement and theory for nominal ambient oxygen mole fractions of 0.265, 0.34, and 0.4.Item ENHANCED DIFFUSIOOSMOSIS AND THERMOOSMOSIS IN POLYELECTROLYTE-BRUSH-FUNCTIONALIZED NANOCHANNELS(2018) Maheedhara, Raja; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the holy grails of nanofluidic systems is to ensure significant flow rates without applying a large pressure gradient. This has motivated researchers to study different mechanisms of liquid transport in nanochannels involving physical effects that exploit the large surface-to-volume ratio of such nanochannels. This thesis will focus on two highly efficient non-pressure-driven flow mechanisms in nanochannels functionalized by grafting the inner walls of nanochannels with end-charged polyelectrolyte (PE) brushes. We study two mechanisms to achieve flow augmentation: (i) ionic diffusioosmosis (IDO), triggered by the application of an external concentration gradient, and (ii) ionic thermoosmosis (ITO), triggered by a temperature gradient. We find a non-intuitive scenario where the flow in nanochannels can be significantly augmented by grafting the nanochannels with PE brushes. Given the difficulty in attaining a desirable flow strength in nanochannels, we anticipate that this thesis will serve as an important milestone in the area of nanofluidics.Item A Method for Measurement of Spatially Resolved Radiation Intensity and Radiative Fraction of Laminar Flames of Gaseous and Solid Fuels(2016) Hamel, Catherine Marie; Stoliarov, Stanislav; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work introduces a new method for determination of the radiation intensity and radiative fraction for axisymmetric laminar diffusion flames for both solid and gaseous fuels by using a modified DSL-R camera employed to collect monochromatic (900 nm) images and a Schmidt-Boelter heat flux gauge. The high spatial resolution provided by the images of the camera allows for a multi-emitter treatment of the 2-6 cm flames. The flame’s radius and intensity are extracted from the images and presented as two curves that are functions of the flame-axis position. Each point on the flame sheet is discretized at pixel-level resolution and treated as a differential emitting surface. Radiation transport equations are formulated and solved numerically to compute a function that relates the camera’s readings to the total radiation heat flux detected by the gauge. The calculation yields spatially resolved radiation intensity information. Integration of this intensity over the flame surface divided by the total heat release rate yields the global radiative fraction. In this work, polyethylene (solid fuel) is studied and three gaseous fuels (methane, propane and acetylene) are studied to validate the methodologyItem The Effect of Package Geometry on Moisture Driven Degradation of Polymer Aluminum Capacitors(2016) Bevensee, Helmut Manfred; Azarian, Michael H.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer aluminum electrolytic capacitors were introduced to provide an alternative to liquid electrolytic capacitors. Polymer electrolytic capacitor electric parameters of capacitance and ESR are less temperature dependent than those of liquid aluminum electrolytic capacitors. Furthermore, the electrical conductivity of the polymer used in these capacitors (poly-3,4ethylenedioxithiophene) is orders of magnitude higher than the electrolytes used in liquid aluminum electrolytic capacitors, resulting in capacitors with much lower equivalent series resistance which are suitable for use in high ripple-current applications. The presence of the moisture-sensitive polymer PEDOT introduces concerns on the reliability of polymer aluminum capacitors in high humidity conditions. Highly accelerated stress testing (or HAST) (110ºC, 85% relative humidity) of polymer aluminum capacitors in which the parts were subjected to unbiased HAST conditions for 700 hours was done to understand the design factors that contribute to the susceptibility to degradation of a polymer aluminum electrolytic capacitor exposed to HAST conditions. A large scale study involving capacitors of different electrical ratings (2.5V – 16V, 100µF – 470 µF), mounting types (surface-mount and through-hole) and manufacturers (6 different manufacturers) was done to determine a relationship between package geometry and reliability in high temperature-humidity conditions. A Geometry-Based HAST test in which the part selection limited variations between capacitor samples to geometric differences only was done to analyze the effect of package geometry on humidity-driven degradation more closely. Raman spectroscopy, x-ray imaging, environmental scanning electron microscopy, and destructive analysis of the capacitors after HAST exposure was done to determine the failure mechanisms of polymer aluminum capacitors under high temperature-humidity conditions.Item Characterization of Non-linear Polymer Properties to Predict Process Induced Warpage and Residual Stress of Electronic Packages(2016) Sun, Yong; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nonlinear thermo-mechanical properties of advanced polymers are crucial to accurate prediction of the process induced warpage and residual stress of electronics packages. The Fiber Bragg grating (FBG) sensor based method is advanced and implemented to determine temperature and time dependent nonlinear properties. The FBG sensor is embedded in the center of the cylindrical specimen, which deforms together with the specimen. The strains of the specimen at different loading conditions are monitored by the FBG sensor. Two main sources of the warpage are considered: curing induced warpage and coefficient of thermal expansion (CTE) mismatch induced warpage. The effective chemical shrinkage and the equilibrium modulus are needed for the curing induced warpage prediction. Considering various polymeric materials used in microelectronic packages, unique curing setups and procedures are developed for elastomers (extremely low modulus, medium viscosity, room temperature curing), underfill materials (medium modulus, low viscosity, high temperature curing), and epoxy molding compound (EMC: high modulus, high viscosity, high temperature pressure curing), most notably, (1) zero-constraint mold for elastomers; (2) a two-stage curing procedure for underfill materials and (3) an air-cylinder based novel setup for EMC. For the CTE mismatch induced warpage, the temperature dependent CTE and the comprehensive viscoelastic properties are measured. The cured cylindrical specimen with a FBG sensor embedded in the center is further used for viscoelastic property measurements. A uni-axial compressive loading is applied to the specimen to measure the time dependent Young’s modulus. The test is repeated from room temperature to the reflow temperature to capture the time-temperature dependent Young’s modulus. A separate high pressure system is developed for the bulk modulus measurement. The time temperature dependent bulk modulus is measured at the same temperatures as the Young’s modulus. The master curve of the Young’s modulus and bulk modulus of the EMC is created and a single set of the shift factors is determined from the time temperature superposition. The supplementary experiments are conducted to verify the validity of the assumptions associated with the linear viscoelasticity. The measured time-temperature dependent properties are further verified by a shadow moiré and Twyman/Green test.Item Polymeric Materials for Hemostatic and Surgical Sealant Applications(2015) Behrens, Adam Michael; Kofinas, Peter; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Commercial hemostatic agents and surgical sealants do not meet the current clinical need. The available options suffer from a variety of shortcomings including high costs, short shelf lives, difficult preparations, and concerns over safety. This work aims to utilize synthetic polymers to develop alternative approaches that have the potential to improve outcomes from traumatic injuries and surgeries while minimizing risk and cost. The first aspect of this research focuses on the development of hemostatic hydrogel particles. These spherical hydrogels with a narrow size distribution were synthesized via inverse suspension polymerization. A cationic monomer was utilized in the hydrogel formulation to facilitate rapid swelling, leading to the formation a physical barrier to blood loss. Coagulation studies demonstrated the ability to cause localized aggregation through charge interactions with erythrocytes while reducing clotting activity in the bulk. This mechanism allows the hydrogel to quickly block blood flow and may mitigate thrombotic complications at distal sites. Hemostatic efficacy was exhibited by decreases in both the time to hemostasis and mass of blood loss in rat liver puncture and tail amputation injury models when compared to compression with gauze alone. The second aspect of this research focuses on the development of a synthetic surgical sealant. This work is centered on the investigation of a polymer fiber mat deposition method called solution blow spinning. This fabrication technique allows for the rapid in situ generation of polymer fibers, offering the ability to conformally deposit polymeric materials directly on the surgical site of interest. Solution blow spinning was utilized to deposit a body temperature responsive, biodegradable polymer blend. Above a critical temperature, the two phase fibrous polymer mat transitioned into a one phase polymer film. This transition resulted in plasticization and promoted polymer-substrate interaction, leading to increased adhesion. Sealant efficacy was demonstrated in a cecal intestinal anastomosis mouse model, where the polymer blend was used to supplement sutures. Both burst pressure and survival rate were significantly improved over the suture-only control.Item A Generalized Model for Wall Flame Heat Flux During Upward Flame Spread on Polymers(2015) Korver, Kevin; Stoliarov, Stanislav; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A current model accurately predicts flame to surface heat flux during upward flame spread on PMMA based on a single input parameter, the mass loss rate. In this study, the model was generalized to predict the heat flux for a broad range of polymers by adding the heat of combustion as a second input parameter. Experimental measurements were conducted to determine mass loss rate during upward flame spread and heat of combustion for seven different polymers. Four types of heat of combustion values were compared to determine which generated the most accurate model predictions. The complete heat of combustion yielded the most accurate predictions (± 4 kW/m2 on average) in the generalized model when compared to experimental heat flux measurements collected in this study. Flame heat flux predictions from FDS direct numerical simulations were also compared to the generalized model predictions in an exploratory manner and found to be similar.Item USE OF 3D PRINTED POLY(PROPYLENE FUMARATE) SCAFFOLDS FOR THE DELIVERY OF DYNAMICALLY CULTURED HUMAN MESENCHYMAL STEM CELLS AS A MODEL METHOD TO TREAT BONE DEFECTS(2014) Wang, Martha Elizabeth Ottenberg; Fisher, John P; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This project investigates the use of a tissue engineering approach of an absorbable polymer, poly(propylene fumarate) (PPF) to provide long term mechanical stability while delivering a bioactive material, precultured human mesenchymal stem cells (hMSC) encapsulated in hydrogel, to repair bone defects. Annually over 2.2 million bone grafting procedures are performed worldwide; however, current treatment options are limited for critically sized and load bearing bone defects. Much progress has been made in development of bone tissue replacements within the field of bone tissue engineering. The combination of a polymer scaffold seeded with cells for the eventual replacement by host tissue has shown significant promise. One such polymer is PPF, a synthetic linear polyester. PPF has been shown to be biocompatible, biodegradable and provide sufficient mechanical strength for bone tissue engineering applications. Additionally PPF is able to be photocrosslinked and therefore can be fabricated into specific geometries using advanced three-dimensional (3-D) rapid prototyping. Current technology to culture and differentiate hMSCs into osteoblasts has been enhanced with the development of the tubular perfusion system (TPS). The TPS bioreactor has been shown to enhance osteoblastic differentiation in hMSCs when encapsulated in alginate beads. Although this system is effective in differentiating hMSCs it lacks the sufficient mechanical strength for the treatment of bone defects. Therefore this work suggests a combination strategy of harnessing the ability of the TPS bioreactor to enhance osteoblastic differentiation with the mechanical properties of poly(propylene fumarate) to develop a porous PPF sleeve scaffold for the treatment of bone defects. This is accomplished through four steps. The first step investigates the cytotoxicity of the polymer PPF. Concurrently the second step focuses on designing, fabricating and characterizing PPF scaffolds. The third step investigates the degradation properties of 3D printed porous PPF scaffolds. The fourth step characterizes alginate bead size and composition for use within the PPF sleeve scaffolds. The successful completion of these aims will develop a functional biodegradable bone tissue engineering strategy that utilizes PPF fabricated scaffolds for use with the TPS bioreactor.Item A NEW CLASS OF HYBRID HYDROGELS(2013) Fernandes, Neville Justine; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid hydrogels are a novel way of combining materials with different properties and retaining their individual functionalities within the same composite gel. Here we attempt to demonstrate how this approach can be used to create hydrogels whose morphologies can be altered depending on external stimuli. First we report the creation of hollow hybrid gels which are similar to the previously created solid hybrid gels but have the advantage of a faster and enhanced response to external stimuli. Two stimuli that we have specifically investigated are temperature and solvent composition. We show how to modify the type and extent of response of the gels, i.e. make them shrink or swell, by changing the composition of the polymer as well as the crosslinker within the gel. Thereafter, we also demonstrate how the responses can be manipulated to change the morphology of the hybrid gel itself.Item Microfluidic Production of Polymeric Functional Microparticles(2012) Jiang, Kunqiang; Raghavan, Srinivasa R; DeVoe, Don L; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on applying droplet-based microfluidics to fabricate new classes of polymeric microparticles with customized properties for various applications. The integration of microfluidic techniques with microparticle engineering allows for unprecedented control over particle size, shape, and functional properties. Specifically, three types of microparticles are discussed here: (1) Magnetic and fluorescent chitosan hydrogel microparticles and their in-situ assembly into higher-order microstructures; (2) Polydimethylsiloxane (PDMS) microbeads with phosphorescent properties for oxygen sensing; (3) Macroporous microparticles as biological immunosensors. First, we describe a microfluidic approach to generate monodisperse chitosan hydrogel microparticles that can be further connected in-situ into higher-order microstructures. Microparticles of the biopolymer chitosan are created continuously by contacting an aqueous solution of chitosan at a microfluidic T-junction with a stream of hexadecane containing a nonionic detergent, followed by downstream crosslinking of the generated droplets by a ternary flow of glutaraldehyde. Functional properties of the microparticles can be easily varied by introducing payloads such as magnetic nanoparticles and/or fluorescent dyes into the chitosan solution. We then use these prepared microparticles as "building blocks" and assemble them into high ordered microstructures, i.e. microchains with controlled geometry and flexibility. Next, we describe a new approach to produce monodisperse microbeads of PDMS using microfluidics. Using a flow-focusing configuration, a PDMS precursor solution is dispersed into microdroplets within an aqueous continuous phase. These droplets are collected and thermally cured off-chip into soft, solid microbeads. In addition, our technique allows for direct integration of payloads, such as an oxygen-sensitive porphyrin dye, into the PDMS microbeads. We then show that the resulting dye-bearing beads can function as non-invasive and real-time oxygen micro-sensors. Finally, we report a co-flow microfluidic method to prepare uniform polymer microparticles with macroporous texture, and investigate their application as discrete immunological biosensors for the detection of biological species. The matrix of such microparticles is based on macroporous polymethacrylate polymers configured with tailored pores ranging from hundreds of nanometers to a few microns. Subsequently, we immobilize bioactive antibodies on the particle surface, and demonstrate the immunological performance of these functionalized porous microbeads over a range of antigen concentrations.