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.

SourceResearch connectionFrom yearThrough year

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

At least 1,189 records · Page 66Linked to original sources

PubMed1991

Time dependent rate of diffusion-influenced ligand binding to receptors on cell surfaces.

The theory of the kinetics of binding of ligands to a sphere partially covered by receptors is extended to provide the full time dependence of the reactive flux. The ligands diffuse to the receptors; the receptors are either fully or partially absorbing. The total flux into the sphere with many receptors is expressed analytically in terms of the flux into a single isolated receptor on the sphere. At steady state, the Berg-Purcell formula is generalized to the case where the binding to a single receptor is only partially diffusion controlled. At short times, the receptors behave independently and the total flux is the sum of the fluxes to the isolated receptors.

Cell Membrane↗

PubMed1997

Transfer and counterconditioning of conditional control in the rabbit nictitating membrane response.

Two experiments using the rabbit nictitating membrane response investigated whether training in one conditional discrimination (A-->X+, B-->X-), enabled the feature cues (A and B) to modulate responding to another CS (Y) trained as a target stimulus in a second conditional discrimination (C-->Y+, D-->Y-). There was near-complete transfer of the feature cue's conditional control, indicating that the feature cue's ability to modulate responding is not based on an association specific to the training target. Experiment 2 also revealed that the role of a stimulus to act as a conditional cue is affected by its ability to act as a simple conditioned excitor or inhibitor. Following initial acquisition of two conditional discriminations, two feature cues were reinforced in a pattern consistent with the initial conditional discrimination (A-->+, B-->-), where as the other two feature cues were reinforced in the reverse pattern to that of the original conditional discrimination (C-->-, D-->+). Subsequent tests revealed that the reversed training of the feature cues interfered with the original conditional discriminations. The results are consistent with theories that the feature cue gains an association with a representation of the emotional attributes of the US, which acts to modulate responding to the target stimulus through a diffuse change in motivational level. However, hierarchical theories of conditional discriminations that assume a lack of CS-specificity may also be able to explain the findings.

Analysis of Variance↗

PubMed1984

Bead centrifuge tube inserts--a semi-quantitative theory.

A semi-quantitative theory is proposed for the sedimentation of particles in centrifuge tubes filled with polymer beads acting as sedimentation aids. The theory concerns the attainment of approximate sedimentation-diffusion equilibrium in minimum time of centrifugation. It is shown empirically that the time to reach this state is approximately proportional to the diameter of the polymer beads as predicted by the theory.

Diffusion↗

PubMed1990

Capillary permeability of 131I-albumin in the resting human forearm.

STUDY OBJECTIVE: The aim was to examine capillary permeability of 131I-albumin in the normal, resting human forearm. DESIGN: A bolus injection of 131I-albumin was injected into the brachial artery, and the residue function was measured by external registration over the forearm tissues. The results were analysed by indicator kinetic "black box" theory using the single injection, residue detection method which is based on indicator diffusion principles. SUBJECTS: Seven normal volunteers participated in the study. MEASUREMENTS AND MAIN RESULTS: The mean capillary extraction fraction of 131I-albumin was 0.023 (SD 0.0056), n = 7, at a mean plasma flow rate of 2.1 (0.34) ml.100 g-1.min-1. At an estimated capillary surface area of 70 cm.g-1 the permeability coefficient was 11.10(-8) cm.s-1. CONCLUSIONS: According to the theories of restricted diffusion and equivalent pores the results are compatible with an equivalent pore radius estimate of 113 A using additional previously published results from experiments with 51Cr-EDTA.

Albumins↗

PubMed1978

Fluorescence energy transfer in the rapid-diffusion limit.

Energy transfer is enhanced by translational diffusion of the donor and acceptor [Steinberg, I. Z. & Katchalski, E. (1968) J. Chem. Phys. 48, 2404-2410]. The effect of diffusion on energy transfer depends on Dtau(0)/s(2), in which D is the sum of the diffusion coefficients of the donor and acceptor, tau(0) is the lifetime of the donor in the absence of transfer, and s is the mean distance between donors and acceptors. In most previous studies, Dtau(0)/s(2) << 1, corresponding to the static limit. We report here steady-state and kinetic fluorescence experiments showing that Dtau(0)/s(2) >> 1, the rapid-diffusion limit, can be attained by using Tb(3+) chelated to dipicolinate as a long-lived energy donor (tau(0) = 2.2 msec). The concentration of rhodamine B, the energy acceptor, resulting in 50% transfer was 0.67 muM, which is three orders of magnitude less than the concentration giving 50% transfer in the static limit. The dependence of the transfer efficiency on diffusion coefficients varying from 5 x 10(-11) to 1.5 x 10(-4) cm(2)/sec, spanning the range from the static limit to the rapid-diffusion limit, is in excellent agreement with theory. It is evident that energy donors with millisecond or longer excited state lifetimes can be used to probe translational motions in membranes and other assemblies. Energy transfer in the rapid diffusion limit is sensitive to the distance of closest approach (a) of the donor and acceptor. For a Tb.(DPA)(3) chelate trapped inside the aqueous space of a membrane vesicle containing eosin phosphatidylethanolamine, a = 10 A. The transverse location of chromophores in model membranes and biological membranes can be determined by this technique.

Journal Article↗

PubMed2002

Fast Kawasaki spin exchange limit of spin-facilitated kinetic Ising models.

We study a Fock-space operator technique for describing the stochastic kinetics of a spin-facilitated kinetic Ising model. We focus in particular on the diffusion (fast Kawasaki exchange) limit in which the kinetics can be described by a single mean field evolution equation. We derive some general criteria for the approximative validity of mean field theory for the case of a nondiverging diffusion coefficient of the local spin states.

Journal Article↗

PubMed2003

Three-component competitive adsorption model for flow-through PAC systems. 1. Model development and verification with a PAC/membrane system.

Natural organic matter (NOM) interferes with the adsorption of trace organic compounds on porous adsorbents such as powdered activated carbon (PAC) by pore blockage and direct competition for adsorption sites. The competitive effect of NOM in flow-through systems in which the retention time of the PAC is greater than the hydraulic retention time of the system can be magnified because NOM from the influent water can continue to adsorb on the PAC retained in the system. As a result, the adsorption capacity and the diffusion coefficient of trace compounds can decrease as NOM from the influent water accumulates. In this study, a dynamic three-component adsorption model was developed to quantitatively describe the removal of a trace compound from water in flow-through PAC processes. The system was simplified by using p-dichlorobenzene (p-DCB) to represent the NOM fraction that competes directly with the target trace organic atrazine for adsorption sites and by using poly(styrene sulfonate) (PSS-1.8k) to represent large, pore-blocking NOM. The model was based on the homogeneous surface diffusion assumption with the adsorption capacity of atrazine being gradually adjusted using a simplified version of the ideal adsorbed solution theory model developed in this study. The surface diffusion coefficients of atrazine and p-DCB were modeled as a function of the surface concentration of the pore-blocking compound, PSS-1.8k. The model was verified experimentally with a PAC/microfiltration (MF) system. The use of single-solute adsorption parameters obtained from batch isotherm and kinetic tests resulted in good model predictions for the adsorption of atrazine and the two model compounds under operating conditions typical of PAC/MF systems. The model will be applied to study various operating conditions and other system parameters of PAC/membrane systems in part 2 of this study.

Adsorption↗

PubMed2004

Nuclear magnetic resonance study of the vapor contribution to diffusion in silica glasses with micrometer pores partially filled with liquid cyclohexane or water.

The contribution of the vapor phase to molecular diffusion in porous silica glass (Vitrapor#5; mean pore diameter 1 micrometer) partially filled with cyclohexane (nonpolar) or water (polar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane increased up to ten times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. A two-phase exchange theory is presented accounting well for all experimental diffusion features. The diffusion behavior in the samples with micrometer pores under investigation here is in contrast to previous findings for the same solvents in a material with nanometer pores (Vycor; mean pore diameter 4 nm) where the fast-exchange limit had to be assumed [Ardelean et al., J. Chem. Phys. 119, 10358 (2003)]. It is concluded that the pore size plays a crucial role for the relevance of molecular exchange limits relative to the experimental diffusion/exchange time.

Journal Article↗

PubMedDate not supplied

Effect of protein adsorption on the transport characteristics of asymmetric ultrafiltration membranes.

The effects of bovine serum albumin adsorption on the transport characteristics of asymmetric poly(ether sulfone) ultrafiltration membranes were determined using polydisperse dextrans with gel permeation chromatography. Actual dextran sieving coefficients were evaluated from observed sieving data for both the clean and preadsorbed membranes using a stagnant film model. The flux dependence of the actual dextran sieving coefficients was used to evaluate the intrinsic membrane hindrance factors for convective (i.e., sieving) and diffusive transport for the different molecular weight dextrans using classical membrane transport theory. Protein adsorption caused a reduction in both dextran sieving and diffusion, with the magnitude of the reduction a function of the dextran molecular weight and pore size. The effects of adsorption on the specific pore area and the membrane porosity were then determined using a recent model for solute transport through asymmetric ultrafiltration membranes. The data indicate that protein adsorption occurs preferentially in the larger membrane pores, causing a greater reduction in solute sieving compared to the membrane hydraulic permeability and porosity than would be predicted on the basis of either a simple pore blockage or pore constriction model.

Adsorption↗

PubMed2002

Diffusion in a granular fluid. II. Simulation.

The linear-response description for impurity diffusion in a granular fluid undergoing homogeneous cooling is developed in the preceding paper. The formally exact Einstein and Green-Kubo expressions for the self-diffusion coefficient are evaluated there from an approximation to the velocity autocorrelation function. These results are compared here to those from molecular-dynamics simulations over a wide range of density and inelasticity, for the particular case of self-diffusion. It is found that the approximate theory is in good agreement with simulation data up to moderate densities and degrees of inelasticity. At higher density, the effects of inelasticity are stronger, leading to a significant enhancement of the diffusion coefficient over its value for elastic collisions. Possible explanations associated with an unstable long wavelength shear mode are explored, including the effects of strong fluctuations and mode coupling.

Journal Article↗

PubMed1996

Multicomponent diffusion in polymeric liquids.

It is shown how the phase-space kinetic theory of polymeric liquid mixtures leads to a set of extended Maxwell-Stefan equations describing multicomponent diffusion. This expression reduces to standard results for dilute solutions and for undiluted polymers. The polymer molecules are modeled as flexible bead-spring structures. To obtain the Maxwell-Stefan equations, the usual expression for the hydrodynamic drag force on a bead, used in previous kinetic theories, must be replaced by a new expression that accounts explicitly for bead-bead interactions between different molecules.

Journal Article↗

PubMed1987

Transverse relaxation of solvent protons induced by magnetized spheres: application to ferritin, erythrocytes, and magnetite.

Since 1/T2 of protons of tissue water is generally much greater than 1/T1 at typical imaging fields, small single-ion contrast agents--such as Gd(DTPA), which make comparable incremental contributions and therefore smaller fractional contributions to 1/T2 compared to 1/T1--are not as desirable for contrast-enhancement as agents that could enhance 1/T2 preferentially. In principle, such specialized agents will only be effective at higher fields because the field dependence (dispersion) of 1/T1 is such that it approaches zero at high fields whereas 1/T2 approaches a constant value. The residual 1/T2 is called the "secular" contribution and arises from fluctuations in time--as sensed by the protons of diffusing solvent or tissue water molecules--of the component of the magnetic field parallel to the static applied field. For solutions or suspensions of sufficiently large paramagnetic or ferromagnetic particles (greater than or equal to 250 A diameter), the paramagnetic contributions to the relaxation rates satisfy 1/T2 much greater than 1/T1 at typical imaging fields. We examine the theory of secular relaxation in some detail, particularly as it applies to systems relevant to magnetic resonance imaging, and then analyze the data for solutions, suspensions, or tissue containing ferritin, erythrocytes, agar-bound magnetite particles, and liver with low-density composite polymer-coated magnetite. In most cases we can explain the relaxation data, often quantitatively, in terms of the theory of relaxation of protons (water molecules) diffusing in the outer sphere environments of magnetized particles. The dipolar field produced by these particles has a strong spatial dependence, and its apparent fluctuations in time as seen by the diffusing protons produce spin transitions that contribute to both 1/T1 and /T2 comparably at low fields; for the larger particles, because of dispersion, the secular term dominates at fields of interest. On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low-density polymer-coated magnetite agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin--for which 1/T2 is unexpectedly large--the source of its large relaxivity must reside in nonideal chemistry of the ferritin core. For blood, it appears that diffusion through intracellular gradients determines 1/T2.

Animals↗

PubMed2001

Drug diffusion mechanism through pH-sensitive hydrophobic/polyelectrolyte hydrogel membranes.

Methylmethacrylate/dimethylaminoethyl methacrylate cross-linked with divinylbenzene is a pH-sensitive hydrogel. We have studied the diffusion mechanisms of drugs with different water solubilities through this hydrogel. A water-soluble model drug (aminopyrine) was used to study the diffusion coefficient changes in different pHs. The results showed a water-content dependent diffusion for this pH-sensitive polycationic hydrogel. However, decreasing the solubility of the drug and increasing the hydrophobic character of the polymer by changing the pH caused a greater affinity (or partition coefficient) between the hydrogel and the drug. Aminopyrine diffusion was shown to follow the free-volume theory, suggesting the 'pore' type mechanism for water soluble drugs, while the 'partition' or 'solution-diffusion' mechanism better described the slow diffusion of water insoluble solutes through this pH-sensitive hydrogel. Comparing the swelling interface number for aminopyrine release through this pH-sensitive hydrogel showed a non-Fickian mechanism in the hydrated form of the hydrogel (pH 1.2), while Fickian in the dehydrated form (pH 7.4).

Aminopyrine↗

PubMed2005

Diffusion of spheres in crowded suspensions of rods.

Translational tracer diffusion of spherical macromolecules in crowded suspensions of rodlike colloids is investigated. Experiments are done using several kinds of spherical tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length L of the rods and the diameter 2a of the tracer sphere is covered by combining several experimental methods: fluorescence correlation spectroscopy for small tracer spheres, dynamic light scattering for intermediate sized spheres, and video microscopy for large spheres. Fluorescence correlation spectroscopy is shown to measure long-time diffusion only for relatively small tracer spheres. Scaling of diffusion coefficients with a/xi, predicted for static networks, is not found for our dynamical network of rods (with xi the mesh size of the network). Self-diffusion of tracer spheres in the dynamical network of freely suspended rods is thus fundamentally different as compared to cross-linked networks. A theory is developed for the rod-concentration dependence of the translational diffusion coefficient at low rod concentrations for freely suspended rods. The proposed theory is based on a variational solution of the appropriate Smoluchowski equation without hydrodynamic interactions. The theory can, in principle, be further developed to describe diffusion through dynamical networks at higher rod concentrations with the inclusion of hydrodynamic interactions. Quantitative agreement with the experiments is found for large tracer spheres, and qualitative agreement for smaller spheres. This is probably due to the increasing importance of hydrodynamic interactions as compared to direct interactions as the size of the tracer sphere decreases.

Journal Article↗

PubMed2002

A free volume microprobe study of water sorption in a contact lens polymer.

The kinetics of moisture uptake in a rigid gas-permeable contact lens material, Fluoroperm60, has been investigated. The free volume changes accompanied by the sorption of water were monitored using the positron lifetime spectroscopy (PLS) technique. The positron results indicated swelling in the initial stages of sorption and the filling of vacant free volume sites with water in the later stages of sorption. The sorption curve suggests that the diffusion process in this polymer follows the dual-mode sorption model. Using this model, it was possible to separate Fickian-controlled diffusion and relaxation-controlled diffusion. Furthermore, the positron results combined with the sorption results indicated that Fujita's free volume theory does not seem to be valid for the diffusion process in the present sample under study. The variation of the permeability as a function of the free volume suggests that the amount of water in a contact lens is a measure of its permeability.

Acrylic Resins↗

PubMed2003

Low-pressure diffusion equilibrium of electronegative complex plasmas.

A self-consistent fluid theory of complex electronegative colloidal plasmas in parallel-plate low-pressure discharge is presented. The self-organized low-pressure diffusion equilibrium is maintained through sources and sinks of electrons, positive and negative ions, in plasmas containing dust grains. It is shown that the colloidal dust grain subsystem strongly affects the stationary state of the discharge by dynamically modifying the electron temperature and particle creation and loss processes. The model accounts for ionization, ambipolar diffusion, electron and ion collection by the dusts, electron attachment, positive-ion-negative-ion recombination, and relevant elastic and inelastic collisions. The spatial profiles of electron and positive-ion-negative-ion number densities, electron temperature, and dust charge in electronegative SiH4 discharges are obtained for different grain size, input power, neutral gas pressure, and rates of negative-ion creation and loss.

Journal Article↗

PubMed2002

Kinetics in one-dimensional lattice gas and Ising models from time-dependent density-functional theory.

Time-dependent density-functional theory, proposed recently in the context of atomic diffusion and nonequilibrium processes in solids, is tested against Monte Carlo simulation. In order to assess the basic approximation of that theory, the representation of nonequilibrium states by a local equilibrium distribution function, we focus on one-dimensional lattice models, where all equilibrium properties can be worked exactly from the known free energy as a functional of the density. This functional determines the thermodynamic driving forces away from equilibrium. In our studies of the interfacial kinetics of atomic hopping and spin relaxation, we find excellent agreement with simulations, suggesting that the method is also useful for treating more complex problems.

Journal Article↗

PubMed1998

Modeling of self-diffusion and relaxation time NMR in multi-compartment systems.

The theory of pulsed field gradient (pfg) NMR applied to molecules in cellular systems which contain different subcellular compartments separated by permeable membranes, acting as diffusion barriers, has been extended. A numerical model of restricted diffusion and magnetization relaxation behavior in pfg-CPMG NMR experiments, based on the Fick's second law of diffusion, is presented. This model is applicable to a wide range of systems and allows the exploration of temporal and spatial behavior of the magnetization with and without the influence of gradient pulses. Results of the numerical experiments show their correspondence to the previously observed ones and demonstrate the importance of the inclusion of the time domain data in analyzing diffusion measurements.

Magnetic Resonance Spectroscopy↗