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Non-Destructive Evaluation of Material System Using Highly Nonlinear Acoustic Waves
Get PDFA chain of granular particles is one of the most studied examples of highly nonlinear systems deriving its response from the nonlinear Hertzian contact interaction between particles. Interest in these systems derives from their tunable dynamic response, encompassing linear, weakly nonlinear, and strongly nonlinear regimes, controlled by varying the static and dynamic load applied. In chains with a very weak (or zero) static precompression, the system supports the formation and propagation of highly nonlinear solitary waves (HNSWs). The dual-nonlinear interaction between particles (i.e., a power-law type contact potential in compression, and zero strength in tension) combined with discreteness of the system, makes the granular system highly tunable. The propagation properties of these waves, such as traveling pulse width, wave speed, number of separated pulses (single or train of pulses), etc., can be controlled by modifying one or many of the parameters, like the particle's dimension, material properties, static and dynamic force amplitude, the type and duration of the initial excitation applied to the system, and/or the periodicity of the chain. The ability to control the wave properties in such chains has been proposed for several different practical engineering applications.
The dynamic properties of these granular chains have been conventionally studied using discrete particle models (DPMs) which consider the particles in the chains as point masses connected by nonlinear Hertzian springs with the neighboring particles. Although, this is a good approximation under proper circumstances, it does not capture many features of the three dimensional elastic particles such as the elastic wave propagation within the particles, the local deformation of the particles in the vicinity of the contact point, the corresponding changes in the contact area, and the collective vibrations of the particles among others. This thesis focuses on the development of a nite element model (FEM)using the commercially available software Abaqus, which takes into account many of these characteristic features. The nite element model discretizes particles by considering them as three-dimensional deformable bodies of revolution and describes the nonlinear dynamic response of one-dimensional granular chains composed of particles with various geometries and orientations. We showed that particles' geometries and orientations provide additional design parameters for controlling the dynamic response of the system, compared to chains composed of spherical particles. We also showed that the tunable and compact nature of these waves can be used to tailor the properties of HNSWs for specfic application, such as information carriers for actuation and sensing of mechanical properties and boundary effects of adjoining media in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM). Using experiments and numerics, we characterized interface dynamics between granular media and adjoining linear elastic media, and found that the coupling produced temporary localization of the incident waves at the boundaries between the two media and their decomposition into reflected waves. We monitored the formation of reflected solitary waves propagating back from the interface and found that their properties are sensitive to the geometric and material properties of the adjoining media. The work done in this research enhances our understanding of the basic physics and tunability of nonlinear granular media, and further establishes a theoretical and numerical foundation
in the applications of HNSWs as information carriers.</p
MEMS for Glaucoma
Get PDFGlaucoma is an eye disease that gradually steals vision. Open angle glaucoma is one of the most common glaucoma forms, in which eye fluid (aqueous humor) produced by the ciliary body cannot be drained away normally by patients’ eyes. The accumulated eye fluid inside the anterior chamber causes high intraocular pressure (IOP), which is transmitted onto the retina in the back of the eyeball (globe), continuously suppressing and damaging the patient’s optic nerves; this may lead to total blindness if not treated properly.
The current most-popular IOP monitoring technique is to use applanation tonometry, which applies applanation force onto the cornea and measures the resulting deformation in order to calculate the IOP. Even though applanation tonometry can provide quite useful information about patients’ IOP, continuous monitoring of IOP is required for ophthalmologists to understand the IOP fluctuation of the patients, something which still cannot be achieved via current applanation approach. In addition, applanation tonometry requires skillful operation performed by well-trained professionals, such as ophthalmologists, making continuous IOP monitoring impractical. In this work, we have developed a telemetric IOP sensor that is capable of monitoring IOP wirelessly and continuously. As the quality factor drops when a telemetric IOP sensor is implanted in the anterior chamber, due to the high loss tangent of the saline-based aqueous humor (~ 0.2) compared to air (0.0), a modified IOP sensor is developed to monitor IOP with sensing coil that is left exposed after implantation in order to avoid interruption from the eye fluid. Another approach is also proposed and tested to demonstrate that the quality factor can also be recovered by covering the sensing coil with low loss tangent materials.
Currently glaucoma is treated mostly by taking oral medications or applying eye drops. However, some glaucoma patients do not respond to those medications. Therefore, another physical approach, using a glaucoma drainage device (GDD), is necessary in order to drain out excessive eye fluid and serve as a long-term way to manage the increased IOP. Current commercially available glaucoma drainage devices do not have reliable valve systems to stop the drainage when the IOP falls into the normal range. Therefore, we have developed a dual-valved GDD to fulfill the “band-pass” flow regulation which drains out eye fluid only when IOP is higher than 20 mmHg, and stops drainage (closes the valve) when IOP is lower than 20 mmHg to prevent hypotony. The key component of GDD is a normally closed (NC) check-valve, which only opens to drain away the excess fluid when the pressure is higher than 20 mmHg. The proposed paradigm of our NC check-valve is to have a couple of parylene-C pre-stressed slanted tethers to provide the desired cracking pressure. The slanted tethers are achieved in this thesis by: 1) slanted photoresist generated by gray-scale photolithography, 2) pop-up mechanism, and 3) self-stiction bonding mechanism. The built-in residual tensile stress can be controlled by mechanical stretching or thermal annealing. The protecting mechanism preventing the unwanted drainage when the eyes experience sudden unpredicted high IOP is achieved by utilizing a normally open (NO) check-valve. A "minimally invasive implantation" procedure is proposed in the thesis to implant the GDD subconjunctivally. The small size of the device allows its insertion using a #19-gauge needle.
To accurately design the desired cracking pressure and also predict the lifetime of the NC check-valve, parylene-C’s mechanical, thermal, and polymer properties are investigated. The results show that the properties of parylene-C are highly process-temperature-dependent and therefore can be tailored by adjusting the thermal annealing process.</p
Atomistic Simulations of Materials: Methods for Accurate Potentials and Realistic Time-Scales
Get PDFThis thesis deals with achieving more realistic atomistic simulations of materials, by developing accurate and robust force-fields, and algorithms for practical time scales.
I develop a formalism for generating interatomic potentials for simulating atomistic phenomena occurring at energy scales ranging from lattice vibrations to crystal defects to high-energy collisions. This is done by fitting against an extensive database of ab initio results, as well as to experimental measurements for mixed oxide nuclear fuels. The applicability of these interactions to a variety of mixed environments beyond the fitting domain is also assessed. The employed formalism makes these potentials applicable across all interatomic distances without the need for any ambiguous splining to the well-established short-range Ziegler-Biersack-Littmark universal pair potential. We expect these to be reliable potentials for carrying out damage simulations (and molecular dynamics simulations in general) in nuclear fuels of varying compositions for all relevant atomic collision energies.
A hybrid stochastic and deterministic algorithm is proposed that while maintaining fully atomistic resolution, allows one to achieve milliseconds and longer time scales for several thousands of atoms. The method exploits the rare event nature of the dynamics like other such methods, but goes beyond them by (i) not having to pick a scheme for biasing the energy landscape, (ii) providing control on the accuracy of the boosted time scale, (iii) not assuming any harmonic transition state theory (HTST), and (iv) not having to identify collective coordinates or interesting degrees of freedom. The method is validated by calculating diffusion constants for vacancy-mediated diffusion in iron metal at low temperatures, and comparing against brute-force high temperature molecular dynamics. We also calculate diffusion constants for vacancy diffusion in tantalum metal, where we compare against low-temperature HTST as well. The robustness of the algorithm with respect to the only free parameter it involves is ascertained.
The method is then applied to perform tensile tests on gold nanopillars on strain rates as low as 100/s, bringing out the perils of high strain-rate molecular dynamics calculations. We also calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we reach the fraction-of-a-second time scale regime. It is found that the activation free energy depends significantly and nonlinearly on the driving force (stress or strain) and temperature, leading to very high activation entropies for surface dislocation nucleation.</p
Localization and Dualities in Three Dimensional Superconformal Field Theories
Get PDFIn this thesis we apply the technique of localization to three-dimensional N=2 superconformal field theories. We consider both theories which are exactly superconformal, and those which are believed to flow to nontrivial superconformal fixed points, for which we consider implicitly these fixed points. We find that in such theories, the partition function and certain supersymmetric observables, such as Wilson loops, can be computed exactly by a matrix model. This matrix model consists of an integral over g, the Lie algebra of the gauge group of the theory, of a certain product of 1-loop factors and classical contributions. One can also consider a space of supersymmetric deformations of the partition function corresponding to the set of abelian global symmetries.
In the second part of the thesis we apply these results to test dualities. We start with the case of ABJM theory, which is dual to M-theory on an asymptotically AdS_4xS^7 background. We extract strong coupling results in the field theory, which can be compared to semiclassical, weak coupling results in the gravity theory, and a nontrivial agreement is found. We also consider several classes of dualities between two three-dimensional field theories, namely, 3D mirror symmetry, Aharony duality, and Giveon-Kutasov duality. Here the dualities are typically between the IR limits of two Yang-Mills theories, which are strongly coupled in three dimensions since Yang-Mills theory is asymptotically free here. Thus the comparison is again very nontrivial, and relies on the exactness of the localization computation. We also compare the deformed partition functions, which tests the mapping of global symmetries of the dual theories. Finally, we discuss some recent progress in the understanding of general three-dimensional theories in the form of the F-theorem, a conjectured analogy to the a-theorem in four dimensions and c-theorem in two dimensions, which is closely related to the localization computation.</p
Elliptic Combinatorics and Markov Processes
Get PDFWe present combinatorial and probabilistic interpretations of recent results in the theory of elliptic special functions (due to, among many others, Frenkel, Turaev, Spiridonov, and Zhedanov in the case of univariate functions, and Rains in the multivariate case). We focus on elliptically distributed random lozenge tilings of the hexagon which we analyze from several perspectives. We compute the N-point function for the associated process, and show the process as a whole is determinantal with correlation kernel given by elliptic biorthogonal functions. We furthermore compute transition probabilities for the Markov processes involved and show they come from the multivariate elliptic difference operators of Rains. Properties of difference operators yield an efficient sampling algorithm for such random lozenge tilings. Simulations of said algorithm lead to new arctic circle behavior. Finally we introduce elliptic Schur processes on bounded partitions analogous to the Schur process of Reshetikhin and Okounkov (
and to the Macdonald processes of Vuletic, Borodin, and Corwin). These give a somewhat different (and faster) sampling algorithm from these elliptic distributions, but in principle should encompass more than just tilings of a hexagon
Non-Invasive in vivo Molecular Imaging of Cancer Nanotherapy Uptake and Response with PET/MRI
Get PDFAlthough researchers have made great strides toward understanding the biological processes underlying cancer pathology, this has not led to major improvements in the management of the disease. Development of new treatments to combat cancer remains imperative. Nanosized therapies show promise to improve tumor treatment response by localizing therapy while reducing treatment-related toxicity. Understanding how nanotherapies are taken up and cause their effects at the intact tumor level in vivo will complement ex vivo histological and in vitro biochemical studies and facilitate the translation of nanotherapy treatments to the clinic. Currently, few in vivo methods exist to study nanotherapy uptake and response at the intact tumor scale. Magnetic resonance imaging (MRI) and positron emission tomography (PET) are imaging methods that provide different but complementary information about the tumor microenvironment and nanotherapy uptake/response. Direct spatiotemporal correlation of PET and MRI data via their simultaneous acquisition has the potential to be powerfully synergistic, especially for the study of physiological processes that are time sensitive or where good spatial coregistration of the multimodal data is important. As the field of hybrid PET/MRI is still in its infancy, with only a handful of active systems worldwide, it is vital that continued PET/MRI technology development be pursued to realize its full potential.
The objective of this thesis is to develop noninvasive, multimodal PET/MRI methods to study the uptake and response of cancer nanotherapies. Three studies were pursued toward this goal. First, we describe the development of a quantitative, small animal simultaneous PET/MRI system that is capable of dynamic, intratumoral imaging. The results show that the system provides quantitative images that are highly correlated with ex vivo autoradiography. The system was able to follow the uptake of a radiolabelled antibody inside the tumor over time, visualizing antibody movement from the vascular space to the tumor mass. Second, we adapted a functional MRI technique, diffusion MRI, to monitor treatment response of the cancer nanotherapy CRLX101. CRLX101-treated animals showed a significant diffusion MRI response within 2 days of treatment, before significant size changes were observed. Modeling of the diffusion MRI data was able to predict the potent antiproliferative effect of CRLX101, commensurate with histological data. Finally, we developed MRI and PET/MRI methods to study the tumor response to the tumor-penetrating peptide iRGD, which has shown good potential to improve cancer nanotherapy uptake. The results show that iRGD can have a variable tumor response, which may be dependent on the tumor microenvironment.
The primary contributions of this thesis work is the development of small animal hybrid PET/MRI technology to enable multimodal intratumoral studies and the development of clinically-applicable imaging methods to monitor the uptake and response of cancer nanotherapies.</p
Engineering of Dengue Virus Refractoriness in Aedes aegypti and Development of an Underdominant Gene Drive System in Drosophila melanogaster
Get PDFVector-borne diseases have a profound impact on world health. The two most well-known and costly diseases are dengue fever and malaria, both spread by mosquito vectors. In the last decade, many new solutions to halting the spread of these diseases have been sought, including vector-mediated disease suppression. The work presented here seeks to generate alleles to effect this suppression, and engineer a drive system to replace the native population. Additional work on systems to keep engineered organisms genetically isolated from native populations has also been carried out. Initial studies in C. elegans investigated use of the transitive nature of RNAi in this species to genetically isolate one population from another. This type of speciation could be used in plant populations to limit gene flow of engineered crops into local environments.
The next series of studies details work on engineering of refractoriness alleles. Dengue virus has several enzymatic activities that are essential for its replicative cycle, including an RNA-dependednt RNA polymerase (RdRp) responsible for synthesizing both the sense and antisense viral genomes and a protease responsible for several essential cleavages of the viral polyprotein. Artificial substrates for these proteins were created to act as sensors, triggering an apoptotic response when viral infection occurs. Several generations of constructs were tested, but so far no completely functional sensor has been generated.
Lastly, a series of underdominant gene drive architectures were built and tested in Drosophila melanogaster. Initial systems utilized a Drosophila cell death protein, Hid, as toxin, and engineered microRNAs designed to target the Hid proteins as antidote. Two toxin-antidote pairs were mismatched and positioned on separate chromosomes so that an organism carrying both chromosomes survives, but an organism carrying only a single chromosome is unviable. Construction of a proof-of-principle in the eye was successful, but work in essential tissues is ongoing. Systems using engineered microRNAs as toxins and resupply of the native protein as antidote were tested in essential tissues. Testing of many components has contributed to the development of these systems, but a complete system has not yet been constructed.</p
Graphene as a Platform for Novel Nanoelectronic Devices
Get PDFGraphene's superlative electrical and mechanical properties, combined with its compatibility with existing planar silicon-based technology, make it an attractive platform for novel nanoelectronic devices. The development of two such devices is reported—a nonvolatile memory element exploiting the nanoscale graphene edge and a field-effect transistor using graphene for both the conducting channel and, in oxidized form, the gate dielectric. These experiments were enabled by custom software written to fully utilize both instrument-based and computer-based data acquisition hardware and provide a simple measurement automation system.
Graphene break junctions were studied and found to exhibit switching behavior in response to an electric field. This switching allows the devices to act as nonvolatile memory elements which have demonstrated thousands of writing cycles and long retention times. A model for device operation is proposed based on the formation and breaking of carbon-atom chains that bridge the junctions. Information storage was demonstrated using the concept of rank coding, in which information is stored in the relative conductance of multiple graphene switches in a memory cell.
The high mobility and two dimensional nature of graphene make it an attractive material for field-effect transistors. Another ultrathin layered material—graphene's insulating analogue, graphite oxide—was studied as an alternative to bulk gate dielectric materials such as Al2O3 or HfO2. Transistors were fabricated comprising single or bilayer graphene channels, graphite oxide gate insulators, and metal top-gates. Electron transport measurements reveal minimal leakage through the graphite oxide at room temperature. Its breakdown electric field was found to be comparable to SiO2, typically 1–3 × 108 V/m, while its dielectric constant is slightly higher, κ ≈ 4.3.
As nanoelectronics experiments and their associated instrumentation continue to grow in complexity the need for powerful data acquisition software has only increased. This role has traditionally been filled by semiconductor parameter analyzers or desktop computers running LabVIEW. Mezurit 2 represents a hybrid approach, providing basic virtual instruments which can be controlled in concert through a comprehensive scripting interface. Each virtual instrument's model of operation is described and an architectural overview is provided.</p
Computational Biases in Decision-Making
Get PDFNeuroeconomics has produced a number of insights into economics, psychology, and neuroscience in its relatively short existence. Here, I show how neuroeconomics can inform these fields through three studies in social decision making and decision making under risk. Specifically, I focus on computational biases inherent in our daily decisions.
First, using functional magnetic resonance imaging (fMRI), I examine how we make decisions for others compared to ourselves. I find that overlapping areas of the ventromedial prefrontal cortex (vmPFC) are involved in both types of decisions, though decisions for others are modulated by areas involved in social cognition. Specifically, activity in the inferior parietal lobule (IPL) encodes a variable measuring the distance between others’ and our own preferences, suggesting that we may anchor our choices for others on our own preferences and attempt to modulate these preferences with what we know about others.
Second, I investigate how visual looking patterns can critically influence the computation and comparison of values. In a first study using eye-tracking, I investigate the relationship between loss aversion and attention and find a correlation between how loss averse subjects are and how long they look at losses vs. gains when evaluating mixed gambles. Importantly, I show that this effect is not due to subjects simply looking longer at items of higher value. In a second study using Mouselab, I show how attention influences multi-attribute choice. I find that the display of different attributes has a significant effect on search among those attributes and, ultimately, choice.</p
A New Approach to the Study of Political Participation
Get PDFThis thesis develops and applies a new theoretical and empirical approach to the study of political participation. It starts with a thorough review of existing theories and evidence of the determinants of political participation. The theories discussed in the literature review are subsequently synthesized using a dual-process account of participation decisions. The central premise of the account is that individuals engage in reasonable deliberation regarding the benefits and costs of participating in political activities, but this reasoning is limited by automatic responses to values, social identifications, and habits that predispose the individual toward participation or abstention. The next chapter develops a new statistical procedure for the study of political participation, based on mixture modeling and simultaneous consideration of involvement in multiple political activities that is consistent with the dual-process account of political participation discussed in the previous chapter. In this model the relationship between underlying utilities and participation probabilities is regulated by a parameter that captures individual propensities toward political participation. After that, the statistical method is applied to survey data from the 1990 American Citizen Participation Study (Verba et al., 1995), where it is used to test a series of hypotheses regarding the impact of resources and civic skills on political participation. Finally, the mixture modeling approach is applied to survey data from the 2008 Cooperative Congressional Elections Study (Ansolabehere, 2008), where it is used to measure the impact of perceptions of economic adversity and support for emergency economic policies on political participation