Control of particle alignment in water by an alternating electric field

We attempted to align a large number of silica particles dispersed in aqueous solution by controlling the alternating electric field between the two electrodes (400 microm apart). Relatively large particles (9.9 microm) were found to align forming strings in the direction parallel to the electric field while relatively small particles (2.0 and 4.9 microm) were observed to align making stripes in the direction perpendicular to the field. The number of stripes formed by particles between the electrodes increased with increasing frequency of the alternating field. This peculiar perpendicular particle alignment appeared when the contribution to particle alignment of electroosmotic flow exceeded that of dielectric polarization and the osmotic flow was found to be stronger around the particles than in the vicinity of the electrode surface.     

Anomalous Kerr effect relaxation in an alternating field

Perturbation theory is used to derive the complex harmonic components (stationary regime) arising in Kerr effect relaxation for an assembly of nonelectrically interacting, polar, and polarizable symmetric-top molecules acted on by a strong dc bias electric field superimposed on a weak ac electric field. The approach starts from a fractional kinetic equation written in configuration space and represents an extension of the Smoluchowski equation to fractional dynamics. This equation is solved in the context of a subdiffusive process characterized by an anomalous exponent alpha ranging from 0 to 1, the Brownian limit. By using a perturbation procedure restricted to the second order in the ac field strength, analytic expressions for the electric birefringence spectra representing the frequency dependence of the first (in omega) and the second (in 2omega) harmonic components are obtained. Various Cole-Cole-like diagrams are presented in order to illustrate the results so obtained and to emphasize the role played by the fractal parameter alpha in the anomalous diffusion collision process. A comparison of our theoretical model with experimental measurements of the ac Kerr effect response of a dilute polymer solution [poly(3-hexylthiophene)] appears to be quite satisfactory.     

Colloidal phase transition driven by alternating electric field

The transverse two-dimensional assembly of colloidal particles near an electrode surface subjected to ac polarization is studied by varying the frequency and field strength in the absence and presence of an added electrolyte. The variation of the translational and bond-orientational correlation functions with frequency suggests the existence of a hexatic phase in which the particles retain the remnants of the crystalline long-range orientational order, but has a liquidlike translational order. The electrohydrodynamic (EHD) flow is analyzed in the light of the existing theoretical models. The equilibrium distribution of particles is considered to be the resultant of the two opposing forces--Stoke's force due to EHD flow and the screened Coulomb interaction between the colloidal particles. Several features of the experimental results are discussed, such as the role played by the EHD flow in the particle aggregation, the dependence of the equilibrium interparticle separation on ionic strength, zeta potential, and particle size.     

Drop formation via breakup of a liquid bridge in an AC electric field

Experimental results are presented for the study of drop formation mechanism in a newly proposed electrohydrodynamic (EHD) method of drop generation in an AC electric field. In the method, a small drop is generated in two stages. A pendant drop is elongated with large oscillation by an electric force in the first stage. Then, it undergoes formation and breakup of a liquid bridge between the upper nozzle and the insulator-coated lower flat plate in the second stage. It is found that there exists a resonant frequency for maximum oscillation, which leads to an efficient drop formation in the latter stage. It is also found that breakup of liquid bridge is accelerated by the electrowetting tension acting on the drop perimeter contacting the insulator-coated flat plate. Thus the whole procedure of drop formation depends heavily on the frequency of AC field and the properties of the insulator such as hydrophilicity, thickness, and the dielectric constant. It is demonstrated that a wide range of drop size, from picoliter to nanoliter, can be obtained by controlling such key parameters without changing the nozzle diameter.     

Assembly of colloidal particles into microwires using an alternating electric field

We have investigated the dielectrophoretic assembly of colloidal gold, carbon black, and carbon nanotubes into electrical wires. The resulting microwires have diameters less than 1 microm, with lengths ranging from 5 microm to 3 mm. Current-voltage curves for these wires indicate an ohmic response, where the resistance is determined by the type of colloid and by the frequency of the alternating field used to grow the wires. The predicted frequency dependence of dielectrophoresis is confirmed by experiment. Measurements of the threshold voltage for initial wire growth are also presented. These experiments demonstrate that a variety of nanoparticles can be assembled into microwires for sensor applications.     

The response of a colloidal suspension to an alternating electric field

This paper is concerned with the calculation of the complex conductivity K* of a suspension, a quantity which may be determined experimentally from the measurement of the alternating current which flows between a pair of electrodes in the suspension due to an alternating voltage difference. A semi-analytic formula is derived for the complex conductivity of a dilute suspension of spherical particles with small dielectric constant which is reasonably accurate for ?-potentials of less than 50 mV. For such suspensions this formula represents a very economical alternative to the exact computer calculation of K* described by DeLacey and White (ref. 2). Although the formula for K* is derived for particles with fixed surface charge, it is shown that the formula can also be applied to a more general class of suspensions, in which the surface charge arises from the dissociation of a single type of surface group.     

Stimulation of tobacco shoot regeneration by alternating weak electric field

Nicotiana tabacum morphogenetic callus was used to study the stimulative effects of external alternating electric currents of low level and low frequency on the cytodifferentiation processes. The N. tabacum calli were grown on standard medium for shoot regeneration and subjected to electric currents (50 Hz, 0.1–50 μA) during the whole period of treatment (30 days). The number of shoots, total mass, DNA and protein content were measured for each callus sample at the end of the experiment. The number of shoots increased by up to 300% for the samples stimulated with 50 μA although no significant changes were noted in total mass, DNA and protein content in the stimulated samples compared to control. We suggest that in the presence of the external electric current, the callus cells become more sensitive to chemical signals (hormones and/or ions) in the culture medium. A series of changes in the activity of membrane components, as a consequence of the modulation of membrane potential by the external electric field, could support our hypothesis.     

Polarization of a diffuse soft particle subjected to an alternating current field

The polarization of a diffuse soft particle submerged in an aqueous electrolyte and subjected to a uniform alternating electric field is theoretically analyzed with the standard electrokinetic model (the Poisson-Nernst-Planck equations). The particle consists of a rigid uncharged core and a charged diffuse polyelectrolytic shell (soft layer) permeable to ions and solvent. Our focus is on the impact of the characteristics of the soft layer including the Donnan potential, the soft layer thickness, and the friction coefficient of the soft layer on the dipole coefficient, characterizing the strength of the polarization. Under the limits of thin double layers and thin polyelectrolytic shells, approximate analytical expressions to evaluate the dipole moment coefficients are derived for high-frequency and low-frequency ranges, respectively. The analytical results are compared and agree favorably with those numerically computed by the standard model. Interestingly, we discover that when the double layer is comparable to the soft layer the dipole moment behaves qualitatively differently at different Donnan potentials. When the Donnan potential is small, the dipole moment decreases as the double layer increases. In contrast, at large Donnan potentials, the dipole moment increases with the increase in the double layer. The distinct responses to Donnan potentials are attributed to the impact of the associated double layer on the charge distribution of mobile ions inside the soft layer. The theoretical model provides a fundamental basis for interpreting the polarization of heterogeneous systems, including environmental or biological colloids or microgel particles.     

Control of particle assisted wetting by an external magnetic field

It is shown that repulsive particles can assist wetting of a water surface by an organic liquid even at a particle density substantially less than a close packed monolayer. By applying external fields, one can change the interparticle interactions from net attractive to net repulsive and thus induce a transition from nonwetting to wetting conditions. This was achieved by applying superparamagnetic polystyrene particles together with a polymerizable organic liquid (trimethylolpropane trimethacrylate) to a water surface in the middle of a solenoid. Passing a current through the solenoid created a magnetic field perpendicular to the interface that polarized the particles and induced repulsive dipole-dipole forces. Without the field, lenses of the organic liquid that included aggregates of particles floating on the water surface were observed. In the presence of the field, the organic liquid and the particles were evenly distributed across the surface. The interparticle distance increases proportional to the square root of the area per particle and is close to the value expected for hexagonal order.     

Study of a d.c. arc in an alternating magnetic field

The distributions of the electron pressure, the temperature, the degree of ionization and the concentration of several elements in the cathode and the anode regions of a d.c. arc to which an external magnetic field is applied are studied. The magnetic field causes oscillations of the arc column which lead to an expansion of the optically integrated image of the discharge on the plane of oscillations. It is shown that the electron pressure and the temperature remain constant over a broad region of the discharge. In the cathode region an intensification of the spectral line-intensities is observed, which is discussed in the paper.     

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.     

Resonant increase of ionic conductance of yttria-stabilized zirconia in an alternating external electric field

Journal of Solid State Electrochemistry - The effect of an external alternating electric field on the diffusion of O2− ions in yttria-stabilized zirconia was studied by the molecular dynamics...     

Simulations of a weakly conducting droplet under the influence of an alternating electric field

We investigate the electrohydrodynamics of an initially spherical droplet under the influence of an external alternating electric field by conducting axisymmetric numerical simulations using a charge-conservative volume-of-fluid based finite volume flow solver. The mean amplitude of shape oscillations of a droplet subjected to an alternating electric field for leaky dielectric fluids is similar to the steady-state deformation under an equivalent root mean squared direct electric field for all possible electrical conductivity ratio and permittivity ratio of the droplet to the surrounding fluid. In contrast, our simulations for weakly conducting media show that this equivalence between alternating and direct electric fields does not hold for . Moreover, for a range of parameters, the deformation obtained using the alternating and direct electric fields is qualitatively different, that is, for low and high , the droplet becomes prolate under alternating electric field but deforms to an oblate shape in the case of the equivalent direct electric field. A parametric study is conducted by varying the time period of the applied alternating electric field, the permittivity and the electrical conductivity ratios. It is observed that while increasing has a negligible effect on the deformation dynamics of the droplet for , it enhances the deformation of the droplet when for both alternating and direct electric fields. We believe that our results may be of immense consequence in explaining the morphological evolution of droplets in a plethora of scenarios ranging from nature to biology.     

DC discharge in an alternating magnetic field as excitation source in emission spectroscopy

Summary The experimental study of the behaviour of a dc discharge in an alternating magnetic field is described. The lower electrode (anode) is a horizontally located chamber electrode. The alternating magnetic field causes oscillations along the lower electrode axis so that the oscillating arc can equably heat the anode. The axial distribution of the spectral line intensities for lead, bismuth, mercury, cadmium, tin and antimony is investigated. The intensity distributions are studied for near-to-cathode and near-to-anode regions of the discharge. The detection limits obtained for pure conditions show that the effects of chamber electrodes and of double arc could be achieved by means of a relatively simple technique.

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Ein Gleichstrombogen im Wechselmagnetfeld als Anregungsquelle in der Emissionsspektrographie

Zusammenfassung Untersuchungen über das Verhalten eines Gleichstrombogens in einem Wechselmagnetfeld werden beschrieben. Die untere Elektrode (Anode) ist eine horizontale Kammerelektrode. Das Wechselmagnetfeld verursacht Bogenschwingungen entlang der Achse der unteren Elektrode und ermöglicht dadurch deren gleichmäßige Heizung. Die axiale Verteilung der Linienintensitäten von Blei, Bismut, Quecksilber, Cadmium, Zinn und Antimon wurde für beide Elektroden untersucht. Die berechneten Nachweisgrenzen zeigen, daß die Wirkung der Kammerelektrode und des Doppelbogens mit einfachen Mitteln erreicht werden kann.

     

Aperiodic capillary electrophoresis method using an alternating current electric field for separation of macromolecules

Switching from direct current (DC) to alternating current (AC) electric fields has provided substantial improvements in various instrument techniques that use electric fields for manipulating with various liquid-based systems. For example, AC fields are now used in both light scattering and electroacoustic instruments for measuring xi-potential, largely replacing more traditional microelectrophoresis techniques that use DC fields. In this paper, we suggest a novel way to make a similar transition in the area of separation techniques, capillary electrophoresis (CE) in particular. Dielectrophoresis is one well-known separation effect in which a drifting motion of particles is produced in a "spatially nonhomogeneous" AC electric field. However, there is another field effect that also causes a similar drift of particles. Instead of a "spatially nonhomogeneous" field, this method relies on a "temporally nonhomogeneous" field, normally referred to as "aperiodic electrophoresis". Despite a number of recently published experimental and theoretical papers describing this effect, it is less well-known than dielectrophoresis. We present a short overview of some of the relevant papers. We point out for the first time the idea that "aperiodic electrophoresis" might be useful for separation of macromolecules. We suggest several new mechanisms that could induce this effect in a sufficiently strong AC electric field. This effect can be used as a basis for a new separation method having several important advantages over traditional CE. We present a simple scheme as an example illustrating this new method.     

Control of delocalization and structural changes by means of an electric field

The strength and, mainly, the direction of a static electric field can be used to control delocalization effects occurring in a non-polar pi-system. The delocalization energy, the weights, and the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations are considered and examined in cis-butadiene, used as model system. The effects of the electric field are detected and evaluated in the basis of natural orbital spaces appropriate to investigate the behavior of one- and poly-electron distributions. The consequences of modifying the delocalization effects on structural changes are also investigated. Full geometry optimizations in both Hartree-Fock and MP2 levels show that the changes in bond lengths, guided by the changes of the behavior of the electronic assembly, can be controlled by means of the electric field.     

Droplet fusion by alternating current (AC) field electrocoalescence in microchannels

We present a system for the electrocoalescence of microfluidic droplets immersed in an immiscible solvent, where the undeformed droplet diameters are comparable to the channel diameter. The electrodes are not in direct contact with the carrier liquid or the droplets, thereby minimizing the risk of cross-contamination between different coalescence events. Results are presented for the coalescence of buffered aqueous droplets in both quiescent and flowing fluorocarbon streams, and on-flight coalescence is demonstrated. The capillary-based system presented here is readily amenable to further miniaturization to any lab-on-a-chip application where the conductivity of the droplets is much greater than the conductivity of the stream containing them, and should aid in the further application of droplet microreactors to biological analyses.     

Recent developments in polymerization by an alternating intra-intermolecular mechanism

Recent developments in polymerization by an alternating intra-intermolecular mechanism are summarized and discussed briefly. The methods used to prove the structure of typical polymers, which in turn established the mechanism of polymerization, are reviewed. A study of larger ring formation by Marvel and Garrison is also summarized. Ring of sizes from 7 to 21 members were formed from α-diolefins in yields of from 4 to 25%. A study of certain unsymmetrical dienes has shown that cyclization occurs in yields up to 60%, but that gelation eventually occurs because of the difference in relative reactivities of the two olefinic double bonds. Copolymerization of certain 1,4-dienes with certain olefins to produce linear copolymers containing six-membered rings is discussed. In an effort to explain the strong driving force to close a six-membered ring during polymerization of 1,6-diolefins, a pronounced interaction between the double bonds, which can be represented by several resonance forms of the molecule, is proposed.     

Triggered release in lipid bilayer-capped mesoporous silica nanoparticles containing SPION using an alternating magnetic field

We report here the on-command cargo controlled delivery using an alternating magnetic field (AMF) from magnetic silica mesoporous supports capped with a lipid bilayer.