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Potential Gradients (potential + gradient)
Selected AbstractsNanocarving of Titania as a Diffusion-Driven Morphological Instability,ADVANCED FUNCTIONAL MATERIALS, Issue 3 2008Doh-Kwon Lee Abstract Under strongly reducing conditions at high temperatures titania develops a specific surface morphology, comprising a regular array of fibers with a diameter in the sub-micrometer range. By a chemical diffusion experiment in a defined oxygen potential gradient it is shown that this surface structuring is caused by a diffusion-driven morphological instability of an advancing reaction front (surface). The kinetics of the process is analyzed in terms of linear transport equations. The conditions for the occurrence of the surface instability are discussed and the required materials properties are analyzed. The observed surface structuring is not restricted to titania, rather it has to occur in all nonstoichiometric compounds with predominant cation mobility. [source] Study of non-Fickian diffusion problems with the potential field in the cylindrical co-ordinate systemINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 5 2003Han-Taw Chen Abstract The present study applies a hybrid numerical scheme of the Laplace transform technique and the control volume method in conjunction with the hyperbolic shape functions to investigate the effect of a potential field on the one-dimensional non-Fickian diffusion problems in the cylindrical co-ordinate system. The Laplace transform method is used to remove the time-dependent terms in the governing differential equation and the boundary conditions, and then the resulting equations are discretized by the control volume scheme. The primary difficulty in dealing with the present problem is the suppression of numerical oscillations in the vicinity of sharp discontinuities. Results show that the present numerical results do not exhibit numerical oscillations and the potential field plays an important role in the present problem. The strength of the jump discontinuity can be reduced by increasing the value of the potential gradient. The propagation speed of mass wave is independent of the potential gradient and the boundary condition. Copyright © 2003 John Wiley & Sons, Ltd. [source] Reentry Site During Fibrillation Induction in Relation to Defibrillation Efficacy for Different Shock WaveformsJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 5 2001Ph.D., RAYMOND E. IDEKER M.D. Reentry Site and Defibrillation Waveform Efficacy.Introduction: Unsuccessful defibrillation shocks may reinitiate fibrillation by causing postshock reentry. Methods and Results: To better understand why some waveforms are more efficacious for defibrillation, reentry was induced in six dogs with 1-, 2-, 4-, 8-, and 16-msec monophasic and 1/1- (both phases 1 msec) 2/2-, 4/4-, and 8/8-msec biphasic shocks. Reentry was initiated by 141 ± 15 V shocks delivered from a defibrillator with a 150- , F capacitance during the vulnerable period of paced rhythm (183 ± 12 msec after the last pacing stimulus). The shock potential gradient field was orthogonal to the dispersion of refractoriness. Activation was mapped with 121 electrodes covering 4 × 4 cm of the right ventricular epicardium, and potential gradient and degree of recovery of excitability were estimated at the sites of reentry. Defibrillation thresholds (DFTs) were estimated by an up-down protocol for the same nine waveforms in eight dogs internally and in nine other dogs externally. DFT voltages for the different waveforms were positively correlated with the magnitude of shock potential gradient and negatively correlated with the recovery interval at the site at which reentry was induced by the waveform during paced rhythm for both internal (DFT = 1719 + 64.5 , V , 11.1RI; R2= 0.93) and external defibrillation (DFT = 3445 + 150 , V , 22RI; R2= 0.93). Conclusion: The defibrillation waveforms with the lowest DFTs were those that induced reentry at sites of low shock potential gradient, indicating efficacious stimulation of myocardium. Additionally, the site of reentry induced by waveforms with the lowest DFTs was in myocardium that was more highly recovered just before the shock, perhaps because this high degree of recovery seldom occurs during defibrillation due to the rapid activation rate during fibrillation. [source] Protein ion-exchange adsorption kineticsAICHE JOURNAL, Issue 7 2001J. A. Wesselingh The kinetics of the adsorption of the protein BSA on the ion exchanger Q-Sepharose FF were measured for several values of the pH and ionic strength, using several techniques. The measurements were best described with a model incorporating both surface and pore diffusion and with the chemical potential gradient as the driving force for diffusion. The surface-diffusion coefficients from this model show an inverse exponential dependency on the binding strength. This dependency can be explained by an activated jump mechanism. The pore-diffusion coefficient is much lower than that in free solution, which is probably caused by a combination of steric and electric exclusion. [source] Molecular Dynamics Simulations of Polymer Translocations,MACROMOLECULAR THEORY AND SIMULATIONS, Issue 5 2004Richard Randel Abstract Summary: Molecular dynamics simulation studies of the translocation of charged homopolymers of length, N, driven by an electric potential gradient through a channel have been performed. We find that the translocation time, ,, displays an inverse power dependence on the temperature of the simulation ,,,,(T,,,T0),7/4, which is in very good agreement with experimental results. In addition, the dependence of , on the driving field strength and the velocity of translocation on the polymer length N have also been obtained. The results suggest that such minimalist models are useful in modelling biological processes and that the molecular dynamics method is a suitable approach for carrying out these simulations. Snapshot of the polymer during the simulation. [source] Hydraulic responses to height growth in maritime pine treesPLANT CELL & ENVIRONMENT, Issue 9 2004S. DELZON ABSTRACT As trees grow taller, decreased xylem path conductance imposes a major constraint on plant water and carbon balance, and is thus a key factor underlying forest productivity decline with age. The responses of stomatal conductance, leaf area: sapwood area ratio (AL : AS) and soil,leaf water potential gradient (,,S,L) to height growth were investigated in maritime pine trees. Extensive measurements of in situ sap flow, stomatal conductance and (non-gravitational) needle water potential (L = ,L , ,wgh) were made during 2 years in a chronosequence of four even-aged stands, under both wet and dry soil conditions. Under wet soil conditions, L was systematically lower in taller trees on account of differences in gravitational potential. In contrast, under dry soil conditions, our measurements clearly showed that L was maintained above a minimum threshold value of ,2.0 MPa independently of tree height, thus limiting the range of compensatory change in ,,S,L. Although a decrease in the AL : AS ratio occurred with tree height, this compensation was not sufficient to prevent a decline in leaf-specific hydraulic conductance, KL (50% lower in 30 m trees than in 10 m trees). An associated decline in stomatal conductance with tree height thus occurred to maintain a balance between water supply and demand. Both the increased investment in non-productive versus productive tissues (AS : AL) and stomatal closure may have contributed to the observed decrease in tree growth efficiency with increasing tree height (by a factor of three from smallest to tallest trees), although other growth-limiting responses (e.g. soil nutrient sequestration, increased respiratory costs) cannot be excluded. [source] Potential-based path planning for robot manipulatorsJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 6 2005Chien-Chou Lin In this paper, a potential-based path-planning algorithm for a high DOF robot manipulator is proposed. Unlike some c-space-based approaches, which often require expensive preprocessing for the construction of the c-space, the proposed approach uses the workspace information directly. The approach computes, similar to that done in electrostatics, repulsive force and torque between objects in the workspace. A collision-free path of a manipulator will then be obtained by locally adjusting the manipulator configuration to search for minimum potential configurations using that force and torque. The proposed approach is efficient because these potential gradients are analytically tractable. Simulation results show that the proposed algorithm works well, in terms of computation time and collision avoidance, for manipulators up to 9 degrees of freedom (DOF). © 2005 Wiley Periodicals, Inc. [source] Diffusion through ordered force fields in nanopores represented by Smoluchowski equationAICHE JOURNAL, Issue 6 2009Fu Yang Wang Abstract The classical Einstein or Fick diffusion equation was developed in random force fields. When the equation is applied to gas transport through coal, significant discrepancies are observed between experimental and simulation results. The explanation may be that the random force field assumption is violated. In this article, we analyze molecular transport driven by both random and ordered (directional) forces in nanopores. When applied to CO2 transport through cone-shaped carbon nano-tubes (CNTs) and Li+ doped graphite pores, computational results show that directional force fields may significantly affect porous media flow. Directional forces may be generated by potential gradients arising from a range of non-uniform characteristics, such as variations in the pore-sizes and in local surface compositions. On the basis of the simulation and experimental results, the Smoluchowski and Fokker-Planck equations, which account for the directional force fields, are recommended for diffusion through ordered force fields in nanopores. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian ShieldJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2007J. A. BALDWIN Abstract A re-evaluation of the P,T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet,clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ,16 kbar and 750 °C, followed by near-isothermal decompression to ,10 kbar. Associated with the eclogite are two types of occurrences of sapphirine-bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1,2 m zone adjacent to the eclogite body with a peak assemblage of garnet,kyanite,quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine,plagioclase and spinel,plagioclase symplectites developed around kyanite. The second variety of sapphirine-bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine,plagioclase, spinel,plagioclase and corundum,plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ,7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid-crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5,10 Myr, indicating that ,25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ,2,5 mm year,1 following eclogite facies metamorphism, after which the rocks resided at mid-crustal levels for 80,100 Myr. [source] Mechanical and corrosion behaviour of a Ti-Al-Nb alloy after deformation at elevated temperaturesMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 12 2008M. V. Popa Abstract The mechanical properties of Ti6Al7Nb alloy deformed and heat treated at elevated temperatures were correlated with its microstructure and corrosion behaviour in Ringer (of different pH values: 2.49, 6.9 and 8.9) and Ringer,Brown solutions. Microstructural analysis revealed a Widmanstatten structure for the alloys deformed at 1100,°C (, field) and structure with , grains at 930,°C (,,+,, field). The thermo-mechanical processing improved the electrochemical behaviour of Ti6Al7Nb alloys, especially their passive state. Open circuit potential variations in time reflected more compact, stable, resistant passive films on the surface of the treated alloys. Open circuit potential gradients simulating the non-uniformities of pH along the implant surface have very low values that cannot generate galvanic corrosion. Corrosion rates and ion release are very much reduced. Impedance spectra were fitted with a two time-constants equivalent circuit for some alloys and with three time-constants equivalent circuit for other alloys. [source] Cellular mechanisms of potassium transport in plantsPHYSIOLOGIA PLANTARUM, Issue 4 2008Dev T. Britto Potassium (K+) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K+ transport proteins, operating over a wide range of K+ concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K+ and other ions such as NH4+ and Na+, the regulation of cellular K+ pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K+ fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K+ acquisition and produce an overview of gene families encoding K+ transporters. [source] Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during droughtPLANT CELL & ENVIRONMENT, Issue 8 2003H. R. SCHULTZ ABSTRACT A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (gmax) and maximum leaf photosynthesis (Amax) values than Grenache at lower predawn leaf water potentials, ,leaf, throughout the season. The ,leaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which ,leaf did not drop significantly below the minimum ,leaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, Kl, incorporating leaf transpiration rate and whole-plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure-flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two- to three-fold larger hydraulic conductance per unit path length (Kh) and a four- to six-fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air-dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry-down. It is hypothesized that the differences in water-conducting capacity of stems and especially petioles may be at the origin of the near-isohydric and anisohydric behaviour of g. [source] Developmental effects of physiologically weak electric fields and heat: An overview,BIOELECTROMAGNETICS, Issue S7 2005Richard D. Saunders Abstract This study summarizes the possible effects on prenatal development of physiologically weak electric fields induced in the body by exposure to extremely low frequency (ELF) electromagnetic fields and of elevated temperature levels that might result from exposure to radiofrequency (RF) radiation. Both topics have been discussed at recent international workshops organized by WHO in collaboration with other bodies. Mammalian development is characterized by a highly ordered sequence of cell proliferation and differentiation, migration, and programmed cell death. These processes, particularly proliferation and migration, are susceptible to a variety of environmental agents including raised maternal temperature. In addition, there is growing evidence that physiologically weak endogenous DC electric fields and ionic currents have a role in guiding developmental processes, including cell orientation and migration, by establishing electrical potential gradients. Disruption of these fields can adversely affect development in amphibian and bird embryos, which are experimentally accessible, and may well do so in mammalian embryos. The extent to which induced ELF electric fields might influence these and other processes that take place during prenatal development, childhood, and adolescence is less clear. Organogenesis, which takes place primarily during the embryonic period, is susceptible to raised maternal temperatures; a large number of studies have shown that RF exposure produces developmental effects that can be attributed to heat. The development of the central nervous system is particularly susceptible to raised temperatures; a reduction in brain size, which results in a smaller head, is one of the most sensitive markers of heat-induced developmental abnormalities and can be correlated with heat-induced behavioral deficits. However, some aspects of CNS development have been less well explored, particularly effects on corticogenesis. In addition, the persistence of CNS developmental sensitivity through to childhood and adolescence is not clear. Bioelectromagnetics Supplement 7:S127,S132, 2005. © 2005 Wiley-Liss, Inc. [source] Redox Properties of a Constructed Wetland: Theoretical and Practical AspectsCHEMISTRY & BIODIVERSITY, Issue 3 2009Abstract Constructed wetlands represent a progressive approach to the wastewater treatment. A fundamental prerequisite of the efficient water quality improvement is the presence of redox potential gradients (connected with the aeration of the system) inside the vegetation bed. Redox properties of a constructed wetland were tested in three longitudinal transects crossing the vegetation bed from the inflow zone to the outflow using diverse indicators (e.g., FeIII/FeII, SO/S2,). Approximately 10,25% of iron was reoxidized in samples taken 10,m from the inflow zone in 2006. Redox processes of iron in artificial (constructed wetland) and natural (peat bog) ecosystems were compared. The peat bog was characterized with higher percentages of FeII (usually ca. 90,100%). Thus, the aeration of the peat land was lower in comparison with the constructed wetland. The constructed wetland efficiently reduced sulfates (average concentrations of 44.7 and 11.2,mg/l at the inflow and the outflow, resp., in 2007). Organics, expressed as CODCr and BOD5, and NH were removed with efficiencies of 86.4, 92.2, and 60.4%, respectively. However, total phosphorus (redox processes play a negligible role in this case) was removed only with 39.6% efficiency. Redox properties of the wetland did not significantly depend on the heterogeneity of the treated wastewater flow. [source] | |||||||||||||||||||