Home About us Contact
|
Home About us Contact | ||
|
|
||
Diffusion Rate (diffusion + rate)
Selected AbstractsDeterminants of Intercountry Internet Diffusion RatesAMERICAN JOURNAL OF ECONOMICS AND SOCIOLOGY, Issue 2 2009Phanindra V. Wunnava This article employs cross-sectional data from 100 countries to analyze the main determinants of intercountry Internet diffusion rates. We set up an empirical model based on strong theoretical foundations, in which we regress Internet usage on variables that capture social, economic, and political differences between these countries. Our results support past findings that economic strength, infrastructure, and knowledge of the English language positively affect Internet connectivity. In addition to these indicators, the openness of a country, tertiary enrollment, and income equality are found to also have a significant positive effect on Internet diffusion. [source] Diffusion models for animals in complex landscapes: incorporating heterogeneity among substrates, individuals and edge behavioursJOURNAL OF ANIMAL ECOLOGY, Issue 5 2008John D. Reeve Summary 1Animals move commonly through a variety of landscape elements and edges in search of food, mates and other resources. We developed a diffusion model for the movement of an insect herbivore, the planthopper Prokelisia crocea, that inhabits a landscape composed of patches of its host plant, prairie cordgrass Spartina pectinata, embedded in a matrix of mudflat or smooth brome Bromus inermis. 2We used mark,release,resight experiments to quantify planthopper movements within cordgrass,brome and cordgrass,mudflat arenas. A diffusion model was then fitted that included varying diffusion rates for cordgrass and matrix, edge behaviour in the form of a biased random walk and heterogeneity among planthoppers (sessile vs. mobile). The model parameters were estimated by maximum likelihood using the numerical solution of the diffusion model as a probability density. Akaike's information criterion (AIC) values were used to compare models with different subsets of features. 3There was clear support for models incorporating edge behaviour and both sessile and mobile insects. The most striking difference between the cordgrass,brome and cordgrass,mudflat experiments involved edge behaviour. Planthoppers crossed the cordgrass,brome edge readily in either direction, but traversed the cordgrass,mudflat edge primarily in one direction (mudflat to cordgrass). Diffusion rates were also significantly higher on mudflat than for cordgrass and brome. 4The differences in behaviour for cordgrass,brome vs. cordgrass,mudflat edges have implications for the connectivity of cordgrass patches as well as their persistence. Higher dispersal rates are expected between cordgrass patches separated by brome relative to mudflat, but patches surrounded by mudflat appear more likely to persist through time. 5The experimental design and diffusion models used here could potentially be extended to any organism where mass mark,recapture experiments are feasible, as well as complex natural landscapes. [source] Influence of crop edges on movement of generalist predators: a diffusion approachAGRICULTURAL AND FOREST ENTOMOLOGY, Issue 1 2002Riccardo Bommarco Abstract 1,Diffusion models were applied to recapture data for the generalist predator Pterostichus cupreus (Coleoptera: Carabidae) moving between two adjacent crop habitats (perennial ley and annual barley) first excluding, and then including, terms in the model quantifying the influences of edges on beetle movements. 2,Adult beetles were released at the centre of experimental 3 × 3 m plots that overlapped the edge separating perennial ley (mixed perennial crop of grass and clover) and annual barley crops, both early and later in the growing season. 3,Mathematical description of the data improved when the attractive or repulsive effects of habitat edges on dispersal were considered in the model. 4,Early in the season, when a sharp habitat edge was present, P. cupreus beetles appeared ,attracted' to ley. 5,Diffusion rates were consistently higher in barley than in ley early in the season, and vice versa late in the season. These patterns were probably linked to variation in prey availability. 6,Despite the risk of experiencing food limitation in the annual crop, our analyses suggest that these beetles regularly move from ley into the early barley habitat and then continue to disperse within the barley, providing opportunities for enhanced biological control of pest species. [source] Localization of deposited polycyclic aromatic hydrocarbons in leaves of PlantagoENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2001Martine I. Bakker Abstract After deposition to foliage, polycyclic aromatic hydrocarbons (PAHs) may remain on the leaf surface, accumulate in the cuticular wax, or diffuse into the remaining interior of the plant. In a field study, the location of deposited PAHs in the leaves of two Plantago species was determined. To this aim, leaves of Plantago major and Plantago media were divided into three fractions. First, the leaves were washed (wash-off fraction), then cuticular wax was extracted (wax fraction). Finally, the remaining leaf material was extracted (interior fraction). The presence of PAHs could be demonstrated in all three fractions. For both plants, the distribution of PAHs over the three fractions changed with molecular weight (mol wt) of the PAHs. The wash-off fraction increased with increasing molecular weight, likely because high molecular-weight PAHs occur predominantly bound to particles, which can be readily washed off from the leaves. In contrast, the amount of PAHs detected in the interior of the leaves decreased with increasing molecular weight. This can be explained by a slow desorption of the PAHs from the particles and a low diffusion rate of the larger molecules. This study shows that washing reduces the amount of high molecular-weight PAHs on plant surfaces. Therefore, washing of leafy vegetables is important to minimize human dietary intake of PAHs. [source] THE DIFFUSIVE SPREAD OF ALLELES IN HETEROGENEOUS POPULATIONSEVOLUTION, Issue 3 2004Garrick T. Skalski Abstract The spread of genes and individuals through space in populations is relevant in many biological contexts. I study, via systems of reaction-diffusion equations, the spatial spread of advantageous alleles through structured populations. The results show that the temporally asymptotic rate of spread of an advantageous allele, a kind of invasion speed, can be approximated for a class of linear partial differential equations via a relatively simple formula, c= 2,rD, that is reminiscent of a classic formula attributed to R. A. Fisher. The parameters r and D, represent an asymptotic growth rate and an average diffusion rate, respectively, and can be interpreted in terms of eigenvalues and eigenvectors that depend on the population's demographic structure. The results can be applied, under certain conditions, to a wide class of nonlinear partial differential equations that are relevant to a variety of ecological and evolutionary scenarios in population biology. I illustrate the approach for computing invasion speed with three examples that allow for heterogeneous dispersal rates among different classes of individuals within model populations. [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] Co3O4 Nanostructures with Different Morphologies and their Field-Emission Properties,ADVANCED FUNCTIONAL MATERIALS, Issue 12 2007B. Varghese Abstract We report an efficient method to synthesize vertically aligned Co3O4 nanostructures on the surface of cobalt foils. This synthesis is accomplished by simply heating the cobalt foils in the presence of oxygen gas. The resultant morphologies of the nanostructures can be tailored to be either one-dimensional nanowires or two-dimensional nanowalls by controlling the reactivity and the diffusion rate of the oxygen species during the growth process. A possible growth mechanism governing the formation of such nanostructures is discussed. The field-emission properties of the as-synthesized nanostructures are investigated in detail. The turn-on field was determined to be 6.4 and 7.7,V,,m,1 for nanowires and nanowalls, respectively. The nanowire samples show superior field-emission characteristics with a lower turn-on field and higher current density because of their sharp tip geometry and high aspect ratio. [source] Laser scanning confocal microscope characterization of dye diffusion in nylon 6 fibers treated with atmospheric pressure plasmasJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008Chunxia Wang Abstract The effect of atmospheric pressure plasma treatment on wettability and dyeability of nylon 6 fibers is investigated. The plasma treatments resulted in an average of 10°,20° decrease in the advancing contact angle and 20°,30° decrease in the receding contact angle. An increased dye diffusion rate of nylon 6 fibers was observed using laser scanning confocal microscope (LSCM). Scanning electron microscope confirmed that the fiber surfaces were roughened, and X-ray photoelectron spectroscopy showed that the polar groups on the fiber surfaces increased after the plasma treatments. As the plasma treatment time increased, a greater degree of etching was achieved and more polar groups such as hydroxyl and carboxyl groups produced on the surfaces of the nylon 6 fibers, leading to a better wettability and thus a better dyeability of the fiber. This study proved that LSCM may be effectively used in detecting the change of dye diffusion rate in nylon fibers treated with plasmas and the mounting medium should have a close refractive index as the fiber to avoid distortion of the fiber cross section image. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Sorption of tannic acid on zirconium pillared clayJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2002P Vinod Abstract Zirconium pillared clay (PILC) was prepared using montmorillonite as the base clay. Adsorption of tannic acid (tannin) was studied by a batch equilibrium technique, as a function of adsorbate concentration, temperature, pH, agitation speed, particle size of the adsorbent and ionic strength. The process of uptake is governed by diffusion controlled first-order reversible rate kinetics. The higher uptake for the pH range 4.0,6.0 was attributed to external hydrogen bonding between phenolic-OH groups of tannin molecules and the hydrogen bonding sites on the clay. The removal of tannin by adsorption was found to be >99.0% depending on the initial concentration in the pH range of 4.0,6.0. The process involves both film and pore diffusion to different extents. The effects of solute concentration, temperature, agitation speed and particle size on the diffusion rate were investigated. Tannin uptake was found to increase with ionic strength due to the compression of diffuse double layers. The applicability of Langmuir and Freundlich isotherm models has been tested. The maximum adsorption capacity of PILC was found to be 45.8,µmol,g,1 of clay and the affinity constant is 2.9,×,10,2,dm3,µmol,1 at 30,°C. Thermodynamic parameters such as ,G,°,,H,° and ,S,° were calculated to predict the nature of adsorption. The isosteric enthalpies of adsorption were also determined and found to decrease with increasing surface coverage. Regeneration with hot water (60,°C) has been investigated for several cycles with a view to recovering the adsorbed tannin and also restoring the sorbent to its original state. Copyright © 2001 Society of Chemical Industry [source] A new gadolinium-based contrast agent for magnetic resonance imaging of brain tumors: Kinetic study on a C6 rat glioma modelJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2001Emmanuel Fonchy Abstract T1 -weighted magnetic resonance imaging (MRI) was used to evaluate the potential interest of a new Gd-based contrast agent, termed P760, to characterize brain tumor heterogeneity and vascularization and to delineate regions containing permeable vessels. The C6 rat glioma model was used as a model of high-grade glioblastoma. The signal enhancement was measured as a function of time in the vascular compartment and in different regions of interest (ROIs) within the tumor after the injection of 0.02 mmol kg,1 of P760. The results were compared to those obtained after the injection of 0.1 mmol kg,1 of Gd-DOTA. We showed that P760, in spite of a Gd concentration five times smaller, produces an enhancement in the blood pool similar to that produced by Gd-DOTA. It was shown that P760 makes possible an excellent delineation of regions containing vessels with a damaged blood-brain barrier (BBB). Images acquired 5,10 minutes after P760 injection showed the location of permeable vessels more accurately than Gd-DOTA-enhanced images. The enhancement produced in the tumor by P760 was, however, less than that produced by Gd-DOTA. The extravasation and/or diffusion rate of P760 in the interstitial medium were found to be strongly reduced, compared to those found with Gd-DOTA. This study suggests that the new contrast agent has promising capabilities in clinical imaging of brain tumors. J. Magn. Reson. Imaging 2001;14:97,105. © 2001 Wiley-Liss, Inc. [source] Geometric design of fluid segments in microreactors using dimensionless numbersAICHE JOURNAL, Issue 4 2006Nobuaki Aoki Abstract In microreactors, reactant fluids are split into many fluid segments and then fed into the reactors to shorten mixing time. Two dimensionless numbers are introduced to represent effects of geometric design factors of fluid segments, such as shapes and arrangements, on reactor performance, namely mixing rate and product yield and selectivity: the ratio of reaction rate to diffusion rate and the aspect ratio of the mean diffusion length in the two-dimensional (2-D) directions in the reactor cross section. Methods to determine these numbers are also proposed. To examine the validity of these numbers on estimating the reactor performance, we compare product yields between each pair of reactors having the different geometric design factors but the same dimensionless numbers using computational fluid dynamics simulations. The results show that these numbers serve as the indices for estimating the reactor performance. Using these numbers, design guidelines for geometry of fluid segments are also discussed. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Three-bed PVSA process for high-purity O2 generation from ambient airAICHE JOURNAL, Issue 11 2005Jeong-Geun Jee Abstract A three-bed PVSA (pressure vacuum swing adsorption) process, combining equilibrium separation with kinetic separation, was developed to overcome the 94% O2 purity restriction inherent to air separation in the adsorption process. To produce 97+% and/or 99+% purity O2 directly from air, the PVSA process with two zeolite 10X beds and one CMS bed was executed at 33.44,45.60 to 253.31 kPa. In addition, the effluent gas from the CMS bed to be used for O2 purification was backfilled to the zeolite 10X bed to improve its purity, recovery, and productivity in bulk separation of the air. PVSA I, which made use of a single blowdown/backfill step, produced an O2 product with a purity of 95.4,97.4% and a recovery of 43.4,84.8%, whereas PVSA II, which used two consecutive blowdown/backfill steps, produced O2 with a purity of 98.2,99.2% and a recovery of 47.2,63.6%. Because the primary impurity in the O2 product was Ar, the amounts of N2 contained in the product were in the range of 4000,5000 ppm at PVSA I and several tens of ppm at PVSA II. A nonisothermal dynamic model incorporating mass, energy, and momentum balances was applied to predict the process dynamics. Using the linear driving force (LDF) model with constant diffusivity for the equilibrium separation bed and a modified LDF model with concentration dependency of the diffusion rate for the kinetic separation bed, the dynamic model was able to accurately predict the results of the experiment. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source] 3-D simulation and visualization of laminar flow in a microchannel with hair-pin curvesAICHE JOURNAL, Issue 7 2004Y. Yamaguchi Abstract The purpose of the present study was to investigate fluidic behavior in a microchannel with hair-pin curves, using a three-dimensional (3-D) computational fluid dynamics simulation, and to observe the 3-D flow pattern, to validate the simulation. The microchannel used was fabricated on a PMMA plate using a flat-end mill. The channel width and depth were 210 and 205 ,m, respectively, and the radius of each hair-pin curve was 500 ,m. Two liquids; purified water and an aqueous solution of 50 ,mol/L fluorescein, were introduced into the microchannel through different inlets and were merged, forming a side-by-side parallel flow in the straight channel. When the average velocity was 25 mm/s, the liquid was thrust outward by centrifugal force and, as a result, the vertical line that crossed the central axis was distorted after passing the first hair-pin curve. At the second hair-pin curve, the centrifugal force was exerted in the opposite direction, and the distorted line returned nearly to an initial vertical line. When the average velocity was 125 mm/s, however, the vertical line, which was distorted at the first hair-pin curve, did not recover to the initial vertical line after the second curve. The interface between the two liquids was permanently waved. The simulation was in good agreement with the experimental data. The results suggest that the diffusion rate through the interface of two liquids in microchannels with hair-pin curves can increase, compared to that in straight microchannels. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1530,1535, 2004 [source] Synthesis of an alkali-swellable emulsion and its effect on the rate of polymer diffusion in poly(vinyl acetate-butyl acrylate) latex filmsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005Jung Kwon Oh Abstract We describe the synthesis and characterization of a weakly cross-linked poly(methacrylic acid- co -ethyl acrylate) alkali-swellable emulsion (ASE), as well as an investigation of its influence on the rate of polymer diffusion in latex films. The films examined were formed from poly(vinyl acetate- co -butyl acrylate) latex particles containing a small amount of acrylic acid as a comonomer. Polymer diffusion rates were monitored by the energy transfer technique. We found that the presence of the ASE component, either in the acid form or fully neutralized by ammonia or sodium hydroxide, had very little effect on the polymer diffusion rate. However, in the presence of 2 wt % NH4 -ASE, there was a small but significant increase in the polymer diffusion rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5632,5642, 2005 [source] Epoxy-functionalized, low-glass-transition-temperature latex.JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2002Abstract This article describes the results of experiments examining the competition between the polymer diffusion rate and the crosslinking rate in low-glass-transition-temperature, epoxy-containing latex films in the presence of a diamine. We examined films formed from donor- and acceptor-labeled poly(butyl acrylate- co -methyl methacrylate- co -glycidyl methacrylate) copolymer latex and studied the influence of several parameters on the growth rate of gel content and the rate of polymer diffusion. These factors include the molecular weight of the latex polymer, the presence or absence of a diamine crosslinking agent, and the cure protocol. The results were compared to the predictions of a recent theory of the competition between crosslinking and polymer diffusion across interfaces. In the initially formed films, polymer diffusion occurs more rapidly than the chemical reaction rate. Therefore, these films fall into the fast-diffusion category of this model. In our system (unlike in the model), the latex polymer has a broad distribution of molecular weights and a distribution of diffusivities. The shortest chains contribute to the early time diffusion that we measure. At later stages of our experiment, slower diffusing species contribute to the signal that we measure. The diffusion time decreases substantially, and we observe a crossover to a regime in which the chemical reaction dominates. The increases in chain branching and gel formation bring polymer diffusion to a halt. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4098,4116, 2002 [source] Formation of High-Quality, Epitaxial La2Zr2O7 Layers on Biaxially Textured Substrates by Slot-Die Coating of Chemical Solution PrecursorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007Sung-Hun Wee Crystallization studies were performed of epitaxial La2Zr2O7 (LZO) films on biaxially textured Ni,3at.%W substrates having thin Y2O3 (10 nm) seed layers. LZO films were deposited under controlled humid atmosphere using reel-to-reel slot-die coating of chemical solution precursors. Controlled crystallization under various processing conditions has revealed a broad phase space for obtaining high-quality, epitaxial LZO films without microcracks, with no degradation of crystallographic texture and with high surface crystallinity. Crack-free and strong c -axis aligned LZO films with no random orientation were obtained even at relatively low annealing temperatures of 850°,950°C in flowing one atmosphere gas mixtures of Ar,4% H2 with an effective oxygen partial pressure of P(O2),10,22 atm. Texture and reflection high-energy electron diffraction analyses reveal that low-temperature-annealed samples have strong cube-on-cube epitaxy and high surface crystallinity, comparable to those of LZO film annealed at high temperature of 1100°C. In addition, these samples have a smoother surface morphology than films annealed at higher temperatures. Ni diffusion rate into the LZO buffer film is also expected to be significantly reduced at the lower annealing temperatures. [source] Kinetics of Miniemulsion Polymerization of Styrene in the Presence of OrganoclaysMACROMOLECULAR MATERIALS & ENGINEERING, Issue 6 2008Zhaohui Tong Abstract The impacts of nanoclays on the miniemulsion polymerization kinetics of styrene were studied. It was found that both RP and the fractional conversion decreased upon increasing the organoclay content in the miniemulsion system. In the presence of nanoclay the molecular weight of polystyrene nanoclay composite is lower and the particle size polydispersity of the final composite latex is greater than that of pure styrene miniemulsion polymerization. The effect of the nanoclays is mainly caused by the destabilization of the miniemulsion by the organoclay particles. The increase in the monomer viscosity and the decrease in the diffusion rate of the monomer and the living polymer inside the monomer droplet also accounts for the reduction in the polymerization rate. [source] Assembly of Polyaniline NanostructuresMACROMOLECULAR RAPID COMMUNICATIONS, Issue 1 2007Jixiao Wang Abstract Polyaniline nanostructures (nanosheets, nanofibers, and nanoparticles) can be assembled at the organic/aqueous interface or in solution by controlling the diffusion rate and the polymerization induction time of aniline. The quality of polyaniline nanostructures is determined by the polymerization solution conditions. Polyaniline nanosheets formation mechanism was proposed. Under certain polymerization conditions, polyaniline nanofibers or/and nanoparticles were obtained. [source] Quantifying oxygen diffusion in paraffin oil used in oocyte and embryo cultureMOLECULAR REPRODUCTION & DEVELOPMENT, Issue 12 2009Yvonne M. StokesArticle first published online: 11 AUG 200 Oxygen diffusion through oil is important in the culture of oocytes and embryos. A diffusion coefficient two orders of magnitude smaller than that of oxygen in water has been thought possible, and this has led to concerns of anoxia in cultures. Using an assay for determining the oxygen consumption rate of embryos and oocytes, along with a mathematical model, it is here shown that the oxygen diffusion rate in paraffin oil at 37°C is about two-thirds of that in water at the same temperature. Although not previously recognised for the assay in question, the geometry is such that anoxia does occur for a period of time in excess of 1,hr and, by the completion of the assay, 30,40% of the medium is anoxic. Hence the quantity of oxygen consumed is less than would be consumed in conditions of plentiful oxygen supply. Nevertheless, using a model with a concentration dependent oxygen consumption rate, the oxygen consumption rate can be estimated. Mol. Reprod. Dev. 76: 1178,1187, 2009. © 2009 Wiley-Liss, Inc. [source] Concentration of aqueous dye solution by freezing and thawingTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Kyuya Nakagawa Abstract The concentration phenomena during freezing and thawing were investigated to study the feasibility of freeze,thaw process as a concentration operation. An aqueous dye solution was employed as a model binary eutectic solution. The localised concentration in the frozen matrix was determined, and the concentration profile of the melting solution during thawing was examined. It was found that the solution obtained during thawing showed higher concentration than the original solution, and the concentration did not correspond to the amount of solute localised in the frozen matrix. It was suggested that the concentration phenomena during thawing would be governed by the melting droplet growth rate at the melting interface and by the diffusion rate of solute from eutectic phases to the droplet. On a observé le phénomène de la concentration durant le gel et le dégel afin de déterminer la faisabilité du processus de gel et de dégel en tant qu'opération de concentration. On a utilisé une solution colorante aqueuse comme solution eutectique binaire de modèle. On a déterminé la concentration localisée dans la matrice congelée et examiné le profil de concentration de la solution durant la décongélation. On a noté que la solution obtenue durant la décongélation présentait une concentration supérieure à la solution d'origine, et que la concentration ne correspondait pas à la quantité de soluté localisée dans la matrice congelée. Il a été suggéré que le phénomène de concentration durant le dégel serait régi par le taux de croissance des gouttelettes issues de la fonte à la surface et par le taux de diffusion du soluté à partir des phases eutectiques à la gouttelette. [source] Sorption kinetics of toluene in humic acid: A computational approachENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 2 2006Yang-Hsin Shih Abstract Molecular dynamics, a computational technique aiming to describe the time-dependent movement of molecules, has been applied to study the sorption kinetics of volatile organic contaminants in soil organic matter. The molecular dynamics simulation results obtain reasonably accurate estimates of diffusion rates and activation energy of the penetration of a volatile organic compound molecule into a model humic substance. The sorption rate of toluene to humic acid decreases with the density of the humic acid matrix and increases with temperature. All the present results indicate that the sorption of toluene into humic acid is mainly diffusion controlled. Finally, the present studies have shown that molecular dynamics of volatile organic compounds in humic substances yields meaningful results, which help in the understanding of diffusion at the molecular level and which facilitate the problem-solving capability for removing the contaminants from the soils. [source] The Influence of Mass Transfer on a Porous Fuel Cell ElectrodeFUEL CELLS, Issue 1-2 2004Y.-P. Sun Abstract A one-dimensional model for a porous fuel cell electrode using a liquid electrolyte with dissolved reactant is presented. The model consists of a Poisson, second-order ordinary differential equation, describing the effect of the electric field and a one-dimensional; Fickian diffusion, second-order ordinary differential equation describing the concentration variation associated with diffusion. The model accounts for mass transport and heterogeneous electrochemical reaction. The solution of this model is by the approximate Adomian polynomial method and is used to determine lateral distributions of concentration, overpotential and current density and overall cell polarisation. The model is used to simulate the effects of important system and operating parameters, i.e. local diffusion rates, and mass transport coefficients and electrode polarisation behaviour. [source] The mechanism of fluid infiltration in peridotites at Almklovdalen, western NorwayGEOFLUIDS (ELECTRONIC), Issue 3 2002O. Kostenko Abstract A major Alpine-type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet,peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite,peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe-like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid-filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (, > 60°) and ,curled up' into discontinuous channels and fluid inclusion arrays following the infiltration event. Re-equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the ,forcefully' introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1,20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (, > 60°) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology. [source] Permeation of Sumatriptan Through Human Vaginal and Buccal MucosaHEADACHE, Issue 2 2000P. Van Der Bijl DSc Continued interest in the various routes by which sumatriptan may be administered prompted us to investigate its passage through buccal mucosa. Because human buccal mucosa is scarce, we proposed using the relatively abundant vaginal mucosa, which has been shown to have comparable diffusion rates for a number of widely varying molecules, as a model of buccal mucosa. In addition, by comparing these two tissues with respect to their permeability to sumatriptan, the human vaginal/buccal mucosa model could be further evaluated. Clinically healthy human vaginal and buccal mucosa specimens were used in the permeability studies. Permeability to sumatriptan was determined using a continuous flow-through diffusion system in the presence and absence of permeation enhancers. No statistically significant differences in permeability could be demonstrated for both mucosae toward sumatriptan. Flux values obtained in the absence and presence of glycodeoxycholate and lauric acid (1:1 molar ratio) to sumatriptan of buccal and vaginal mucosa, respectively, were not significantly different. The results obtained further support the hypothesis of the vaginal/buccal mucosal in vitro permeability model and suggest that this model may be used in conjunction with various absorption enhancers. Further studies on the buccal route of absorption of sumatriptan are thus warranted. [source] Diffusion models for animals in complex landscapes: incorporating heterogeneity among substrates, individuals and edge behavioursJOURNAL OF ANIMAL ECOLOGY, Issue 5 2008John D. Reeve Summary 1Animals move commonly through a variety of landscape elements and edges in search of food, mates and other resources. We developed a diffusion model for the movement of an insect herbivore, the planthopper Prokelisia crocea, that inhabits a landscape composed of patches of its host plant, prairie cordgrass Spartina pectinata, embedded in a matrix of mudflat or smooth brome Bromus inermis. 2We used mark,release,resight experiments to quantify planthopper movements within cordgrass,brome and cordgrass,mudflat arenas. A diffusion model was then fitted that included varying diffusion rates for cordgrass and matrix, edge behaviour in the form of a biased random walk and heterogeneity among planthoppers (sessile vs. mobile). The model parameters were estimated by maximum likelihood using the numerical solution of the diffusion model as a probability density. Akaike's information criterion (AIC) values were used to compare models with different subsets of features. 3There was clear support for models incorporating edge behaviour and both sessile and mobile insects. The most striking difference between the cordgrass,brome and cordgrass,mudflat experiments involved edge behaviour. Planthoppers crossed the cordgrass,brome edge readily in either direction, but traversed the cordgrass,mudflat edge primarily in one direction (mudflat to cordgrass). Diffusion rates were also significantly higher on mudflat than for cordgrass and brome. 4The differences in behaviour for cordgrass,brome vs. cordgrass,mudflat edges have implications for the connectivity of cordgrass patches as well as their persistence. Higher dispersal rates are expected between cordgrass patches separated by brome relative to mudflat, but patches surrounded by mudflat appear more likely to persist through time. 5The experimental design and diffusion models used here could potentially be extended to any organism where mass mark,recapture experiments are feasible, as well as complex natural landscapes. [source] Analysis of video images from a gas,liquid transfer experiment: a comparison of PCA and PARAFAC for multivariate image analysisJOURNAL OF CHEMOMETRICS, Issue 7 2003Stephen P. Gurden Abstract The use of chemical imaging is a developing area which has potential benefits for chemical systems where spatial distribution is important. Examples include processes in which homogeneity is critical, such as polymerizations, pharmaceutical powder blending and surface catalysis, and dynamic processes such as the study of diffusion rates or the transport of environmental pollutants. Whilst single images can be used to determine chemical distribution patterns at a given point in time, dynamic processes can be studied using a sequence of images measured at regular time intervals, i.e. a movie. Multivariate modeling of image data can help to provide insight into the important chemical factors present. However, many issues of how best to apply these models remain unclear, especially when the data arrays involved have four or five different dimensions (height, width, wavelength, time, experiment number, etc.). In this paper we describe the analysis of video images recorded during an experiment to investigate the uptake of CO2 across a free air,water interface. The use of PCA and PARAFAC for the analysis of both single images and movies is described and some differences and similarities are highlighted. Some other image transformation techniques, such as chemical mapping and histograms, are found to be useful both for pretreatment of the raw data and for dimensionality reduction of the data arrays prior to further modeling. Copyright © 2003 John Wiley & Sons, Ltd. [source] Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland CaledonidesJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2001H. M. Lang Abstract The Sanddal mafic-ultramafic complex (SMUK) is a cluster of variably eclogitised mafic and ultramafic bodies that comprise the westernmost known eclogite facies locality in the North-East Greenland eclogite province (NEGEP). Although there are no true eclogites in the SMUK, we interpret three distinct textural types of plagioclase replacement to record sequential stages in adjustment of SMUK olivine gabbro-norites to eclogite facies conditions. The earliest stage, in which plagioclase was replaced by omphacite/spinel symplectite before nucleation of garnet (Type 1A & 1B) has not previously been described. Documentation of this texture provides clear evidence that, at least in some cases, garnet nucleation is delayed relative to nucleation of omphacite and is a rate-limiting step for eclogitisation. Type 1C domains were produced by scattered nucleation of garnet in the same sample. In Type 2 domains, plagioclase was replaced by a layered corona with an outer layer of garnet, an inner layer of omphacite and an interior of inclusion-rich plagioclase. In Type 3 domains, the omphacite layer was overgrown by the garnet rim, and omphacite is preserved only as inclusions in garnet. In more coarse grained leucogabbros, recrystallization was more complete, plagioclase replacement textures were less localised, and could not be divided into distinct stages. Plagioclase replacement in SMUK samples was not isochemical, and required diffusion of at least Mg and Fe from replacement of mafic phases in the surroundings. Strong compositional gradients in garnet reflect disequilibrium and were controlled by the different diffusion rates of Mg/Fe and Ca, different local chemical environments, and progress of the plagioclase breakdown reaction. The presence of small amounts of hydrous minerals (amphibole, phlogopite and clinozoisite) in local equilibrium in plagioclase domains of most SMUK samples indicates that a small amount of H2O was present during high pressure metamorphism. [source] ZSM-11 membranes: Characterization and pervaporation performanceAICHE JOURNAL, Issue 2 2002Shiguang Li High-quality boron and aluminium-substituted ZSM-11 membranes were prepared on porous tubular supports to separate alcohols from water by pervaporation. The C1,C3 alcohols were preferentially separated from aqueous solutions through the B-ZSM-11 membrane, and the maximum flux was 1.7 kg/m2·h for a 5 wt.% methanol/water mixture. The alcohol fluxes decreased as the carbon number increased. The methanol/water separation selectivity decreased with pervaporation temperature, but other alcohol/water selectivities increased. All selectivities increased with decreasing alcohol feed concentration. In a range of 1 to 50 wt. % alcohol, the separation selectivities at 333 K for the C1,C3 linear alcohols were higher than vapor/liquid equilibrium selectivities. The highest selectivities observed for methanol/water, ethanol/water, 1-propanol/water, and 2-propanol/water were 28, 97, 34, and 26, respectively, at 1 wt. % alcohol feed concentrations. The separation selectivities were based on both preferential adsorption of alcohols and differences in diffusion rates. [source] Synthesis of an alkali-swellable emulsion and its effect on the rate of polymer diffusion in poly(vinyl acetate-butyl acrylate) latex filmsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005Jung Kwon Oh Abstract We describe the synthesis and characterization of a weakly cross-linked poly(methacrylic acid- co -ethyl acrylate) alkali-swellable emulsion (ASE), as well as an investigation of its influence on the rate of polymer diffusion in latex films. The films examined were formed from poly(vinyl acetate- co -butyl acrylate) latex particles containing a small amount of acrylic acid as a comonomer. Polymer diffusion rates were monitored by the energy transfer technique. We found that the presence of the ASE component, either in the acid form or fully neutralized by ammonia or sodium hydroxide, had very little effect on the polymer diffusion rate. However, in the presence of 2 wt % NH4 -ASE, there was a small but significant increase in the polymer diffusion rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5632,5642, 2005 [source] Water diffusion heterogeneity index in the human brain is insensitive to the orientation of applied magnetic field gradients,MAGNETIC RESONANCE IN MEDICINE, Issue 2 2006Kevin M. Bennett Abstract The , diffusion-weighted imaging (DWI) method was developed to study heterogeneous water diffusion in the human brain using magnetic resonance imaging (MRI). An advantage of this model is that it does not require an assumption about the shape of the intravoxel distribution of apparent diffusion rates, and it has a calculable relationship to this distribution. The ,- DWI technique is useful for detecting microstructural tissue changes associated with brain tumor invasion, and may be useful for directing therapy to invading tumor cells. In previous work, ,- DWI was performed with magnetic field gradients applied along a single direction in order to avoid artificially introducing a source of heterogeneity to the decay. However, it is known that restricted diffusion is anisotropic in the brain, and the ,- DWI method must take this into account to be complete. In this work the relationship between the applied magnetic field gradients and the fitted stretched-exponential model parameters was studied in the human brain. It was found the distributed diffusion coefficient (DDC) varies with the direction of applied gradients, while the heterogeneity index , is relatively direction-insensitive. It is proposed that in clinical use, maps of , can be created using diffusion-weighting gradients applied in a single direction that reflect the tissue heterogeneity. Magn Reson Med, 2006. Published 2006 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||