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 1,027 records · Page 57Linked to original sources
PubMed1992

Gas dispersion in volume-cycled tube flow. I. Theory.

A mathematical theory is derived for the dispersion of a contaminant bolus introduced into a fully developed volume-cycled oscillatory pipe flow. The convection-diffusion equation is solved for a tracer gas bolus by expressing the local concentration field as a series expansion of derivatives of the area-averaged concentration. The local, as well as the area-averaged, concentration is determined for a uniform initial slug or Gaussian bolus. The effect of various flow parameters such as Womersley parameter, Schmidt number, and tidal volume is investigated. The overall dispersion is characterized by a time-averaged effective diffusion coefficient, which for long duration coincides with previous dispersion theories based on a constant linear axial concentration profile. The effective diffusion coefficient can be determined from the local time history of concentration, independent of the spatial location or the initial tracer bolus. Furthermore the local peaks of the concentration-time curve follow a decaying curve dictated by the time-averaged effective diffusion coefficient. Thus the theory is directly applicable for dispersion measurements in oscillatory tube flows, a basis for the pulmonary airways application, as shown by Gaver et al. (J. Appl. Physiol. 72: 321-331, 1992).

Bronchi↗
PubMed1992

Molecular mechanisms of intracellular calcium excitability in X. laevis oocytes.

Following receptor activation in Xenopus oocytes, spiral waves of intracellular Ca2+ release were observed. We have identified key molecular elements in the pathway that give rise to Ca2+ excitability. The patterns of Ca2+ release produced by GTP-gamma-S and by inositol 1,4,5-trisphosphate (IP3) are indistinguishable from receptor-induced Ca2+ patterns. The regenerative Ca2+ activity is critically dependent on the presence of IP3 and on the concentration of intracellular Ca2+, but is independent of extracellular Ca2+. Broad regions of the intracellular milieu can be synchronously excited to initiate Ca2+ waves and produce pulsating foci of Ca2+ release. By testing the temperature dependence of wavefront propagation, we provide evidence for an underlying process limited by diffusion, consistent with the elementary theory of excitable media. We propose a model for intracellular Ca2+ signaling in which wave propagation is controlled by IP3-mediated Ca2+ release from internal stores, but is modulated by the cytoplasmic concentration and diffusion of Ca2+.

Animals↗
PubMed1999

Colloidal Stability of Polymer Colloids with Variable Surface Charge.

Colloidal stability of two latexes with variable charge surface groups has been studied at a range of pH. Experimental log W vs log[electrolyte] plots were fitted using the DLVO theory, and values for the diffuse potential, psi(d), and the Hamaker constant, A, were obtained. Reasonable tendency was found for the diffuse potential of both latexes, decreasing when the surface charge decreased. However, the Hamaker constant, which was expected to be constant over the range of pH covered, also showed a decreasing behavior with decreasing charge. Several theories for the electric repulsive potential have been used to try to explain this paradoxing trend, although none of them was successful. Finally, two explanations are proposed, one based on coion adsorption and the other on the hairy layer model, although the first one seems to be the most probable on the basis of thermodynamics considerations. Copyright 1999 Academic Press.

Journal Article↗
PubMed2005

General theory for migrational voltammetry. Strong influence of diversity in redox species diffusivities on charge reversal electrode processes.

High sensitivity of the microelectrode response to the difference between the substrate and the product diffusion coefficients is predicted for the charge reversal processes. This effect is anticipated from the general theoretical model developed for the diffusional-migrational transport to microelectrodes. The model predicts the voltammetric wave heights for any type of electrode processes carried out in the presence of any number and concentration of nonelectroactive ions. It involves changes in diffusion coefficients of the redox species and assumes no homogeneous complications. Handy, analytical expressions for the limiting current and limiting potential can be derived for a system of a univalent product and univalent ions of supporting electrolyte. This case covers charge reversal processes of the following type: S(z) --> P(+/-) + ne (n + z = sgn(n), absolute value(n) > or = 2). It has been shown that under migrational conditions the change in the ratio of the product and the substrate diffusivities (D(P)/D(S)) by as little as 10% results in significant changes in the voltammetric wave height. For 2-e charge reversal processes, a 10% increase in D(P) versus D(S) leads to a drop in the voltammetric wave height of 18.3% compared to that calculated for equal diffusion coefficients. The reversed change, i.e., the 10% decrease of the D(P) value with respect to D(S), increases the voltammetric wave height by 30.5% compared to that obtained for equal diffusivities. The theoretical predictions were confronted with our recent experimental results obtained for the 2-e oxidation of sodium (6,8-diferrocenylmethylthio)octanoate, which process can be classified as the charge reversal reaction. The best fit was obtained for D(P)/D(S) equal to 0.71.

Journal Article↗
PubMed2003

Thermal diffuse scattering in sub-angstrom quantitative electron microscopy-phenomenon, effects and approaches.

This paper reviews the recent progress in the following areas. (1) In quantitative high-resolution transmission electron microscopy, the theoretically calculated images usually give better contrast than the experimentally observed ones although all of the factors have been accounted for. This discrepancy is suggested due to thermal diffusely scattered (TDS) electrons, which were not included in the image calculation. The contribution from TDS electrons is especially important if the image resolution is approaching 0.1 nm and beyond with the introduction of Cs corrected microscopes. A more rigorous multislice theory has been developed to account for this effect. (2) We proved that the off-axis holography is an ideal energy filter that even filters away the contribution made by TDS electrons in the electron wave function, but conventional high-resolution microscopy do contain the contribution made by phonon scattered electrons. (3) In electron scattering, most of the existing dynamical theories have been developed under the first order diffuse scattering approximation, thus, they are restricted to cases where the lattice distortion is small. A formal dynamical theory is presented for calculating diffuse scattering with the inclusion of multiple diffuse scattering. By inclusion of a complex potential in dynamical calculation, a rigorous proof is given to show that the high order diffuse scattering are fully recovered in the calculations using the equation derived under the distorted wave Born approximation, and more importantly, the statistical time and structure averages over the distorted crystal lattices are evaluated analytically prior numerical calculation. This conclusion establishes the basis for expanding the applications of the existing theories. (4) The 'frozen lattice' model is a semi-classical approach for calculating electron diffuse scattering in crystals arisen from thermal vibration of crystal atoms. Based on a rigorous quantum mechanical phonon excitation theory, we have proved that the frozen lattice mode is an excellent approximation and no detectable error would be possible under normal experimental conditions.

Journal Article↗
PubMed1992

Interaction of blood flow, diffusive transport and cell metabolism in isovolemic anemia.

1) High blood flow can compensate for half-normal hematocrit, leaving the rate at which O2 is offered to the capillaries unchanged. Nevertheless, intracellular PO2 is lower in anemia, indicating impaired diffusive transport. 2) Anemia increases O2 flux per red cell and decreases functional capillary surface area. These changes increase flux density and the extracellular component of resistance to diffusive O2 transport, in accord with current theory (Federspiel and Popel, 1986; Groebe, 1990; Hellums, 1977). 3) Maintenance of diffusive flux in presence of anemia required a larger delta PO2 between Hb and Mb, and higher intracellular O2 conductance brought about by greater Mb-facilitated diffusion. Both compensations depend on lower PmbO2. 4) PmbO2 and creatine charge fall with increasing VO2 and ATP demand. These responses, as well as adaptive changes in redox help maintain VO2 in the presence of a lower O2 drive on electron transport. 5) Greater engagement of reserves of both transport and metabolism limits the range of aerobic performance in anemia. 6) The match between the transcapillary and mitochondrial O2 fluxes depends on interaction of transport and metabolism as a system.

Anemia↗
PubMed1978

Effects of abnormal hemoglobin concentration in human blood on membrane diffusing capacity of the lung and on pulmonary capillary blood volume.

In 13 patients with chronic anemia or polycythemia but normal lung function, and in 7 healthy students a statistically significant correlation between hemoglobin concentration and membrane diffusing capacity of the lung for CO was found. A membrane diffusing constant resulting from extrapolation for hemoglobin concentration 0 g/ was 13.4% of the predicted value (100% for Hb = 14.5 g%). A change of 1 g hemoglobin/100 ml changes membrane diffusing capacity for 6.3% of the predicted value. This result confirms the theory of Koyama and Mochizuki that the diffusing capacity of the pulmonary membrane is a function of the hemoglobin concentration of pulmonary capillary blood. In 3 healthy subjects with normal hemoglobin concentration the effect of DL/VA inhomogenities in the lung on single-breath diffusing capacity and its subdivisions was examined. An increase of DLCO for 20% of predicted normal at decreasing apnea time (tA = 5 sec) resulted from an increased pulmonary capillary blood volume for 50--90% of the predicted value (at tA = 10 sec), whereas membrane diffusing capacity remained unchanged over the range of apnea times. The reasons for this finding are still unknown; alteration of pulmonary perfusion at changing apnea time is taken into account.

Anemia↗
PubMed1979

Spatial patterns for an interaction-diffusion equation in morphogenesis.

A certain interaction-diffusion equation occurring in morphogenesis is considered. This equation is proposed by Gierer and Meinhardt, which is introduced by Child's gradient theory and Turing's idea about diffusion driven instability. It is shown that slightly asymmetric gradients in the tissue produce stable striking patterns depending on its asymmetry, starting from uniform distribution of morphogens. The tool is the perturbed bifurcation theory. Moreover, from a mathematical point of view, the global existence of steady state solutions with respect to some parameters is discussed.

Cell Differentiation↗
PubMed1983

Electric dichroism of DNA. Influence of the ionic environment on the electric polarizability.

In order to test the diffuse ion atmosphere polarization model recently developed by us, the effects of ionic strength, titrating with Mg2+ and Co(NH3)3+6, and coion charge on the electric polarizability of short fragments of DNA are investigated. The results are consistent with the predictions of the theory and show that the diffuse ion atmosphere polarization contributes significantly to the overall orientation of DNA. At low ionic strengths, we attempt to separate the total dipole moment into two components: one that agrees well with the Debye-Hückel ion atmosphere calculations, while the other, presumably due to condensed counterion polarization, appears to be substantially independent of the ionic strength. At higher salt concentrations, however, a simple separation into dipole components is not possible, perhaps due to a significant coupling of ion flows between the diffuse atmosphere and the condensed counterion layer.

Animals↗
PubMed2006

On measurement of molecular and thermal diffusion coefficients in multicomponent mixtures.

We have developed the theory for using the deflection of laser beams of various wavelengths to determine molecular and thermal diffusion coefficients in multicomponent mixtures. In the past, simultaneous determination of molecular and thermal diffusion coefficients have only been achieved for binaries. Our procedure is faster and more accurate than the current techniques to determine molecular diffusion coefficients. The analysis for an N-component mixture requires deflections from (N - 1) beams of different wavelengths. We show in an example that the molecular and thermal diffusion coefficients can be determined reliably in a ternary mixture.

Journal Article↗
PubMed2005

Improving hollow fiber dialyzer efficiency with a recirculating dialysate system. I: Theory and applicability.

The mathematical theory that underlies a novel non-regenerated recirculating dialysate system (RDS) for improving diffusive clearance in hemodialyzers is presented. The theory states the conditions that hemodialyzers must meet to be suitable in RDS optimization. We have verified the applicability of the RDS for several Cuprophan and polysulfone (PS) commercial dialyzers, showing that PS (synthetic) membranes achieve the highest increments of diffusive clearance. A numerical simulation analysis over more general conditions defined by the dimensionless groups of the system demonstrated that the highest diffusive clearance improvements are achieved in dialyzers operating with a low value of the diffusive mass-transfer area/blood flow rate ratio. This study has provided the base for the assessment of the performance of the RDS as compared to several high-efficiency systems, presented in Part II of this work [M. Prado, L. M. Roa, A. Palma, and J. A. Milan, Ann. Biomed. Eng. (2004) submitted].

Blood Chemical Analysis↗
PubMed2004

Diffuse electrical injury: a study of 89 subjects reporting long-term symptomatology that is remote to the theoretical current pathway.

Historically, tissue damage from electrical contact was thought to arise from resistive heating of tissues along the current pathway. The modern view has accepted that tissue damage can result from cellular rupture (electroporation) induced by the presence of an electric field. There remain electrical injuries that defy explanation by either theory. In rare electrical contacts, diffuse symptomatology arises that is neither proportionate to the electrical contact nor does it occur along the theoretical linear pathway of the current from entry point to exit point. Disproportionate, remote electrical injury is most notable when the contact voltage is low (120 and 240 V). Symptoms occur, absent diagnostic evidence, that defy explanation as organic injury. A Web-based interactive survey was used to locate and query individuals suffering from rarely occurring responses to electrical contact. The results of the study suggest that there is a common symptomatology that is neither linked to voltage nor loss of consciousness at the time of contact.

Accidents↗