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Molecular Diffusion (molecular + diffusion)
Selected AbstractsUnified Internal Architecture and Surface Barriers for Molecular Diffusion of Microporous Crystalline Aluminophosphates,ANGEWANDTE CHEMIE, Issue 38 2010Lukasz Karwacki Keine Sterne in ALPO-5: Sternmuster in konfokalen Fluoreszenzmikroskopie(CFM)-Bildern von großen Kristallen mesoporöser AlPO-5-Materialien stammen von Barrieren der Moleküldiffusion im Inneren der Kristalle (siehe Bild) und nicht von sternförmigen Unterkristallen, wie aus Untersuchungen mit CFM, fokussierten Ionenstrahlen, Rückstreuelektronenbeugung und Rasterkraftmikrokopie geschlossen wurde. [source] Motional smearing of electrically recovered couplings measured from multipulse transientsCONCEPTS IN MAGNETIC RESONANCE, Issue 3 2001Scott A. Riley Abstract The measurement of residual dipolar and quadrupolar coupling constants in the liquid phase by using an electric field to destroy the isotropic nature of molecular tumbling is complicated by charge-induced turbulent motion. In many cases this motion is due to charge injection at electrode surfaces, an effect that leads to an apparent removal of electrically recovered anisotropic spectral splittings when measured from a spin-echo envelope modulation produced by a train of radio frequency (rf) pulses. To understand this averaging, the effect of quadrupolar couplings and enhanced molecular diffusion on free-induction, spin-echo, and Carr,Purcell signals is analytically determined in the special case of homogeneous rf pulses. Additional signal damping due to rf inhomogeneity and coupling constant heterogeneity is determined by numerically extending the kernel formalism introduced by Herzog and Hahn to understand spin diffusion in solids. Finally, the merit of the numerical approach is tested by comparison with analytical results for homogeneous rf pulses and experimental results for perdeuterated nitrobenzene involving inhomogeneous rf pulses and coupling heterogeneity. © 2001 John Wiley & Sons, Inc. Concepts Magn Reson 13: 171,189, 2001 [source] pH-Responsive Nanoporous Silica Colloidal MembranesADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Olga Schepelina Abstract Free-standing colloidal membranes (nanofrits) with varied thickness and nanopore size are fabricated and modified with pH-responsive poly(2-(dimethylamino)ethyl methacrylate) brushes. The polymer-modified nanofrits demonstrate excellent gating behavior for molecular diffusion: in the presence of acid, the diffusion rate of positively charged species significantly decreases. Increasing the polymer length and membrane thickness and decreasing the nanopore size leads to the complete acid-controlled gating of the membranes. [source] Small-scale fluid motion mediates growth and nutrient uptake of Selenastrum capricornutumFRESHWATER BIOLOGY, Issue 6 2006T. A. WARNAARS Summary 1. A fluid-flow reactor using submersible speakers was constructed to generate small-scale fluid motion similar to conditions measured in open water environments; flow was quantified by particle image velocimetry. Additionally a Couette-type rotating cylinder was used to generate shear flows; flow was quantified using an optical hotwire probe and torque measurements. Growth rates of the green alga Selenastrum capricornutum were determined from changes in cell counts and viability was tested using the fluorogenic probe fluoresceine diacetate. 2. Evidence that fluid motion directly affects growth rates was obtained as a significant difference between growth in a moving versus non-moving fluid. A near 2-fold increase in growth rate was achieved for an energy dissipation rate of , = 10,7 m2 s,3; a rate common in lakes and oceans. The onset of the viability equilibrium, identified as the day of the test period when the number of active cells equalled non-active cells, was delayed by 2 days for moving fluid conditions as compared with a non-moving fluid. 3. Nutrient uptake was determined by a decrease in the bulk fluid concentration and cellular phosphorus concentration was also estimated. The thickness of the diffusive sublayer surrounding a cell, a zone dominated by molecular diffusion, was estimated. Increasing fluid motion was found to decrease the thickness of this layer. The Sherwood number (ratio of total mass flux to molecular mass flux) showed that advective flux surrounding cells dominated molecular diffusion flux with regard to Péclet numbers (ratio of advective transport to molecular diffusion transport). Fluid motion facilitated uptake rates and resulted in increased growth rates, compared with no-flow conditions. The rate-of-rotation and the rate-of-strain in a moving fluid equally mediated the diffusive sublayer thickness surrounding the cells. Our study demonstrates that small-scale fluid motion mediates algal growth kinetics and therefore should be included in predictive models for algal blooms. [source] Numerical modelling of 3D fluid flow and oxygen isotope exchange in fractured media: spatial distribution of isotope patternsGEOFLUIDS (ELECTRONIC), Issue 4 2007C. SAVARD Abstract An understanding of fluid flow, mass transport and isotopic exchange in fractured rock is required to understand the origin of several geological processes including hydrothermal mineral deposits. The numerical model HydroGeoSphere simulates 3D advection, molecular diffusion, mechanical dispersion and isotopic exchange in a discretely fractured porous media, and can be used to better understand the processes of mass transport and isotopic exchange in fractured rocks. Study of 18O isopleth patterns for different types of fractures and fracture networks with a range of structural complexity and hydraulic properties shows that fracture properties and geometry control mass transport and isotopic exchange. The hydraulic properties, as well as the density, spacing, and connectivity of fractures determine the isotopic patterns. Asymmetries in the geometry of oxygen isotope patterns could be used to determine the direction of hydrothermal fluid flow. [source] Determination of Transverse Dispersion Coefficients from Reactive Plume LengthsGROUND WATER, Issue 2 2006Olaf A. Cirpka With most existing methods, transverse dispersion coefficients are difficult to determine. We present a new, simple, and robust approach based on steady-state transport of a reacting agent, introduced over a certain height into the porous medium of interest. The agent reacts with compounds in the ambient water. In our application, we use an alkaline solution injected into acidic ambient water. Threshold values of pH are visualized by adding standard pH indicators. Since aqueous-phase acid-base reactions can be considered practically instantaneous and the only process leading to mixing of the reactants is transverse dispersion, the length of the plume is controlled by the ratio of transverse dispersion to advection. We use existing closed-form expressions for multidimensional steady-state transport of conservative compounds in order to evaluate the concentration distributions of the reacting compounds. Based on these results, we derive an easy-to-use expression for the length of the reactive plume; it is proportional to the injection height squared, times the velocity, and inversely proportional to the transverse dispersion coefficient. Solving this expression for the transverse dispersion coefficient, we can estimate its value from the length of the alkaline plume. We apply the method to two experimental setups of different dimension. The computed transverse dispersion coefficients are rather small. We conclude that at slow but realistic ground water velocities, the contribution of effective molecular diffusion to transverse dispersion cannot be neglected. This results in plume lengths that increase with increasing velocity. [source] Interpretation of the enhancement of field-scale effective matrix diffusion coefficient in a single fracture using a semi-analytical power series solutionHYDROLOGICAL PROCESSES, Issue 6 2009Tai-Sheng Liou Abstract A power series solution for convergent radial transport in a single fracture (PCRTSF) is developed. Transport processes considered in PCRTSF include advection and hydrodynamic dispersion in the fracture, molecular diffusion in the matrix, diffusive mass exchange across the fracture-matrix interface, and mixing effects in the injection and the extraction boreholes. An analytical solution in terms of a power series in Laplace domain is developed first, which is then numerically inverted by de-Hoog et al.'s algorithm. Four dimensionless parameters determine the behaviour of a breakthrough curve (BTC) calculated by PCRTSF, which are, in the order of decreasing sensitivity, the matrix diffusion factor, two mixing factors, and the Peclet number. The first parameter is lumped from matrix porosity, effective matrix diffusion coefficient, fracture aperture, and retardation factors. Its value increases as the matrix diffusion effect becomes significant. A non-zero matrix diffusion factor results in a , 3/2 slope of the tail of a log,log BTC, a common property for tracer diffusion into an infinite matrix. Both mixing factors have equal effects on BTC characteristics. However, the Peclet number has virtually no effect on BTC tail. PCRTSF is applied to re-analyse two published test results that were obtained from convergent radial tracer tests in a discrete, horizontal fracture in Silurian dolomite. PCRTSF is able to fit the field BTCs better than the original channel model does if a large matrix diffusion coefficient is used. Noticeably, the ratio of field-scale to lab-scale matrix diffusion coefficients can be as large as 378. This enhancement of the field-scale matrix diffusion coefficient may be ascribed to the presence of a degraded zone at the fracture-matrix interface because of karstic effects, or to flow channeling as a result of aperture heterogeneity. Copyright © 2009 John Wiley & Sons, Ltd. [source] Monitoring of glucose permeability in monkey skin in vivo using Optical Coherence TomographyJOURNAL OF BIOPHOTONICS, Issue 1-2 2010Mohamad G. Ghosn Abstract Topical trans-dermal delivery of drugs has proven to be a promising route for treatment of many dermatological diseases. The aim of this study is to monitor and quantify the permeability rate of glucose solutions in rhesus monkey skin noninvasively in vivo as a primate model for drug diffusion. A time-domain Optical Coherence Tomography (OCT) system was used to image the diffusion of glucose in the skin of anesthetized monkeys for which the permeability rate was calculated. From 5 experiments on 4 different monkeys, the permeability for glucose-20% was found to be (4.41 ± 0.28) 10,6 cm/sec. The results suggest that OCT might be utilized for the noninvasive study of molecular diffusion in the multilayered biological tissues in vivo. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Fat Migration in Chocolate: Diffusion or Capillary Flow in a Particulate Solid?,A Hypothesis PaperJOURNAL OF FOOD SCIENCE, Issue 7 2004J. M. Aguilera ABSTRACT: The exact mechanism of fat and oil migration in chocolate and chocolate coatings is still unknown. Nevertheless, the so-called "diffusion equation" derived from Fick's 2nd law has been extensively used to model the phenomenon, giving the impression that molecular diffusion is the single transport mechanism. We propose that chocolate may be microstructurally regarded as a particulate medium formed by an assembly of fat-coated particles (for example, cocoa solids, sugars crystals, and milk powder). Within this matrix the liquid fraction of cocoa fat (which increases with temperature) is likely to move under capillary forces through interparticle passages and connected pores. Based on available evidence (microstructure, kinetic data, temperature dependence of liquid fat fraction, and so on) we demonstrate that capillary forces may have an important role to play in bulk flow of liquid fat and oils. The Lucas-Washburn equation for capillary rise fits available data under most reported experimental conditions. Detailed microstructural analysis in actual products as well as data on key parameters (surface tension, contact angle, viscosity) is necessary to confirm this hypothesis. Bulk flow due to capillary effects, highly disregarded in structured foods, should be considered as a mass transfer mechanism in liquid-filled porous or particulate foods. [source] Functional MR imaging of the female pelvisJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2007Takashi Koyama MD Abstract Recent developments in MR techniques have magnified the roles and potential of MRI in the female pelvis. This article reviews the techniques and clinical applications of functional MRI (fMRI) of the female pelvis, including cine MRI, diffusion-weighted MRI (DWI), and dynamic contrast-enhanced (DCE)-MRI. Cine MRI is a useful tool for evaluating uterine contractility, including sustained contraction and peristalsis, in a variety of conditions and gynecologic disorders, and for evaluating pelvic-floor weakness. DWI can demonstrate abnormal signals in pathologic foci based on differences in molecular diffusion. It also enables the quantitative evaluation of the apparent diffusion coefficient (ADC), which may be useful for distinguishing malignant from benign tissues and monitoring therapeutic outcome. DCE-MRI has the potential to improve tumor detection and local staging, and can also provide quantitative information about perfusion of the tumor, which may be useful for both monitoring therapeutic effects and predicting therapeutic outcome. Understanding the roles played by functional MR techniques in the female pelvic region is beneficial not only for determining clinical applications, but also for developing further investigations with MRI. J. Magn. Reson. Imaging 2007;25:1101,1112. © 2007 Wiley-Liss, Inc. [source] Experimental study of reactive chaotic flows in tubular reactorsAICHE JOURNAL, Issue 8 2005C. Boesinger For many reactive processes, fluid mixing has a significant effect on the rate of a chemical reaction and on the quality of the product. Mixing in a chaotic flow reactor is a promising phenomenon to control and optimize chemical processes. The effect of three-dimensional (3-D) chaotic flow advection on mixing efficiency and on chemical reaction advancement is examined. An experimental comparison is made, for low Reynolds number flows, between two tubular reactors made of successive bends, with the same number of bends (equal to 80) mounted in different configurations : a helical configuration (for regular flow) and a chaotic flow configuration with bends in perpendicular planes. We show that the mixing and the chemical reaction (for an instantaneous bimolecular chemical reaction) are more efficient in the chaotic flow reactor than in the helical reactor. The different effects of chaotic advection, molecular diffusion and reaction are discussed in view of laboratory findings. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source] The apparent dependence of the diffusion coefficient of N-acetylaspartate upon magnetic field strength: evidence of an interaction with NMR methodologyNMR IN BIOMEDICINE, Issue 8 2003David N. Guilfoyle Abstract An inverse relationship between applied magnetic field strength and the apparent diffusion coefficient (ADC) of several important brain metabolites including N -acetyl- l -aspartate (NAA), choline and creatine, measured in vivo using proton magnetic resonance spectroscopy (MRS), has been reported. In this investigation, using phantom studies of NAA at magnetic field strengths of 3 and 7,T, these observations have been verified under controlled MRS conditions in vitro, and the ADC of NAA has been found to vary inversely with magnetic field strength, decreasing at a rate of 2.5%/T at 20°C. We have also assessed whether the effect is a function of a systemic bias in methodology, or if the effect is actually on the rate of molecular diffusion. This was done using an MRS-independent method for measurement of molecular diffusion in NAA phantoms at 0, 0.025 and 7,T applied magnetic field strengths. As a result, it has been demonstrated that the observed apparent magnetic field dependence of the ADC of NAA is a consequence of the NMR measurement and is apparently not a real effect on molecular diffusion. Copyright © 2003 John Wiley & Sons, Ltd. [source] Interfacial adhesion and molecular diffusion in melt lamination of wood sawdust/ebonite NR and EPDMPOLYMER COMPOSITES, Issue 3 2009W. Yamsaengsung Adhesion mechanisms and peel strengths of wood/ebonite NR-EPDM laminates were investigated. Three different chemical coupling agents: namely; N-(, aminoethyl)-,-aminopropyl-triethoxysilane (AAS), 3-methacryloxypropyl trimethoxysilane (ACS), and Bis-(3-triethoxylpropyl) tetrasulfan (Si69) were introduced into the wood/NR composites to enhance an interaction between wood sawdust (SD) particles and NR molecules, and to improve the adhesion strength between the SD/NR and EPDM layers. The quantitative evidences were given to explain the changes in the adhesion or peel strengths of the SD/NR-EPDM laminates through scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDS). The experimental results indicated that the suitable cure time and cure temperature for SD/NR-EPDM melt-laminates were the tc90 of SD/NR composites and 140°C, respectively. The Si69 coupling agent was found to be the most effective coupling agent as compared with AAS and ACS coupling agents. The Si69 of 0.5 wt% was recommended for the optimizations of the tensile modulus of the SD/NR composites and the peel strength of the SD/NR-EPDM laminates. The diffusion level between the SD/NR and EPDM layers could be quantitatively substantiated by determining the sulfur content transfer from the SD/NR layer to the EPDM layer. The diffusion and entanglement of molecular chains from the SD/NR to the EPDM layer initiated the co-crosslinking reaction which played an important role on the changes in the interfacial strength in the SD/NR-EPDM melt-laminates. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers [source] Preparation and reflectance properties of new cholesteric liquid crystalline copolymers containing cholesteryl groupPOLYMER ENGINEERING & SCIENCE, Issue 5 2009Jinbao Guo In this study, a new family of side-chain cholesteric liquid crystal copolymers derived from cholesterol substituents were synthesized. The chemical structures of the copolymers were validated by FTIR and 1H NMR. A wide-band reflective polarizer from the above copolymers with stable optical properties was developed. The pitch gradient was formed by molecular diffusion between adjacent liquid crystal layers based on these cholesteric copolymers with different pitch lengths, and the pitch gradient was further stabilized by UV crosslinking of the polymerizable monomers during the diffusion process. The stability of these systems as a function of time was investigated. The polymer network was validated by scanning electron microscopy and this network played an important role in stabilizing the pitch gradient of the system. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers [source] Wärme- und Feuchtetransport in VakuumisolationspaneelenBAUPHYSIK, Issue 6 2008Andreas Beck Prof. Dr. Wegen ihrer extrem niedrigen Wärmeleitfähigkeit und der damit verbundenen Möglichkeiten, mit geringen Bauteildicken hochwertig zu dämmen, haben Vakuumisolationspaneele (VIP) innerhalb kurzer Zeit Verbreitung im Bauwesen gefunden , insbesondere in Situationen, in denen andere Dämmstoffe aus Platzgründen nicht in Frage kommen. Da noch keine Langzeiterfahrungen über die Alterung der Paneele vorhanden sind, muss beim Bemessungswert der Wärmeleitfähigkeit im Moment allerdings noch mit hohen Sicherheitszuschlägen gerechnet werden, vor allem wegen des über die Nutzungsdauer stattfindenden Gaseintrags. Besonders von Interesse ist hierbei das Verhalten von aus der Umgebung eindiffundierendem Wasserdampf. Während die Auswirkungen von trockenen Gasen genau bezifferbar sind, überlagern sich beim Stoff- und Wärmetransport durch Wasser verschiedene Vorgänge, die die Wärmeleitung im VIP deutlich erhöhen, aber messtechnisch nur schwer erfassbar sind. Daher wurde ein theoretisches Modell entwickelt, das den Wärmestrom auf Grundlage der kinetischen Gastheorie für Molekularströmung und Oberflächendiffusion beschreibt und darüber hinaus Rückschlüsse auf die Diffusionseigenschaften des Kernmaterials zulässt. Heat and Moisture Transport in Vacuum Insulation Panels. Due to their extremely low thermal conductivity, vacuum insulation panels (VIP) allow for high standard thermal insulation with slim building components. Within few years, this led to widespread use especially in building situations where space is limited. As there are no long-term experiences concerning the thermal characteristics of VIPs, however, their rated values of thermal conductivity are notably higher than the actual measured values at present, which primarily represents effects of degradation caused by gases infiltrating the panel. Most significant is the influence of water vapour diffusing into the VIP. Whereas the effects of dry gases are well known and can be exactly quantified, water vapour causes different processes of heat and matter transfer which increase thermal conduction within the VIP considerably, but which cannot be separated accurately by means of measuring. Thus a theoretical model was developed which describes heat flux basing on the kinetic theory of gases for molecular diffusion and surface diffusion. It moreover provides information about the diffusion characteristics of the core material. [source] Perfusion seeding of channeled elastomeric scaffolds with myocytes and endothelial cells for cardiac tissue engineeringBIOTECHNOLOGY PROGRESS, Issue 2 2010Robert Maidhof Abstract The requirements for engineering clinically sized cardiac constructs include medium perfusion (to maintain cell viability throughout the construct volume) and the protection of cardiac myocytes from hydrodynamic shear. To reconcile these conflicting requirements, we proposed the use of porous elastomeric scaffolds with an array of channels providing conduits for medium perfusion, and sized to provide efficient transport of oxygen to the cells, by a combination of convective flow and molecular diffusion over short distances between the channels. In this study, we investigate the conditions for perfusion seeding of channeled constructs with myocytes and endothelial cells without the gel carrier we previously used to lock the cells within the scaffold pores. We first established the flow parameters for perfusion seeding of porous elastomer scaffolds using the C2C12 myoblast line, and determined that a linear perfusion velocity of 1.0 mm/s resulted in seeding efficiency of 87% ± 26% within 2 hours. When applied to seeding of channeled scaffolds with neonatal rat cardiac myocytes, these conditions also resulted in high efficiency (77.2% ± 23.7%) of cell seeding. Uniform spatial cell distributions were obtained when scaffolds were stacked on top of one another in perfusion cartridges, effectively closing off the channels during perfusion seeding. Perfusion seeding of single scaffolds resulted in preferential cell attachment at the channel surfaces, and was employed for seeding scaffolds with rat aortic endothelial cells. We thus propose that these techniques can be utilized to engineer thick and compact cardiac constructs with parallel channels lined with endothelial cells. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010 [source] Mixing in Sub-micron DuctsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2004E.B. Nauman Abstract This paper considers a class of fluidic devices, anticipated to become important in the near future, where characteristic channel dimensions are in the range 0.1 to 1.0 microns. Typical current applications of microfluidics have device sizes of 10 to 100 micron, this is sufficiently small to force laminar flow but not so small that molecular diffusion is a dominant factor. In the smaller devices contemplated here, diffusion is important and existing mixing strategies and correlations are no longer applicable. Novel results and interesting complexities are discussed for reactive, single and two phase flows in sub-micron channels. [source] Bayesian analysis of single-molecule experimental dataJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 3 2005S. C. Kou Summary., Recent advances in experimental technologies allow scientists to follow biochemical processes on a single-molecule basis, which provides much richer information about chemical dynamics than traditional ensemble-averaged experiments but also raises many new statistical challenges. The paper provides the first likelihood-based statistical analysis of the single-molecule fluorescence lifetime experiment designed to probe the conformational dynamics of a single deoxyribonucleic acid (DNA) hairpin molecule. The conformational change is initially treated as a continuous time two-state Markov chain, which is not observable and must be inferred from changes in photon emissions. This model is further complicated by unobserved molecular Brownian diffusions. Beyond the simple two-state model, a competing model that models the energy barrier between the two states of the DNA hairpin as an Ornstein,Uhlenbeck process has been suggested in the literature. We first derive the likelihood function of the simple two-state model and then generalize the method to handle complications such as unobserved molecular diffusions and the fluctuating energy barrier. The data augmentation technique and Markov chain Monte Carlo methods are developed to sample from the posterior distribution desired. The Bayes factor calculation and posterior estimates of relevant parameters indicate that the fluctuating barrier model fits the data better than the simple two-state model. [source] | ||||||||||||||||||||||