High Potential Gradients Research Articles

Gated-charge force microscopy for imaging a surface-acoustic-wave-induced charge in a depleted one-dimensional channel

Gated-charge force microscopy (GCFM) generates images of subsurface induced charge with single-electron charge resolution and zero sensitivity to background charge, for the noninvasive study of confined quantum electronic systems. By gating the radio-frequency signal applied to transducers, which generate surface-acoustic waves (SAWs), we use GCFM to generate images of a GaAs depleted one-dimensional channel and provide insight to SAW-induced transport. Unexpected SAW-induced subsurface charge was imaged at points of high potential gradient. This suggests that electrons depopulating a SAW minimum due to a high potential gradient rapidly lose energy and interact with following SAW minima.

Dielectrophoretic and Electrothermal Effects at Alternating Current Heated Disk Microelectrodes

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for "jet boiling" observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.

The olive tree: a paradigm for drought tolerance in Mediterranean climates

Abstract. Olive trees (Olea europaea L.) are commonly grown in the Mediterranean basin where prolonged droughts may occur during the vegetative period. This species has developed a series of physiological mechanisms, that can be observed in several plants of the Mediterranean macchia, to tolerate drought stress and grow under adverse climatic conditions. These mechanisms have been investigated through an experimental campaign carried out over both irrigated and drought-stressed plants in order to comprehend the plant response under stressed conditions and its ability to recover. Experimental results show that olive plants subjected to water deficit lower the water content and water potentials of their tissues, establishing a particularly high potential gradient between leaves and roots, and stop canopy growth but not photosynthetic activity and transpiration. This allows the continuous production of assimilates as well as their accumulation in the various plant parts, so creating a higher root/leaf ratio if compared to well-watered plants. Active and passive osmotic adjustment due to the accumulation of carbohydrates (in particular mannitol and glucose), proline and other osmolytes have key roles in maintaining cell turgor and leaf activities. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and a light-dependent inactivation of the photosystem II occurs. Finally, the activities of some antioxidant enzymes involved in the scavenging of activated oxygen species and in other biochemical pathways increase during a period of drought. The present paper provides an overview of the driving mechanisms adopted by olive trees to face drought stress with the aim of better understanding plant-soil interactions.

Effect of Joule heating on electrokinetic transport

The Joule heating (JH) is a ubiquitous phenomenon in electrokinetic flow due to the presence of electrical potential gradient and electrical current. JH may become pronounced for applications with high electrical potential gradients or with high ionic concentration buffer solutions. In this review, an in-depth look at the effect of JH on electrokinetic processes is provided. Theoretical modeling of EOF and electrophoresis (EP) with the presence of JH is presented and the important findings from the previous studies are examined. A numerical study of a fused-silica capillary PCR reactor powered by JH is also presented to extend the discussion of favorable usage of JH.

Ordered Nanogroove Arrays on n-TiO2 with a Variation of the Groove Depth, Formed by Self-Organized Photoetching

Photoelectrochemical etching of n-TiO2 (rutile) single crystals produced ordered arrays of nanogrooves with the spacing of about 300 nm over a macroscopically wide area of 0.5 × 0.5 cm at the surface without any use of imprints or templates. Moreover, the photoetching by irradiation with patterned light intensities gave nanogrooves with a desired depth at a desired location, interesting from the point of view of nanoscience and nanotechnology. A plausible model is proposed for the formation of ordered arrays of nanogrooves in terms of self-organized photoetching in which autocatalytic accelerated photoetching at the bottom of the grooves, caused by an excess concentration of photogenerated holes due to a high potential gradient, is coupled with its retardation by a downward shift in the flat-band potential of n-TiO2 at the bottom of the grooves, induced by an increase in the solution pH due to accelerated photoreactions.

Processing, stability and oxygen permeability of Sr(Fe, Al)O 3-based ceramic membranes

Minor additions of alumina into perovskite-type SrFe 0.7Al 0.3O 3− δ , a composition close to the solid solution formation limits in SrFe 1− x Al x O 3− δ system, result in decreasing thermal expansion and increasing oxygen permeability. The improved sinterability of SrFe 0.7Al 0.3O 3-based composite with 3 wt.% Al 2O 3 addition enables to fabricate high-quality tubular membranes for the methane conversion reactors. No essential degradation in the performance of SrFe 0.7Al 0.3O 3− δ membranes under air/CH 4 or air/H 2 H 2O gradients at 973–1223 K was observed during 200–700 h. The stable operation under high oxygen chemical potential gradients is possible due to surface-limited oxygen transport, indicated by the dependencies of oxygen permeability on the membrane thickness. Applying porous layers of the same composition, synthesized via cellulose-precursor technique, onto the permeate-side surface leads to substantially higher oxygen fluxes. For a model reactor with the surface-modified SrFe 0.7Al 0.3O 3− δ membrane and commercial Ni/Al 2O 3 catalyst, the CH 4 conversion rate achieved 90–97% at 1073–1123 K, when the CO selectivity was almost 100%.

Comment on the comment by M. J. Siegert on “A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars” by N. S. Duxbury et al.

[1] There is some debate on the origin of subglacial Lake Vostok and whether the water within the present subglacial lake system contains biota that survived the buildup of the Antarctic ice sheet. One theory suggests that Lake Vostok existed as a preglacial lake before glaciation of the continent at around 15 Ma ago, survived the subsequent period of ice sheet growth, and remained stable beneath the thick ice cover to the present day [Duxbury et al., 2001]. Another hypothesis challenges this view by stating that the early phase of build-up would have resulted in ice grounding throughout the trough which the lake now occupies [Siegert, 2004]. [2] According to Duxbury et al. [2001], the lake could have survived the growth of the ice sheet as long as the preglacial lake was more than 53 m deep. In their model, subglacial Lake Vostok is considered as an approximate closed system. However, if Lake Vostok existed as a preglacial lake, the closure of the Lake Vostok system must have happened between the initiation of the ice sheet and the establishment of a stable ice cover in East Antarctica [Siegert, 2004]. Therefore the dynamic evolution of the ice sheet and its interaction with the preglacial lake should be accounted for in determining whether subglacial Lake Vostok is a direct remnant of a preglacial surficial lake. [3] Siegert [2004] demonstrates that the preglacial lake could not have survived the buildup of the Antarctic ice sheet: water flow beneath an ice mass is controlled by the hydraulic potential gradient, Pg = rigas + (rw ri) gab, where ri and rw are the density of ice and water, and as and ab are the surface and basal slope, respectively [Shreve, 1972]. If the magnitude of surface slope is larger than one tenth the basal slope, water can be driven out of a topographic depression and flow ‘‘uphill.’’ Two icesheet model studies [Huybrechts, 1993; DeConto and Pollard, 2003] show that during the inception and growth of the Antarctic ice sheet, the ice sheet margin was situated close to Lake Vostok. The steep margin of the ice sheet across Lake Vostok implies high subglacial hydraulic potential gradients, which led Siegert [2004] to conclude that basal water is evacuated as the steep ice sheet margin progresses over the lake. However, none of the above models take into account the interaction of the ice sheet with the underlying lake, whether it be a preglacial or a subglacial lake, and only consider an ice sheet more or less frozen to the underlying bedrock. [4] Below I present a physical mechanism that allows for subglacial Lake Vostok to survive the buildup of the ice sheet, by taking into account the interaction of the ice sheet with the preglacial/subglacial lake. This hypothesis is supported by the fact that the surface of a preglacial lake as well as the interface between a subglacial lake and the overriding ice sheet can be regarded as a slippery spot.

Arabian-Anatolian Plate Movements and Related Trends in Southeast Turkey's Oilfields

Two important tectonic phases generated by African and Arabian plate movements influenced the evolution of trends in SE Turkey's oilfields. The main phase occurred during the late Cretaceous (Maastrichtian) time, while the second phase reactivated the existing trends in Miocene. Paleogeological evolutions reveal that SE Turkey was tectonically stable from Paleozoic up to the end of Cretaceous, during which the structural trends were oriented in a WSW-ENE direction, contrarily, under the influence of opening of the Red Sea area, the existing trends rotated in a counterclockwise direction and oriented in a SW-NE direction. This rotational tectonic event is evidenced by SW-NE alignments of SE Turkey's oilfields in subsurface and Karacadag basaltic expensions and appearence of major faults, like the Dead Sea fault, Adiyaman fault, and Akcakale fault, on surface. Subsurface interpretations indicate that, contrarily to the previous works, which considered the Dead Sea fault and Adiyaman fault separate tectonic features from each other and also attached them to the East Anatolian fault, the Adiyaman fault is the continuity of the Dead Sea fault through the Adiyaman, Diyarbakir and Selmo areas. The Akcakale fault, which generated the Karacadag basaltic flows, is another tectonic element and has influenced the exploration activities in SE Turkey. Moreover, detailed interpretations of well data reveal that the SW-NE structural trends also improved reservoir conditions by creating fractures and dolomitizations in reservoir rocks, and the high geothermal and potential (piezometric) gradients and porosity trends are also aligned in the same direction.

The Performance of Electrical Methods for Assessing the Integrity of Geomembrane Liners in Landfill Caps and Waste Storage Ponds

Geomembrane leak location methods have been established based on injecting electrical current through the liner into the surrounding soil and then, using pairs of electrodes above the liner, high potential gradients associated with current leakage may be identified. Recently, permanent electrode arrays have been installed at a number of sites as a means of long term monitoring of the integrity of a landfill liner. There has, however, been no scientific study to date which addresses the performance of such systems. Here, we utilise a physical scale model of a landfill liner∕cap and investigate the usefulness of grid based and boundary electrode arrays for detecting multiple leaks in a landfill liner or cap. We compare the two electrode geometries under a series of controlled electrical leak experiments and demonstrate how both these arrangements can detect and locate leaks in a geomembrane. We also demonstrate some of the limitations of the method. For example, present data processing schemes can give misleading results when there are multiple leaks. The influence of variation in resistivity of the soil above or below the liner and also boundary current leakage on the performance of the technique is also studied. The results suggest that these two factors, unless they are accounted for, can have a significant impact on the results and subsequently affect the practical value of electrical leak detection systems.

Use of mixed conducting membranes to produce hydrogen by water dissociation

We have studied the production of hydrogen by water dissociation at moderate temperatures (700–900°C) with novel mixed-conducting membranes. Hydrogen production rates were investigated as a function of temperature, water partial pressure, membrane thickness, and oxygen chemical potential gradient across the membranes. The hydrogen production rate increased with both increasing moisture concentration and oxygen chemical potential gradient. A hydrogen production rate of 6 cm 3( STP)/ min cm 2 was measured with a 0.10-mm-thick membrane at 900°C and 50 vol% water vapor on one side of the membrane and 80% hydrogen (balance helium) on the other side. Hydrogen was used as a model gas on one side of the membrane to establish a high oxygen potential gradient; however, another reducing gas, methane, was substituted in one experiment to maintain the high oxygen potential gradient. The hydrogen production rate increased with decreasing membrane thickness, but surface kinetics played an important role as membrane thickness decreased.

P-Type electronic conductivity, oxygen permeability and stability of La2Ni0.9Co0.1O4+δElectronic supplementary infromation (ESI) available: further experimental data for the oxygen permeability, total conductivity and Seebeck coefficient of La2Ni0.9Co0.1O4+δ. See http://www.rsc.org/suppdata/jm/b3/b300357d/

The oxygen permeability, total conductivity and Seebeck coefficient of La2Ni0.9Co0.1O4+δ were studied in the oxygen partial pressure range of 10−16 Pa to 50 kPa at 973–1223 K. The conductivity of La2Ni0.9Co0.1O4+δ is predominantly p-type electronic within the whole p(O2) range in which the K2NiF4-type structure exists. Thermally-activated mobility, the values of which are 0.02–0.08 cm2 V−1 s−1, and the p(O2) dependencies of electron-hole transport suggest a small-polaron conduction mechanism. Oxygen permeability of dense La2Ni0.9Co0.1O4+δ membranes, with an apparent activation energy of 192 kJ mol−1 in oxidising conditions, is limited by both bulk ionic conductivity and the surface exchange rate. The role of surface processes in limiting permeation is also significant under air/H2–H2O gradients and increases with decreasing temperature. The stability boundary of the La2Ni0.9Co0.1O4+δ phase at low oxygen pressures is similar to that of undoped lanthanum nickelate, which allows stable operation of nickelate membranes under high oxygen chemical potential gradients, such as air/10% H2–90% N2, at 973 K. At temperatures above 1000 K, the decomposition products form blocking layers on the membrane surface causing degradation of the membrane performance with time. The average thermal expansion coefficient of La2Ni0.9Co0.1O4+δ ceramics, calculated from dilatometric data in air, is 12.8 × 10−6 K−1 at 400–1265 K.

Liquid crystalline pharmacogel based enhanced transdermal delivery of propranolol hydrochloride

A novel pharmacogel was developed for the enhanced transdermal delivery of propranolol hydrochloride (PH). The synthesized prodrugs, propranolol palmitate hydrochloride (PPH) and propranolol stearate hydrochloride (PSH) self-assembled to form gel simply upon mixing alcoholic solution of prodrug with an aqueous solution in a specified ratio. By varying the ratio of prodrug, alcohol and water, three-component phase diagram was constructed which revealed isotropic–gel–vesicular dispersion regions, respectively concomitant to increasing the ratio of water. The gel phase is termed ‘Pharmacogel’ and exhibits birefringence under plane-polarized light corroborating the presence of lamellar liquid crystals. The pharmacogel by virtue of high chemical potential gradient and improved physicochemical properties showed the enhanced in-vitro skin permeation flux of 51.5±3.7 and 42.5±3.1 μg/cm 2/h from PPH and PSH gel, respectively, as compared to 1.9±0.1 μg/cm 2/h for control; and decrease in lag time (1.8 and 2.8 h for PPH and PSH gel, respectively) compared to control (7.6 h) was observed. The admixing of egg lecithin (EL) in increasing ratio concomitantly decreased the flux values to 31.7±2.1 μg/cm 2/h (at a mole ratio of 50:50 PPH:EL) and increased the lag time. In the gel containing 50% EL, the addition of span 40 and cholesterol slightly reduced the permeation while sodium deoxycholate and Tween-80 improved it. The plasma drug levels following transdermal application of control were low ( C max=23 ng/ml) while in PPH gel, it increased with time reaching C max of 94 ng/ml at 8 h post-application of PPH gel ( C max of 75 ng/ml at 12 h post application of PL5 gel) and maintained for longer times. The AUC 0–32 h for PPH gel was much higher (1968 ng h/ml) than control (AUC 0–18 h was 239 ng h/ml), while EL mixed gel also showed better absorption (AUC 0–32 h was 1707 ng h/ml). The gel formulations also caused less irritation than control, while mixed gel showed least irritation. This novel self-assembled pharmacogel providing high transdermal permeation with many variables to regulate the delivery is therefore having a great potential in percutaneous delivery.

Change of migration time and separation window accompanied by field-enhanced sample stacking in capillary zone electrophoresis

When field-enhanced sample stacking was used in capillary zone electrophoresis (CZE) analysis of cations, the decrease of migration time and the reduction of separation window was observed with increase of sample plug length. A simple equation expressing the migration velocity in the stacking process was derived to explain the above phenomenon. From experiments and theoretical consideration, we confirmed that this effect was caused by the higher potential gradient and larger eletroosmotic flow (EOF) mobility at the sample plug than those at the supporting electrolyte. A mathematical model appropriate for the computer simulation of such a system was studied considering the experimental results, and it was concluded that electroosmotic velocity (v(eof)) should be introduced to the equation of continuity as a constant.

Mechanism of ionization of organic compounds on tip emitters

The current-voltage characteristics of organic compounds at a tip platinum emitter have been studied in 0.2-1.0 V/A fields. It is shown that the parameter (V-ϕ) does not affect the formation of the desorption ion barrier in the field ionization of atoms and molecules under a high potential gradient providing electron tunneling to the Fermi level of the emitter material and, therefore, the probability of the ion desorption cannot explicitly depend on this parameter.

Numerical simulation for capillary electrophoresis: II. Relaxation effect of potential gradient in capillary zone electrophoresis

Using our simulation program for CE based on partial differential equations of unsteady state, we found the phenomena at the initial stage of the capillary zone electrophoretic process that a transient zone with very high potential gradient was produced at the front-end of the sample plug and consequently the potential gradient of the rest part without samples decreased. The potential gradient of the zone and the rest part gradually reached the averaged potential gradient (relaxation of potential gradient). This phenomena causes delay of migration time to some degree affecting its reproducibility. This effect is due to discontinuities of the CE system and low mobility of a counter ion in a background electrolyte (BGE). Therefore, in order to improve the delay, discontinuities of solution should be minimized or a counter ion with high mobility in BGE should be selected.

Soluble Solids Concentrations in Pecan Liquid Endosperm of Several Pecan Clones

Variability in soluble solids concentration (SSC, °Brix) in liquid endosperm (LE) among individual pecan [Carya illinoinensis (Wangenh.) K. Koch] fruits and among fruits from different trees and cultivars using a sugar refractometer was determined at College Station, Texas, in 1997. Repeatability of readings from LE from the same fruit was excellent. Fruits from the same tree did not vary for SSC, but significant differences among clones were common. Soluble solids concentration appears to decrease as the fruit matures. The SSC values for two full-sib clones (one susceptible to water split and one resistant to water split) were similar. This information discounts the possibility that high osmotic water potential gradients alone induce the water split phenomenon. A wide range of SSC percents was recorded. A low of 0.5% was recorded for LE from a `Houma' fruit, while 6.1% was recorded for LE from a fruit from a drought-stressed `Burkett' tree.

A field-compatible method for interpolating biopotentials.

Mapping of bioelectric potentials over a given surface (e.g., the torso surface, the scalp) often requires interpolation of potentials into regions of missing data. Existing interpolation methods introduce significant errors when interpolating into large regions of high potential gradients, due mostly to their incompatibility with the properties of the three-dimensional (3D) potential field. In this paper, an interpolation method, inverse-forward (IF) interpolation, was developed to be consistent with Laplace's equation that governs the 3D field in the volume conductor bounded by the mapped surface. This method is evaluated in an experimental heart-torso preparation in the context of electrocardiographic body surface potential mapping. Results demonstrate that IF interpolation is able to recreate major potential features such as a potential minimum and high potential gradients within a large region of missing data. Other commonly used interpolation methods failed to reconstruct major potential features or preserve high potential gradients. An example of IF interpolation with patient data is provided to illustrate its applicability in the actual clinical setting. Application of IF interpolation in the context of noninvasive reconstruction of epicardial potentials (the "inverse problem") is also examined.

Surface discharge plasma induced by spontaneous polarization switching

A flashover plasma has been induced by spontaneous polarization switching on a polar surface of the ferroelectric crystal triglycine sulphate (TGS). The effect has not been observed in the paraelectric phase. The threshold switching voltage for a surface flashover ignition was as low as 100 V for pulsed and ac voltage. A mechanism of plasma initiation on a ferroelectric surface is proposed. It is assumed that the plasma is ignited by electron emission initiated by polarization switching. Subsequent electron avalanching occurs as a result of the high potential gradient along the ferroelectric surface caused by inhomogeneous polarization switching. Electrons and ions with energies up to several hundreds of eV were been recorded due to a high surface potential of the switched ferroelectric.

Calculation of Dielectric Constants near Polyelectrolytes in Solution

The recent application of numerical Poisson−Boltzmann methods to the determination of the electrostatic potential and counterion distributions around polyelectrolytes such as DNA has prompted the determination of accurate solvent dielectric constants. The previous assumption for proteins of using a constant value of about 80 for the solvating environment appears inadequate when dealing with the much higher potential gradients and local ion concentrations of charged polyelectrolytes in solution. Approximations using lower dielectric values near 30 at the surface and increasing away to a bulk value of 78.5 have been incorporated into modern finite-difference Poisson−Boltzmann techniques and have led to more reasonable estimates of counterion distributions. However, choosing this dielectric constant “field” has been more a matter of guessing than scientific analysis. To put these techniques on a firmer foundation, we present a simple calculational procedure for determining dielectric constants near the surfa...

Regional regularization of the electrocardiographic inverse problem: a model study using spherical geometry.

This study examines the use of a new regularization scheme, called regional regularization, for solving the electrocardiographic inverse problem. Previous work has shown that different time frames in the cardiac cycle require varying degrees of regularization. This reflects differences in potential magnitudes, gradients, signal-to-noise ratio (SNR), and locations of electrical activity. One might expect, therefore, that a single regularization parameter and a uniform level of regularization may also be insufficient for a single potential map of a single time frame because in one map there are regions of high and low potentials and potential gradients. Regional regularization is a class of methods that subdivides a given potential map into functional "regions" based on the spatial characteristics of the potential ("spatial frequencies"). These individual regions are regularized separately and recombined into a complete map. This paper examines the hypothesis that such regionally regularized maps are more accurate than if all regions were taken together and solved with an averaged level of regularization. In a homogeneous concentric spheres model, Legendre polynomials are used to decompose a torso potential map into a set of submaps, each with a different degree of spatial variation. The original torso map is contaminated with data noise, or geometrical error or both, and regional regularization improves the epicardial potential reconstruction by up to 25% [relative error (RE)]. Regional regularization also improves the reconstructed location of peaks. A practical goal is to extend the application of this method to the realistic torso geometry, but because Legendre decomposition is limited to geometries with spherical symmetry, other methods of map decomposition must be found. Singular value decomposition (SVD) is used to decompose the maps into component parts. Its individual submaps also have different levels of spatial variation; moreover, it is generalizable to any vector, does not require spherical symmetry, and is extremely efficient numerically. Using SVD decomposition for regional regularization, significant improvement was achieved in the map quality in the presence of data noise.