Browsing by Subject "Aggregation"
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Item Open Access Air-stable n-channel diketopyrrolopyrrole-diketopyrrolopyrrole oligomers for high performance ambipolar organic transistors(American Chemical Society, 2016) Mukhopadhyay, T.; Puttaraju, B.; Senanayak, S. P.; Sadhanala, A.; Friend, R.; Faber, H. A.; Anthopoulos, T. D.; Salzner, U.; Meyer A.; Patil, S.n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm2 V-1 s-1 with an on/off ratio exceeding 104. Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.Item Open Access The benefits of state aggregation with extreme-point weighting for assemble-to-order systems(Institute for Operations Research and the Management Sciences (INFORMS), 2018) Nadar, Emre; Akçay, A.; Akan, M.; Scheller Wolf, A.We provide a new method for solving a very general model of an assemble-toorder system: multiple products, multiple components that may be demanded in different quantities by different products, batch production, random lead times, and lost sales, modeled as a Markov decision process under the discounted cost criterion. A control policy specifies when a batch of components should be produced and whether an arriving demand for each product should be satisfied. As optimal solutions for our model are computationally intractable for even moderately sized systems, we approximate the optimal cost function by reformulating it on an aggregate state space and restricting each aggregate state to be represented by its extreme original states. Our aggregation drastically reduces the value iteration computational burden. We derive an upper bound on the distance between aggregate and optimal solutions. This guarantees that the value iteration algorithm for the original problem initialized with the aggregate solution converges to the optimal solution. We also establish the optimality of a lattice-dependent base-stock and rationing policy in the aggregate problem when certain product and component characteristics are incorporated into the aggregation/disaggregation schemes. This enables us to further alleviate the value iteration computational burden in the aggregate problem by eliminating suboptimal actions. Leveraging all of our results, we can solve the aggregate problem for systems of up to 22 components, with an average distance of 11.09% from the optimal cost in systems of up to 4 components (for which we could solve the original problem to optimality).Item Open Access Characterization of exact lumpability for vector fields on smooth manifolds(Elsevier, 2016) Horstmeyer, L.; Atay, F. M.We characterize the exact lumpability of smooth vector fields on smooth manifolds. We derive necessary and sufficient conditions for lumpability and express them from four different perspectives, thus simplifying and generalizing various results from the literature that exist for Euclidean spaces. We introduce a partial connection on the pullback bundle that is related to the Bott connection and behaves like a Lie derivative. The lumping conditions are formulated in terms of the differential of the lumping map, its covariant derivative with respect to the connection and their respective kernels. Some examples are discussed to illustrate the theory. © 2016 Published by Elsevier B.V.Item Open Access Componentwise bounds for nearly completely decomposable Markov chains using stochastic comparison and reordering(Elsevier, 2005) Pekergin, N.; Dayar T.; Alparslan, D. N.This paper presents an improved version of a componentwise bounding algorithm for the state probability vector of nearly completely decomposable Markov chains, and on an application it provides the first numerical results with the type of algorithm discussed. The given two-level algorithm uses aggregation and stochastic comparison with the strong stochastic (st) order. In order to improve accuracy, it employs reordering of states and a better componentwise probability bounding algorithm given st upper- and lower-bounding probability vectors. Results in sparse storage show that there are cases in which the given algorithm proves to be useful. © 2004 Elsevier B.V. All rights reserved.Item Open Access Decompositional analysis of Kronecker structured Markov chains(Kent State University, 2008) Bao, Y.; Bozkur, I. N.; Dayar, T.; Sun, X.; Trivedi, K. S.This contribution proposes a decompositional iterative method with low memory requirements for the steadystate analysis ofKronecker structured Markov chains. The Markovian system is formed by a composition of subsystems using the Kronecker sum operator for local transitions and the Kronecker product operator for synchronized transitions. Even though the interactions among subsystems, which are captured by synchronized transitions, need not be weak, numerical experiments indicate that the solver benefits considerably from weak interactions among subsystems, and is to be recommended specifically in this case. © 2008, Kent State University.Item Open Access A handheld microfluidic device for whole blood coagulation measurement using erythrocyte aggregation(Chemical and Biological Microsystems Society, 2020) Işıksaçan, Ziya; Erel, O; Elbüken, ÇağlarWe present a portable platform that enables coagulation time measurement from a drop of whole blood at the point-of-care by optical investigation of erythrocyte aggregation. The assay was demonstrated for the evaluation of both intrinsic and extrinsic pathways of the coagulation cascade.Item Open Access Inflation targeting: an indirect approach to assess the direct impact(Pergamon Press, 2010) Yigit, T. M.It is quite difficult to assess the benefits of inflation targeting (IT) since its immediate effect will be on inflation expectations, an unobserved variable. Due to lack of comprehensive data on inflation expectations, most studies so far concentrated on the impact of IT either on observable variables like output, unemployment, and inflation or compared post-IT surveys of IT countries with non-IT countries. In our study, we focus on a yet unanswered question, i.e., how the expectations change with the adoption of IT. We suggest that heterogeneous inflation expectations lead to long memory in actual inflation, and IT, if successful, should decrease this persistence by concentrating the public's expectations toward the announced target. Empirical results confirm our hypothesis with a reduction in inflation memory after the adoption of IT in almost all eight developed countries in our sample. © 2010 Elsevier Ltd.Item Open Access Lumpability of linear evolution equations in banach spaces(American Institute of Mathematical Sciences, 2017) Atay, F. M.; Roncoroni, L.We analyze the lumpability of linear systems on Banach spaces, namely, the possibility of projecting the dynamics by a linear reduction opera-tor onto a smaller state space in which a self-contained dynamical description exists. We obtain conditions for lumpability of dynamics defined by unbounded operators using the theory of strongly continuous semigroups. We also derive results from the dual space point of view using sun dual theory. Furthermore, we connect the theory of lumping to several results from operator factoriza-tion. We indicate several applications to particular systems, including delay differential equations. © 2017, American Institute of Mathematical Sciences. All rights reserved.Item Open Access A microfluidic erythrocyte sedimentation rate analyzer using rouleaux formation kinetics(Springer Verlag, 2017-03) Isiksacan, Z.; Asghari, M.; Elbuken, C.Red blood cell aggregation is an intrinsic property of red blood cells that form reversible stacked structures, also called rouleaux, under low shear rates. Erythrocyte sedimentation rate (ESR), commonly performed in clinics, is an indirect inflammation screener and a prognostic test for diseases. We have recently developed a microfluidic system for rapid measurement of ESR from 40 µl whole blood employing the aggregation dynamics. In this work, we propose the use of an aggregation inducer, dextran polyglucose, for the preparation of multiple blood samples with differing aggregation dynamics. Using these samples, we characterized the performance of the system with three aggregation indices and under varying experimental conditions. Additionally, using the same underlying principle, we improved the system for ESR measurement using both venipuncture and fingerprick whole blood samples depending on the user needs. The results demonstrate that the system performs equally well with both samples, which validates the compatibility of the system for both laboratory and point-of-care applications where venous and capillary blood are the primary samples, respectively. The detailed characterization presented in this study legitimates the feasibility of the system for ultrafast and facile measurement of ESR in clinics and diverse off-laboratory settings.Item Open Access Silver nitrate/oligo(ethylene oxide) surfactant/mesoporous silica nanocomposite films and monoliths(Academic Press, 2001) Samarskaya, O.; Dag, Ö.A lyotropic, liquid crystalline (LC) phase of a silver nitrate/oligo(ethylene oxide), water, and acid mixture was used for one-pot synthesis of mesoporous silica materials in which Ag+ ions are uniformly distributed. We established that the AgNO3-to-surfactant mole ratio is very important in a 50 wt% surfactant/water system to preserve the hexagonal LC phase before and after the addition of the silica source. Below a 0.6 AgNO3-to-surfactant mole ratio, the mixture is liquid crystalline and serves as a template for silica polymerization. However, between 0.6 and 0.8 AgNO3-to-surfactant mole ratios, one must control the composition of the mixture during the polymerization processes. Above a 0.8 mole ratio, Ag+ ions undergo phase separation from the reaction mixture by complexing with the surfactant molecules. The resulting silica materials obtained from AgNO3/surfactant ratio above 0.8 have anisotropy but without a hexagonal mesophase. Here, we establish a AgNO3 concentration range in which the LC phase is preserved to template the synthesis of mesoporous silica, and we discuss the structural behavior of the mixtures at AgNO3/surfactant mole ratios of 0.00-2.00, using POM, PXRD, FTIR, and UV-Vis absorption spectroscopy. © 2001 Academic Press.Item Open Access Steady-state analysis of Google-like stochastic matrices(Bilkent University, 2007) Noyan, Gökçe NilMany search engines use a two-step process to retrieve from the web pages related to a user’s query. In the first step, traditional text processing is performed to find all pages matching the given query terms. Due to the massive size of the web, this step can result in thousands of retrieved pages. In the second step, many search engines sort the list of retrieved pages according to some ranking criterion to make it manageable for the user. One popular way to create this ranking is to exploit additional information inherent in the web due to its hyperlink structure. One successful and well publicized link-based ranking system is PageRank, the ranking system used by the Google search engine. The dynamically changing matrices reflecting the hyperlink structure of the web and used by Google in ranking pages are not only very large, but they are also sparse, reducible, stochastic matrices with some zero rows. Ranking pages amounts to solving for the steady-state vectors of linear combinations of these matrices with appropriately chosen rank-1 matrices. The most suitable method of choice for this task appears to be the power method. Certain improvements have been obtained using techniques such as quadratic extrapolation and iterative aggregation. In this thesis, we propose iterative methods based on various block partitionings, including those with triangular diagonal blocks obtained using cutsets, for the computation of the steady-state vector of such stochastic matrices. The proposed iterative methods together with power and quadratically extrapolated power methods are coded into a software tool. Experimental results on benchmark matrices show that it is possible to recommend Gauss-Seidel for easier web problems and block Gauss-Seidel with partitionings based on a block upper triangular form in the remaining problems, although it takes about twice as much memory as quadratically extrapolated power method.Item Open Access Stochastic comparison on nearly completely decomposable Markov chains(Bilkent University, 2000) Alparslan, Denizhan N.This thesis presents an improved version of a componentwise bounding algorithm for the steady state probability vector of nearly completely decomposable Markov chains. The given two-level algorithm uses aggregation and stochastic comparison with the strong stochastic (st) order. In order to improve accuracy, it employs reordering of states and a better componentwise probability bounding algorithm given st upper- and lower- bounding probability vectors. A thorough analysis of the algorithm is implemented in sparse storage and its implementation details are given. Numerical results on an application of wireless Asynchronous Transfer Mode network show that there are cases in which the given algorithm proves to be useful in computing bounds on the performance measures of the system. An improvement in the algorithm that must be considered to obtain better bounds on performance measures is also presented at the end.Item Open Access The effect of exosomes released from apheresis platelet concentrates under the impact of gamma irradiation and storage time upon platelet aggregation and hemostasis(Edizioni SIMTI, 2023) Bal, Salih H.; Sağdilek, E.; Karaçay, M.; Kızmaz, Muhammed A.; Kumaş, Levent T.; Can, Fatma E.; Yıldırım, M.; Canavar-Yıldırım, Tuğçe; Koşay-Gülkaya, D.; Yöyen-Ermiş, D.; Budak, F.; Heper, Y.