Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “diffusion theory”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,459 records · Page 81Linked to original sources
PubMed2002

Diffusive partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone.

This paper proposes the theory and practical application of a new partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone, which is based on diffusion. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone to form a point source at the injection point. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. The test has been evaluated in laboratory sand columns contaminated with dodecane. NAPL in saturations of 0.8-4% of the total porosity have been quantified in a wide range of different water contents. Actual and measured NAPL saturations calculated as an average from four different tracer pairs agreed within +/-30%. The new method was successfully used for repeated NAPL quantification in a large-scale field lysimeter contaminated with artificial kerosene. This rapid and inexpensive test is potentially of value for site investigations especially in combination with soil gas measurements, because it requires similar equipment. Possible applications are source delineation and repeated NAPL quantification in situ during a remediation.

Air↗
PubMed1986

Evidence that pyrene excimer formation in membranes is not diffusion-controlled.

Kinetic and steady-state measurements of pyrene fluorescence in a variety of model membranes are evaluated in terms of the theory of collisional excimer formation. In the region of 10(-3)-0.1 M pyrene, molecular fluorescence decay in membranes is biphasic and the two component lifetimes do not depend on the pyrene concentration. The lifetime data are consistent with the rate constant for collisional excimer formation being of the order 10(6) M-1 X s-1 or less. The concentration dependence of the component amplitudes is inconsistent with the theory of collisional excimer formation and suggests that pyrene exists in two forms in membranes: a slowly diffusing monomeric form and an aggregated form. The component of molecular fluorescence decay associated with aggregated pyrene is highly correlated with steady-state excimer fluorescence, suggesting that excimer fluorescence in membranes arises from aggregated pyrene in which excimers are formed by a static rather than a collisional mechanism. It is suggested that the concentration dependence of excimer to molecular fluorescence intensity ratios in membranes is related to the equilibrium constant for exchange between monomeric and aggregated pyrene forms rather than to the collisional excimer formation rate constant.

Chloroplasts↗
PubMed2002

Comparison of filamin A-induced cross-linking and Arp2/3 complex-mediated branching on the mechanics of actin filaments.

We compared the effects of human filamin A (FLNa) and the activated human Arp2/3 complex on mechanical properties of actin filaments. As little as 1 FLNa to 800 polymerizing actin monomers induces a sharp concentration-dependent increase in the apparent viscosity of 24 microm actin, a parameter classically defined as a gel point. The activated Arp2/3 complex, at concentrations up to 1:25 actins had no detectable actin gelation activity, even in the presence of phalloidin, to stabilize actin filaments against debranching. Increasing the activated Arp2/3 complex to actin ratio raises the FLNa concentration required to induce actin gelation, an effect ascribable to Arp2/3-mediated actin nucleation resulting in actin filament length diminution. Time lapse video microscopy of microparticles attached to actin filaments or photoactivation of fluorescence revealed actin filament immobilization by FLNa in contrast to diffusion of Arp2/3-branched actin filaments. The experimental results support theories predicting that polymer branching absent cross-linking does not lead to polymer gelation and are consistent with the observation that cells deficient in actin filament cross-linking activity have unstable surfaces. They suggest complementary roles for actin branching and cross-linking in cellular actin mechanics in vivo.

Actin-Related Protein 2↗
PubMed1982

Simultaneous self-association and diffusion of phenol in isooctane.

The impact of self-association on mass transport was studied. The model system chosen was the diffusion of phenol through an immobilized layer of isooctane. In the theoretical development, self-associated phenol contributed to diffusion, with the fluxes being interdependent because of the self-association equilibrium. Predictions from theory were then compared with experimental results. It was shown that self-association can significantly affect flux of diffusing species.

Chemical Phenomena↗
PubMed2001

Gastric adenocarcinoma: pathomorphology and molecular pathology.

Two types of gastric adenocarcinoma can be distinguished histopathologically: the diffuse and the intestinal type. Molecular pathology supports this theory by showing differences in the genetic pathways of both tumor types. In addition to known pathomorphological factors of prognosis, e.g., depth of tumor infiltration, number of lymph node metastases and resection margins, a few genes have been suggested to have prognostic impact in gastric carcinoma. Clinically relevant molecules whose expression or structure is altered include the plasminogen activator (uPA) and its inhibitor PAI-1 (plasminogen activator inhibitor type 1), the cell cycle regulator cyclin E, epidermal growth factor (EGF), the apoptosis inhibitor bcl-2, the cell adhesion molecule E-cadherin, and the multifunctional protein beta-catenin. Gene amplification and protein overexpression of the growth factor receptors c-erbB-2 and K-sam may be prognostic factors for intestinal-type and diffuse-type gastric cancer, respectively. In addition, genetic instability is commonly seen. There has long been evidence for a genetic predisposition to gastric cancer by epidemiological studies and case reports. Very recently, germ line mutations of E-cadherin have been identified that are responsible for a dominantly inherited form of diffuse-type gastric cancer and could be used to identify individuals that are at high risk.

Adenocarcinoma↗
PubMed1995

Muscle O2 transport and O2 dependent control of metabolism.

Whether maximal oxygen uptake (VO2max) during exercise in healthy subjects reflects limitation of the supply of oxygen to muscle mitochondria or an inability to use oxygen beyond some biochemical limit remains debated. This paper summarizes evidence for the hypothesis that VO2max is limited by O2 supply. How the many steps in the pathway for oxygen interact to determine maximum oxygen supply is analyzed, resulting in a mathematical model of predictive value. A major component of this model is the diffusional transport of oxygen out of the muscle microcirculation. The proportionality between VO2max and the capillary to mitochondrion partial pressure gradient (as FIO2 is altered) supports this hypothesis, but does not exclude the existence of convective heterogeneity of blood flow as a factor contributing to the limitation of VO2max. Although there is no direct method for measuring convective heterogeneity, altering hemoglobin P50 permits distinguishing diffusive from convective elements in limiting VO2max, and both the underlying theory and the results are summarized herein. Our conclusion is that VO2max in health appears limited by the supply of oxygen to the mitochondria, which in turn, depends upon the integrated interaction among all of the diffusive and convective steps in the oxygen pathway between the environment and the mitochondria.

Blood Pressure↗
PubMed2005

Trace metal measurements in low ionic strength synthetic solutions by diffusive gradients in thin films.

In view of conflicting reports regarding the performance of DGT in low ionic strength solutions (I < 1 mM), further investigations have been carried out. Minimal washing of the diffusive gel and deployment in 1.0 and 10 mM NaNO3 solutions containing Cu and Cd gave the theoretical response of 1 for [C](DGT)/[C](SOLN), where [C](DGT) is the concentration of metal measured by DGT and [C](SOLN) is the concentration of metal measured directly in the solution by an appropriate analytical method. Erroneously high values for [C](DGT)/[C](SOLN) were obtained when these same gels were deployed at I = 0.1 mM, presumably due to a net negative charge on the gel, attributable to the presence of initiation products of polymerization. However, washing the diffusive gels completely, where the storage solution pH equaled that of deionized water, gave values of approximately 0.5 for [C](DGT)/[C](SOLN) from deployments at I = 0.1 mM, consistent with the lower measured value of the diffusion coefficients at this ionic strength. These results can be explained by the presence of a net positive charge on the gel when it is exhaustively washed, which reduces the effective diffusion coefficient of metal ions by changing their concentration at the gel-solution interface (Donnan partitioning). Diffusive gel equilibration experiments showed the presence of low capacity sites capable of binding metals irrespective of ionic strength. This binding within the diffusive gel does not affect most DGT measurements, as short (4 h) deployments at concentrations of 10 ppb gave theoretical results. Incomplete washing of the resin-gel caused a 5-15% measurement error and a decrease in precision, even at ionic strengths of 10 mM. A high level of accuracy and precision (typically <5%) was maintained during all aspects of this work, even at ionic strengths of 0.1 mM, in contrast to previous results. This is attributable to three factors: (1) exhaustive washing and conditioning protocols, (2) improvements to the DGT sampling device, and (3) low and reproducible blanks due to ultraclean handling procedures. Provided effective diffusion coefficients measured at the same ionic strength are used, the established DGT theory is obeyed irrespective of ionic strength.

Cadmium↗
PubMed2001

Theoretical description of transdermal transport of hydrophilic permeants: application to low-frequency sonophoresis.

Application of ultrasound enhances transdermal transport of drugs (sonophoresis). The enhancement may result from enhanced diffusion due to ultrasound-induced skin alteration and/or from forced convection. To understand the relative roles played by these two mechanisms in low-frequency sonophoresis (LFS, 20 kHz), a theory describing the transdermal transport of hydrophilic permeants in both the absence and the presence of ultrasound was developed using fundamental equations of membrane transport, hindered-transport theory, and electrochemistry principles. With mannitol as the model permeant, the role of convection in LFS was evaluated experimentally with two commonly used in vitro skin models- human cadaver heat-stripped skin (HSS) and pig full-thickness skin (FTS). Our results suggest that convection plays an important role during LFS of HSS, whereas its effect is negligible when FTS is utilized. The theory developed was utilized to characterize the transport pathways of hydrophilic permeants during both passive diffusion and LFS with mannitol and sucrose as two probe molecules. Our results show that the porous pathway theory can adequately describe the transdermal transport of hydrophilic permeants in both the presence and the absence of ultrasound. Ultrasound alters the skin porous pathways by two mechanisms: (1) enlarging the skin effective pore radii, or (2) creating more pores and/or making the pores less tortuous. During passive diffusion, both HSS and FTS exhibit the same skin effective pore radii (r = 28 +/- 13 A). In contrast, during LFS, r within HSS is greatly enlarged (r > 125 A), whereas r within FTS does not change significantly (23 +/- 10 A). The observed different roles of convection during LFS across HSS and FTS can be attributed to the different degrees of structural alteration that these two types of skin undergo during LFS.

Administration, Cutaneous↗
PubMed1984

Theoretical interpretation of extraction (in brain) of peptides including concentration variations.

The transport properties of several peptides across blood-brain barrier (BBB) have been investigated theoretically in terms of simple diffusion and facilitated diffusion processes. Comparison of the calculated results from the simple diffusion and the experimental data reveals the presence of the facilitated diffusion of these substances which we have conceived of as a carrier-mediated process. The values of the partition coefficients f for these peptides were in the range 7 X 10(-4) less than or equal to f less than or equal to 200 X 10(-4). The calculated f values gave permeabilities, Ps, in lipids between 10(-7) less than or equal to Ps less than or equal to 14 X 10(-7) cm/s. These values were then used to estimate the extraction for peptides from simple diffusion alone which vary from 0.3 to 3.5% compared with the experimental extraction (0.4-12%) indicating the inadequacy of the simple diffusion alone to explain the experimental data. As for the carrier-mediated facilitated diffusion process we have used the activated-complex theory. The extraction in this case depends on the maximal rate of transport (Tmax)f and the reciprocal of the affinity constant Kt for the transport of peptides through BBB. We have deduced that (Tmax)f approximately 0.46 X 10(-3) pmol/g X s and Kt approximately 0.35 nM for Met-enkephalin (Met-ENK), Leu-enkephalin (Leu-ENK), glutathione, carnosine, alpha-MSH and MIF and (Tmax)f approximately 10 X 10(-3) pmol/g X s and Kt approximately 7 nM for AVP, beta LT, beta E and alpha E to explain the observed results. We have also obtained the quantitative variation of extraction with concentration of peptides in the brain-capillary and have established that the extraction decreases with increasing concentration of peptides, tending to a small constant value at high concentrations. It has been inferred that carrier-mediated facilitated diffusion is important for the transport of peptides across BBB.

Animals↗
PubMed1994

Morphological bifurcations involving reaction-diffusion processes during microtubule formation.

Nonlinear chemically dissipative mechanisms have been proposed as providing a possible underlying process for some aspects of biological self-organization, pattern formation, and morphogenesis. Nonlinearities during the formation of microtubular solutions result in a chemical instability and bifurcation between pathways leading to macroscopically self-organized states of different morphology. The self-organizing process, which contains reactive and diffusive contributions, involves chemical waves and differences in microtubule concentration in the sample. Patterns of similar appearance are observed at different distance scales. This behavior is in agreement with theories of chemically dissipative systems.

Birefringence↗
PubMed1994

Computations of post-inductive dynamics in axolotl heart formation.

This paper reports modelling of heart localization in the axolotl (Ambystoma mexicanum). The region of heart specification in the mesoderm defined by classical induction from the endoderm is larger than the area of final myocardial differentiation. For localizing the area of differentiation within the area of specification, we postulate a reaction-diffusion system that arises within the mesoderm in response to induction from the endoderm. This mechanism generates a spatial pattern for two chemicals, an activator and an inhibitor, corresponding to the area of myocardial differentiation. We postulate a diffusible chemical rescuer, which is absent in the cardiac lethal mutant, and which is a precursor to the reaction-diffusion mechanism. The activator, inhibitor, rescuer, and product of endodermal induction are presented in an enzyme mechanism with rate equations similar to the Gierer-Meinhardt equations. These equations were solved numerically in both one and two spatial dimensions. We have attained quantitative agreement with the experimental data for sizes of tissue regions and for times to heartbeat. Experiments modelled include wild-type heart localization as well as both in vitro and in vivo rescue of cardiac lethal mesoderm with wild-type mesoderm. Based upon the parameters necessary to model heart localization, we make a series of predictions. We predict: a specific profile for the endodermal inducer gradient; the possibility of producing multiple hearts in vivo; and a greater contribution to the heart from the wild-type mesoderm for in vivo transplants with cardiac lethal mesoderm. We make some suggestions as to the possible chemical nature of the substances in the model. We indicate that the inhibitory field and mechanochemical theories are probably not as promising as reaction-diffusion for the mechanism of heart localization.

Ambystoma mexicanum↗
PubMed2005

Competing failure risk analysis using evidence theory.

Safety systems are important components of high-consequence systems that are intended to prevent the unintended operation of the system and thus the potentially significant negative consequences that could result from such an operation. This presentation investigates and illustrates formal procedures for assessing the uncertainty in the probability that a safety system will fail to operate as intended in an accident environment. Probability theory and evidence theory are introduced as possible mathematical structures for the representation of the epistemic uncertainty associated with the performance of safety systems, and a representation of this type is illustrated with a hypothetical safety system involving one weak link and one strong link that is exposed to a high temperature fire environment. Topics considered include (1) the nature of diffuse uncertainty information involving a system and its environment, (2) the conversion of diffuse uncertainty information into the mathematical structures associated with probability theory and evidence theory, and (3) the propagation of these uncertainty structures through a model for a safety system to obtain representations in the context of probability theory and evidence theory of the uncertainty in the probability that the safety system will fail to operate as intended. The results suggest that evidence theory provides a potentially valuable representational tool for the display of the implications of significant epistemic uncertainty in inputs to complex analyses.

Journal Article↗
PubMed1985

A thermodynamic treatment of passive monitors.

Previous mathematical descriptions of sampling using passive monitors have used Fick's First Law of diffusion and the assumption that the concentration of adsorbate in the vapor phase above the sorbent is zero. This paper shows that by introducing a simplified expression for the equilibrium vapor pressure, behavior more nearly resembling that observed for passive monitors is predicted. The theory can also be applied to the case of loss of sample from a diffusive monitor. Experimental evidence is also provided which demonstrates that the theory adequately describes the observed results.

Air Pollutants↗
PubMed2003

Controlling oscillator coherence by delayed feedback.

We demonstrate that the coherence of a noisy or chaotic self-sustained oscillator can be efficiently controlled by the delayed feedback. We develop a theory of this effect, considering noisy systems in the Gaussian approximation. We obtain a closed equation system for the phase diffusion constant and the mean frequency of oscillation. For weak feedback and strong noise, the theory is in good agreement with the numerics. We discuss possible applications of the effect for the synchronization control.

Journal Article↗
PubMed2002

Self-diffusion rates in Al from combined first-principles and model-potential calculations.

Monovacancy diffusion alone dominates over diffusion due to divacancies and interstitials in Al for all temperatures up to the melting point. Deviations from a single Arrhenius dependence are due to anharmonicity. The conclusion is based on a combination of theoretical methods, from density functional theory to thermodynamic integration, without fitting to experimental data. The calculated diffusion rate agrees with experimental data over 11 orders of magnitude.

Journal Article↗
PubMed2005

Supply pre-emption, not concentration reduction, is the mechanism of competition for nutrients.

Concentration reduction theory is the leading theory regarding the mechanism of competition for nutrients in soils among plants, yet it has not been rigorously tested. Here we used a spatially explicit, fine-scale grid-based model that simulated diffusion and plant uptake of nutrients by plants in soil to test whether concentration reduction theory was appropriate for terrestrial plant competition for nutrients. In the absence of competition, increasing the rate of diffusion allows a plant to maintain positive growth rates below the lowest average concentration to which it can reduce nutrients in soil solution (R*). As such, differences among plants in the reduction of soil moisture, which here primarily affects nutrient diffusion, can cause R* to predict competitive success incorrectly. The stronger competitor for nutrients captures the largest proportion of the nutrient supply by ensuring nutrients contact its roots before those of a competitor. Although the metric derived from concentration reduction theory, R*, might have predictive power for competitive outcomes in terrestrial ecosystems, this evidence suggests that plants outcompete other plants for nutrients by pre-empting the supply, not reducing the average concentration.

Computer Simulation↗
PubMed2002

Generalized Taylor-Aris dispersion in discrete spatially periodic networks: microfluidic applications.

A theory is presented for the lumped parameter, convective-diffusive transport of individual, noninteracting Brownian solute particles ("macromolecules") moving within spatially periodic, solvent-filled networks--the latter representing models of chip-based microfluidic chromatographic separation devices, as well as porous media. Using graph-theoretical techniques, the composite medium is conceptually decomposed into a network of channels (the edges) through which the solute is transported by a combination of molecular diffusion and either "piggyback" entrainment within a flowing solvent or an externally applied force field acting upon the solute molecules. A probabilistic choice of egress channel for a solute particle exiting the intersection (vertex) of the channels is furnished by an imperfect mixing model. A spatially periodic, Taylor-Aris-like "method-of-moments" scheme is applied to this transport model, leading to discrete matrix equations for computing the network-scale particle velocity vector U(*) and dispersivity dyadic D(*) in terms of the prescribed microscale transport parameters and network geometry characterizing the basic unit cell of which the spatially periodic device is comprised. The ensuing algebraic equations governing the vertex-based, discrete unit-cell "fields" P(0)(infinity)(i) and B(i) (i=1,2,...,n), whose paradigmatic summations yield U(*) and D(*), constitute discrete analogs of classical continuous macrotransport phenomenological parameters, P(0)(infinity)(r) and B(r), with r a continuous position vector defined within the unit cell. The ease with which these discrete calculations can be performed for complex networks renders feasible parametric studies of potential microfluidic chip designs, particularly those pertinent to biomolecular separation schemes. Application of this discrete theory to the dispersion analysis of pressure-driven flow in spatially periodic serpentine microchannels is shown to accord with existing results previously derived using classical continuous macrotransport theory.

Journal Article↗
PubMed1996

Tissue distribution kinetics as determinant of transit time dispersion of drugs in organs: application of a stochastic model to the rat hindlimb.

A stochastic theory of drug transport in a random capillary network with permeation across the endothelial barrier is coupled with a model of tissue residence time of drugs assuming radial intratissue diffusion. Axial diffusion is neglected both in tissue as well as in the radially well-mixed vascular phase. The convective transport through the microcirculatory network is characterized by an experimentally determined transit time distribution of a nonpermeating vascular indicator. This information is used to identify three adjustable model parameters characterizing permeation, diffusion, and steady-state distribution into tissue. Predictions are made for the influence of distribution volume, capillary permeability, and tissue diffusion on transit time distributions. The role of convection (through the random capillary network), permeation, and diffusion as determinants of the relative dispersion of organ transit times has been examined. The relationship to previously proposed models of capillary exchange is discussed. Results obtained for lidocaine in the isolated perfused hindleg in rats indicate that although the contribution of intratissue diffusion to the dispersion process is relatively small in quantitative terms, it has a pronounced influence on the shape of the impulse response curve. The theory suggests that the rate of diffusion in muscle tissue is about two orders of magnitude slower than in water.

Anesthetics, Local↗