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At least 1,297 records · Page 72Linked to original sources
PubMed2006

Improved theory of cyclical electrical field flow fractionation.

Previously reported theories for cyclical electrical field flow fractionation (CyElFFF) are severely limited in that they do not account for diffusion, steric, or electric double layer effects. Experiments have shown that these theories overpredict the retention of particles in CyElFFF. In this work, we present a model for prediction of steric, diffusion, and electrical effects. The electrical double layer effects are treated using a lumped electrical circuit model that accounts for the field shielding by the electrical double layer formed at the electrode-carrier interface. The electrical effects are shown to dominate retention times and outweigh the contributions of diffusion and particle size. Detailed results from the simulations are presented in this work, and a comparison between the theoretical and experimental results obtained from the retentions of polystyrene particle standards is presented in this paper. The models are shown to correctly predict the retention of the polystyrene standards in CyElFFF with a reasonable error, while existing models are shown to have significant failings.

Electricity↗
PubMed2002

Iterative renormalization group for anomalous dimension in a nonlinear diffusion process.

We construct a classical successive method, the Picard method in integral equation theory, to make an iterative algorithm with the renormalization group (RG) approach to calculate the anomalous dimension in a nonlinear diffusion equation. We find our result improves than the original RG work because we begin with the var epsilonth RG solution, not the trivial fixed-point solution.

Journal Article↗
PubMed1993

Hindered diffusion of high molecular weight compounds in brain extracellular microenvironment measured with integrative optical imaging.

This paper describes the theory of an integrative optical imaging system and its application to the analysis of the diffusion of 3-, 10-, 40-, and 70-kDa fluorescent dextran molecules in agarose gel and brain extracellular microenvironment. The method uses a precisely defined source of fluorescent molecules pressure ejected from a micropipette, and a detailed theory of the intensity contributions from out-of-focus molecules in a three-dimensional medium to a two-dimensional image. Dextrans tagged with either tetramethylrhodamine or Texas Red were ejected into 0.3% agarose gel or rat cortical slices maintained in a perfused chamber at 34 degrees C and imaged using a compound epifluorescent microscope with a 10 x water-immersion objective. About 20 images were taken at 2-10-s intervals, recorded with a cooled CCD camera, then transferred to a 486 PC for quantitative analysis. The diffusion coefficient in agarose gel, D, and the apparent diffusion coefficient, D*, in brain tissue were determined by fitting an integral expression relating the measured two-dimensional image intensity to the theoretical three-dimensional dextran concentration. The measurements in dilute agarose gel provided a reference value of D and validated the method. Values of the tortuosity, lambda = (D/D*)1/2, for the 3- and 10-kDa dextrans were 1.70 and 1.63, respectively, which were consistent with previous values derived from tetramethylammonium measurements in cortex. Tortuosities for the 40- and 70-kDa dextrans had significantly larger values of 2.16 and 2.25, respectively. This suggests that the extracellular space may have local constrictions that hinder the diffusion of molecules above a critical size that lies in the range of many neurotrophic compounds.

Animals↗
PubMed2006

Photoinduced electron transfer and geminate recombination in the group head region of micelles.

A pump-probe spectroscopic study of photoinduced forward electron transfer and geminate recombination between donors and acceptors located in the head group regions of micelles is presented. The hole donor is octadecyl-rhodamine B (ODRB) and the hole acceptor is N,N-dimethyl-aniline (DMA). The experiments are conducted as a function of the DMA concentration in the dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In spite of the fact that the absorptions of both the ODRB radical and ground state bleach spectrally overlap with the ODRB excited state absorption, a procedure that makes it possible to determine the geminate recombination dynamics is presented. These experiments are the first to measure the dynamics of geminate recombination in micelles, and the experiments have two orders of magnitude better time resolution than previous studies of forward transfer. The experimental data are compared to statistical mechanics theoretical calculations of both the forward transfer and the geminate recombination. The theory includes important aspects of the topology of the micelle and the diffusion of the donor-acceptors in the micelle head group region. A semiquantitative but nonquantitative agreement between theory and experiments is achieved.

Aniline Compounds↗
PubMed1974

Restricted diffusion in biophysical systems. Experiment.

The pulsed-gradient spin echo nuclear magnetic resonance (PGSENMR) technique was used to measure restricted diffusion of water in three types of animal tissue: human blood plasma and red cells; rat and rabbit heart; rat and rabbit liver. Characteristic lengths (L) for restriction of diffusion are estimated from dependence on the measuring time. Limitations on the range of observable restrictive lengths (1.5-15 mum) are discussed.The decrease in diffusivity due to 1 mum alumina powder (volume fraction = 0.18) in glycerin/water mixtures agrees with the Wang theory assuming spherical particles and no hydration. The characteristic length (L approximately 4 mum) is larger than the particle size (1 mum) or separation (1.8 mum). Comparison of the diffusivities in tissues at short diffusion times with the Wang theory indicates some bound or trapped water.For packed red blood cells, a restriction (L approximately 2.3 mum) was attributed tothe red cell membrane. A permeability p approximately 0.014 cm/s may be estimated from the decrease in diffusivity. Average values of diffusivity ratio in heart were: 0.36 +/- 0.02 for rat; and 0.26 +/- 0.03 for rabbit; and in liver: 0.25 +/- 0.01 for rat; 0.25 +/- .04 for 10-day old rabbit; and 0.195 +/- 0.03 for 2-yr old rabbit. A restriction (L approximately 2.7 mum) in rat liver probably results from the mitochondria.

Aluminum↗
PubMedDate not supplied

Flow, diffusion, and thermal convection in percolation clusters: NMR experiments and numerical FEM/FVM simulations.

Percolation objects were fabricated based on computer-generated, two- or three-dimensional templates. Random-site, semi-continuous swiss cheese, and semi-continuous inverse swiss-cheese percolation models above the percolation threshold were considered. The water-filled pore space was investigated by NMR imaging and, in the presence of a pressure gradient, NMR velocity mapping. The fractal dimension, the correlation length, and the percolation probability were evaluated both from the computer-generated templates and the corresponding NMR spin density maps. Based on velocity maps, the percolation backbones were determined. The fractal dimension of the backbones turned out to be smaller than that of the complete cluster. As a further relation of interest, the volume-averaged velocity was calculated as a function of the probe volume radius. In a certain scaling window, the resulting dependence can be represented by a power law the exponent of which was not yet considered in the theoretical literature. The experimental results favorably compare to computer simulations based on the finite-element method (FEM) or the finite-volume method (FVM). Percolation theory suggests a relationship between the anomalous diffusion exponent and the fractal dimension of the cluster, i.e., between a dynamic and a structural parameter. We examined interdiffusion between two compartments initially filled with H2O and D2O, respectively, by proton imaging. The results confirm the theoretical expectation. As a third transport mechanism, thermal convection in percolation clusters of different porosities was studied with the aid of NMR velocity mapping. The velocity distribution is related to the convection roll size distribution. Corresponding histograms consist of a power law part representing localized rolls, and a high-velocity cut-off for cluster-spanning rolls. The maximum velocity as a function of the porosity clearly visualizes the percolation transition.

Computer Simulation↗
PubMed2000

DNA electrophoresis in a monodisperse porous medium.

Electrophoresis of DNA migrating in an ordered matrix is studied and compared with classical agarose gel electrophoresis. A well-defined migration medium is obtained by crystallization of monodisperse silica spheres. Electrophoretic mobility of DNA is measured with fluorescence recovery after photobleaching experiments. The main result is that, as it was the case for gel electrophoresis, diffusion and electrophoretic mobility of DNA in such a medium are well described with reptation theories.

Base Pairing↗
PubMed2004

Diffuse scattering provides material parameters and electron density profiles of biomembranes.

Fully hydrated stacks of DOPC lipid bilayer membranes generate large diffuse x-ray scattering that corrupts the Bragg peak intensities that are used in conventional biophysical structural analysis, but the diffuse scattering actually contains more information. Using an efficient algorithm for fitting extensive regions of diffuse data to classical smectic liquid crystalline theory we first obtain the compressional modulus B= 10(13) erg/ cm(4), which involves interactions between membranes, and the bending modulus K(c) =8x 10(-13) erg of the membranes. The membrane form factor F ( q(z) ) is then obtained for most values of q(z) up to 0.8 A(-1). The electron density profile rho(z) is obtained by fitting models to F( q(z) ). Constraining the models to conform to other measurements provides structural quantities such as area A=72.1+/-0.5 A(2) per lipid at the interface.

Absorptiometry, Photon↗
PubMed2005

Free-volume dynamics in glasses and supercooled liquids.

A free-volume theory is developed based on the defect diffusion model (DDM). In addition, positronium annihilation lifetime spectroscopy (PALS) ortho-positronium free-volume and intensity data are presented for poly(propylene glycol) with a molecular weight of 4000 (PPG 4000) in both the glassy and liquid states and dielectric relaxation and electrical conductivity data are reported for PPG 4000 in the liquid state. The DDM is used to interpret all of the data for PPG 4000 and previously reported PALS and dielectric relaxation data for glycerol. It is shown that while the PPG 4000 data exhibit a preference for the three-halves power law, the data for glycerol favor the first power (standard Vogel-Fulcher-Tammann) law. Good agreement between the DDM and the experimental results is found for all of the electrical data and the PALS free-volume data. While reasonable agreement is also found for the PALS intensity data for PPG 4000, a discrepancy exists between the experimental PALS intensity data and theory for glycerol. For the electrical conductivity for PPG 4000, a transition is observed at the same temperature (about 1.4 T(g) where T(g) is the glass transition temperature) where the PALS free volume changes from steeply rising with temperature to approximately independent of temperature. The same behavior is observed at about 1.5 T(g) for previously reported dielectric relaxation and PALS data for glycerol. Model parameters are presented that show the dominance of mobile single defects above (1.4-1.5) T(g) and the dominance of immobile clustered single defects below T(g) . Finally, a coherent picture of glasses and glass-forming liquids is presented based on the theory and results of the experiments.

Journal Article↗
PubMed2001

Transport tensors in perfectly aligned low-density fluids: self-diffusion and thermal conductivity.

The modified Taxman equation for the kinetic theory of low-density fluids composed of rigid aspherical molecules possessing internal degrees of freedom is generalized to obtain the transport tensors in a fluid of aligned molecules. The theory takes care of the shape of the particles exactly but the solution has been obtained only for the case of perfectly aligned hard spheroids within the framework of the first Sonine polynomial approximation. The expressions for the thermal-conductivity components have been obtained for the first time whereas the self-diffusion components obtained here turn out to be exactly the same as those derived by Kumar and Masters [Mol. Phys. 81, 491 (1994)] through the solution of the Lorentz-Boltzmann equation. All our expressions yield correct results in the hard-sphere limit.

Journal Article↗
PubMed1997

Experimental and Theoretical Diffusivities of Cd and Sr in Hydrous Ferric Oxide

Oxides of manganese, aluminum, and especially iron are important sorbents for inorganic contaminants. The sorption process can be characterized by two steps. The first step is a rapid, reversible reaction between the bulk aqueous phase and external surfaces. The slow, second step is the rate limiting mechanism wherein the contaminant diffuses through small pores along surface sites. Isotherm and constant boundary condition studies were conducted to evaluate the sorption process. Best fit experimental surface diffusivities ranged from 10(-14) to 6 x 10(-13) cm2/s. Using site activation theory and assuming a sinusoidal potential field along the pore surface, theoretical surface diffusion coefficients were estimated from the adsorption enthalpy.

Journal Article↗
PubMed1987

A method for estimating lateral diffusion coefficients in membranes from steady-state fluorescence quenching studies.

The Stern-Volmer theory, in which the quantum yield ratio (Io/I) depends linearly on the quencher concentration, will typically be inapplicable to fluorescence quenching in membranes. Numerical analysis shows that diffusion-controlled quenching results in a nonlinear concentration dependence for diffusion coefficients less than or of the order of 10(-6) cm2 s-1 and probe fluorescence lifetimes in the region of 10-100 ns. Lateral diffusion coefficients in membranes are typically overestimated an order of magnitude or more by the Stern-Volmer theory. An alternative empirical method is presented, which represents nonlinear concentration curves by a single parameter linear approximation determined by a least-squares analysis. The fitting parameter, P, depends on the interaction distance, the membrane thickness, the maximum extent of quenching and, in the case of biexponential probe fluorescence decay, the fluorescence kinetic parameters. P is presented in tabular form for a useful range of these parameters. The method is used to estimate diffusion coefficients for plastoquinone and plastoquinol from pyrene fluorescence quenching in soya bean phosphatidylcholine liposomes. It is found that the diffusion coefficients are nearly equal and in the region of 1.3-3.5 X 10(-7) cm2 s-1 for interaction radii of 1.5-0.5 nm, respectively.

Diffusion↗
PubMed2003

Proteins in cerebrospinal fluid and blood: barriers, CSF flow rate and source-related dynamics.

Cerebrospinal fluid (CSF) routine analysis for diagnosis of neurological diseases is based on the concepts for discrimination of blood-derived and brain-derived immunoglobulin fractions in CSF. The actual molecular flux/CSF flow theory of the blood/CSF barrier function, which founded the hyperbolic discrimination lines in quotient diagrams, is derived from the laws of molecular diffusion combined with CSF flow rate. It emerged from this theory that the decrease of CSF flow rate is sufficient to explain quantitatively the increase of CSF protein concentrations as observed in many neurological diseases. With this concept of CSF flow rate as the modulator of the normal and pathological blood-CSF barrier function, we got for the first time a theoretical frame work to explain also quantitatively the dynamics of brain-derived proteins and their source related (neurons and glial cells or leptomeningal cells) differences. The review of the anatomical, physiological and biophysical knowledge points to the new interpretations: The changing albumin quotient is an indicator of changing CSF flow rate and not for a morphological "leakage" of the blood-brain barrier. As an application of these concepts the dynamics of brain-derived molecules in blood are discussed with two examples: beta trace protein, flowing with CSF into venous blood, and neuron-specific enolase, passing from tissue into blood the opposite direction of serum proteins, again a gradient-dependent protein diffusion across the intact blood vessel wall.

Animals↗
PubMed1997

Transforming research and practice with the human becoming theory.

This article focuses on the human becoming theory as a guide to the transformation of research and practice. New knowledge gained from human becoming research guides the nurse in practice. Different ways of being in true presence in living the human becoming theory are discussed.

Diffusion of Innovation↗
PubMed2001

Effect of a range of microbial polysaccharides on the diffusion of manganese ions using spatially resolved NMR relaxometry.

In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases the rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion coefficient of a model cation, Mn(2+,) through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn(2+) in solution was measured by recording the change in the spin-spin (T(2)) relaxation time of water (1)H over time in compartments either side of a polymer layer. This approach provides a sensitive, in situ, non-invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2-2.5x) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysaccharide chitosan reduced the rate of Mn(2+) diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface.

Journal Article↗
PubMed2003

Theory of the viscosity of supercooled liquids and the glass transition: fragile liquids.

A statistical mechanical theory is presented for viscosity of relatively low molecular weight organic liquids which are supercooled down to the glass transition temperature. In this theory a relation resembling the Stokes-Einstein relation between the viscosity and self-diffusion coefficient of supercooled liquids and an expression for the self-diffusion coefficient are augmented by a suitably constructed semiempirical generic van der Waals equation of state that makes it possible to calculate the free volume. The theory accounts in excellent accuracy for viscosities and self-diffusion coefficients of fragile liquids over the entire range of temperature experimentally investigated. According to the theory, vitrification occurs when the free volume available for translational molecular motion falls below a critical value.

Journal Article↗
PubMed2004

Symmetric diffusion equations, barodiffusion, and cross-diffusion in concentrated liquid mixtures.

In models of diffusion in multicomponent mixtures, the current practice is to derive equations for an isobaric system. The equations are nonsymmetric in relation to the components of the mixture, and the concentration of solvent is assumed to be governed by the conservation of mass instead of its own corresponding diffusion equation. For concentrated mixtures, the solvent component is selected arbitrarily, which makes interpretation of the experimental data dependent on the choice of the interpreter. In this work, we derive a symmetric system of equations, made possible by the introduction of a spontaneously produced secondary pressure gradient. The effect of that pressure gradient is barodiffusion (barophoresis), defined by the force expressed as the secondary pressure gradient multiplied by the molecular volume. The model also considers the cross-diffusion (diffusiophoresis) that results from the hydrodynamic stresses associated with the local concentration-induced pressure gradient in liquid layers surrounding individual molecules. The resulting system of diffusion equations, which contains the secondary pressure gradient and component concentrations, is applied to a binary (nonionic) mixture of benzene and 1,2-dichloroethane. The steady-state system is placed in a uniform force field, and the effect of the secondary pressure gradient on the field-induced migration is discussed. Fluctuation dynamics in a system with no external force field is also discussed. The numerical results predict the establishment of lower concentration gradients compared to standard theory. Also, the predicted concentration dependence in the effective diffusion coefficient measured by dynamic light scattering is different compared to standard theory. Finally, experiments are proposed to further evaluate differences between the new model and the standard approach.

Journal Article↗