Browsing by Subject "Rheology"

Now showing 1 - 12 of 12
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    Cell Response to Silica Gels with Varying Mechanical Properties
    (2013-07) Lefebvre, Molly
    Sol-gel encapsulation has a variety of applications in biotechnology and medicine: creating biosensors, biocatalysts, and bioartificial organs. However, encapsulated cell viability is a major challenge. Consequently, interactions between cells and their 3D microenvironment were studied through rheological, metabolic activity, and extraction studies to aid in the development of new gel protocols. The cells were encapsulated in variations of three silica sol-gels with varying stiffness. It was hypothesized that the cell viability and the amount of extracted cells would depend on gel stiffness. For two gels, there was no apparent correlation between the gel stiffness and the cell viability and extracted cell quantity. These gels did strongly depend on the varying gel ingredient, polyethylene glycol. The third gel appeared to follow the hypothesized correlation, but it was not statistically significant. Finally, one gel had a significantly longer period of cell viability and higher quantity of extracted cells than the other gels.
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    Characterizing the flow behavior of protein and excipient solutions using Dripping-onto-Substrate extensional rheometry
    (2023-02) Lauser, Kathleen
    Injectable protein medications are lifesaving therapies for patients with cancer, COVID-19, and autoimmune diseases. However developing stable, concentrated protein therapies canbe challenging due to regulations requiring small volumes and viscosities. Accordingly,many therapies are administered intravenously at hospitals, requiring long, expensive stays.Developing ultra-concentrated (>150 mg/mL) medications that can be self-administered subcutaneously can reduce costs and improve flexibility for patients. However ultra-concentrated formulations often suffer from high viscosities and poor stability at rest and under flow. Further, protein medications can denature and lose efficacy when injected due to strong extensional “stretching” forces. These extensional flows can be more detrimental to protein structure and function than shear flows, although thorough studies of protein extensional rheology are limited due to volume constraints and instrumentation challenges. Further, pharmaceutical excipients – molecules like polymers or surfactants which are added to reduce shear viscosity and stabilize formulations – can produce complex flow effects in extension. The first goal of this thesis is to build and validate a novel instrument to measure extensional rheology and associated material properties of protein and protein-excipient solutions for the first time. To do so, a modified dripping-onto-substrate (DoS) extensional rheology device was designed and created. DoS is an extensional rheological technique that creates a semi-stable liquid bridge from a single drop. In time, the liquid bridge self-thins due to inertial, surface tension, viscous and elastic forces. The evolution of the liquid bridge radius is captured with high-speed imaging, which can then be fit to extract rheological parameters. The modified DoS instrument enables measurements of low-viscosity solutions in pure extensional flows in 10 μL or less, overcoming traditional limitations associated with measuring protein solutions in extension. This technique was validated using several test fluids with well-known literature values prior to measuring protein solutions. The second goal was to understand the flow behavior and potential synergistic or antagonistic effects between proteins and pharmaceutical excipients in extensional flows. Model protein ovalbumin (OVA) solutions with added FDA-approved excipients poloxamer 188 (P188), polysorbate 20 (PS20), or polysorbate 80 (PS80) were examined using the DoS technique. OVA is similar in size to insulin, which has previously demonstrated injection-dependant flow behavior. However promisingly, OVA-only solutions up to 300 mg/mL protein exhibited rapid thinning and breakup behavior characteristic of low viscosity fluids. Conversely, excipients typically added to prevent protein aggregation at restor in shear flow appeared to cause detrimental behavior in injection-like flows. P188, a poloxamer that is primarily composed of unimers in solution, demonstrated rapid thinning at low concentrations but transitioned to weakly elastic behavior at higher concentrations. P188 addition was required to observe elasticity in combined P188/OVA conditions since OVA alone did not demonstrate elasticity at any studied concentration. Compared to P188, PS20, and PS80 are smaller molecules and form micelles at the studied conditions. Although PS20 and PS80 are structurally similar, differences in surface activity result in observed flow differences at low concentrations for PS and PS/OVA solutions. However, at higher concentrations, PS20 and PS80 behavior becomes statistically identical due to crowded solution effects. The final goal was to examine the effect of substrate spreading behaviors on capillary-driven thinning, which is not well-explored, particularly for low-viscosity solutions. While capillary-driven thinning progresses in DoS experiments, fluid spreads on the substrate; these differences in spreading or other instrument parameters can lead to variations in the liquid bridge breakup times and rheological parameters. These discrepancies in behavior can be reconciled by correlating to the dimensionless Weber number as well as other fluid and instrument parameters such as aspect ratio or drop volume. Further, computing spreading distances and Weber numbers of spreading can elucidate the importance of Marangoni stresses in capillary thinning experiments for surface-active macromolecules. The results of this thesis demonstrate the first capillary thinning rheological measurements of protein excipient solutions and create a methodology for measuring future protein/excipient combinations. Identifying differences in flow behavior between formulations is important for pharmaceutical development to create more stable therapies in extensional flows. Additionally, an understanding of capillary thinning and spreading dependence can explain variation in DoS experiments and lead to more accurate comparisons of experiment results across samples and concentrations. While this thesis focused on protein-excipient solutions, many of the leanings on methodology are generally relevant to low-viscosity fluids, which can be useful for the broader rheological community.
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    Cocontinuous polymer blends: controlling morphology via interfacial modification and rheology
    (2014-03) Hedegaard, Aaron Thomas
    Cocontinuous polymer blends are formed by melt blending two or more immiscible polymers to form multiple continuous interpenetrated networks. These are non-equilibrium structures where the morphology is determined by a combination of processing conditions, interfacial properties, and rheology. Thermodynamic instability causes the morphology to coarsen during annealing. Furthermore, a thorough understanding of the conditions and mechanism of cocontinuity formation has not been developed, and predictive models are empirical and frequently contradictory. This thesis seeks to advance the field of immiscible polymer blends by providing insight to two critical questions. First, can a better understanding of the role of interfacial stabilization on cocontinuity be developed? Second, can morphological predictions based on rheology be improved? Concerning interfacial stabilization, this thesis approaches the problem via reactive blending and interfacially localized clay nanoparticles. The effectiveness of reactive blending was found to be heavily dependent on the molecular weight of the reactive polymers. Also, the formation of a copolymer brush at the interface was able to prevent coarsening due to a compression of that brush when interfacial area decreased. Nanoclays, when interfacially localized, were found to also prevent coarsening by jamming at the interface, and the combined compatibilization mechanism of reaction and clay was found to achieve the smallest phase sizes. As an application, these compatibilized blends were also tested as gas separation membranes. Concerning the predictions of cocontinuity, various models from the literature were tested against experimental data to determine the center of the compositions that resulted in cocontinuity. It was found that models based on droplet packing worked best, though they gave no information concerning the range of cocontinuous compositions. Various mechanisms and rules of thumb were developed from the present work to provide much-needed insight for predicting the relative size of these ranges. This study also investigated the role of extensional viscosity on cocontinuity by blending with long-chain branched polymers, where it was found that strain hardening branched polymers significantly broadened the range of cocontinuity. This demonstrated a shortcoming in the existing predictive models, which have only considered shear rheology when predicting cocontinuity.
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    Data for DNA fragmentation in a steady shear flow
    (2022-09-23) Qiao, Yiming; Ma, Zixue; Onyango, Clive; Cheng, Xiang; Dorfman, Kevin D; qiao0017@umn.edu; Qiao, Yiming; University of Minnesota Dorfman Research Lab
    We have determined the susceptibility of T4 DNA (166 kilobase pairs, kbp) to fragmentation under steady shear in a cone-and-plate rheometer.
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    Effects of Dough Conditioners on Rheology and Bread Quality of Intermediate Wheatgrass
    (2018-07) Banjade, Jaya
    Studies have shown the detrimental effect of agricultural practices on the environment. One solution to combat those problems would be to focus on alternatives that would lead to sustainable environmental benefits, like cultivating perennial crops. While annual crops are dominating current agricultural production, cultivating perennial crops would contribute to several environmental benefits like reduced nitrogen losses, and soil erosion. With expanding global food insecurity, using perennial crops for food would offer an alternate to diminishing food supply. Among the several perennial crops screened for domestication, intermediate wheatgrass (Thinopyrum intermedium, IWG) has been considered a promising crop to be used as food. The aim of this study was to evaluate IWG of same genetic material, cultivated at two location in Minnesota- Rosemount (RM) and Roseau (RS) for chemical and functional characteristics of dough and breads as affected by refinement (bran removal) and the use of dough conditioners. Five dough conditioners were used- wheat protein isolate, (WPI), vital wheat gluten (VWG), ascorbic acid (AA), Powerbake (a commercial enzyme mix) (PB) and transglutaminase (TG). While IWG kernels were studied for kernel physical properties, IWG flour at three refinement levels - 0 %bran (0B), 50 %bran (50B) and 100 %bran was investigated for proximate composition and dietary fiber following respective standard methods. Dough extensibility and resistance to extension were measured with the texture analyzer equipped with Kieffer extensibility rig, and dough stickiness was measured with a texture analyzer equipped with Chen-Hoseney stickiness cell. Baked breads were evaluated for dimensions, specific volume, crumb firmness, and crumb grain characteristics. Controls consisted of annual hard red winter wheat (W) and IWG dough without conditioners (N). IWG kernels were thinner, with lower weight, volume and bulk density in comparison to wheat. Results from proximate composition indicated an increased fat, protein and ash content with increasing bran concentration, and a decrease in moisture and carbohydratecontents. While there was no difference between IWG and wheat at 0B for moisture and carbohydrate, for the remaining two bran concentrations, wheat had higher moisture and carbohydrate, and lower protein, fat and ash content than IWG. IWG had higher dietary fiber content than wheat at 50 and 100B refinement levels, the difference attributed to insoluble dietary fiber, as no differences was observed in soluble fiber between wheat and IWG at all bran concentrations. At all bran concentrations, extensibility of wheat dough was higher than for the dough made with IWG from both locations. Adding dough conditioners did not improve extensibility for any samples. Some differences were noted between the two locations- 50B-N, 50B and 100B with WPI, 100B with VWG, 50B and 100B with AA, 50B and 100B with PB and 0B and 100B with TG. At 0B and 50B, resistance to extension of wheat dough was higher than for dough made with IWG from both locations, however, for 100B, IWG from RM N, with AA, and PB and IWG from RS N, with WPI, VWG, AA and PB were different. TG increased resistance to extension for IWG from RM at 0B and 50B; for IWG from RS at all refinement levels. At all bran concentrations, stickiness of wheat dough was lower than for dough made with IWG from both locations. Adding PB and TG to 100B IWG from RM reduced stickiness to match values of wheat, however, no such effect was seen in IWG from RS. Adding WPI and VWG reduced stickiness of 0B IWG samples from both locations, in addition, TG also reduced stickiness of 0B RM IWG dough. While no conditioners reduced stickiness of 50B IWG from RS; WPI, VWG, PB and TG reduced stickiness of 50B IWG from RM. While dough conditioners did not reduce stickiness of 100B samples from RM; WPI, VWG and TG reduced stickiness of 100B samples from RS. Bread results indicated a negative effect of bran on dimensions, specific volume and crumb grain characteristics. While WPI, VWG, AA and PB improved or did not change the bread dimensions, TG always reduced them. The effect of dough conditioners was more pronounced for length and width than for height; indicating IWG expanded more than rose. While none of the conditioners increased the specific volume of RS IWG samples at any refinement level, PB increased the volume of 0B IWG from RM. TG decreased the specific volume of all samples. At 0B concentration, controls and breads with WPI and VWG demonstrated collapse when in oven. A noticeable surface smoothing effect was observed for 0B samples with AA and PB. AA and PB improved the crumb grain properties with uniform air cells distribution for 0B samples. Bran negatively affected the air cells count, and adding dough conditioners did not improve the crumb grain characteristics. While there was no effect of TG on 0B samples; cell count, cell area and cell size decreased with TG addition for higher bran contents. The breads were unacceptably dense and the effect was pronounced at higher bran concentrations. This work provides insight on ways to improve functionality and product quality of IWG breads. AA and PB produced loaf of consistent appearance with smoother surface and uniformly distributed gas cells in the crumb. WPI and VWG exhibited expansion before the dough collapsed and thus, the loafs were unable to hold gas. Adding starch or other functional ingredients to increase viscosity would help in retaining the gas, and is thus recommended. This research would facilitate future efforts towards using IWG as a standalone flour for breads, as well as help breeders for markers selections towards developing IWG bread flour.
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    Experimental Investigation of Bio-sealants Used for Pavement Preservation and Development of a New Strength Test for Asphalt Binders at Low Temperature
    (2017-10) Ghosh, Debaroti
    Surface treatment using sealants as a mean of pavement preservation is an important tool for cost-effectively extending service life of pavement. Sealants have become an important tool for cost-effectively extending the service life pavements. Due to the combined negative effects of asphalt aging and thermal cracking, it is always more challenging to choose an appropriate preservation technique for pavements built in cold-regions. Asphalt aging and thermal cracking negatively affect pavements built in cold climates. Therefore, it is important to evaluate the effects of sealants in laboratory conditions before application in the field to ensure effective performance. However, preservation activities cannot effectively address major distresses, such as low-temperature cracking, that can occur when the pavement was built from the very beginning with less durable materials. Therefore, an essential requirement to mitigate low-temperature cracking of pavements for asphalt materials used in the construction of pavement built in cold- regions is ensuring proper fracture properties of the asphalt materials used in construction. This study has two parts. In the first part, a laboratory evaluation of the effects of adding bio-sealants to both asphalt binder and mixture is performed. The goal is to obtain relevant properties of treated asphalt materials to understand the mechanism by which sealants improve pavement performance. For asphalt binders, a dynamic shear rheometer and a bending beam rheometer were used to obtain rheological properties of treated and untreated asphalt binders. For asphalt mixtures, field cores from both untreated and treated sections were collected and thin beam specimens were prepared from the cores to compare the creep and strength properties of the field-treated and laboratory-treated mixture. It is observed that the oil-based sealants have a significant softening effect on the control binder compared to the water-based sealant and traditional emulsion. Oil-based sealants increased rutting and fatigue potential of the binder and helped the low-temperature cracking resistance. For asphalt mixtures, different trends are observed for the field samples compared to the laboratory prepared samples. Similar to binder results, significant differences are observed between the asphalt mixtures treated with oil-based and water-based sealants, respectively. Additional analyses were performed to better understand the sealant effects. Fourier transform infrared spectroscopy (FTIR) analysis showed that the sealant products could not be detected in mixture samples collected from the surface of the treated section. Semi-empirical Hirsch model was able to predict asphalt mixture creep stiffness from binder stiffness. The results of a distress survey of the test sections correlated well with the laboratory findings. In the second part, a news binder strength testing method is proposed with the goal to provide an effective tool for selecting asphalt binders that are crack resistant. A modified Bending Beam Rheometer (BBR) is used to perform three-point bending strength tests, at constant loading rate, on asphalt binder beams at low temperature. Based on the results, a protocol for selecting the most crack resistant material from binders with similar rheological properties is proposed.
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    Graft Polymer Physics
    (2010-05) Haugan, Ingrid
    Graft polymers have polymeric side chains grafted onto a common backbone and exhibit extended conformations due to steric repulsion from densely grafted side chains. The ability to modulate conformation, and thus material performance, has made graft polymers a rich area of research in the last decade. This thesis expands the fundamental understanding of the physical properties of graft polymers in order to aid in the design of novel materials and focuses in three areas: rheology, thermodynamics, and mechanical properties. First, the effect of grafting density on the linear viscoelasticity of graft polymers is investigated. We demonstrate that graft polymers experience the same relaxational modes as linear polymers and their viscoelastic behavior can be described by the same Rouse and reptation theories. The experimental data is compared to scaling models to determine the conformation of the graft polymers, and a new model is proposed to better capture the behavior of experimentally relevant graft polymers. Next, the thermodynamics of densely grafted bottlebrush block polymers is explored. Bottlebrush block polymers were prepared with homopolymer side chains added in blocks along the backbone, varying side chain and backbone length. Their order-disorder transition temperatures were measured by temperature controlled small-angle X-ray scattering. The bottlebrush block polymers display a higher segregation strength compared to linear diblock polymers at the order-disorder transition, indicative of the shielding of the segments near the backbone. The segregation strength at the order-disorder transition decreased with increasing side chain and backbone length. Finally, the mechanical properties of graft polymers with diblock side chains are studied in an attempt to produce tough and sustainable polylactide plastics. The addition of a rubber domain initially toughens the polylactide but the polymers still undergo physical aging and become brittle over time; the time to brittle failure is found to be strongly dependent on the rubber content of the graft block polymers. Pre-straining of the polymers is used to produce stronger and tougher plastics that do not embrittle upon aging.
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    Polymer Solutions in Complex Flows: Fibrils, Filaments, and Flocs
    (2020-08) Metaxas, Athena
    The behavior of polymers in solution under complex physicochemical and hydrodynamic flow fields is of interest to a variety of industrial processes, such as polymer processing and water treatment. In this thesis, two main areas are presented: (1) self-association of polymer chains in flow, resulting in the formation of fibrils and filaments, and (2) association of polymer chains with suspended particulate, resulting in the formation of flocs. In the first area, extensional properties of methylcellulose (MC) solutions were characterized using millimeter scale capillary thinning and micrometer scale filament stretching methods. The addition of NaCl to MC solutions results in self-assembly of a fraction of the MC chains into MC fibrils, which imparts elastic characteristics to the solution. Capillary Breakup Extensional Rheometry (CaBER) studies demonstrate the extensional relaxation time and extensional viscosity increased with increasing MC concentration in the presence of salt. Likewise, microfluidic filament stretching studies demonstrate the extensional viscosity increased with increasing NaCl concentration. By reducing the characteristic length scale of the thinning filament, the microfluidic platform enabled new measurements extensional properties for low molecular weight and low viscosity MC solutions. In the second area, the assembly of charged polymers, or polyelectrolytes, with bentonite clay into flocs was studied in complex flow fields using macroscale Taylor-Couette (TC) flows. A custom-built TC cell allowed for injection of the polyelectrolyte solution into the particle-laden flow to investigate in-situ floc nucleation and growth in varied hydrodynamic flow states. Faster floc growth rates and decreased 2-D perimeter-based fractal dimensions were observed for higher order flow states, indicating improved mass transfer of the polymer flocculant and shear rounding of the flocs, respectively. Additionally, the effects of ionic strength and polyelectrolyte molecular weight on flocculation in the TC cell were investigated. Smaller flocs were formed with increasing ionic strength, due to the role of charge screening on the initial bentonite aggregate size and polyelectrolyte chain persistence length and conformation in solution. Overall, this thesis seeks to provide additional understanding of how polymers assemble in solution under a variety of physicochemical conditions and flow, which can inform predictive processing capabilities and performance.
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    Strategies to Create Electrically Conductive Polymer/Graphene Composites
    (2021-08) Kou, Yangming
    Conductive polymer composites, typically constructed by melt compounding conductive fillers into a polymer matrix, enjoy specialized applications such as electrostatic discharge protection. Graphene nanoplatelets (GNPs) exhibit high inherent electrical conductivity and geometric anisotropy, thus require much lower loading (< 1 wt%) in a polymer matrix to achieve electric percolation while preserving good melt processability. However, due to their relative high cost, it is desirable to further reduce GNP loading while enhancing the polymer/GNP composite electrical conductivity. In this thesis, I demonstrate two formulation strategies to attain conductive polymer composites by controlling GNP localization in cocontinuous polymer blends using both miscible and immiscible systems. For the miscible system, poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) blends are selected. By first compounding PMMA, SAN, and GNP together at lower temperature and then inducing PMMA/SAN spinodal decomposition by heating, I create spatially regular, cocontinuous domains where GNPs preferentially localize within the thermodynamically preferred SAN-rich phase and form conductive networks. I develop a quantitative transmission electron microscopy (TEM) image analysis method to quantify both the polymer domain size and GNP localization. Dielectric measurements show that quiescent annealing improves particle connectivity of the GNP network, leading to further enhancement in electrical conductivity to ~ 10^[-8] S/cm at 1 wt% GNP concentration. For the immiscible system, polylactide/poly(ethylene-co-vinyl acetate) (PLA/EVA) blends are selected. PLA/GNP masterbatches are melt compounded with the EVA homopolymer. Since GNPs preferentially wet the EVA phase, they transfer from PLA to EVA but become kinetically trapped at the interface, as confirmed by electron microscopy. I achieve an ultra-low percolation threshold of 0.048 wt% GNPs and obtain blends with electrical conductivities of ~ 10^[-5] S/cm at 0.5 wt% GNP concentration. Rheology, in-situ dielectric measurements, and TEM imaging after nonlinear shearing and extensional deformations all show that interfacial GNP network remains at the PLA/EVA interface. Moreover, high electrical conductivity is maintained during a wide range of melt compounding times, between 2–10 minutes. In addition to cocontinuous blends, this thesis also addresses practical challenges related to homopolymer-based conductive composites. The effect of electric field-induced conductivity enhancement and dielectric breakdown due to electrical treeing formation within EVA/GNP composites is studied through in-situ measurement of the electrical conductivity. Furthermore, the relationship between shear rheology, filler dispersion, and electrical conductivity of industrially produced conductive polymer composites is studied. These analytical techniques allow for understanding of composite characteristics, enabling industrial partners to quickly determine which conductive fillers are best suited for the construction of conductive polymer composites.
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    Thermomechanics and hydrology of a detachment shear zone.
    (2012-07) Gottardi, Raphaël
    The Raft River-Grouse Creek-Albion metamorphic core complex (NW Utah) is bound to the east by the Miocene Raft River detachment shear zone that is localized in a \textasciitilde 100 m thick quartzite mylonite. By performing combined structural, microstructural, 40Ar/39Ar geochronology, and oxygen and hydrogen stable isotope geochemistry of the well-exposed quartzite mylonite, we are provided with insight on the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complex. Microstructural, electron backscattered diffraction, strain, and vorticity results show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers (Compton, 1980; Wells, 2001; Sullivan 2008). The results fit in a model of "necking" of the shear zone proposed by Wells (2001) and Sullivan (2008). Microstructural evidences (quartz microstructures and deformation lamellae) suggest that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement and eventually seismic failure. Two empirically calibrated paleopiezometers, the quartz recrystallized grain size paleopiezometer of Hirth et al. (2001), and a deformation lamellae spacing based paleopeizometer of Koch and Christie (1981), show very similar results, indicating that the shear zone in question developed under stress ranging from 40 MPa to 60 MPa. Application of a quartzite dislocation creep flow law (Hirth et al., 2001) reveals that the detachment shear zone quartzite mylonite developed at a strain rate between 10E-12 and 10E-14 s-1. We suggest that compressed geothermal gradient (Gottardi et al., 2011), produced by a combination of ductile shearing, heat advection and enhanced cooling by meteoric fluids, can trigger significant mechanical instabilities, and strongly influences the rheology of the detachment shear zone. Combined geochronological and stable isotope data of quartz/muscovite pairs from the quartzite mylonite reveal that ductile deformation, and infiltration of meteoric fluids in the detachment shear zone, occurred between 26 and 20 Ma. 40Ar/39Ar release spectra are complex, but plateau ages decrease systematically from 31.1 +/- 0.8 Ma at the top to 20.2 +/- 0.6 Ma at the bottom of the quartzite mylonite. Throughout the studied area, hydrogen stable isotope values of syn-kinematic muscovite are low, ranging from -123 permil to 88 permil suggesting that meteoric fluids were infiltrating the detachment shear zone over the time scale of mylonite formation. Hydrogen stable isotope analyses from both muscovite and fluid inclusions show that the fluid infiltrating the detachment shear zone was meteoric in origin, with a low D/H and low 18O/16O composition. Quartz and muscovite oxygen isotope analyses show different degree of oxygen isotope depletion, suggesting different time-integrated interaction of the minerals with meteoric fluid; these fluids would be channelized in preferential layers, or shear zones, within the deforming system. The variability in oxygen stable isotope of both quartz and muscovite can be explained by variations in permeability in the basement units (confined versus diffuse flow), and strain variations along the transport direction of detachment shear zone (from flattening to constriction), resulting in different fluid-rock exchange patterns. Based on our geochemical analyses and other published data, we conduct continuum - scale (i.e., large-scale, partial-bounceback) lattice-Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross-section of a metamorphic core complex. The simulations investigate the effects of crustal and fault permeability and buoyancy driven flow on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between fluid and rock. The results show that fluid migration to mid- to lower-crustal levels has to be fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rock. High fault/crust permeability contrasts lead to channelized flow in the fault and shear zones, while lower contrasts allow leakage of the fluids from the fault into the crust. Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show profound oxygen depletion (starting value of &delta18O = +13 permil down to 4 permil concentrated along the faults and shear zone, similar to field data.
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    Transfer and Filling of Non-Newtonian Liquids in Printing and Coating Processes
    (2019-11) Wu, Jyun-Ting
    Non-Newtonian liquids are omnipresent in a variety of industrial settings and natural systems. While complex rheology is commonly encountered for multi-component inks in liquid-based printing and coating applications, much remains to be explored regarding their influence on the flow dynamics. In this thesis we study several model problems to advance our understanding of how non-Newtonian behavior affects (i) transfer of liquid between two surfaces and (ii) filling of liquid into a cavity, which are two important processes in printing and coating techniques and are relevant to numerous technologies. For liquid transfer dominated by the relative vertical motion between two surfaces, we study the stretching of a liquid bridge between either two flat plates or a flat plate and a cavity using finite-element simulations. We find that the influence of rate-dependent rheology primarily occurs near the less-wettable surface for cases of two plates and mainly occurs near the flat plate for cases involving a cavity due to stronger interface deformation there. We further examine the influence of shear thinning and strain hardening on liquid transfer between two flat plates using flow visualizations. We observe that shear thinning can be exploited to significantly enhance liquid transfer from a less-wettable plate to a more-wettable one, and that strain hardening results in a stabilized thin liquid thread but has little effect on the amount of liquid transferred. For liquid transfer dominated by the relative horizontal motion between a flat plate and a cavity, our numerical results suggest that the fraction of liquid left in the cavity collapses onto a master curve with three regimes distinguished by the ratio of the driving forces for flow to the resistance controlling contact-line motion. For Newtonian liquids, we find that the second regime is characterized by a power-law relationship similar to that observed for liquid-film withdrawal. We find that shear thinning improves cavity emptying compared to Newtonian liquids by aiding contact-line motion through reduced viscosities and has larger power-law exponents in the second regime, and that shear thickening leads to the opposite. To study cavity filling we consider liquid confined between a flat plate and a cavity. We find that shear thinning reduces entrapped air compared to Newtonian liquids in general. For flows driven purely by a pressure gradient, shear thinning improves filling by producing a difference of viscosity gradients near the contact line between two surfaces. For flows driven by a pressure gradient and horizontal plate motion, shear thinning benefits filling by enhancing contact-line motion along the cavity.
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    Uniaxial Extensional Behavior of A–B–A Thermoplastic Elastomers: Structure-Properties Relationship and Modeling
    (2015-05) Martinetti, Luca
    At service temperatures, A–B–A thermoplastic elastomers (TPEs) behave similarly to filled (and often entangled) B-rich rubbers since B block ends are anchored on rigid A domains. Therefore, their viscoelastic behavior is largely dictated by chain mobility of the B block rather than by microstructural order. Relating the small- and large-strain response of undiluted A–B–A triblocks to molecular parameters is a prerequisite for designing associated TPE-based systems that can meet the desired linear and nonlinear rheological criteria. This dissertation was aimed at connecting the chemical and topological structure of A–B–A TPEs with their viscoelastic properties, both in the linear and in the nonlinear regime. Since extensional deformations are relevant for the processing and often the end-use applications of thermoplastic elastomers, the behavior was investigated predominantly in uniaxial extension. The conceptual basis of the theories underlying each topical area was explained while the emphasis was kept on fundamental principles and the molecular viewpoint. The analysis herein is independent from the specific choice of the constituent blocks and thus applies to any microphase-segregated thermoplastic elastomer of the A–B–A type. The unperturbed size of polymer coils is one of the most fundamental properties in polymer physics, affecting both the thermodynamics of macromolecules and their viscoelastic properties. Literature results on poly(D,L-lactide) (PLA) unperturbed chain dimensions, plateau modulus, and critical molar mass for entanglement effect in viscosity were reviewed and discussed in the framework of the coil packing model. Self-consistency between experimental estimates of melt chain dimensions and viscoelastic properties was discussed, and the scaling behaviors predicted by the coil packing model were identified. Contrary to the widespread belief that amorphous polylactide must be intrinsically stiff, the coil packing model and accurate experimental measurements undoubtedly support the flexible nature of PLA. The apparent brittleness of PLA in mechanical testing was attributed to a potentially severe physical aging occurring at room temperature and to the limited extensibility of the PLA tube statistical segment. The linear viscoelastic response of A–B–A TPEs was first examined at temperatures where the A domains are glassy. Characteristic length scales and tube model parameters were determined, and the role of the glassy A domains on the entangled rubbery B network was assessed. Thermo-rheological complexity, observed near and below Tg,A, was attributed to augmented motional freedom of the B block ends at the corresponding A/B interfaces, in harmony with the theoretical treatment of thermo-rheological complexity for two-phase materials developed by Fesko and Tschoegl. When the magnitude of the steepness index was taken into account, the shift behavior was analogous to the response measured for pure B melts. Building upon the procedure proposed by Ferry and co-workers for entangled and unfilled polymer melts, a new method was developed to extract the matrix monomeric friction coefficient ζ0 from the linear response behavior of a filled system in the rubber-glass transition region, and to estimate the size of Gaussian submolecules. Stress relaxation beyond the path equilibration time was found qualitatively and quantitatively compatible with dynamically undiluted arm retraction dynamics of entangled dangling structures (originating either from a fraction of triblock chains having one end residing outside A domains or from diblock impurities). By employing tube models and rubber elasticity theories, suitably modified to account for microphase-segregation, the linear elastic behavior across the rubbery plateau and up to the entanglement time was modeled, and a simple analytical expression relating the Langley trapping factor with the fraction of entangled and unentangled dangling structures of the material was obtained. The critical-gel-like behavior typical of A–B–A TPEs at service temperatures approaching Tg,A was analyzed in terms of a power-law distribution of relaxation times derived from the wedge distribution, shown to be equivalent to Chambon–Winter's critical gel model and to the mechanical behavior of a fractional element. A relation between the observed power-law exponent and molecular structure was established. The measured low-frequency response, originating from the incipient glass transition of the A domains, was exploited and extrapolated to lower frequencies via a sequential application of the fractional Maxwell model and the fractional Zener model. With only a few, physically meaningful material parameters a realistic description of the A–B–A self-similar relaxation was obtained over a frequency range much broader than the experimental window and not accessible via time-temperature superposition. The relationship between large-strain response and network structure of A–B–A triblocks was investigated, by examining (1) the effect of linear relaxation mechanisms on the tensile behavior, (2) the sources of elastic and viscoelastic nonlinearities, and (3) the strain rate dependence of the ultimate properties. Because of the numerous typos that appear in the original papers as well as in a recent Macromolecules review, a detailed analysis of the Edwards–Vilgis slip-link model was performed and the main steps leading to the determination of the chemical and topological contributions to the reduced stress were outlined. After establishing an operational definition of initial modulus for critical-gel-like materials subjected to start-up extensional tests, it was possible to determine the relationship between the dimensionless stress in tensile tests at constant strain rate and the step-strain extensional damping function. Based on the molecular picture of the strain-induced structural changes gained from exposing time and strain effects, the governing mechanism of rupture was identified with ductile/fragile rupture of A domains. To the best of our knowledge, this is the first experimental evidence linking the strain rate dependence of ultimate properties of triblock TPEs to the strain-induced glass-rubber transition of the domains. In addition, experimental results on the ultimate properties of A–B–A/B–A blends were consistent with this mechanism of rupture. For the first time in the literature, the complex high-dimensional rheological signature of chewing gum was analyzed, especially in response to nonlinear and unsteady deformations in both shear and extension. A unique rheological fingerprint was obtained that is sufficient to provide a new robust definition of chewing gum that is independent of specific molecular composition.