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At least 793 records · Page 44Linked to original sources
PubMed1998

Stochastic resonance in coupled nonlinear dynamic elements.

We investigate the response of a linear chain of diffusively coupled diode resonators under the influence of thermal noise. We also examine the connection between spatiotemporal stochastic resonance and the presence of kink-antikink pairs in the array. The interplay of nucleation rates and kink speeds is briefly addressed. The experimental results are supplemented with simulations on a coupled map lattice. We furthermore present analytical results for the synchronization and signal processing properties of a Phi(4) field theory and explore the effects of various forms of nonlinear coupling. (c) 1998 American Institute of Physics.

Journal Article↗
PubMed1995

The structure of copper-nitrite reductase from Achromobacter cycloclastes at five pH values, with NO2- bound and with type II copper depleted.

High resolution x-ray crystallographic structures of nitrite reductase from Achromobacter cycloclastes, undertaken in order to understand the pH optimum of the reaction with nitrite, show that at pH 5.0, 5.4, 6.0, 6.2, and 6.8, no significant changes occur, other than in the occupancy of the type II copper at the active site. An extensive network of hydrogen bonds, both within and between subunits of the trimer, maintains the rigidity of the protein structure. A water occupies a site approximately 1.5 A from the site of the type II copper in the structure of the type II copper-depleted structure (at pH 5.4), again with no other significant changes in structure. In nitrite-soaked crystals, nitrite binds via its oxygens to the type II copper and replaces the water normally bound to the type II copper. The active-site cavity of the protein is distinctly hydrophobic on one side and hydrophilic on the other, providing a possible path for diffusion of the product NO. Asp-98 exhibits thermal parameter values higher than its surroundings, suggesting a role in shuttling the two protons necessary for the overall reaction. The strong structural homology with cupredoxins is described.

Alcaligenes↗
PubMed1999

Starch gelatinization under thermal stress.

The behavior under thermal stress of starch dispersed in water was studied by differential scanning calorimetry (DSC) to estimate the heat transported through the aqueous medium in gelatinization, and to characterize the range of gelatinization temperatures. In DSC scanning mode, the endotherm of 10% starch in aqueous dispersion showed the tracing of gelatinization at between 67 and 80 degrees C, having an onset at approximately 69 degrees C. In the isothermal mode, characteristically distinct isothermal heat flow profiles were revealed. It was hypothesized that the thermal influx proposed as being analogous to the diffusion process may affect the profiles. The profiles were transformed and nonlinearly fitted according to the square root of time model to characterize a so-called t-parameter, which was related to mean square displacement of molecular distribution. The t-parameter of starch in excess of water decreased compared to that of water only. The plot of difference in these t-parameters, expressed as delta, against temperature showed a dramatically decreased delta at the temperature between 66.7 and 75.2 degrees C, which coincided with the findings from scanning mode DSC. It was further hypothesized that the decreased delta may be due to the gelatinizing process. According to the theory of polymer solution, the critical temperature (theta) at 75.2 degrees C, where the free energy became theoretically negative, i.e., the starch became spontaneously dissolved, was drawn. This theta was located within the range of gelatinizing temperatures. It was deduced that starch polymer may have dissolved during gelatinization. The dissolution from acetaminophen tablets prepared by starch paste was lower compared with that of negative controls (without paste). Moreover, the paste prepared at gelatinizing temperature (70 degrees C) seemed to inhibit acetaminophen dissolution from tablet matrices more than that prepared at subgelatinizing temperature (50 degrees C).

Algorithms↗
PubMed2004

Flux-matching theory of particle charging.

A new flux-matching theory is formulated and applied to the study of particle charging by ions. Assuming that the ion-particle interaction includes the Coulomb + polarization forces the collisionless kinetic equation is solved and the ion concentration profile in the free-molecule zone (at the distances less than the ion mean free path) is found. This profile is then matched to that derived from the solution of the diffusion equation, which describes the ion transport outside the free-molecule zone. Three matching parameters are introduced: the ion flux, the matching distance, and the ion density at the matching distance; and three conditions are formulated for fixing these parameters: (i) the constancy of the total ion flux, (ii) the continuity of the ion concentration profile, and (iii) the continuity of the derivative of the ion concentration profile. The charging efficiencies are expressed in terms of their free-molecule values, the ion diffusivity in the carrier gas, and the ion thermal velocity. This approach is applied for calculating the efficiencies of particle charging in the transition regime (the particle size is comparable to the ion mean free path and the Coulomb length). The corrections due to ion-carrier gas interaction to the particle-ion recombination rate are shown to remain finite even for very small particles, whereas in the case of particle-ion repulsion the contribution of ion-molecular collisions to the rate of particle charging is suppressed in the free-molecule regime.

Journal Article↗
PubMed2003

Steric interactions in cellular structures formed in a water/oil/surfactant/cosurfactant mixture.

Dynamics of dense microemulsion droplets forming ordered cellular phases as temperature increases are investigated by neutron spin echo spectroscopy (NSE). The shape fluctuations of droplets are shown using a specific contrast. Their relaxation time tau(f) is obtained by analysis of the NSE curves, which also reveals the short-range Brownian motion (diffusion constant D0) of the "caged" droplets. The thermal dependence of D0 and tau(f) supports the notion of entropically driven interdroplet steric repulsion stabilizing the cellular phases.

Journal Article↗
PubMed2001

Diffuse X-ray scattering from benzil, C(14)H(10)O(2): analysis via automatic refinement of a Monte Carlo model.

A recently developed method for fitting a Monte Carlo computer-simulation model to observed single-crystal diffuse X-ray scattering has been used to study the diffuse scattering in benzil, diphenylethanedione, C(6)H(5)-CO-CO-C(6)H(5). A model involving 13 parameters consisting of 11 intermolecular force constants, a single intramolecular torsional force constant and a local Debye-Waller factor was refined to give an agreement factor, R = [summation operator omega(Delta I)(2)/summation operator omega I(obs)(2)](1/2), of 14.5% for 101,324 data points. The model was purely thermal in nature. The analysis has shown that the diffuse lines, which feature so prominently in the observed diffraction patterns, are due to strong longitudinal displacement correlations. These are transmitted from molecule to molecule via a network of contacts involving hydrogen bonding of an O atom on one molecule and the para H atom of the phenyl ring of a neighbouring molecule. The analysis also allowed the determination of a torsional force constant for rotations about the single bonds in the molecule. This is the first diffuse scattering study in which measurement of such internal molecular torsion forces has been attempted.

Journal Article↗
PubMed2004

New problems of particle transfer in ferrocolloids: magnetic Soret effect and thermoosmosis.

The paper deals with critical reviewing of the experiments on thermodiffusion in ferrocolloids. The observed magnetic Soret effect is much stronger than that predicted theoretically. It is shown that the main reason of that is the influence of the magnetic field on mass diffusion. Besides, some measurements are affected by uncontrolled thermal and solutal magnetic convection. In porous media, when macroscopic convection is suppressed, thermodiffusion is accompanied by thermoosmosis as well as by a microconvective mass transfer induced by particle magnetophoresis on filter grains.

Journal Article↗
PubMed2006

Nanostructure in hybrid thermosets with interpenetrating networks and its effect on properties.

Hybrid resins composed of vinylester (VE) and aliphatic amine (Al-Am)-cured cycloaliphatic epoxy (Cal-EP) were produced and their morphology and properties (toughness, water uptake) determined. According to atomic force microscopy (AFM) results achieved on physically etched (ion eroded) specimens the hybrid resins possessed a nanoscaled interpenetrating network (IPN) structure, the characteristics of which depended on the VE/EP ratio. Characteristics of the IPN morphology, viz strand width and mean roughness data increase with increasing amount of the EP component. The fracture mechanical data (K(Q) and G(Q)) pass a maximum as a function of the VE/EP ratio, similar to that of the phase segregation term (alpha). The latter was deduced from dynamic-mechanical thermal analysis (DMTA) traces. The water sorption and diffusion behavior of the interpenetrated VE/EP hybrids were controlled by the relative amount of the EP (Cal-EP+Al-Am) used.

Diffusion↗
PubMed1996

Difference in the enhancing effects of ultrasound on the skin permeation of polar and non-polar drugs.

The effect of ultrasound (150 kHz, 111 mW/cm2) on the permeability of isosorbide dinitrate (ISDN) and antipyrine (ANP) through excised hairless rat skin was evaluated using an Arrhenius plot. The permeability coefficients of ISDN across skin (at various temperatures) in the presence and absence of ultrasound were virtually isolinear on the Arrhenius plot. It has been suggested that the temporal increase in the ISDN flux, which was observed when ultrasound was applied in our previous study, was only a result of the thermal effect of ultrasound, i.e., an increase in the temperature of the donor solution. On the other hand, ultrasound influenced the Arrhenius plot of ANP, suggesting that the enhancement effect for ANP permeation could be not explained only by the thermal effect of ultrasound. In addition, the effective diffusion (D) and partition coefficients (K) of ISDN and ANP were estimated using their skin permeation profiles across the ultrasonic pretreated skin. The coefficients of ISDN with ultrasonic pretreatment were comparable to those without pretreatment. On the other hand, the D value of ANP with ultrasonic pretreatment was increased about 4 times by ultrasonic pretreatment, in spite of an insignificant change in the K value. These results suggest that the ultrasound used in the present study increased the effective diffusivity across the aqueous region in the stratum corneum to enhance the skin permeation of the polar compound, ANP.

Animals↗
PubMed2003

Photothermal analysis of polymeric dye laser materials excited at different pump rates.

The photothermal properties and heat diffusion of polymeric lasers, made up from solutions of Rhodamine 6G in solid matrices of poly(2-hydroxyethyl methacrylate) with different amounts of the cross-linking monomer ethylene glycol dimethacrylate and copolymers of 2-hydroxyethyl methacrylate and methyl methacrylate have been studied through photothermal deflection spectroscopy. The heat load that is due to the pumping process was quantified as a function of the pump excitation repetition frequency (0.25-10 Hz), determining the time-dependent temperature changes at different locations within the laser matrix. A theoretical model, which reproduces these changes with high accuracy, was developed on the basis of the heat-diffusion equation of optically dense fluids. The observed thermal effects became important for impairing the laser stability at pump repetition frequencies higher than 1 Hz. In addition, the irreversible optical changes produced in the laser matrices at high pump fluence values (>1 J/cm2) were also analyzed. These effects originate, most likely, from a two-step photothermal mechanism.

Journal Article↗
PubMed1992

Determination of Haemophilus influenzae and Haemophilus parainfluenzae susceptibility to ampicillin and other antibiotics.

A sample comprising 40 H. influenzae and 74 H. parainfluenzae strains was used to verify methods for determining susceptibility to antibiotics. Modified Levinthal agar proved to be suitable for the agar dilution and agar diffusion method, while brain heart infusion with the thermally released components of sheep blood (X and V factor) and lysed horse blood performed well in the dilution micromethod. The iodometric method served well for beta-lactamase production. A substantial proportion of strains was resistant to penicillin, erythromycin, roxitromycin and sulfamethoxazole. Ampicillin susceptibility was of crucial importance. Resistance was largely due to beta-lactamase production. Since there are ampicillin-resistant strains which fail to produce beta-lactamase, it is necessary either to determine the MIC value or use a disk with 2 micrograms ampicillin. A disk containing 10 micrograms ampicillin may yield a false positive result.

Ampicillin Resistance↗
PubMed2006

[Synthesis and spectroscopy of K7[P2Mo4W13M(H2O)O61] (M= Cr, Fe) and their organic composite materials].

The Dawson heteropoly complexes K7[P2Mo4W13M(H2O)O61] (M= Cr(1), Fe(2)) and their organic composite materials formed by (E)-N-butyl-4-(2-(4'-dimethylaminophenyl)ethenyl)pyridinium cation and compound 1 (or 2) with the stoichiometry of (C19H25N2)(6)K3[P2Mo4W13MO62](M=Cr(3), Fe(4)) were synthesized and characterized by elemental analyses, thermogravimetric and differential thermal analyses, and IR, UV-Vis, solid diffuse reflectance UV-Vis-near IR, X-ray photoelectron and fluorescence spectroscopy. The results of spectra show that compounds 1 and 2 have Dawson structure, inorganic-organic composite materials are formed between the hemicyanine cations and the heteropoly anions, and the charge transfer occurs from the hemicyanine cations to the heteropoly anions in 3 and 4 complexes. The interaction between the hemicyanine cations and the heteropoly anions in the solid powder is stronger than that in the fluid solution. The compounds reported in this paper are interesting as nonlinear optical materials.

English Abstract↗
PubMed1988

[Resuscitation and extravascular lung fluid in cardiac surgery].

Cardiovascular arrest may be followed by severe respiratory insufficiency due to an increase in the pressure in the pulmonary vascular system, an alteration in capillary permeability, or both. Extracorporeal circulation (ECC), on the other hand, can lead to a change in capillary integrity ('capillary leakage') caused by the unphysiologic perfusion patterns and/or activation of various mediator systems. Pulmonary hyperhydration (increased extravascular lung water [EVLW]) seems to be the most important factor limiting pulmonary function in this situation. This retrospective study was designed to investigate the influence of resuscitation in the period before ECC on pulmonary EVLW thereafter. Eight coronary surgery patients who had to be resuscitated in the period before the start of ECC due to cardiocirculatory arrest were compared to 8 patients without resuscitation selected randomly (control group). There were no differences between the two groups with regard to age, catheterization data, and ECC conditions. All patients underwent identical monitoring, including estimation of EVLW. Lung water measurement was performed by the thermal dye technique with indocyanine green (= non-diffusible indicator) dissolved in ice-cold dextrose (= diffusible indicator) and a bedside microprocessor before resusciation and after ECC (15 min, 45 min, 5h). Simultaneously, hemodynamics and parameters of pulmonary gas exchange were monitored. Baseline values of EVLW were comparable in both groups. After ECC a transient increase in EVLW could be demonstrated in the controls, indicating an altered fluid flux even in 'uncomplicated' courses; 5 h after ECC lung water content had again reached baseline values. In contrast, there was a significant increase in EVLW in the 'complicated group' immediately after ECC (+2.60 ml/kg) and 5 h after ECC (+1.38 ml/kg); in consequence, the paO2 was significantly decreased (-180 mmHg) while Qs/Qt was increased (+6.79%). It is concluded that the combination of two factors that potentially damage pulmonary tissue and increase lung water content (reanimation due to circulatory arrest and extracorporeal circulation) lead to a significant increase in extravascular lung water combined with a deterioration of pulmonary function, resulting in severe respiratory failure.

Capillary Permeability↗
PubMed1999

Analysis of changes in MR properties of tissues after heat treatment.

To characterize changes in the MR parameters of tissues due to thermal coagulation, a series of T(1), T(2), diffusion, and magnetization transfer measurements were performed on a variety of ex vivo tissues: murine slow twitch skeletal muscle, murine cardiac muscle, murine cerebral hemisphere, bovine white matter, murine liver tissue, bovine retroperitoneal adipose tissue, hen egg white, human prostate and human blood. Standardized heat treatments were performed for each tissue type, over the temperature range from 37 degrees C to 90 degrees C. For all tissues, changes in each MR measurement resulting from thermal coagulation were observed above a threshold temperature of approximately 60 degrees C. These changes are explained based on biophysical knowledge of thermal damage mechanisms and the MR properties of normal tissues, and are particularly relevant for interpreting the changes in image contrast that are observed when MRI is used to guide and monitor thermal coagulation therapy procedures. Magn Reson Med 42:1061-1071, 1999.

Adipose Tissue↗
PubMed2003

Heat transport in proximity structures.

We study heat and charge transport through a normal diffusive wire coupled with a superconducting wire over the region smaller than the coherence length. Because of partial Andreev reflection of quasiparticles from the interface, the subgap thermal flow is essentially suppressed and approaches zero along with energy, which is specific for diffusive structures. Whereas the electric conductance shows a conventional reentrance effect, the thermal conductance kappa rapidly decreases with temperature which qualitatively explains the results of recent experiments. In the Andreev interferometer geometry, kappa experiences full-scale oscillations with the order parameter phase difference.

Journal Article↗
PubMed1984

Diffusional arteriovenous shunting in the heart.

Previous indicator dilution experiments in isolated blood-perfused dog hearts suggested that there was intramyocardial diffusional shunting of water relative to a flow-limited solute, antipyrine. Two sets of studies have been done to assess the importance of this shunting, since it implies the possibility of a diffusional bypass for oxygen and other substances, which may be important in ischemia. Nonconsumed tracers were used to show the phenomenon. In the first set, bolus injections of 133Xe dissolved in saline were made into the coronary inflow and the tracer content of the organ recorded by an external gamma detector. The initial Xe washout was disproportionately rapid at low flows, and the late phase was also relatively retarded. In the second set, boluses of cool saline containing indocyanine green were injected into the coronary arterial inflow while coronary sinus outflow dilution curves were recorded via a thermistor and a dye densitometer over a wide range of flows. The thermal curves showed emergence of heat preceding the dye; the degree of precession was much greater at low flows, and, unlike the dye curves, the thermal dilution curves showed dramatic differences in shape at different flows. A model for diffusional countercurrent exchange shows similar changes in residue curves and outflow dilution curves. The conclusion is that there is diffusional shunting of small lipid-soluble molecules whose diffusion coefficients in tissue are high. While the shunting of heat is great, the shunting of soluble gases will not be large and that of normal substrates will be negligible.

Animals↗
PubMed1999

Self-diffusion of grains in a two-dimensional vibrofluidized bed.

The analogy of vibrofluidized granular beds with a thermal gas of hard discs has been tested. Analysis of the mean squared displacement behavior of the grains allowed comparison of the measured diffusion with the predicted value at a particular combination of granular temperature and packing fraction. High speed photography, image analysis, and particle tracking software enabled accurate location of the grains. Appropriate analysis of the three mean squared displacement regimes, ballistic, diffusive, and crossover between the two extremes, allowed both the diffusion coefficient and the granular temperature to be measured at the same packing fraction. Broad agreement between Chapman-Enskog theory relating temperature to self-diffusion and observation was observed up to packing fractions of eta approximately equal to 0.7. At higher packing fractions the grains showed evidence of caging and jump diffusion, with the observed diffusion rapidly diverging from that predicted by theory. Measurement of self-diffusion coefficients and subsequent use of kinetic theory was found to be an accurate method to determine the granular temperature for intermediate packing fractions (eta=0.4-0.6), and would be particularly suitable for those situations where the time resolution of the experimental facility is insufficient to resolve the speed of the grain between collisions.

Journal Article↗
PubMed1999

Specular and diffuse scattering of highly aligned phospholipid membranes.

We present a quantitative study of specular and diffuse (non-specular) x-ray and neutron reflectivity from highly aligned phospholipid membranes deposited on solid substrates. The height-height correlation function could be obtained from the diffuse scattering without further model assumptions. The results differ significantly from the linear theory of smectic elasticity. We argue that the diffuse scattering is dominated by static liquid-crystalline defects, rather than thermal fluctuations.

1,2-Dipalmitoylphosphatidylcholine↗