Thermal-field emission flicker (1/f) noise and diffusive equilibrium density fluctuations.
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Journal Article↗
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Thermal effect on uniform increase in pulmonary diffusing capacity during exercise.
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Adult↗
Phase-field modeling of binary alloy solidification with coupled heat and solute diffusion.
A phase-field model is developed for simulating quantitatively microstructural pattern formation in solidification of dilute binary alloys with coupled heat and solute diffusion. The model reduces to the sharp-interface equations in a computationally tractable thin-interface limit where (i). the width of the diffuse interface is about one order of magnitude smaller than the radius of curvature of the interface but much larger than the real microscopic width of a solid-liquid interface, and (ii). kinetic effects are negligible. A recently derived antitrapping current [Phys. Rev. Lett. 87, 115701 (2001)]] is used in the solute conservation equation to recover precisely local equilibrium at the interface and to eliminate interface stretching and surface diffusion effects that arise when the solutal diffusivities are unequal in the solid and liquid. Model results are first compared to analytical solutions for one-dimensional steady-state solidification. Two-dimensional thermosolutal dendritic growth simulations with vanishing solutal diffusivity in the solid show that both the microstructural evolution and the solute profile in the solid are accurately modeled by the present approach. Results are then presented that illustrate the utility of the model for simulating dendritic solidification for the large ratios of the liquid thermal to solutal diffusivities (Lewis numbers) typical of alloys.
Journal Article↗
Water translational motion at the bilayer interface: an NMR relaxation dispersion measurement.
Nuclear magnetic relaxation rates for water protons in aqueous palmitoyloleoylphosphatidylcholine vesicle suspensions containing different nitroxide free radical spin labels are reported as a function of magnetic field strength corresponding to proton Larmor frequencies from 10 kHz to 30 MHz. Under these conditions the water proton relaxation rate is determined by the magnetic coupling between the water protons and the paramagnetic nitroxide fixed on the phospholipid. This coupling is made time-dependent by the relative translational motion of the water proton spins past the nitroxide radical. Using theories developed by Freed and others, we interpret the NMR relaxation data in terms of localized water translational motion and find that the translational diffusion constant for water within approximately 10 A of the phospholipid surface is 6 x 10(-10) m2 s(-1) at 298 K. Similar results are obtained for three different nitroxide labels positioned at different points on the lipid. The diffusion is a thermally activated process with an activation energy only slightly higher than that for bulk water.
Cyclic N-Oxides↗
Structural and thermal characterization of ternary complexes of piroxicam and alanine with transition metals: uranyl binary and ternary complexes of piroxicam. Spectroscopic characterization and properties of metal complexes.
Ternary Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II) complexes with piroxicam (Pir) drug (H2L1) and dl-alanine (Ala) (HL2) and also the binary UO2(II) complex with Pir were studied. The structures of the complexes were elucidated using elemental, IR, molar conductance, magnetic moment, diffused reflectance and thermal analyses. The UO2(II) binary complex was isolated in 1:2 ratio with the formula [UO2(H2L)2](NO3)2. The ternary complexes were isolated in 1:1:1 (M:H2L1:L2) ratios. The solid complexes were isolated in the general formulae [M(H2L)(L2)(Cl)n(H2O)m].yH2O (M=Fe(III) (n=2, m=0, y=1), Co(II) (n=1, m=1, y=2) and Ni(II) (n=1, m=1, y=0)); [M(H2L)(L2)](X)z.yH2O (M=Cu(II) (X=AcO, z=1, y=0), Zn(II) (X=AcO, z=1, y=3) and UO2(II) (X=NO3, z=1, y=2)). Pir behaves as a neutral bidentate ligand coordinated to the metal ions via the pyridine-N and carbonyl-O groups, while Ala behaves as a uninegatively bidentate ligand coordinated to the metal ions via the deprotonated carboxylate-O and amino-N. The magnetic and reflectance spectral data show that the complexes have octahedral geometry except Cu(II) and Zn(II) complexes have tetrahedral structures. The thermal decomposition of the complexes was discussed in relation to structure, and the thermodynamic parameters of the decomposition stages were evaluated.
Alanine↗
Critical oxide thickness for efficient single-walled carbon nanotube growth on silicon using thin SiO2 diffusion barriers.
The ability to integrate carbon nanotubes, especially single-walled carbon nanotubes, seamlessly onto silicon would expand their range of applications considerably. Though direct integration using chemical vapor deposition is the simplest method, the growth of single-walled carbon nanotubes on bare silicon and on ultrathin oxides is greatly inhibited due to the formation of a noncatalytic silicide. Using X-ray photoelectron spectroscopy, we show that silicide formation occurs on ultrathin oxides due to thermally activated metal diffusion through the oxide. Silicides affect the growth of single-walled nanotubes more than multi-walled nanotubes due to the increased kinetics at the higher single-walled nanotube growth temperature. We demonstrate that nickel and iron catalysts, when deposited on clean silicon or ultrathin silicon dioxide layers, begin to form silicides at relatively low temperatures, and that by 900 degrees C, all of the catalyst has been incorporated into the silicide, rendering it inactive for subsequent single-walled nanotube growth. We further show that a 4-nm silicon dioxide layer is the minimum diffusion barrier thickness that allows for efficient single-walled nanotube growth.
Catalysis↗
Bacterial swimming strategies and turbulence.
Most bacteria in the ocean can be motile. Chemotaxis allows bacteria to detect nutrient gradients, and hence motility is believed to serve as a method of approaching sources of food. This picture is well established in a stagnant environment. In the ocean a shear microenvironment is associated with turbulence. This shear flow prevents clustering of bacteria around local nutrient sources if they swim in the commonly assumed "run-and-tumble" strategy. Recent observations, however, indicate a "back-and-forth" swimming behavior for marine bacteria. In a theoretical study we compare the two bacterial swimming strategies in a realistic ocean environment. The "back-and-forth" strategy is found to enable the bacteria to stay close to a nutrient source even under high shear. Furthermore, rotational diffusion driven by thermal noise can significantly enhance the efficiency of this strategy. The superiority of the "back-and-forth" strategy suggests that bacterial motility has a control function rather than an approach function under turbulent conditions.
Bacteria↗
Effect of total percent polyacrylamide in capillary gel electrophoresis for DNA sequencing of short fragments. A phenomenological model.
Polyacrylamide capillary gels were prepared with constant (5% C) cross-linker concentration and with total acrylamide concentration ranging from 2.5 to 6% T. At each acrylamide concentration, peak spacing was constant for DNA sequencing fragments ranging from 25 to 250 nucleotides in length. Peak spacing increased linearly with the total acrylamide concentration. The intercept of the retention time vs. fragment length plot was independent of % T. Ferguson plots were constructed for short DNA fragments; the polyacrylamide pore size falls in the 2.5 to 3.5 nm range for the gels studied. Theoretical plate count is independent of total acrylamide concentration; longitudinal diffusion, and not thermal gradients, limit the plate count. A phenomenological model is presented that predicts retention time, plate count, and resolution for sequencing fragments ranging in size from 25 to 250 bases and gels that range from 2.5 to 6% total acrylamide.
Acrylic Resins↗
Minimum entropy production in photosynthesis.
In this paper a flux-coupling model of photosynthesis is presented. By requiring minimum entropy production, it is found that the photosynthetic efficiency is essentially given by the square root of D/lambda. D and lambda are the diffusion coefficient and thermal conductivity of the rate-limiting processes in the chloroplast, respectively. For experimental values of D and lambda, the efficiency is found to be 2.4-7.5%, with a likely value of 6.1%, whereas C4-plants are known to have an efficiency of 6.2%. We conclude that the process of photosynthesis is in quantitative agreement with the principle of minimum entropy production.
Diffusion↗
Dielectric analysis in the characterization of amorphous pharmaceutical solids. 1. Molecular mobility in poly(vinylpyrrolidone)-water systems in the glassy state.
The effect of water on the relaxation behavior below the glass transition temperature (beta-relaxation) of an amorphous powder, poly(vinylpyrrolidone) (PVP, MW 30,000), was studied by subjecting the sample to dielectric analysis in the frequency range from 20 Hz to 20 kHz. The material stored at 0% relative humidity (RH) (containing 0.05% w/w H2O) exhibited a frequency dependent second-order beta-relaxation (T beta = -56 degrees C at 500 Hz). The peak frequency-temperature data could be fitted to the Arrhenius equation, yielding an activation energy (Ea) of 36.5 kJ mol-1. Water was found to significantly lower T beta, increase the dielectric loss, and increase Ea. The initial decrease in T beta was found to be quite significant, as little as 7% w/w H2O lowering T beta by 26 degrees C, followed by a more gradual decrease. PVP exposed to 69% RH (containing approximately 31% w/w H2O) exhibited T beta at -104 degrees C with an activation energy of 46.3 kJ mol-1. The observations that the beta relaxation was poorly visible when the water content was 0.05% w/w and that the change in Ea was from a low to a high value as the temperature is decreased suggest that thermally activated rotational diffusion of water molecules plays a major role in the beta-relaxation of PVP containing moderate to high water contents. The rate of increase in activation energy as a function of H2O/PVP mole ratio exhibited a minimum at unity, suggesting that water binding to one site on PVP has a distinct effect on the activation energy.
Calorimetry, Differential Scanning↗
Relaxometry of brain: why white matter appears bright in MRI.
The remarkable success of magnetic resonance imaging of adult brain relates to the unusually large ratio of the longitudinal relaxation rates 1/T1 of white and gray matter, approximately 2:1 at physiological temperature and traditional imaging fields. Several investigators have conjectured that myelin is the source of the greater 1/T1 of white matter without, however, suggesting details of the molecular mechanisms responsible. From measurements of the magnetic field dependence of 1/T1 (NMRD profiles) of adult and neonatal gray and white matter at 5 and 35 degrees C, we find a thermally activated contribution to the NMRD profile of adult white matter that is not present in the profiles of either adult gray or neonatal gray and white matter. We attribute this contribution to myelin and develop a quantitative model that accounts for the unique relaxation behavior of myelinated white matter. We find that myelin water, 15% of the total, has a relatively short T1 that arises from an unexpectedly large interaction with myelin lipid; when cast in terms of an interaction over the entire myelin bilipid-water interface, it is sevenfold greater than the analogous protein-water interfacial interaction. Its magnitude remains to be accounted for, but cholesterol, known to alter the relaxation rates of lipid protons, may play an important role. The contribution of myelin to 1/T1 at physiological temperatures is attributed to thermally activated transmembrane diffusion of water and, hence, more rapid mixing of axonal and the rapidly relaxing myelin water molecules.
Adult↗
Novel primitive swallowing reflex: facial receptor distribution and stimulus characteristics.
We recently described a primitive swallowing reflex: swallowing as a response to a puff of air administered to the face. To identify the facial afferent distribution of this response, the necessary characteristics of the stimulus, and the role of the infant's antecedent behavior, we studied 13 infants who had demonstrated this reflex. We evaluated nine infants by clinically observing for swallowing in response to a total of 135 stimulus applications. All nine had consistently positive responses to the maxillary-ophthalmic area and to the maxillary-mandibular area. Two had consistent responses to stimulation of the mandibular area alone; these were positive. Six had consistent responses to stimulation of the ophthalmic area alone; these were negative. Four infants, evaluated by manometric documentation of swallowing (a total of 137 stimulus applications) demonstrated 47 of 79 (59%) positive responses to stimuli applied to facial areas including any parts of the lips, but only 7 of 28 (25%) positive responses to stimuli applied to facial areas excluding all parts of the lips (chi-square P = 0.002). Light touch to any facial area, including the cornea, failed to produce a swallow in any infant. Crying and sleep were incompatible with the reflex. This newly identified primitive swallow reflex seems to require diffuse stimulation, possibly thermal, to the perioral area of the face in an awake and noncrying infant.
Deglutition↗
Carbon NMR used in probing the exchange of ethanol with water in water-saturated cement pastes.
The exchange of water by ethanol in two water-saturated cement pastes has been investigated by carbon NMR. The two cement pastes differed only in their thermal history. The diffusion of ethanol into the cement paste was shown to be described by Fickian diffusion, assuming one-dimensional diffusion under perfect sink boundary conditions. The diffusion coefficients were calculated to be (1.28 +/- 0.14) 10(-7) cm2/s for the virgin cement sample and (4.38 +/- 0.57) 10(-7) m2/s for the preheated cement sample (preheated at 105 degrees C for 12 h), respectively. The measurements indicate an extensive exchange between water and ethanol.
Construction Materials↗
Intercalation of [Cr(C2O4)3]3 - complex in mg,al layered double hydroxides.
A Mg,Al layered double hydroxide (LDH) with [Cr(C(2)O(4))(3)](3)(-) anions in the interlayer has been synthesized following two different routes: reconstruction from a mildly calcined Mg,Al-carbonate LDH, and anion exchange from a Mg,Al-nitrate LDH. The solids prepared have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and UV-vis/DR (diffuse reflectance) spectroscopies, thermal methods, nitrogen adsorption at -196 degrees C, and FT-IR monitoring of pyridine adsorption. The results obtained indicate that the most appropriate method is anion exchange, leading to a well crystallized LDH with an interlayer spacing of 10 A. Due to the high pH value (>8) of the solution in the reconstruction method, however, a polyphasic system is obtained, where, in addition to a phase with the LDH structure, amorphous magnesium oxalate and chromium oxohydroxides are also formed due to hydrolysis of the complex. The interlayer complex is stable up to 200 degrees C, but the layered structure is stable up to 330 degrees C, probably because of the presence of interlayer oxalate anions formed during decomposition of the complex. Calcination leads to oxidation of Cr(3+) ions to the six-valent state, which reverts to Cr(3+) when the calcination temperature is further increased.
Journal Article↗
An NMR investigation of CO tolerance in a Pt/Ru fuel cell catalyst.
We report the first combined application of solid-state electrochemical NMR (EC NMR), cyclic voltammetry (CV), and potentiostatic current generation to investigate the topic of the ruthenium promotion of MeOH electro-oxidation over nanoscale platinum catalysts. The CV and EC NMR results give evidence for two types of CO: CO on essentially pure Pt and CO on Pt/Ru islands. There is no NMR evidence for rapid exchange between the two CO populations. CO molecules on the primarily Pt domains behave much like CO on pure Pt, with there being little effect of Ru on the Knight shift or on Korringa relaxation. In sharp contrast, COs on Pt/Ru have highly shifted (13)C NMR resonances, much weaker Korringa relaxation, and, at higher temperatures, they undergo thermally activated surface diffusion. For CO on Pt, the correlation observed between the 2pi* Fermi level local density of states and the steady-state current suggests a role for Ru in weakening the Pt-CO bond, thereby increasing the CO oxidation rate (current). The combined EC NMR/electrochemistry approach thus provides new insights into the promotion of CO tolerance in Pt/Ru fuel cell catalysts, in addition to providing a novel route to investigating promotion in heterogeneous catalysis in general.
Journal Article↗
A Kirkwood-Buff derived force field for methanol and aqueous methanol solutions.
A force field for the simulation of methanol and aqueous methanol mixtures is presented. The force field was specifically designed to reproduce the experimental Kirkwood-Buff integrals as a function of methanol mole fraction, thereby ensuring a reasonable description of the methanol cosolvent and water solvent activities. Other thermodynamic and physical properties of pure methanol and aqueous methanol solutions, including the density, enthalpy of mixing, translational diffusion constants, compressibility, thermal expansion, and dielectric properties, were also well reproduced.
Journal Article↗
On the coupling between molecular diffusion and solvation shell exchange.
The connection between diffusion and solvent exchanges between first and second solvation shells is studied by means of molecular dynamics simulations and analytic calculations, with detailed illustrations for water exchange for the Li(+) and Na(+) ions, and for liquid argon. First, two methods are proposed which allow, by means of simulation, to extract the quantitative speed-up in diffusion induced by the exchange events. Second, it is shown by simple kinematic considerations that the instantaneous velocity of the solute conditions to a considerable extent the character of the exchanges. Analytic formulas are derived which quantitatively estimate this effect, and which are of general applicability to molecular diffusion in any thermal fluid. Despite the simplicity of the kinematic considerations, they are shown to well describe many aspects of solvent exchange/diffusion coupling features for nontrivial systems.
Journal Article↗
Pulsating wave propagation in reactive flows: flow-distributed oscillations
Stationary waves in a reactive flow with equal transport coefficients were recently generated by passing the oscillating Belousov-Zhabotinsky reaction medium through a tubular packed bed reactor while keeping the concentrations constant at the inflow boundary [M. Kaern and M. Menzinger, Phys. Rev. E 60, 3471 (1999)]. Here we study the effects of oscillatory boundary conditions, and observe traveling wave fronts that propagate in either a pulsating or a steady manner. The present experiment is isothermal and conditions are such that all species have identical transport properties. This excludes rapid thermal or activator diffusion and the wave pulsation appears to be induced by an essentially kinematic mechanism. Our experimental findings are supported by numerical simulations of the full reaction-diffusion-advection system using a realistic kinetic model. Finally, the kinematic essence of the mechanism inducing wave pulsation is captured in an iterative one-variable map.
Journal Article↗