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Filtro de difusão anisotrópica anômala como método de melhoramento de imagens de ressonância magnética nuclear ponderada em difusão; Anisotropic anomalous filter as image enhancement method to nuclear magnetic resonance diffusion weighted imaging

Senra Filho, Antonio Carlos da Silva
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 25/07/2013 PT
Relevância na Pesquisa
36.43%
Métodos de suavização através de processos de difusão é frequentemente utilizado como etapa prévia em diferentes procedimentos em imagens. Apesar da difusão anômala ser um processo físico conhecido, ainda não é aplicada à suavização de imagens como a difusão clássica. Esta dissertação propõe e relata a implementação e avaliação de filtros de difusão anômala, tanto isotrópico quanto anisotrópico, como um método de melhoramento em imagens ponderadas em difusão (DWI) e imagens de tensor de difusão (DTI) dentro do imageamento por ressonãncia magnética (MRI). Aqui propõe-se generalizar a difusão anisotrópica e isotrópica com o conceito de difusão anômala em processamento de imagens. Como metodologia implementou-se computacionalmente as equações de difusão bidimensional e aplicou às imagens MRI para avaliar o seu potencial como filtro de melhoramento. Foram utilizadas imagens de ressonância magnética de aquisição DTI em voluntários saudáveis. Os resultados obtidos neste estudo foram a verificação que métodos baseados em difusão anômala melhoram a qualidade em processamento das imagens DTI e DWI quando observadas medidas de qualidade como a relação sinal ruído (SNR) e índice de similaridade estrutural (SSIM)...

Estudos de difusão e trocas iônicas de vanádio, cromo, manganês e ferro no berilo incolor; Studies of diffusion and ionic exchanges of vanadium, chromium, manganese, and iron in colorless beryl

Mittani, Juan Carlos Ramírez
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 05/12/2002 PT
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Neste trabalho a difusão de V, Cr, Mn e Fe por troca iônica, eletrodifusão e implante iônico foi efetuada na amostra natural de berilo incolor (goshenita). As medidas de termoluminescência (TL), ressonância spin paramagnética (EPR) e absorção ótica (AO) foram feitas na amostra natural com a finalidade de comparar as amostras apos a difusão em alguns casos a observação da variação das propriedades medidas. A amostra natural após a irradiação - delta com 10 Gy apresentou 3 picos TL a aproximadamente 150, 250 e 320ºC. Para altas doses o primeiro pico aumentou muito sua intensidade, sobrepondo-se sobre os outros dois picos de temperaturas altas. A amostra natural apresentou sinais EPR referentes aos íons Mn POT.2+ localizados na posição de Al POT.3+ e Fe POT.3+ no canal do cristal. As amostras irradiadas com altas doses de até 10 POT.6 Gy, além dos sinais mencionados, apresentaram novos sinais referentes ao CH IND.3, CO POT.-IND.3, H POT.GRAUS e Fe POT.3+. Este último íon, localizado em um ambiente tetraédrico, tem sinais na região de campos magnéticos baixos. As medidas de obsorção ótica na amostra natural apresentaram bandas muito intensas das moléculas de H IND.2O, situadas no canal, o que indica que os canais dessas amostras estão entupidos destas. A banda típica na região de 820 nm devido ao Fe POT.2+ no canal foi...

Coordinate and time-dependent diffusion dynamics in protein folding

Oliveira, Ronaldo J.; Whitford, Paul C.; Chahine, Jorge; Leite, Vitor B. P.; Wang, Jin
Fonte: Academic Press Inc. Elsevier B.V. Publicador: Academic Press Inc. Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 91-98
ENG
Relevância na Pesquisa
36.46%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); We developed both analytical and simulation methods to explore the diffusion dynamics in protein folding. We found the diffusion as a quantitative measure of escape from local traps along the protein folding funnel with chosen reaction coordinates has two remarkable effects on kinetics. At a fixed coordinate, local escape time depends on the distribution of barriers around it, therefore the diffusion is often time distributed. on the other hand, the environments (local escape barriers) change along the coordinates, therefore diffusion is coordinate dependent. The effects of time-dependent diffusion on folding can lead to non-exponential kinetics and non-Poisson statistics of folding time distribution. The effects of coordinate dependent diffusion on folding can lead to the change of the kinetic barrier height as well as the position of the corresponding transition state and therefore modify the folding kinetic rates as well as the kinetic routes. Our analytical models for folding are based on a generalized Fokker-Planck diffusion equation with diffusion coefficient both dependent on coordinate and time. Our simulation for folding are based on structure-based folding models with a specific fast folding protein CspTm studied experimentally on diffusion and folding with single molecules. The coordinate and time-dependent diffusion are especially important to be considered in fast folding and single molecule studies...

Configuration-dependent diffusion can shift the kinetic transition state and barrier height of protein folding

Chahine, Jorge; Oliveira, Ronaldo J.; Leite, Vitor B. P.; Wang, Jin
Fonte: Natl Acad Sciences Publicador: Natl Acad Sciences
Tipo: Artigo de Revista Científica Formato: 14646-14651
ENG
Relevância na Pesquisa
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We show that diffusion can play an important role in protein-folding kinetics. We explicitly calculate the diffusion coefficient of protein folding in a lattice model. We found that diffusion typically is configuration- or reaction coordinate-dependent. The diffusion coefficient is found to be decreasing with respect to the progression of folding toward the native state, which is caused by the collapse to a compact state constraining the configurational space for exploration. The configuration- or position-dependent diffusion coefficient has a significant contribution to the kinetics in addition to the thermodynamic free-energy barrier. It effectively changes (increases in this case) the kinetic barrier height as well as the position of the corresponding transition state and therefore modifies the folding kinetic rates as well as the kinetic routes. The resulting folding time, by considering both kinetic diffusion and the thermodynamic folding free-energy profile, thus is slower than the estimation from the thermodynamic free-energy barrier with constant diffusion but is consistent with the results from kinetic simulations. The configuration- or coordinate-dependent diffusion is especially important with respect to fast folding, when there is a small or no free-energy barrier and kinetics is controlled by diffusion. Including the configurational dependence will challenge the transition state theory of protein folding. The classical transition state theory will have to be modified to be consistent. The more detailed folding mechanistic studies involving phi value analysis based on the classical transition state theory also will have to be modified quantitatively.

Diffusion and swelling of CO2/light oil mixtures using pressure decay and CT-scan = : Difusão e inchamento de misturas de CO2/óleo leve usando as técnicas de queda de pressão e de tomografia computadorizada; Difusão e inchamento de misturas de CO2/óleo leve usando as técnicas de queda de pressão e de tomografia computadorizada

Susana Vasconcelos Araújo
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 21/02/2014 PT
Relevância na Pesquisa
36.44%
O CO2 utilizado na recuperação avançada em reservatórios de petróleo é um importante agente de injeção devido a suas características de miscibilidade e de inchamento. No que tange a miscibilidade, a difusão molecular é o processo que descreve a mistura natural de fluidos miscíveis, e seu principal parâmetro é o coeficiente de difusão molecular. O objetivo deste estudo é medir o coeficiente de difusão bem como o fator de inchamento do CO2 em óleos leves em diferentes condições experimentais envolvendo pressão, temperatura e composição do óleo. Com intuito de obter o coeficiente de difusão, duas técnicas experimentais foram realizadas em paralelo: uma primeira técnica conhecida como decaimento de pressão e outra chamada de tomografia computadorizada. Esta última, apesar de ainda estar em processo de investigação, permitiu também a obtenção do fator de inchamento nas condições experimentais selecionadas. O óleo estudado é um óleo leve proveniente dos reservatórios do pré-sal no Brasil. Os testes foram efetuados em uma célula para altas pressões especialmente desenvolvida para esses ensaios que envolvem tomografia computadorizada. As pressões e temperatura variaram de 2.76 MPa a 28.96 MPa a 293.15 K e somente 10.34 MPa a 341.15 K. A difusão molecular é considerada um mecanismo chave que controla a miscibilidade entre o óleo e o gás. Sabendo que o coeficiente de difusão determina a taxa de transferência de massa durante o processo...

Oxygen diffusion in pure and doped ZnO

Sabioni,Antônio Claret Soares; Ramos,Marcelo José Ferreira; Ferraz,Wilmar Barbosa
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2003 EN
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Oxygen diffusion coefficients in pure and doped ZnO polycrystals were determined by means of the gas-solid isotope exchange method using the isotope 18O as oxygen tracer. The diffusion experiments were performed from 900 to 1000 °C, under an oxygen pressure of 10(5) Pa. After the diffusion annealings, the 18O diffusion profiles were determined by secondary ion mass spectrometry. The results of the experiments show that oxygen diffusion in Li-doped ZnO is similar to the oxygen diffusion in pure ZnO, while in Al-doped ZnO the oxygen diffusion is enhanced in relation to that observed in pure ZnO, in the same experimental conditions. Based on these results is proposed an interstitial mechanism for oxygen diffusion in ZnO. Moreover, it was found that oxygen grain-boundary diffusion is ca. 3 to 4 orders of magnitude greater than oxygen volume diffusion in pure and doped ZnO, which means that the grain-boundary is a fast path for oxygen diffusion in ZnO.

Oxygen diffusion in Bi2O3-doped ZnO

Sabioni,Antônio Claret Soares; Daniel,Antônio Márcio J.M.; Ferraz,Wilmar Barbosa; Pais,Rafael Witter Dias; Huntz,Anne-Marie; Jomard,François
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2008 EN
Relevância na Pesquisa
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In order to clarify the influence of Bi-doping on oxygen diffusion in ZnO, the bulk and grain boundary oxygen diffusion coefficients were measured in Bi2O3-doped ZnO polycrystals by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The experiments were performed on ZnO sintered samples containing 0.1, 0.3 and 0.5 mol% Bi2O3. The diffusion annealings were performed at 942, 1000 and 1092 °C, in an Ar+18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase in the oxygen diffusion in the Bi2O3-doped ZnO, when compared to the oxygen diffusion in the undoped ZnO polycrystal under the same experimental conditions, both in bulk and in grain-boundaries. Moreover, it was observed that the higher the Bi2O3 concentration, the higher the oxygen diffusion. These results suggest that the incorporation of Bi2O3 increases the interstitial oxygen concentration which agrees with an interstitial diffusion mechanism both in bulk and in grain-boundaries.

Productivity Effects of Information Diffusion in Networks

Aral, Sinan; Brynjolfsson, Erik; Van Alstyne, Marshall
Fonte: Cambridge, MA; Alfred P. Sloan School of Management. Massachusetts Institute of Technology Publicador: Cambridge, MA; Alfred P. Sloan School of Management. Massachusetts Institute of Technology
Tipo: Trabalho em Andamento
EN_US
Relevância na Pesquisa
36.43%
We examine the drivers of diffusion of information through organizations and the effects on performance. In particular, we ask: What predicts the likelihood of an individual becoming aware of a strategic piece of information, or becoming aware of it sooner? Do different types of information exhibit different diffusion patterns, and do different characteristics of social structure, relationships and individuals in turn affect access to different kinds of information? Does better access to information predict an individual’s ability to complete projects or generate revenue? We hypothesize that the dual effects of content and structure jointly predict the diffusion path of information, and ultimately performance. To test our hypotheses, we characterize the social network of a medium sized executive recruiting firm using accounting data on project co-work relationships and ten months of email traffic observed over two five month periods. We identify two distinct types of information diffusing over this network – ‘event news’ and ‘discussion topics’ – by their usage characteristics, and observe several thousand diffusion processes of each type of information from their original first use to their varied recipients over time. We then test the effects of network structure and functional and demographic characteristics of dyadic relationships on the likelihood of receiving each type of information and receiving it more quickly. Our results demonstrate that the diffusion of news...

Dynamics of Information Diffusion

Kim, Minkyoung
Fonte: Universidade Nacional da Austrália Publicador: Universidade Nacional da Austrália
Tipo: Thesis (PhD)
EN
Relevância na Pesquisa
36.5%
Real diffusion networks are complex and dynamic, since underlying social structures are not only far-reaching beyond a single homogeneous system but also frequently changing with the context of diffusion. Thus, studying topic-related diffusion across multiple social systems is important for a better understanding of such realistic situations. Accordingly, this thesis focuses on uncovering topic-related diffusion dynamics across heterogeneous social networks in both model-driven and model-free ways. We first conduct empirical studies for analyzing diffusion phenomena in real world systems, such as new diffusion in social media and knowledge transfer in academic publications. We observe that large diffusion is more likely attributed to interactions between heterogeneous social networks as if they were in the same networks. Thus, external influences from out-of-the-network sources, as observed in previous work, need to be explained with the context of interactions between heterogeneous social networks. This observation motivates our new conceptual framework for cross-population diffusion, which extends the traditional diffusion mechanism to a more flexible and general one. Second, we propose both model-driven and model-free approaches to estimate global trends of information diffusion. Based on our conceptual framework...

Multi-directional diffusion weighted imaging: Implementation, verification and clinical application; Multidirektionale dissusionsgewichtete MR-Bildgebung: Implementierung, Verifizierung und klinische Anwendung

Gorczewski, Kamil Marek
Fonte: Universidade de Tubinga Publicador: Universidade de Tubinga
Tipo: Dissertação
EN
Relevância na Pesquisa
36.49%
A water diffusion in a human brain can be used to analyze fibers within the white matter. The basis of the method is a measurement of a MR signal attenuation due to the water particles motion in a presence of a magnetic field gradient. The signal loss is related to the free path that a particle can travel. The obstacles keep the signal from the water spins coherent. Therefore, the attenuation of MR signal is stronger in the directions of the axons. The information about the directional structure of the white matter can be extracted from the data with various methods. Those approaches can be divided into two groups: a model-based and direct measurements. Both groups have their own advantages and disadvantages. The model-based approach like for example a diffusion tensor imaging (DTI) or a spherical harmonics deconvolution (SHD) can shorten the acquisition time and give high resolution images. However, the additional information from the acquisition have to be provided by some prior knowledge (response function in SHD) or assumptions (diffusion function can be described by a tensor). The results from those methods are biased by the first choice of the input functions. At the other hand, a direct measurement is free from the mentioned above defects...

Sensibilidade in vitro de isolados de Clostridium difficile: comparação de duas metodologias (disco-difusão e ágar-diluição); Susceptibility in vitro of isolates of Clostridium difficile: comparison of two methodologies (disk-diffusion and agar-dilution)

Fraga, Edmir Geraldo de Siqueira
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 16/07/2015 PT
Relevância na Pesquisa
36.43%
Introdução: O Clostridium difficile é um bacilo Gram-positivo, anaeróbio estrito, formador de esporos, que produz toxinas que podem causar diarreia, colite pseudomembranosa, dilatação do cólon, sepse e até morte. Nos últimos anos o quadro clínico e epidemiológico das infecções por Clostridium difficile tem se modificado e as limitações das opções terapêuticas tornaram-se mais evidentes. Objetivo Primário: Comparar as metodologias de disco-difusão e ágar-diluição na detecção de sensibilidade/resistência de isolados de Clostridium difficile. Objetivos Secundários: Avaliar prospectivamente o perfil de sensibilidade/resistência de isolados clínicos hospitalares de Clostridium difficile provenientes de seis hospitais terciários da cidade de São Paulo e fornecer evidências para fundamentar o diagnóstico e o tratamento empírico das diarreias causadas por Clostridium difficile. Métodos: utilizamos os métodos de disco-difusão e ágar-diluição, de acordo com os critérios estabelecidos pelo CLSI e EUCAST. Resultados: Os coeficientes de correlação observados entre os diâmetros dos halos de inibição e Concentração Inibitória Mínima foram abaixo do esperado tornando inviável o método de disco-difusão para determinação de sensibilidade aos antimicrobianos nitazoxanida...

Estudo da difusão entre a fase intermetálica  Υ-TiAl e Fe / Aço; Study of the diffusion between ?-TiAl intermetallic phase and Fe / Steel

Silva, Paulo Ricardo Soares Azevedo e
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 25/08/2015 PT
Relevância na Pesquisa
36.42%
Compostos intermetálicos baseados em Aluminetos de Titânio são considerados promissores para aplicações estruturais em altas temperaturas. Eles apresentaram, principalmente, baixa massa específica (3,2 - 4,2 g/cm3), alto ponto de fusão (1400 - 1500 °C) e alta resistência mecânica entre 600 e 950 °C. O estudo de difusão entre ?-TiAl e materiais ricos em ferro é importante para observar fenômenos e obter dados relevantes a junção desses materiais com o Aço - a liga metálica mais versátil conhecida. Componentes como turbocompressores automotivos serão beneficiados com o desenvolvimento e a aplicação das ligas TiAl, sendo um aspecto importante a junção do rotor de TiAl com o eixo de aço do componente. O presente trabalho foi executado em três etapas: obtenção do intermetálico monofásico ?-TiAl; estudo de pares de difusão ?-TiAl/Fe; e estudo de pares de difusão ?-TiAl/Aço. Ligas com composições Ti50Al50 e Ti47Al53 foram confeccionadas e caracterizadas por microscopia ótica, microscopia eletrônica de varredura e microanálise EDS, difratometria de raios-X e ensaios de dureza. As fases ?-TiAl e ?2-Ti3Al foram observadas em diversas microestruturas de acordo com a composição química e tratamento térmico da amostra. Os parâmetros de rede e as frações volumétricas das fases ?-TiAl e ?2-Ti3Al foram determinados por difração de raios-X através do refinamento pelo método de Rietvelt. A dureza média encontrada nas amostras TiAl foi de 272 a 340 HV500. A liga Ti47Al53 tratada termicamente a 1200 °C por 7 dias apresentou-se monofásica ?-TiAl e foi a escolhida para os estudos de pares de difusão. Caracterização microestrutural por MO e MEV/EDS...

Kombinatorische Wirkstofftestung - Entwicklung eines neuartigen diffusionsgestützten Bioassays und Einfluss von Kinaseinhibitoren auf die T-Zell Signaltransduktion; Combinatorial drug testing - development of a novel diffusion based bioassay and influence of kinase inhibitors on the T-cell signalling pathway.

Ganser, Alexander
Fonte: Universität Tübingen Publicador: Universität Tübingen
Tipo: Dissertation; info:eu-repo/semantics/doctoralThesis
DE_DE
Relevância na Pesquisa
36.45%
In der kombinatorischen Substanztestung steigen, mit gebräuchlichen Mikro-titerplatten-Methoden, die benötigten Pipettierschritte mit der Anzahl der verwendeten Konzentrationen und Substanzen exponentiell an. Die Verminderung von Pipettierschritten in kombinatorisch angelegten Experimenten bedeutet daher eine wichtige methodische Weiterentwicklung. In der klinischen Anwendung von Chemotherapeutika sind empirische Daten immer noch Grundlage für die Behandlungspläne von Krebspatienten. Dabei werden Subpopulationen von Patienten, die weniger oder gar nicht auf eine bestimmte Substanzkombination reagieren, nicht ausreichend berücksichtigt. Der Ansatz der Testmethode, welche in dieser Dissertation vorgestellt wird, mit der individuelles Ansprechen von Patienten auf Wirkstoffe vorhergesagt werden kann, soll helfen die Behandlung von Krebspatienten in der Zukunft zu verbessern. Um die genannten Verbesserungen zu erreichen, wird das Phänomen der Diffusion ausgenutzt, welche in dem dafür neu entwickelten sogenannten "Diffusion Device" Anwendung findet. Durch die orthogonale Anordnung zweier Diffusionsrichtungen wird hierbei die Möglichkeit geschaffen, stufenlos unendlich viele Mischungs-verhältnisse zu erreichen. Im Rahmen dieser Dissertation wurde die Funktionsweise und Anwendung des Diffusion Device optimiert und mit einem zellbasierten Bioassay verbunden. Dazu wurde der Diffusion Device zuerst auf alle für die Diffusion wichtigen Parameter untersucht und validiert. Danach wurden chemotherapeutische Substanzen im Diffusion Device auf Zelllinien und primäre Zellisolate getestet. Bei den dabei implementierten Bioassays handelt es sich um einen annexinbasierten Apoptosenachweis und einen MTT-Zytoxizitätstest. Prioritäten der Arbeit lagen auf der einfachen Handhabung und dem geringen Gebrauch von Resourcen und technischem Equipment. Ebenfalls wichtig war die Möglichkeit der parallelisierten Anwendung der Devices...

Study of the diffusion in polymer solutions and hydrogels by NMR spectroscopy and NMR imaging

Wang, Yu Juan
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
EN
Relevância na Pesquisa
36.45%
Afin d'étudier la diffusion et la libération de molécules de tailles inférieures dans un gel polymère, les coefficients d'auto-diffusion d'une série de polymères en étoile avec un noyau d'acide cholique et quatre branches de poly(éthylène glycol) (PEG) ont été déterminés par spectroscopie RMN à gradient de champ pulsé dans des solutions aqueuses et des gels de poly(alcool vinylique). Les coefficients de diffusion obtenus ont été comparés avec ceux des PEGs linéaires et dendritiques pour étudier l'effet de l'architecture des polymères. Les polymères en étoile amphiphiles ont des profils de diffusion en fonction de la concentration similaires à leurs homologues linéaires dans le régime dilué. Ils diffusent plus lentement dans le régime semi-dilué en raison de leur noyau hydrophobe. Leurs conformations en solution ont été étudiées par des mesures de temps de relaxation spin-réseau T1 du noyau et des branches. L'imagerie RMN a été utilisée pour étudier le gonflement des comprimés polymères et la diffusion dans la matrice polymère. Les comprimés étaient constitués d'amidon à haute teneur en amylose et chargés avec de l'acétaminophène (de 10 à 40% en poids). Le gonflement des comprimés, ainsi que l'absorption et la diffusion de l'eau...

Quantifying diffusion in biofilms : from model hydrogels to living biofilms

Golmohamadi, Mahmood
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
EN
Relevância na Pesquisa
36.53%
Les biofilms sont des communautés de microorganismes incorporés dans une matrice exo-polymérique complexe. Ils sont reconnus pour jouer un rôle important comme barrière de diffusion dans les systèmes environnementaux et la santé humaine, donnant lieu à une résistance accrue aux antibiotiques et aux désinfectants. Comme le transfert de masse dans un biofilm est principalement dû à la diffusion moléculaire, il est primordial de comprendre les principaux paramètres influençant les flux de diffusion. Dans ce travail, nous avons étudié un biofilm de Pseudomonas fluorescens et deux hydrogels modèles (agarose et alginate) pour lesquels l’autodiffusion (mouvement Brownien) et les coefficients de diffusion mutuels ont été quantifiés. La spectroscopie par corrélation de fluorescence a été utilisée pour mesurer les coefficients d'autodiffusion dans une volume confocal de ca. 1 m3 dans les gels ou les biofilms, tandis que les mesures de diffusion mutuelle ont été faites par cellule de diffusion. En outre, la voltamétrie sur microélectrode a été utilisée pour évaluer le potentiel de Donnan des gels afin de déterminer son impact sur la diffusion. Pour l'hydrogel d'agarose, les observations combinées d'une diminution du coefficient d’autodiffusion et de l’augmentation de la diffusion mutuelle pour une force ionique décroissante ont été attribuées au potentiel de Donnan du gel. Des mesures de l'effet Donnan (différence de -30 mV entre des forces ioniques de 10-4 et 10-1 M) et l'accumulation correspondante d’ions dans l'hydrogel (augmentation d’un facteur de 13 par rapport à la solution) ont indiqué que les interactions électrostatiques peuvent fortement influencer le flux de diffusion de cations...

Étude de l’effet d'agents potentiellement perturbateurs de la structure des biofilms sur la diffusion des macromolécules dans les biofilms de Streptococcus mutans : cas de l’EDTA et de l’aspirine

Madoda-Nsiambote, Doudou
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
FR
Relevância na Pesquisa
36.46%
Le but de ce travail de mémoire était d'explorer des moyens pour augmenter la perméabilité des biofilms de Streptococcus mutans aux macromolécules en utilisant des agents potentiellement perturbateurs de la structure des biofilms. L’acide éthylènediamine tétraacétique (EDTA) ainsi que l’acide acétylsalicylique (aspirine) sont les agents perturbateurs choisis. Le changement de perméabilité des biofilms de S. mutans a été déterminé en mesurant les coefficients de diffusion globale du polyéthylène glycol (PEG) et de diffusion locale de dextrans. Les coefficients de diffusion globale ont été mesurés par spectroscopie infrarouge avec un échantillonnage par réflexion totale atténuée (ATR) alors que la spectroscopie par corrélation de fluorescence (SCF) a été utilisée pour la mesure des coefficients de diffusion locale. Les résultats ont démontré que l’incorporation de l’EDTA à une concentration de 7.5 (m/v) % dans la solution de diffusion permet d’améliorer les propriétés de transport du PEG dans les biofilms en augmentant sa pénétrabilité et son coefficient de diffusion globale. Par contre, aucune variation n’a été constatée dans la valeur du coefficient de diffusion locale de dextran fluorescent. Cette différence peut être expliquée...

Action collective patronale : les mécanismes de diffusion entre les niveaux sectoriel et local

Roussel Boudreau, Nathalie
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
FR
Relevância na Pesquisa
36.46%
Cette recherche s'intéresse à l'acteur patronal organisé, encore peu étudié en Amérique du Nord. Pourtant, cet acteur est fortement organisé au Québec et il exerce une influence reconnue sur les politiques publiques et les relations industrielles. Cette recherche vise à mieux comprendre la logique d’action des employeurs et les lieux où ils exercent leur influence. Plus important encore, la recherche s’interroge sur les mécanismes de diffusion utilisés par les associations patronales pour transmettre à leurs membres des orientations et des lignes directrices à adopter. Tout comme pour l’acteur syndical qui doit développer sa capacité représentative (Dufour, Hege, Levesque et Murray, 2009), nous croyons qu’il en est de même pour l’acteur patronal. Bref, cette étude cherche à comprendre comment les associations patronales vont s'assurer que leurs membres adoptent des pratiques en lien avec les positions défendues dans les institutions du marché du travail et dans la sphère des politiques publiques. Notre question de recherche est la suivante : Quels sont les mécanismes développés par les associations patronales pour diffuser leurs orientations en matière de politiques publiques et de relations du travail en vue d’influencer les pratiques locales de gestion de leurs membres? Au plan théorique...

Higher Order Tensor Diffusion Imaging of Complex White Matter Fiber Tracts

Murphy, Nicole E.
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Tese de Doutorado
Publicado em //2012
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Resolving multiple fiber orientations within one imaging voxel is critical for accurate quantification of tissue microstructure and faithful reconstruction of the complex neural fiber pathways. It is recognized that the diffusion process in brain tissues is generally non-Gaussian and recently there have been multiple methods developed specifically to account for this non-Gaussian property. Many of these efforts, however, have been largely focused on improving fiber tracking algorithms rather than providing a quantitative assessment of the deviation from Gaussian diffusion. Consequently, fast and quantitative assessment of this non-Gaussian property is critically needed. Developing a quantitative measurement of the non-Gaussian diffusion processes may provide powerful tools to assess changes in white matter that are due to axonal damage or degeneration. Generalized diffusion tensor imaging (GDTI) with the use of higher order tensors (HOT) provides one such method to identify, characterize and visualize underlying fiber structures. An optimal scan protocol, which estimates the fourth-order diffusion tensor, is needed in order to maximize the efficiency and speed of the data acquisition. This optimization was investigated for a phantom that models crossing white matter fibers and compared to in vivo scans. The minimum number of b-values...

A Search for MRI diffusion coefficient standards

Yuan, Hongmei
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Dissertação
EN_US
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Diffusion coefficients (D) can be readily measured by nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) instruments. Operators of these instruments often utilize standards with known diffusion coefficients to rapidly and conveniently test the performance of the NMR or MRI system to measure diffusion. A variety of these standards have been proposed in the scientific literature. This thesis describes a diffusion standard based on water constrained by container geometry, specifically water between tightly packed, parallel glass fiber filaments. The restricted diffusion of water in this environment gives a diffusion coefficient which is selectable by the choice of data acquisition parameters. Thus, one standard can be used to achieve multiple diffusion coefficients and replaces the need for multiple diffusion standards. Educational training was performed on a 300 MHz NMR spectrometer located at Rochester Institute of Technology (RIT). As a part of this training, pulsed magnetic field gradient strengths were calibrated and diffusion coefficients (D) measured for a series of silicone oils of different viscosities. Diffusion coefficient values for a small diameter test phantom were measured on a 600 MHz NMR spectrometer with stimulated echo pulse sequence at 25°C. A predictable behavior between the apparent diffusion coefficient and gradient separation (δ) value in the sequence was observed. Diffusion coefficient values were measured for a larger diameter phantom using a 1.5 T imager at 20°C using echo-planar imaging sequence and confirmed to follow the same D vs. δ behavior. Based on these observations...

Determination of effective water vapor diffusion coefficient in PEMFC gas diffusion layers

LaManna, Jacob
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
EN_US
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36.47%
Proton exchange membrane fuel cells have emerged as one of the leaders for the replacement of fossil fuel powered internal combustion engines. Water removal from the cell is one of the top concerns regarding fuel cell performance for transportation applications. During lower power output or high temperature operation, water removal in the vapor phase can dominate. The rate of water vapor diffusion through the porous cathode gas diffusion layer (GDL) of the fuel cell is limited by the porosity and tortuosity formed by the solid fiber matrix. In this work an experimental apparatus is designed to measure the rate of water vapor diffusion across the GDL to determine an effective diffusion coefficient. The effects of microporous layer (MPL) coating, GDL thickness, and polytetrafluoroethylene (PTFE) loading on the diffusion coefficient is demonstrated. Commercially available diffusion media are tested and include Mitsubishi Rayon Corp. Grafil U-105 series, SGL Sigracet® 25, 35, and 10 series, and Toray TGP-H-120 series. Standard corrections, such as the Bruggeman correction, used in fuel cell literature are found to overpredict the effective diffusion coefficient for the GDL. The MPL was found to produce a significant resistance to water vapor diffusion due to its smaller pore diameters...