Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item Dynamics of Elastic Capsules in Cross-Junction and T-Junction Microfluidic Channels(2017) Mputu udipabu, Pompon; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we investigate via numerical computations the dynamicsof elastic capsules (made from a thin strain-hardening elastic membrane) in two microfluidic channels of cross-junction and T-junction geometries. For the cross-junction microfluidic channel, we consider an initially spherical capsule with a size smaller than the cross-section of the square channels comprising the cross-junction, and investigate the effects of the capsule size, flow rate, and lateral flow rates on the transient dynamics and deformation of low-viscosity and equiviscous capsules. In addition, we also study the effects of viscosity ratio on the transient capsule dynamics and deformation. Our investigation shows that the intersecting lateral flows at the cross-junction act like a constriction. Larger capsules, higher flow rates and higher intersecting lateral flows result in stronger hydrodynamic forces that cause a significant capsule deformation, i.e., the capsule’s length increases while its height decreases significantly. The capsule obtains different dynamic shape transitions due to the asymmetric shape of the cross-junction. Larger capsules take more time to pass through the cross-junction owning to the higher flow blocking. As the viscosity ratio decreases, the capsule’s transient deformation increases and tail formation develops transiently, especially for low-viscosity capsules owing to the normal-stress effects of the surrounding fluid on the capsule’s interface. However, the viscosity ratio does not affect much the capsule velocity due to a weak inner circulation. Our findings suggest that the tail formation of low-viscosity capsule may promote membrane breaking and thus drug release of pharmaceutical capsules in the microcirculation. Furthermore, we investigate via numerical computations the motion of an elastic capsule (made from an elastic membrane obeying the strain-hardening Skalak law) flowing inside a microfluidic T-junction device. In particular, we consider the effects of the capsule size, flow rate, lateral flow rate, and fluid viscosity ratio on the motion of the capsule in the T-junction micro-channel. As the capsule’s initial lateral position increases, the capsule moves faster and reaches different final lateral positions. As the capsule size increases, the gap between the capsule’s surface and the channel wall decreases. This results in the development of stronger hydrodynamic forces and a decrease in the capsule velocity due to flow blocking. As the capsule size increases, there is a small lateral migration towards the micro-channel centerline, which is the low-shear region of the T-junction micro-channel. This migration is in agreement with experimental and numerical studies on non-inertial lateral migration of vesicles in bounded Poiseuille flow by Coupier et al. [13] who showed that the combined effects of the walls and of the curvature of the velocity profile induce a lateral migration toward the centerline of the channel. As the capillary number Ca increases, the stronger hydrodynamic forces cause the capsule to extend along the flow direction (i.e., the capsule’s length Lx increases as the capsule enters the T-junctions and decreases as the capsule exits the T-junction). There is a small lateral migration away from the micro-channel centerline as the flow rate Ca increases. The capsule lateral position zc, main-flow velocity Ux and migration velocity Uz are practically not affected by the fluids viscosity ratio λ. As the channel’s lateral flow rate increases, the capsule migrates downwards towards the bottom of the device. Our findings on the lateral migration in the T-junction micro-channel suggest that there is a great potential for designing a T-junction microfluidic device that can be used to manipulate artificial and biological capsules.Item Synthesis of Novel Alkaline Polymer Electrolyte for Alkaline Fuel Cell Applicaitons(2012) Luo, Yanting; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Development of the intrinsically OH- conductive polymeric electrolyte (alkaline polymer electrolyte, APE) is the critical component to enable the wide application of alkaline fuel cell (AFC) technology. Alkaline polymer electrolyte fuel cell (APEFC) based on AFC technology has been revived recently for applications in transportation and portable electronic devices due to its advantages of using non-noble metal catalysts, faster oxygen reduction in alkaline medium, and compact design. The research described in this dissertation aims to synthesize a novel APE, with controlled ionic conductivity and mechanical strength to achieve high fuel cell power density and long durability. Most APEs synthesized up to now use a modification of existing engineering polymer backbones, which are very difficult to balance its mechanical properties with its ionic conductivities. In this research, we copolymerized APE precursor polymers, namely poly (methyl methacrylate-co-butyl acrylate-co vinylbenzyl chloride) (PMBV) from three functional monomers, methyl methacrylate (MMA), butyl acrylate (BA) and vinylbenzyl chloride (VBC), where VBC was the functional group that was attached with trimethylamine (TMA) and was the OH- carrier after ion-exchanging. MMA was used for mechanical support and BA was used to alleviate the brittleness coming from MMA and VBC. We synthesized alkaline polymer electrolytes from bottom-up polymerization of these selected functional monomers using free radical solution and miniemulsion copolymerization techniques. By miniemulsion copolymerization, the properties of the obtained APEs could be precisely controlled by tuning the (1) monomer ratio, (2) glass transition temperature (Tg), (3) molecular weight (MW), and (4) crosslinking the copolymer. The increase in Tg was realized by eliminating BA from monomers, which was a low Tg component. MW was optimized through investigating binary copolymerization kinetics factors (initiator and surfactant). For crosslinking, the newly obtained poly (methyl methacrylate-co-vinylbenzyl chloride) (PMV) was crosslinked as a semi-interpenetrating network (s-IPN) to reduce water uptake and thus enhanced the mechanical strength in a humidified environment for APEFCs. After the optimization, our best quaternized PMBV (QPMBV) series APE membranes could reach a maximum power density of 180 mW/cm2 and the crosslinked QPMV APE could last 420 hours on APEFCs, which was among the best overall performance in APE technologies. In the future, we propose to use fluorinated polymer monomers to redesign the polymer backbone. Another direction in the design of APEs is to reselect the possible functional OH- carrier groups to make APEs more chemically and mechanically stable in a high pH environment. And last but not least, atomic force spectroscopy (AFM) is proposed to observe the APE nanostructure, the ionic conductive path, and the local mechanical strength by applying a small voltage between the tip and stage.