Control of spiral breakup by an alternating advective field

The control of spiral breakup due to Doppler instability is investigated. It is found that applying an alternating advective field with suitable amplitude and period can prevent the breakup of spiral waves. Further numerical simulations show that the growing meandering behavior of a spiral tip caused by decreasing the excitability of the medium can be efficiently suppressed by the alternating advective field, which inhibits the breakup of spiral waves eventually.     

Electrochemically induced chemomechanical bending of bilayered ion-exchange membranes

A bilayered ion-exchange membrane consisting of poly(styrene-co-4-vinylpyridinium ion) (anion-exchange membrane) and a gel-like mixture of poly(vinyl alcohol) and poly(acrylic acid) (cation-exchange membrane) was prepared. The bilayered membrane strip, one end of which is fixed, is placed between two carbon electrodes in a cell which contains KCl solution of 0.01 mol kg⁻¹ molality. If the electric field is applied from the cation-exchange layer side to the anion-exchange layer side, the cation-exchange layer becomes the K⁻ form and is elongated. On the contrary, if the electric field is reversed, dissociation of water into H⁺ and OH⁻ occurs at the interface between the two layers. Consequently, the carboxyl group becomes the acid form, resulting in contraction of the layer. The anion-exchange layer, which contains a strong base group, becomes either the Cl form or the OH form with changing the electric field, but there is no detectable change in volume. Thus, if the electric field applied to the membrane is periodically reversed, periodical bending of the bilayered membrane is observed.     

Electro-hydrodynamic micro-fluidic mixer

Fluid mixing in microchannels is needed for many applications ranging from bio-arrays to micro-reactors, but is typically difficult to achieve. A simple geometry micro-mixer is proposed based on the electro-hydrodynamic (EHD) force present when the fluids to be mixed have different electrical properties and are subjected to an electric field. The electrodes are arranged so that the electric field is perpendicular to the interface between the two fluids, creating a transversal secondary flow. The technique is demonstrated experimentally using the flow of two liquids with identical viscosity and density, but different electrical properties. The volume flow rate and average velocity are 0.26 microl s(-1) and 4.2 mm s(-1), respectively, corresponding to a Reynolds number Re= 0.0174. The effect of a continuous (DC) electric field and two alternating (AC)- sinusoidal and square - electric fields is explored. At the appropriate parameter values, very good mixing takes place in less than 0.1 s, over a very short distance (within a fraction of the width 250 microm of the electrodes).     

Control and modeling of the dielectrophoretic assembly of on-chip nanoparticle wires

Suspensions of metallic nanoparticles in water were assembled via the action of an alternating electric field (dielectrophoresis) into wires of micrometer thickness. Two modes of microwire assembly, one through the bulk of the suspension and one as half-cylinders on the glass surface between the electrodes, were identified. The operating conditions responsible for the two assembly modes were recognized. The control of the process parameters allows making, for example, straight single connectors or massively parallel arrays of microwires on the surface of the chip, which can be extracted in dry form. The microwire assembly process was modeled using finite element electrostatic calculations. The direction of growth can be guided by introducing conductive islands or particles in the suspension. The experiments, supported by electrostatic calculations, show that the wires grow in the direction of highest field intensity, "automatically" making electrical connections to the objects between the electrodes. The results point the way to controlled dielectrophoretic assembly of nanoparticles into on-chip electrical connectors, switches, and networks.     

First-principles molecular dynamics study on aqueous sulfuric acid solutions

The properties of aqueous sulfuric acid have been studied employing density functional theory-based molecular dynamics simulations in conjunction with norm-conserving pseudopotentials. The simulations were carried out for two different concentrations whose molar concentrations were fixed at 0.84 and 10.2 mol/l. The structural features of aqueous sulfuric acid solutions show a strong dependency on the concentration. The Gr?tthuss-type proton transfer mechanism is not effectively operative at the higher concentration because of the broken hydrogen bond network of water induced by ions generated by the dissociation of sulfuric acid. In addition, to evaluate electrical properties, we carried out a simulation that takes an electric field into account. Results are compared with those of the simulation undertaken with no external electric field.     

Electroosmosis through a Cation-Exchange Membrane: Effect of an ac Perturbation on the Electroosmotic Flow

Electroosmosis experiments through a cation-exchange membrane have been performed using NaCl solutions in different experimental situations. The influence of an alternating (ac) sinusoidal perturbation, of known angular frequency and small amplitude, superimposed to the usual applied continuous (dc) signal on the electroosmotic flow has been studied. The experimental results show that the presence of the ac perturbation affects the electroosmotic flow value, depending on the frequency of the ac signal and on the solution stirring conditions. In the frequency range studied, two regions have been observed where the electroosmotic flow reaches a maximum value: one at low frequencies (Hz); and another at frequencies of the order of kHz. These regions could be related to membrane relaxation phenomena.     

Electroconformational coupling (ECC): An electric field induced enzyme oscillation for cellular energy and signal transductions

Previous work has shown that membrane ATPases can extract free energy from applied oscillating electric fields for doing chemical work, e.g. to synthesize ATP from ADP and P_i or to transport Rb and Na ions against their respective electrochemical gradient. Data of these experiments are briefly reviewed. Electroconformational Coupling (ECC) is used to interpret these results. Computer analysis of a four state cyclic enzyme mechanism reproduces many experimental features. It is shown that a coulombic interaction between an enzyme and an alternating electric field (ac) can cause the enzyme to oscillate between different conformational states. If the frequency of the applied field matches the kinetic characteristics of the system and the amplitude matches the energy level required for inducing productive catalytic cycling, a phenomenological resonance between catalytic reaction and the periodic field is generated. A condition necessary for achieving energy coupling is the kinetic bias arising from the binding energy of the ligand. Analysis indicates that only dynamic electric fields, i.e. oscillating or fluctuating fields, can propel the cyclic reaction of the enzyme catalysis, and thus be effective for transducing energy. A stationary transmembrane electric field must be modulated, e.g. by opening and closing of an ion channel, to become oscillatory in order o produce the same effect. We propose that ECC is a fundamental process of cellular energy and signal transductions. Here, many membrane associated events are reduced to Michaelis-Menten types of enzyme catalytic reactions and they are thus amenable to the quantitative analysis of chemical kinetics.     

Fluid streaming above interdigitated electrodes in dielectrophoresis experiments

For the investigation of alternating current electrokinetic effects, a system is presented that allows for the simultaneous observation of fluid flow above and around microelectrodes in all three directions in space. Beside the usual microscopical view from top, lateral observation through the same objective is made possible by two small mirrors that are placed next to the electrodes. Fluid flow and movement of fluorescent nanoparticles above interdigitated electrodes are monitored by fluorescence microscopy and digital imaging and are further analysed by image processing. Field frequencies are varied from 10 Hz to 1 GHz at up to 10V(rms) . Electrical conductivity of the fluid is monitored in situ in the actual measuring chamber.     

Topography changes of rhodium electrodes induced by the application of fast periodic potential routines

The surface structure of polycrystalline rhodium electrodes in contact with aqueous sulfuric acid was modified by chemical etching with hot concentrated acid or by applying fast square waves with an upper potential equal to 1.55 V and a lower potential within the –0.75 V to –0.35 V range. Polycrystalline rhodium and chemical etched electrodes were characterized by voltammetry, Cu underpotential deposition (upd) and X-ray diffraction. For electrofaceted surfaces were used voltammetry, Cu upd and SEM, revealing that two modified rhodium electrodes exhibit similar voltammetric characteristics as those found for Rh(111) and Rh(110) single crystals, and a third surface with an equal distribution of (110) and (111) planes. In addition, the upd of Cu on those surfaces corroborated the existence of those crystallographic planes. SEM micrographs show surface structures with a high density of terraces and steps. A mechanism of faceting is proposed. Electronic Publication     

不同电极材料和不同酸介质对3-甲基吡啶电氧化的影响

在以质子交换膜为隔膜的电解池内,通过3-甲基吡啶在PbO₂/Ti、SnO₂/Ti、石墨和MnO₂/Ti电极上的电氧化研究发现,在硫酸溶液中,PbO₂电极是催化活性最高的工作电极.通过3-甲基吡啶在硫酸、高氯酸、磷酸和乙酸介质中的电氧化研究发现,对于PbO₂电极,硫酸是最适合的介质.利用循环伏安实验和恒电位电解实验,研究了电氧化条件和电催化活性,比较了各种条件下的电流效率和选择性.     

Electrical impedance spectroscopy characterisation of conducting membranes: I. Theory

An electrical impedance spectroscopy (EIS) method for measuring changes in the electrical properties of synthetic membranes is investigated as a possible way of monitoring, in situ, the separation performance of these membranes including membrane fouling. Unlike other EIS methods, which require traditional electrodes in the feed and permeate solutions, alternating current is injected directly into the membrane via external electrical contacts with the edges of the membrane. A metal layer sputtered onto the surface of the membrane can be used to enhance its conduction properties. The impedance models of these systems is shown to be sensitive to membrane surface properties, including porosity, as well as electrical properties of solutions and the interfacial regions between the membrane surfaces and the solutions. The investigation indicates that fouling along the surface of the membrane might be more readily detectable than inside the pores.     

Delay-induced inward and outward spiral waves in oscillatory medium

Inward and outward spiral waves as well as inward target waves are induced by local delay feedback in a reaction-diffusion system exhibiting a Turing hexagon pattern spontaneously. The system gives rise to large-amplitude sinusoidal oscillations when the inward spiral waves are observed. Compared with the inward spirals, the outward spiral waves usually possess longer wavelength and exhibit larger amplitude relaxation oscillations. Varying the feedback parameters continuously, the inward spiral waves are transformed into outward spirals through a turbulent state. The dispersion analysis about the delayed system reveals that there is an unstable band on the dispersion curve, where the turbulent state arises. The dispersion curve is divided into two parts by the unstable band. The inward spiral waves exist in the left part with negative group velocity, i.e., domega/dk<0, while the outward spiral waves are stable in the right part with domega/dk>0 (omega is frequency and k wavenumber).     

Model of a confined spherical cell in uniform and heterogeneous applied electric fields

Cells exposed to electric fields are often confined to a small volume within a solid tissue or within or near a device. Here we report on an approach to describing the frequency and time domain electrical responses of a spatially confined spherical cell by using a transport lattice system model. Two cases are considered: (1) a uniform applied field created by parallel plane electrodes, and (2) a heterogeneous applied field created by a planar electrode and a sharp microelectrode. Here fixed conductivities and dielectric permittivities of the extra- and intracellular media and of the membrane are used to create local transport models that are interconnected to create the system model. Consistent with traditional analytical solutions for spherical cells in an electrolyte of infinite extent, in the frequency domain the field amplification, G(m) (f) is large at low frequencies, f<1 MHz. G(m) (f) gradually decreases above 1 MHz and reaches a lower plateau at about 300 MHz, with the cell becoming almost "electrically invisible". In the time domain the application of a field pulse can result in altered localized transmembrane voltage changes due to a single microelectrode. The transport lattice approach provides modular, multiscale modeling capability that here ranges from cell membranes (5 nm scale) to the cell confinement volume ( approximately 40 microm scale).     

Cells with Manipulated Functions: New Perspectives for Cell Biology,Medicine, and Technology

Exposure to electrical fields can reversibly increase the electrical conductivity and permeability of a cell membrane, which regulates and directs the exchange of materials and information between the cell and its environment. If cell membranes (or artificial lipid membranes) are exposed to a field pulse of high intensity and short duration (ns to μs), local electrical breakdown occurs in them. This electrical breakdown is associated with a large permeability change in the membrane, which is such that substances or particles (up to the size of genes) which cannot normally permeate through the membrane, are able to traverse the membrane into the cell. The original properties of the membrane are restored within μs to min, depending on the experimental conditions and the membrane properties. Electrical breakdown in the zone of contact between the membranes of cells (or lipid vesicles), which have been made to adhere to each other by the action of weak inhomogeneous alternating electrical fields, leads to fusion of these cells with formation of a single cell having new functional characteristics. The electrical fusion method is very mild, and the yield of fused cells is high. The electrically induced fusion and entrapment of membrane-impermeable substances and genes in cells provide a new tool for the productions of a wide range of cells with manipulated functions, which could be used (or are being used) for the solution of a number of problems in cell biology, medicine and technology. The application of electrical membrane breakdown to clinical diagnostics, the development of cellular carrier systems for the selective transport of drugs to a site of action within the organism and the potential applications of electrically induced fusion for breeding salt-tolerant crop plants for converting solar energy into ethanol, for synthesizing natural materials and manipulating genes, are described.     

Influence of Aprotic Solvent on Selectivity of an Amperometric Sensor with Nafion Membrane

This paper presents the results of investigation on selectivity of the sulfur dioxide amperometric sensor with Nafion membrane in the presence of carbon monoxide and nitrogen dioxide as the interferents. There have been compared selectivity coefficients, for the sensors containing the following internal electrolytes: solution of sulfuric acid (concentration 5 mol dm^?3) in pure water (A) and solution of sulfuric acid (concentration 5 mol dm^?3) in mixed solvent dimethylsulfoxide‐water with an DMSO: H₂O mole ratio of 1 : 2 (B). Values of the selectivity coefficients have been calculated based on the calibration curves. Analysis of both calibration curves and selectivity coefficients plays a significant role in optimization of a working point of a particular sensor. The investigated sensor operates in a three‐electrode system, where the working and counter electrodes are vacuum sublimation deposited on the membrane surface.     

Polarographic study of Mo(VI) in 0.1–5 M sulfuric acid solutions

Reduction mechanisms of polarographic reduction waves of Mo(VI) in 0.1–5 M sulfuric acid solutions are described. Three reduction waves are observed when the concentration of sulfuric acid is >3 M. From the results of coulometry and the catalytic behavior of Mo(V), it is concluded that three different reduction mechanisms of Mo(VI) to Mo(V) are present and that two separate reductions of Mo(VI) to Mo(V) and of Mo(V) to Mo(III) are involved at the potential of the third wave. The presence of three reduction mechanisms of Mo(VI) to Mo(V) in sulfuric acid α 3 M seems to indicate the existence of three different chemical species of Mo(VI). Two of these three species are different from the present in 0.1 M sulfuric acid.     

Determination of germanium(IV) in perchloric acid solution by a.c. polarography and differential pulse polarography in the presence of 3,4-dihydroxybenzaldehyde

Germanium(IV) at trace levels can be determined in perchloric acid solution containing 3,4-dihydroxybenzaldehyde by alternating current and differential pulse polarography with dropping mercury and hanging mercury drop electrodes. The results are comparable by the two methods. The possible interferences of some elements are discussed. The serious interference of lead(II) can be prevented by addition of EDTA in hydrochloric acid medium. Interference of selenium can be avoided by precipitation with potassium iodide. The method is applied to silver—germanium alloys.     

Drift of spiral waves controlled by a polarized electric field

The drift behavior of spiral waves under the influence of a polarized electric field is investigated in the light that both the polarized electric field and the spiral waves possess rotation symmetry. Numerical simulations of a reaction-diffusion model show that the drift velocity of the spiral tip can be controlled by changing the polarization mode of the polarized electric field and some interesting drift phenomena are observed. When the electric field is circularly polarized and its rotation follows that of the spiral, the drift speed of the spiral tip reaches its maximal value. On the contrary, opposite rotation between the spiral and electric field locks the drift of the spiral tip. Analytical results based on the weak deformation approximation are consistent with the numerical results. We hope that our theoretical results will be observed in experiments, such as the Belousov-Zhabotinsky reaction.     

Novel electrophoresis mechanism based on synchronous alternating drag perturbation

We present a novel means of transporting molecules in solution by applying a zero-time-average alternating motive force to the molecules, and perturbing the molecular drag coefficient synchronously with the applied force, thus causing a net drift in a direction determined by the phase of the alternating drag perturbation relative to the alternating force. We apply an electrophoretic form of the method to transport and concentrate DNA in a gel, such that all molecules migrate on average away from the nearest electrode and toward a central region. Since an electrode does not occupy this central region, this method presents the possibility of transporting and focusing DNA and other charged molecules in regions free from electrodes and the associated electrochemistry.     

Infrared reflection absorption spectroscopy of the sulfuric acid anion adsorbed on Pd(S)-[n(111) x (111)] electrodes

Adsorption of the sulfuric acid anion (HSO4- or SO42-) has been studied on Pd(S)-[n(111) x (111)] electrodes (n = 2, 3, 5, 9, 20, infinity) using in situ infrared reflection absorption spectroscopy (IRAS). A single band is observed around 1200 cm(-1) on all the electrodes. The band is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with three- or onefold geometry. This result differs from the case of Pt-stepped surfaces on which two IRAS bands are observed around 1200 and 1100 cm(-1). The maximum coverage of the sulfuric acid anion is enhanced with the increase of the terrace width. The surfaces with n more than 3 have similar IRAS band shifts (dv/dE). Pd-stepped surfaces, for which the terrace is wide enough for the anion adsorption, adsorb the anion on the terrace rather than the step.