Showing papers on "Electric potential published in 2002"

Lubrication theory for electro-osmotic flow in a microfluidic channel of slowly varying cross-section and wall charge

Abstract: Electro-osmotic flow is a convenient mechanism for transporting fluid in microfluidic devices. The flow is generated through the application of an external electric field that acts on the free charges that exist in a thin Debye layer at the channel walls. The charge on the wall is due to the particular chemistry of the solid–fluid interface and can vary along the channel either by design or because of various unavoidable inhomogeneities of the wall material or because of contamination of the wall by chemicals contained in the fluid stream. The channel cross-section could also vary in shape and area. The effect of such variability on the flow through microfluidic channels is of interest in the design of devices that use electro-osmotic flow. The problem of electro-osmotic flow in a straight microfluidic channel of arbitrary cross-sectional geometry and distribution of wall charge is solved in the lubrication approximation, which is justified when the characteristic length scales for axial variation of the wall charge and cross-section are both large compared to a characteristic width of the channel. It is thereby shown that the volume flux of fluid through such a microchannel is a linear function of the applied pressure drop and electric potential drop across it, the coefficients of which may be calculated explicitly in terms of the geometry and charge distribution on the wall. These coefficients characterize the ‘fluidic resistance’ of each segment of a microfluidic network in analogy to the electrical ‘resistance’ in a microelectronic circuit. A consequence of the axial variation in channel properties is the appearance of an induced pressure gradient and an associated secondary flow that leads to increased Taylor dispersion limiting the resolution of electrophoretic separations. The lubrication theory presented here offers a simple way of calculating the distortion of the flow profile in general geometries and could be useful in studies of dispersion induced by inhomogeneities in microfluidic channels.

Thermally activated plastic flow of metals and ceramics with an electric field or current

Abstract: The effects of high density electric current pulses (103–106 A cm−2) on the flow stress of metals at low homologous temperatures and of a modest external electric field on the flow stress of fine-grained oxides at high temperatures is presented. The results in both cases are evaluated in terms of thermally-activated plastic deformation processes. In the case of the metals, the influence of an electron wind on each of the parameters in the equation for the thermally-activated motion of dislocations was determined, the largest effect being on the pre-exponential. The derived electron wind push coefficient was one or more orders of magnitude larger than the value normally accepted for the electron drag coefficient. In the case of the oxides, the substantial effect of an applied electric field on the flow stress was evaluated in terms of its influence on the electrochemical potential of vacancies in the space-charge cloud adjacent to the grain boundaries. Both the derived space-charge cloud width and the electric potential/stress parameter Δ∅/Δσ are in reasonable accord with theoretical predictions.

Dependence of induced transmembrane potential on cell density, arrangement, and cell position inside a cell system

Abstract: A nonuniform transmembrane potential (TMP) is induced on a cell membrane exposed to external electric field. If the induced TMP is above the threshold value, cell membrane becomes permeabilized in a reversible process called electropermeabilization. Studying electric potential distribution on the cell membrane gives us an insight into the effects of the electric field on cells and tissues. Since cells are always surrounded by other cells, we studied how their interactions influence the induced TMP. In the first part of our study, we studied dependence of potential distribution on cell arrangement and density in infinite cell suspensions where cells were organized into simple-cubic, body-centered cubic, and face-centered cubic lattices. In the second part of the study, we examined how induced TMP on a cell membrane is dependent on its position inside a three-dimensional cell cluster. Finally, the results for cells inside the cluster were compared to those in an infinite lattice. We used numerical analysis for the study, specifically the finite-element method (FEM). The results for infinite cell suspensions show that the induced TMP depends on both cell volume fraction and cell arrangement. We established from the results for finite volume cell clusters and layers, that there is no radial dependence of induced TMP for cells inside the cluster.

Quasi-classical versus non-classical spectral asymptotics for magnetic schrödinger operators with decreasing electric potentials

Abstract: We consider the Schrodinger operator H(V) on L2 (ℝ2) or L2(ℝ3) with constant magnetic field, and electric potential V which typically decays at infinity exponentially fast or has a compact support. We investigate the asymptotic behaviour of the discrete spectrum of H(V) near the boundary points of its essential spectrum. If the decay of V is Gaussian or faster, this behaviour is non-classical in the sense that it is not described by the quasi-classical formulas known for the case where V admits a power-like decay.

A model‐derived storm time asymmetric ring current driven electric field description

Abstract: [1] Low-latitude ionospheric and near-Earth magnetospheric electric fields are calculated from model results of the storm time asymmetric ring current. These fields are generated from subauroral field-aligned currents out of the ionosphere in the midnight sector and into the ionosphere on the dayside. The currents balance the divergence of the asymmetric ring current, which is the dominant component of the ring current during main phase and early recovery phase of magnetic storms. The basic shape of the electric potential pattern is described, both in the ionosphere and in the magnetosphere. It is found that intense ring current injection events can create potential differences up to 200 kV and can create local electric fields in the nightside magnetosphere >5 mV m−1. The magnitudes and locations of the most intense electric fields are quite consistent with observations of subauroral (ionospheric) and near-Earth (magnetospheric) electric fields during magnetic storms. In addition, a relationship between the magnitude of the low-latitude electric potential and the Dst* index is described. This relationship can be used to predict the location and size of strong low-latitude electric fields and currents. The presented electric field results are derived from model output, such that there is no feedback of the calculated fields back into the model.

Charged polymer membrane translocation

Abstract: We study the process of charged polymer translocation, driven by an external electric potential, through a narrow pore in a membrane We assume that the number of polymer segments, m, having passed the entrance pore mouth, is a slow variable governing the translocation process Outside the pore the probability that there is an end segment at the entrance pore mouth, is taken as the relevant parameter In particular we derive an expression for the free energy as a function of m, F(m) F(m) is used in the Smoluchowski equation in order to obtain the flux of polymers through the pore In the low voltage regime we find a thresholdlike behavior and exponential dependence on voltage Above this regime the flux depends linearly on the applied voltage At very high voltages the process is diffusion limited and the flux saturates to a constant value The model accounts for all features of the recent experiments by Henrickson et al [Phys Rev Lett 85, 3057 (2000)] for the flux of DNA molecules through an α-hemolysin pore as a function of applied voltage

Models of two‐dimensional embedded thin current sheets from Vlasov theory

Abstract: [1] This paper uses the steady state Vlasov theory to obtain an explicit description of the properties of thin current sheets in two space dimensions. The method is general; in particular, it is not restricted to local Maxwellian distribution functions. The treatment includes chaotic particle motion, and the plasma is treated as quasi-neutral. Such general equilibria are necessary for modeling observed structures in the Earth's magnetotail. In particular, this applies to thin current sheets embedded in wider sheets. Abandoning local Maxwellian distribution functions implies that the electric potential can no longer be transformed to zero such that additional efforts are required to include the electric field. Critical input parameters are the gyroscales of the different particle species and the ion/electron temperature ratio. The results illustrate how finite ion gyroscales affect the current sheet structure. Further aspects that are analyzed include the contributions of electrons and ions to the total current density, Hall currents, the occurrence of significant electric fields and the role of the ion/electron temperature ratio. The necessity of electric potentials in the self-consistent structure of thin current sheets of the present type may provide a link between the presence or formation of thin current sheets and auroral arcs.

A magnetohydrodynamic chaotic stirrer

Abstract: A magnetohydrodynamic (MHD) stirrer that exhibits chaotic advection is investigated experimentally and theoretically. The stirrer consists of a circular cavity with an electrode (C) deposited around its periphery. Two additional electrodes (A) and (B) are deposited eccentrically inside the cavity on the bottom. The cavity is positioned in a uniform magnetic field that is parallel to the cylinder's axis, and it is filled with a weak electrolyte solution. Fluid motion is induced in the cavity by applying a potential difference across a pair of electrodes. A closed-form, analytical solution is derived for the MHD creeping flow field in the gap between the two eccentric cylinders. A singular solution is obtained for the special case when the size of the inner electrode shrinks to a point. Subsequently, passive tracers' trajectories are computed when the electric potential differences are applied alternately across electrodes AC and BC with period T. At small periods T, the flow is regular and periodic in most of the cavity. As the period increases, so does the complexity of the motion. At relatively large periods, the passive tracer experiences global chaotic advection. Such a device can serve as an efficient stirrer. Since this device has no moving parts, it is especially suitable for microfluidic applications. This is yet another practical example of a modulated, two-dimensional Stokes flow that exhibits chaotic advection.

A new finite-element formulation for electromechanical boundary value problems

Abstract: In this paper, a new finite-element formulation for the solution of electromechanical boundary value problems is presented. As opposed to the standard formulation that uses scalar electric potential as nodal variables, this new formulation implements a vector potential from which components of electric displacement are derived. For linear piezoelectric materials with positive definite material moduli, the resulting finite-element stiffness matrix from the vector potential formulation is also positive definite. If the material is non-linear in a fashion characteristic of ferroelectric materials, it is demonstrated that a straightforward iterative solution procedure is unstable for the standard scalar potential formulation, but stable for the new vector potential formulation. Finally, the method is used to compute fields around a crack tip in an idealized non-linear ferroelectric material, and results are compared to an analytical solution. Copyright © 2002 John Wiley & Sons, Ltd.

Electric measurements of charged sprays emitted by cone-jets

Abstract: We use time-of-flight and energy analysis techniques to measure in a vacuum the charge, specific charge and stopping potential of primary and satellite droplets generated by electrosprays of tributyl phosphate solutions. This information, of interest in itself, is subsequently analysed to obtain the following relevant parameters of the jet emanating from the Taylor cone: the velocity of the fluid at the breakup point, the voltage difference between the liquid cone and jet breakup location, and the most probable wavelength for varicose breakup. A large fraction of the electrospray needle voltage is used to accelerate the jet. Indeed, for the solutions of lowest electrical conductivities studied here, the voltage difference between electrospray needle and jet breakup location becomes approximately 90% of the needle voltage. In addition, the pressure of the jet fluid at the breakup point is negligible compared to its specific kinetic energy. The specific charge distribution function of the main droplets produced in the varicose breakup is remarkably narrow. Hence, the limiting and commonly accepted case of varicose breakup at constant electric potential is not consistent with this experimental observation. On the other hand, a scenario in which the electric charge is bound to the jet surface seems to be a good approximation to simulate the effect of charge on capillary breakup. It is also found that the effect of viscosity on the formation of droplets is paramount in electrosprays of moderate and high electrical conductivity. We expect that these measurements will guide the analytical modelling of cone-jets.

Interfacial feedback dynamics in polymer light-emitting electrochemical cells

Abstract: We present modeling studies to demonstrate how incorporating ions into organic light-emitting diodes improves carrier injection. The simulations show that the electric potential is dropped preferentially at the electrodes, thereby narrowing injection barriers. The studies provide insight into the interfacial feedback dynamics that regulate carrier injection and are relevant to the optimization of light-emitting electrochemical cells for display purposes.

Removal of metabolic components from blood

Abstract: An electrophoresis system for removing or reducing concentration of a metabolic component from blood or plasma of a subject, the system comprising: (a) a cathode in a cathode zone; (b) an anode in an anode zone, the anode disposed relative to the cathode so as to be adapted to generate an electric field in an electric field area therebetween upon application of an electric potential between the cathode and the anode; (c) a first ion-permeable barrier having a defined pore size and pore size distribution disposed in the electric field area; (d) a second ion-permeable barrier having a defined pore size and pore size distribution disposed between the cathode zone and the first barrier so as to define a treatment chamber therebetween; (e) means adapted for cooling blood or plasma from the subject; (f) means adapted to provide dialysate to the cathode zone and the anode zone; and (g) means adapted to provide blood or plasma from the subject to the treatment chamber; wherein, upon application of the electric potential, a metabolic component from the blood or plasma is caused to move through at least one barrier into at least one of the cathode or anode zones.

Physics-based analytical modeling of potential and electrical field distribution in dual material gate (DMG)-MOSFET for improved hot electron effect and carrier transport efficiency

Abstract: We propose a new two-dimensional (2-D) analytical model of a dual material gate MOSFET (DMG-MOSFET) for reduced drain-induced barrier lowering (DIBL) effect, merging two metal gates of different materials, laterally into one. The arrangement is such that the work function of the gate metal near the source is higher than the one near the drain. The model so developed predicts a step-function in the potential along the channel, which ensures screening of the drain potential variation by the gate near the drain. The small difference of voltage due to different gate material keeps a uniform electric field along the channel, which in turn improves the carrier transport efficiency. The ratio of two metal gate lengths can be optimized along with the metal work functions and oxide thickness for reducing the hot electron effect. The model is verified by comparison to the simulated results using a 2-D device simulator ATLAS over a wide range of device parameters and bias conditions.

Exciton states and optical transitions in colloidal CdS quantum dots: Shape and dielectric mismatch effects

Abstract: We present a theoretical investigation of electron-hole and exciton energy spectra as well as oscillator strengths of optical transitions in colloidal CdS quantum dots (QD's) with spherical and tetrahedral shape. The Coulomb potential energy of the electron-hole system is treated taking into account the dielectric mismatch at the QD boundaries. Calculation of electron-hole energy spectrum and Coulomb potential energy in tetrahedral QD's is carried out using the finite difference method. It is shown that the bulk Coulomb potential energy with the dielectric constant of QD leads to lower exciton energy levels as compared to the Coulomb potential, which includes electron-hole interaction and self-action energies. The Coulomb potential changes the electron-hole pair energies without dielectric confinement contributions in such a way that the exciton ground state becomes active for optical transitions in dipole approximation for both tetrahedral and spherical QD's while the lowest electron-hole pair energy level is active for tetrahedral and passive for spherical QD's. The exciton binding energy in both types of QD's is enhanced by a factor of 2 in the presence of the dielectric mismatch. It is proven that the inclusion of the real QD shape and dielectric mismatch is important not only for the quantitative analysis but also for the qualitative description of optical properties of colloidal CdS QD's.

Perpendicular nonlinear waves in an electron–positron–ion plasma

Abstract: Waves propagating perpendicular to a magnetic field in a plasma consisting of electrons, positrons, and ions are studied theoretically and numerically. In a three component plasma, there appears a frequency domain in which the magnetosonic waves cannot propagate; thus, we have two separate modes below the electron cyclotron frequency. Their dispersion relations are discussed. Then, Korteweg–de Vries equations are derived for these modes. A solitary wave of the low-frequency mode has a soliton width 1–103 times as large as the electron skin depth and has an electric potential 1–102 times as large as that in an electron–ion plasma; both of them increase with decreasing ion density. A solitary wave of the high-frequency mode has a soliton width of the order of the electron skin depth and has negligibly small electric potential. Three-fluid simulations show that the low-frequency mode solitary pulse can emit high-frequency mode solitons, if the amplitude of the original pulse is large and the ion density is low.

Dynamic diffractive optical transform

Abstract: A dynamic diffractive optical transform. An electric field pattern is created across a body of material, the material being characterized in that it has an optical transmission property which varies in response to of an electric potential applied across a portion thereof. The electric field pattern is created such that the resulting profile of the transmission property is an arbitrary shape which produces a desired diffraction pattern that may not be physically realizable in conventional refractive optics or is a Fresnel lens-like construct derived from a refractive optical element. This is done by selectively applying electric potentials to transparent electrode pairs having liquid crystal material therebetween and preferably relatively small proportions in relation to the relevant wavelength of light, so as to create variations in phase delay that are aperiodic, have other than fifty percent spatial duty factor or have multiple levels of phase delay. The transform is embodied in an optical scanner, an adaptive lens, and an optical switch.

Imaging of electron potential landscapes on Au(111).

Abstract: The Hohenberg-Kohn theorem states that the ground state electron density completely determines the external potential acting on an electron system. Inspired by this fundamental theorem, we developed a novel approach to map directly the electron potential in surface systems: linear response theory applied to the total electron density as measured with scanning tunneling microscopy determines the external potential. Potential imaging is demonstrated for the s-p derived surface state on Au(111), where the "herringbone" reconstruction induces a periodic potential modulation, the details of which are revealed by our technique.

Numerical simulation of the steady-state deformation of a smart hydrogel under an external electric field

Abstract: In this paper, we develop a numerical model based on a multiphasic theory to simulate the deformation response of a hydrogel strip immersed into an acidic solution under an external electric field. The deformation response consists of complicated mechano-electrochemical behaviours including mechanical effects (pressure and diffusive drag), chemical effects (concentration, chemical potential, osmotic pressure) and electric effects (electric field intensity, electric potential). The complexly coupled nonlinear governing equations are numerically solved using a recently developed meshless radial basis function method. We analyse the major factors which influence the swelling/shrinking behaviours of the hydrogel strip. The numerical results show good agreement with the experimental measurements.

Column-row addressable electric microswitch arrays and sensor matrices employing them

Abstract: The present invention relates generally to fabricating two-terminal electric microswitches comprising thin semiconductor films and using these microswitches to construct column-row (x-y) addressable microswitch matrices. These microswitches are two terminal devices through which electric current and electric potential (or their derivatives or integrals) can be switched on and off by the magnitude or the polarity of the external bias. The microswitches are made from semiconducting thin films in a electrode/semiconductor/electrode, thin film configuration. Column-row addressable electric microswitch matrices can be made in large areas, with high pixel density. Such matrices can be integrated with a sensor layer with electronic properties which vary in response to external physical conditions (such as photon radiation, temperature, pressure, magnetic field and so on), thereby forming a variety of detector matrices.

The ion-acoustic soliton: A gas-dynamic viewpoint

Abstract: The properties of fully nonlinear ion-acoustic solitons are investigated by interpreting conservation of total momentum as the structure equation for the proton flow in the wave. In most studies momentum conservation is regarded as the first integral of the Poisson equation for the electric potential and is interpreted as being analogous to a particle moving in a pseudo-potential well. By adopting an essentially gas-dynamic viewpoint, which emphasizes momentum conservation and the properties of the Bernoulli-type energy equations, the crucial role played by the proton sonic point becomes apparent. The relationship (implied by energy conservation) between the electron and proton speeds in the transition yields a locus—the hodograph of the system–which shows that, in the first half of the soliton, the electrons initially lag behind the protons until the charge neutral point is reached, after which they run ahead of the protons. The system reaches an equilibrium point (the center of the soliton) before the p...

Observation of magnetoelectric linear birefringence

Abstract: We report the first experimental observation of optical linear birefringence induced in molecular liquids by crossed electric and magnetic fields perpendicular to the direction of light propagation. The optical axes coincide with the external fields, and the strength is bilinear in the electric and magnetic fields.

An accurate modelling of piezoelectric multi-layer plates

Abstract: The present work proposes a new approach to laminated plates with piezoelectric layers based on a refinement of the electric potential as function of the thickness coordinate and accounting for shearing correction of the elastic displacement. The approach deals with the combination of an equivalent single-layer approach for the mechanical displacement with a layerwise-type modelling of the electric potential considered as an additional degree of freedom. Such an approach offers flexibility in accommodating electric conditions at the layer interfaces. The equations governing the force, moment and electric charge resultants of the laminated piezoelectric plate are then deduced from a variational formulation involving mechanical surface loads or prescribed electric potential on the top and bottom faces of the plate as well as at layer interfaces. A particular attention is devoted to the interface conditions which are enforced by using Lagrange multipliers in the variational principle. Emphasis is placed on the performances, advantages and limitations of the present approach. The quality of the predictions of the global and local responses (the through-the-thickness variation of elastic displacements, stresses, electric potential and induction) is quantified for particular structures of practical interest such as piezoelectric bimorph, bilayer structure and piezoelectric sandwich undergoing applied density of force and electric potential. Moreover, comparisons of the results provided by the refined approach to those of finite element computations and simplified model are also presented. The comparisons assess of the effectiveness of the present laminated piezoelectric plate model that improves, in significant way, the predictions given by a simplified approach.

Variational calculations for the energy levels of confined two-electron atomic systems

Abstract: An accurate variational calculation has been performed for the ground-state-energy values of confined two-electron isoelectronic series from He to Ar16+. The confinement is obtained by embedding the ion in an overall charge neutral environment like that of a plasma. The confinement potential is chosen as that of a screened Coulomb potential between charges, obtained from a Debye model. The wave function is expanded in terms of product basis sets involving interparticle coordinates. The energy levels are found to be less bound with an increase of the screening parameter and ultimately become unstable. One- and two-particle moments have been calculated for the first time under such screening. The study is expected to throw new light on the behavior of the energy levels of foreign atoms embedded in an overall neutral environment which can be treated like a plasma.

Quasi-classical versus non-classical spectral asymptotics for magnetic Schroedinger operators with decreasing electric potentials

Abstract: We consider the Schroedinger operator H on L^2(R^2) or L^2(R^3) with constant magnetic field and electric potential V which typically decays at infinity exponentially fast or has a compact support. We investigate the asymptotic behaviour of the discrete spectrum of H near the boundary points of its essential spectrum. If the decay of V is Gaussian or faster, this behaviour is non-classical in the sense that it is not described by the quasi-classical formulas known for the case where V admits a power-like decay.

The Coupling between the Hydration and Double Layer Interactions

Abstract: The electric potential and the polarization between two charged, flat surfaces immersed in water are calculated without the usual assumption that the polarization is proportional to the electric fi...

Effective electron mobility reduced by remote charge scattering in high-κ gate stacks

Abstract: The mobility reduction in high-κ gate stacks is discussed in terms of remote charge scattering. The mobility is recovered by growing an additional layer of SiO2 at the interface between the high-κ dielectric and the silicon substrate. A good explanation for this finding is that the Coulomb potential which is induced by the fixed charge at the dielectric/SiO2 interface is responsible for the scattering of electrons.