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PubMed2000

From a profiled diffuser to an optimized absorber

The quadratic residue diffuser was originally designed for enhanced scattering. Subsequently, however, it has been found that these diffusers can also be designed to produce exceptional absorption. This paper looks into the absorption mechanism of the one-dimensional quadratic residue diffuser. A theory for enhanced absorption is presented. Corresponding experiments have also been done to verify the theory. The usefulness of a resistive layer at the well openings has been verified. A numerical optimization was performed to obtain a better depth sequence. The results clearly show that by arranging the depths of the wells properly in one period, the absorption is considerably better than that of a quadratic residue diffuser.

Journal Article↗
PubMed2005

Kinetics of proton diffusion in the regimes of fast and slow exchange between the membrane surface and the bulk solution.

The phenomenological model developed in our recent publications [9,10] is used to investigate the kinetics of proton diffusion from a source to a detector on the membrane surface. In most cases the observed kinetics shows a single diffusional maximum with the exponential ascending front and the power-law descending tail. The kinetics depends on the distance between the source and the detector. If the detector is located inside the proton collecting antenna, the kinetics corresponds to the surface diffusion at the times near the maximum and shortly thereafter, and it turns into the bulk diffusion kinetics at longer times, after the equilibrium is established between the membrane surface and the bulk solution. If the detector is located outside the antenna, the kinetics corresponds to the bulk diffusion at all times where the signal is nonvanishing. What is seen at locations near the antenna radius depends on the exchange regime. In the regime of fast exchange between the surface and the bulk as compared to the bulk diffusion, the kinetics shows a single peak whose location is intermediate between the peaks for the surface and bulk diffusion. In the regime of slow exchange there are two maxima corresponding to the surface and bulk diffusion. In buffered solutions the antenna radius decreases with increasing buffer concentration, which changes the kinetics from the surface to bulk diffusion. The theory is applied to interprete recent experiments on a phospholipid membrane [25]. It is found that (i) the fast exchange regime is operating since only a single maximum is observed; (ii) the shift of the maximum toward longer times with increasing buffer concentration is a manifestation of the transition from the surface to bulk diffusion kinetics.

Algorithms↗
PubMed2005

Instructional technology adoption of medical faculty in teaching.

Despite large investment by higher education institutions in technology for faculty and student use, instructional technology is not being integrated into instructions in the higher education institutions, including medical education institutions. While diffusion of instructional technologies has been reached a saturation point with early adopters of technology, it has remained limited among mainstream faculty. This study explored technology adoption patterns and perceptions of medical faculty about barriers and incentives to technology adoption in teaching.Complete data was obtained from 155 participants by using survey methodology and analyzed on the basis of theories of diffusion of innovation.Findings provided evidence for limited adoption of relatively new tools associated with instruction into mainstream faculty. Inadequate hardware for students and faculty, lack of reward structure, insufficient traing oppurtunites were identified as major barriers to faculty technology adoption.

Education, Medical↗
PubMed2005

Theory of frequency and phase synchronization in a rocked bistable stochastic system.

We investigate the role of noise in the phenomenon of stochastic synchronization of switching events in a rocked, overdamped bistable potential driven by white Gaussian noise, the archetype description of stochastic resonance. We present an approach to the stochastic counting process of noise-induced switching events: starting from the Markovian dynamics of the nonstationary, continuous particle dynamics, one finds upon contraction onto two states a non-Markovian renewal dynamics. A proper definition of an output discrete phase is given, and the time rate of change of its noise average determines the corresponding output frequency. The phenomenon of noise-assisted phase synchronization is investigated in terms of an effective, instantaneous phase diffusion. The theory is applied to rectangular-shaped rocking signals versus increasing input-noise strengths. In this case, for an appropriate choice of the parameter values, the system exhibits a noise-induced frequency locking accompanied by a very pronounced suppression of the phase diffusion of the output signal. Precise numerical simulations corroborate very favorably our analytical results. The novel theoretical findings are also compared with prior ones.

Action Potentials↗
PubMed1982

The sodium currents of nerve under voltage clamp as heterogeneous kinetics. A model that is consistent with possible kinetic behavior.

A model is presented which explains in Na+ currents of voltage-clamped nerve as resulting from a heterogeneous initiation of a sequential kinetic process. This is in analogy with the heterogeneity of the kinetics of other dielectric relaxations. The results suggest that: (1) The kinetic processes responsible for the voltage response occur within the membrane rather than at the surface; (2) The heterogeneity is due to simultaneous thermal diffusion and electric field-induced charge migration: (3) The slow turnoff upon prolonged depolarization is a voltage-independent, thermally controlled process; (4) The fast turnoff upon instantaneous repolarization is the reverse of the turning-on process. All the kinetic parameters depend on the transmembrane potential in accord with the possible behavior expected from activated-state theory. The diffusion coefficient of the charged species in the membrane as found from the data agrees with that found by photobleaching experiments on general proteins in membranes. The charge on the molecule responsible for the heterogeneous "gating' can be calculated unambiguously from the data.

Electric Conductivity↗
PubMed2001

Maillard reaction kinetics in model preservation systems in the vicinity of the glass transition: experiment and theory.

Rates of reactant consumption for the Maillard reaction between lysine and glucose were measured for a noncrystallizing trehalose-sucrose-water matrix in the glass transition region. At temperatures above the glass transition temperature (T(g)), the consumption rates showed Arrhenius temperature dependence with activation energies of 135 and 140 kJ mol(-1) for lysine and glucose, respectively. Finite reaction rates were observed for glassy samples that were faster than that of one of the nonglassy samples. A comparison of experimental results with predicted diffusion-controlled reaction rate constants indicated that the reaction was reaction-controlled at temperatures above T(g) and approached the diffusion-influenced regime in the glassy state. The needs for further research on reactant diffusivity, the theory of the orientation dependence of reactivity, and a detailed understanding of the reaction mechanism and kinetics were identified.

Chemical Phenomena↗
PubMed1982

Pulmonary blood flow, diffusing, capacity and tissue volume by rebreathing: theory.

The determination of pulmonary capillary blood flow (Q), diffusing capacity (D) and tissue volume (Vtis) from rebreathing equilibration kinetics is studied on mathematical models. The validity of a continuously ventilated two-compartment model (model II) is analyzed with reference to a cyclically ventilated model (model III) and compared to a one-compartment model (model I). Use of model II leads to slight overestimation of Q and D, and to underestimation of Vtis whereas model I yields considerable underestimation of D, Q and Vtis. The following procedures are recommended: (1) Rebreathing data should be analyzed on the basis of model II. (2) Q and D should be calculated from the rate constant of fast (k1) and slow (k2) exponential component and extrapolated zero-time intercept of K2, based on end-expired gas fractions, of the gas under study (e.g. C2H2, CO). (3) Addition of an insoluble inert gas (e.g. He) is required only for Vtis. (4) Soluble inert gases with blood-gas partition coefficient (lambda) less than 1 (e.g. C2H2, CHC1F2) applied at low concentration should be used for determination of Q and Vtis. (5) A high rebreathing ventilation is recommended.

Blood Gas Analysis↗
PubMed2000

Diffusion reconsidered: variation and transformation in biomedical practice, a case study from Mexico.

This paper focuses on biomedical transformations in an economically developing nation and poses the question, what are the variant and invariant aspects of biomedical practice, using empirical data from Mexico? I propose that when infectious disease predominated, biomedicine may have been uniformly practiced the world over, but with the ascent of chronic disease of unknown etiology, certain aspects of biomedical practice, especially diagnoses and causality, become reinterpreted culturally and idiosyncratically. The findings of the case study also illuminate current theories of globalization and earlier anthropological theories of diffusion as well as issues concerning the social constructivist perspective.

Anthropology, Cultural↗
PubMed2001

Spatiotemporal dynamics of charged species in the afterglow of plasmas containing negative ions.

The spatiotemporal evolution of charged species densities and wall fluxes during the afterglow of an electronegative discharge has been investigated. The decay of a plasma with negative ions consists of two stages. During the first stage of the afterglow, electrons dominate plasma diffusion and negative ions are trapped inside the vessel by the static electric field; the flux of negative ions to the walls is nearly zero. During this stage, the electron escape frequency increases considerably in the presence of negative ions, and can eventually approach free electron diffusion. During the second stage of the afterglow, electrons have disappeared, and positive and negative ions diffuse to the walls with the ion-ion ambipolar diffusion coefficient. Theories for plasma decay have been developed for equal and strongly different ion (T(i)) and electron (T(e)) temperatures. In the case T(i)=T(e), the species spatial profiles are similar and an analytic solution exists. When detachment is important in the afterglow (weakly electronegative gases, e.g., oxygen) the plasma decay crucially depends on the product of negative ion detachment frequency (gamma(d)) and diffusion time (tau(d)). If gamma(d)tau(d)>2, negative ions convert to electrons during their diffusion towards the walls. The presence of detached electrons results in "self-trapping" of the negative ions, due to emerging electric fields, and the negative ion flux to the walls is extremely small. In the case T(i)<<T(e), the spatiotemporal dynamics is more complicated due to the presence of negative ion density fronts. During the afterglow, although negative ions diffuse freely in the plasma core, the negative ion fronts propagate towards the chamber walls with a nearly constant velocity. The evolution of ion fronts in the afterglow of electronegative plasmas is important, since it determines the time needed for negative ions to reach the wall, and thus influence surface reactions in plasma processing.

Journal Article↗
PubMed1998

The atmospheric cosmic- and solar energetic particle radiation environment at aircraft altitudes.

Galactic cosmic rays interact with the solar wind, the earth's magnetic field and hadron, lepton and photon fields at aircraft altitudes. In addition to cosmic rays, energetic particles generated by solar activity bombard the earth from time to time. These particles, while less energetic than cosmic rays, also produce radiation fields at aircraft altitudes which have qualitatively the same properties as atmospheric cosmic rays. We have used a code based on transport theory to calculate atmospheric cosmic-ray quantities and compared them with experimental data. Agreement with these data is seen to be good. We have then used this code to calculate equivalent doses to aircraft crews. We have also used the code to calculate radiation doses from several large solar energetic particle events which took place in 1989, including the very large event that occurred on September 29th and 30th of that year. The spectra incident on the atmosphere were determined assuming diffusive shock theory.

Aerospace Medicine↗
PubMed1992

Role of diffusion in nonaqueous enzymology. 1. Theory.

Using a set of standard equations, we have calculated the role of internal and external mass transfer in limiting the rate of enzyme-catalysed reactions in anhydrous organic solvents and supercritical fluids. We have shown that enzyme particles suspended in anhydrous organic solvents will be subject to increasing diffusional limitation as the enzyme activity and particle size increase. Using particle dimensions, as measured by scanning electron microscopy, we have prepared a series of graphs that will enable investigators to determine whether their combination of particle size and activity will result in internal or external diffusional limitations. We have shown that supercritical fluids can be expected to enhance the activity of enzymes in nonaqueous environments as a result of the high diffusivity of the bulk solvent. The plots also clearly indicate that enzyme particles in supercritical fluids require nearly two orders of magnitude less agitation than those suspended in conventional solvents in order to overcome any external mass transfer limitations.

Catalysis↗
PubMed2006

Molecular theory of thermal conductivity of the Lennard-Jones fluid.

In this paper the thermal conductivity of the Lennard-Jones fluid is calculated by applying the combination of the density-fluctuation theory, the modified free volume theory of diffusion, and the generic van der Waals equation of state. A Monte Carlo simulation method is used to compute the equilibrium pair-correlation function necessary for computing the mean free volume and the coefficient in the potential-energy and virial contributions to the thermal conductivity. The theoretical results are compared with our own molecular dynamics simulation results and with those reported in the literature. They agree in good accuracy over wide ranges of density and temperature examined in molecular dynamics simulations. Thus the combined theory represents a molecular theory of thermal conductivity of the Lennard-Jones fluid and by extension simple fluids, which enables us to compute the nonequilibrium quantity by means of the Monte Carlo simulations for the equilibrium pair-correlation function.

Journal Article↗
PubMed1985

Strong electrolyte continuum theory solution for equilibrium profiles, diffusion limitation, and conductance in charged ion channels.

The solution for the ion flux through a membrane channel that incorporates the electrolyte nature of the aqueous solution is a difficult theoretical problem that, until now, has not been properly formulated. The difficulty arises from the complicated electrostatic problem presented by a high dielectric aqueous channel piercing a low dielectric lipid membrane. The problem is greatly simplified by assuming that the ratio of the dielectric constant of the water to that of the lipid is infinite. It is shown that this is a good approximation for most channels of biological interest. This assumption allows one to derive simple analytical expressions for the Born image potential and the potential from a fixed charge in the channel, and it leads to a differential equation for the potential from the background electrolyte. This leads to a rigorous solution for the ion flux or the equilibrium potential based on a combination of the Nernst-Planck equation and strong electrolyte theory (i.e., Gouy-Chapman or Debye-Huckel). This approach is illustrated by solving the system of equations for the specific case of a large channel containing fixed negative charges. The following characteristics of this channels are discussed: anion and mono- and divalent cation conductance, saturation of current with increasing concentration, current-voltage relationship, influence of location and valence of fixed charge, and interaction between ions. The qualitative behavior of this channel is similar to that of the acetylcholine receptor channel.

Animals↗
PubMed1982

Role of diffusion in ligand binding to macromolecules and cell-bound receptors.

The association and dissociation rates of partially diffusion-controlled bimolecular reactions are considered. A simple expression for the equilibrium constant is derived using classical statistical mechanics. The relationship is established between the Collins-Kimball treatment, which is based on the "radiation" boundary condition involving an intrinsic rate constant k, and the kinetic scheme A + B in equilibrium A . . . B in equilibrium AB where A . . . B is an encounter complex. It is shown that with the appropriate choice of the interaction potential, Debye's expression for the association rate constant becomes identical to that obtained using the radiation boundary condition if k is evaluated using Kramers' theory of diffusive barrier crossing. Finally, the competitive binding of ligand to a spherical cell, whose surface is partially covered by multiple reactive sites, is studied by treating the cell as a partially reacting sphere.

Animals↗
PubMed2005

Diffusion-reaction in branched structures: theory and application to the lung acinus.

An exact "branch by branch" calculation of the diffusional flux is proposed for partially absorbed random walks on arbitrary tree structures. In the particular case of symmetric trees, an explicit analytical expression is found which is valid whatever the size of the tree. Its application to the respiratory phenomena in pulmonary acini gives an analytical description of the crossover regime governing the human lung efficiency.

Animals↗
PubMed2001

Inherent-structure dynamics and diffusion in liquids.

The self-diffusion constant D is expressed in terms of transitions among the local minima (inherent structures, IS) of the N-body potential-energy surface or landscape, and their correlations. The formulas are evaluated and tested against simulation in the supercooled, unit-density Lennard-Jones liquid. The approximation of uncorrelated IS-transition (IST) vectors D0, greatly exceeds D for the highest T, but merges with simulation at reduced T approximately 0.50, close to the estimated mode-coupling temperature T(c). Since uncorrelated IST's are associated with a hopping mechanism, the condition D approximately D0 provides a new way to identify the crossover to hopping. The results suggest that theories of diffusion in deeply supercooled liquids may be based on weakly correlated IST's.

Journal Article↗
PubMed2006

Electrophoresis of diffuse soft particles.

A theory is presented for the electrophoresis of diffuse soft particles in a steady dc electric field. The particles investigated consist of an uncharged impenetrable core and a charged diffuse polyelectrolytic shell, which is to some extent permeable to ions and solvent molecules. The diffuse character of the shell is defined by a gradual distribution of the density of polymer segments in the interspatial region separating the core from the bulk electrolyte solution. The hydrodynamic impact of the polymer chains on the electrophoretic motion of the particle is accounted for by a distribution of Stokes resistance centers. The numerical treatment of the electrostatics includes the possibility of partial dissociation of the hydrodynamically immobile ionogenic groups distributed throughout the shell as well as specific interaction between those sites with ions from the background electrolyte other than charge-determining ions. Electrophoretic mobilities are computed on the basis of an original numerical scheme allowing rigorous evaluation of the governing transport and electrostatic equations derived following the strategy reported by Ohshima, albeit within the restricted context of a discontinuous chain distribution. Attention is particularly paid to the influence of the type of distribution adopted on the electrophoretic mobility of the particle as a function of its size, charge, degree of permeability, and solution composition. The results are systematically compared with those obtained with a discontinuous representation of the interface. The theory constitutes a basis for interpreting electrophoretic mobilities of heterogeneous systems such as environmental or biological colloids or swollen/deswollen microgel particles.

Biophysics↗
PubMed2001

Impact of polymer tether length on multiple ligand-receptor bond formation.

The promoters of cell adhesion are ligands, which are often attached to flexible tethers that bind to surface receptors on adjacent cells. Using a combination of Monte Carlo simulations, diffusion reaction theory, and direct experiments (surface force measurements) of the biotin-streptavidin system, we have quantified polymer chain dynamics and the kinetics and spatial range of tethered ligand-receptor binding. The results show that the efficiency of strong binding does not depend solely on the molecular architecture or binding energy of the receptor-ligand pair, nor on the equilibrium configuration of the polymer tether, but rather on its "rare" extended conformations.

Biotin↗