Presence Of AC Electric Field Research Articles

Cu doped ZnO nanoparticles: Correlations between tuneable optoelectronic, antioxidant and photocatalytic activities

Microstructural, optical and electronic properties of Cu doped ZnO nanoparticles with a general formula Zn1-xCuxO (x = 0.0, 0.1 and 0.2) prepared via wet chemical co-precipitation method were investigated thoroughly. Both the free radicals scavenging properties and photocatalytic activities of synthesized nanoparticles were explored. Formation of pure hexagonal wurtzite crystal structure of entire samples were identified by recorded x-ray diffraction patterns. For determining both the mean crystallite size and intrinsic microstrain of all the samples, several standard methods were utilized and a comparative study was introduced. Average size, morphology and shape regularity of synthesized samples were examined using HRTEM images. Optical direct band gap estimated from Tauc plots was noted to reduce with increasing Cu concentration in ZnO nanoparticles. Collected FTIR spectra at 300 K also validated the incorporation of Cu ions and their complete dissolution in the host ZnO crystal structure. It is also found that the hopping of electrons is the fundamental charge conduction process for all the samples in presence of ac electric field. FDTD simulation clearly predicts increase in electric field intensity with increased doping of Cu. There could be a plasmonic coupling between particles at the hotspot region that increases the electric field intensities. BET study revealed that the 10 % Cu doped ZnO sample has highest specific surface area which implies it contains more active sites for taking part in rapid photodegradation of toxic dyes compared to other samples. It was observed that the 10 % Cu doped ZnO nanoparticles were more efficient in degrading toxic dyes and the degradation pathway of all the samples obeyed first order rate kinetics model. Based on estimated bandgap of nanoparticles, a possible mechanism for the Rhodamine B (RhB) and Methylene blue (MB) dye degradation is suggested. All the prepared samples also exhibited notable antioxidant properties. Consequently, this study clearly shows that the Cu doped ZnO nanoparticles are the effective nanomaterial for photocatalytic and antioxidant applications.

Jetting and Droplet Formation Driven by Interfacial Electrohydrodynamic Effects Mediated by Solitons in Liquid Crystals.

Solitons are highly confined, propagating waves that arise from nonlinear feedback in natural (e.g., shallow and confined waters) and engineered systems (e.g., optical wave propagation in fibers). Solitons have recently been observed in thin films of liquid crystals (LCs) in the presence of ac electric fields, where localized LC director distortions arise and propagate due to flexoelectric polarization. Here we report that collisions between LC solitons and interfaces to isotropic fluids can generate a range of interfacial hydrodynamic phenomena. We find that single solitons can either generate single droplets or, alternatively, form jets of LC that subsequently break up into organized assemblies of droplets. We show that the influence of key parameters, such as electric field strength, LC film thickness, and LC-oil interfacial tension, map onto a universal state diagram that characterizes the transduction of soliton flexoelectric energy into droplet interfacial energy. Overall, we reveal that solitons in LCs can be used to focus the energy of nonlocalized electric fields to generate a new class of nonlinear electrohydrodynamic effects at fluid interfaces, including jetting and emulsification.

Quantifying Order during Field-Driven Alignment of Colloidal Semiconductor Nanorods.

Aligning large populations of colloidal nanorods (NRs) into ordered assemblies provides a strategy for engineering macroscopic functional materials with strong optical anisotropy. The bulk optical properties of such systems depend not only on the individual NR building blocks but also on their meso- and macroscale ordering, in addition to more complex interparticle coupling effects. Here, we investigate the dynamic alignment of colloidal CdSe/CdS NRs in the presence of AC electric fields by measuring concurrent changes in optical transmission. Our work identifies two distinct scales of interaction that give rise to the field-driven optical response: (1) the spontaneous mesoscale self-assembly of colloidal NRs into structures with increased optical anisotropy and (2) the macroscopic ordering of NR assemblies along the direction of the applied AC field. By modeling the alignment of NR ensembles using directional statistics, we experimentally quantify the maximum degree of order in terms of the average deviation angle relative to the field axis. Results show a consistent improvement in alignment as a function of NR concentration─with a minimum average deviation of 36.2°─indicating that mesoscale assembly helps facilitate field-driven alignment of colloidal NRs.

Coupling of long-wavelength density fluctuations to orientations in cellulose nanocrystal suspensions under external fields.

Motivated by the development of cellulose-based functional materials, we investigate the microscopic dynamics of suspensions of cellulose nanocrystals (CNCs) at different ionic strengths, both in the absence and in the presence of AC electric fields and for various temperatures. A concentration of 5 wt% of the CNCs is chosen for which the dispersions are in the full chiral-nematic state at low ionic strengths. Dynamic light scattering is used to characterize the wave vector-dependent decay rates of number-density fluctuations. Contrary to an isotropic suspension, the dispersion relations (the wave vector dependence of the correlation-function decay rates) as obtained by means of depolarized light scattering are found to exhibit anomalous behavior. The dispersion relations, both without and with an external field, exhibit minima at small wave vectors, which is attributed to coupling of translational motion to the orientation of the CNCs, shown in the chiral-nematic state. The location of the minima is found to weakly depend on ionic strength and shifts significantly towards larger wave vectors upon applying an external electric field for sufficiently high ionic strengths. Finally, preliminary results are presented for smaller length-scale density fluctuations (at larger wave vectors) as a function of temperature, revealing the anisotropic mobilities in the chiral-nematic state of CNCs.

Empirical modeling coupled with pore blocking for predicting cake formation of electric field effects on oily waste water cross-flow microfiltration

Cake formation is the most important phenomenon to reduce cross-membrane flux in cross-flow microfiltration although pore blocking and concentration polarization (CP) phenomena affect it. Electric field may be used to enhance process efficiency and decreased fouling phenomena in the process. An empirical model is developed here taking into consideration the cake formation effects on permeate flux reduction in the presence of AC electric field. A semi-empirical model is also developed in the presence and absence of electric field at two cases; with and without considering membrane structure in the modeling. It is the first time that electric field effects on microfiltration performance are modeled macroscopically and used with pore blocking phenomenon simultaneously. The predicted permeate fluxes by the models are compared against experimental permeate fluxes in absence and presence of electric field. Comparison of the model based predictions against experimental results indicate semi-empirical model precision with a maximum error of 8.9% for the case without and 3.6% for the case with pore blocking consideration and a maximum error for empirical model of 11.9%. Effects of different electric potentials and distances between the electrodes on CP layer thickness and oil concentration on membrane surface are also studied. The predicted results showed that the CP layer thickness is decreased by 60.9% through applying an electric potential of 500 V and also decreased by 65.2% through decreasing the distance between the electrodes from 2.3 to 1.3 mm at 200 V. The oil concentration on the membrane surface is also decreased by 72.3% as a result of applying the electric field while it is decreased by 19.2% when the electrodes distance is decreased from 2.3 to 1.3 mm at 200 V.

Analytical study of Whistler mode waves for relativistic plasma with AC electric field in inner magnetosphere of Saturn

The population of plasma present in the rotationally dominated inner magnetosphere of Saturn is identified by the observed plasma waves in the magnetosphere. Whistler mode emissions along with electrostatic cyclotron emissions often with harmonics are a common feature of Saturnian inner magnetosphere. We present the outcomes of a study of very large amplitude Whistler mode waves characteristics inside the magnetosphere of radial distance of less than 15 $$\hbox {R}_{\mathrm{s}}$$ . Whistler mode waves with temperature anisotropy in the magnetosphere of Saturn have been studied in the present work. Observations of Whistler mode emissions from Cassini Radio and Plasma Wave Science instruments have been obtained. Whistler mode waves were investigated using the method of characteristic solution by kinetic approach, in the presence of AC field. The observations made by space probes Voyager 1 and 2 and Cassini, launched by NASA, showed that charged particles are trapped in planet’s magnetic field lines. In 2004, the Cassini encounter with Saturn revealed that magnetosphere of Saturn exhibits Maxwellian distribution. So, the dispersion relation, real frequency and growth rate were evaluated using ring distribution function. Effect of AC frequency, temperature anisotropy, energy and number density of particles was found. Temperature anisotropies greater than one ( $$\hbox {T}_\bot {/}\hbox {T}_{\Vert } >1$$ ) and pancake-like distribution can generate Whistler mode emissions of the warmer plasma population. The study also extended to oblique propagation of Whistler mode waves in presence of AC electric field. However, when relativistic factor $${{\upbeta } }=\sqrt{\hbox {1}-\frac{\hbox {v}^{\mathrm {2}}}{\hbox {c}^{\mathrm {2}}}}$$ increases, growth rate decreases. Through comprehensive mathematical analysis, it was found that when Whistler mode waves propagate parallel to the intrinsic magnetic field of Saturn, its growth is enhanced more than in the case of oblique propagation. Results are also discussed while computing the rate with which the wave grows for a particular wavenumber.

Effect of external electrode arrangements on R-134a two-phase flow regimes in the presence of AC electric fields

In this study, two-phase flow regime redistribution in a horizontal tube within which two-phase flow of refrigerant R-134a is passing under the influence of high-voltage AC electric fields is experimentally investigated. The experimental study deals with the effects of external position for double electrodes, external arrangements for triple electrodes and using alternative refrigerant. A theoretical Steiner tube two-phase flow pattern map for external double electrode arrangement in the horizontal tube under flow conditions with mass fluxes of 30–530kg/m2s, inlet qualities from 0% to 90% and fixed applied voltage of 9.8kV is developed. The results show that by intensifying the applied electric field and increasing the vapor quality, the volume of the liquid splashed on the tube wall increases. It is found that electrode arrangements substantially affect on the two-phase flow pattern. Based on these results, because of the difference between the created electric field of the various electrode arrangements, the number of generated bubbles and their density in arrangement #2 is significantly less than arrangement #1. Also the results indicate that because of the uniformity of electric field, the number of the generated bubbles and their density in electrode arrangement #5 is more than all of the electrode arrangements. Eventually, the effect of using refrigerant R-600a rather than R-134a is studied. It was observed that because of low difference between liquid and vapor electric permittivity for R-600a, applying AC high-voltage on R-600a two-phase flow has not any impact on two-phase flow regime redistribution.

Numerical investigation of electrohydrodynamic instability in a vertical porous layer

The electrohydrodynamic instability of a vertical dielectric fluid saturated Brinkman porous layer whose vertical walls are maintained at different temperatures is considered. An external AC electric field is applied across the vertical porous layer to induce an unstably stratified electrical body force. The stability eigenvalue equation is solved numerically using the Chebyshev collocation method. The presence of inertia is found to instill instability on the system and the value of modified Darcy–Prandtl number PrD at which the transition from stationary to travelling-wave mode takes place is independent of the AC electric field but increases considerably with an increase in the value of Darcy number Da. The presence of AC electric field promotes instability but its effect is found to be only marginal. Although the flow is stabilizing against stationary disturbances with increasing Da, its effect is noted to be dual in nature if the instability is via travelling-wave mode. The streamlines and isotherms for various values of physical parameters at their critical state are presented and analyzed. Besides, energy norm at the critical state is also computed and found that the disturbance kinetic energy due to surface drag, viscous force and dielectrophoretic force have no significant effect on the stability of fluid flow.

Dielectrophoretic motion and deformation of a liquid drop in an axisymmetric non-uniform AC electric field

Use of dielectrophoresis as a tool to maneuver particles (biological, rigid, fluid, etc.) or their carrier drops is prevalent in various microfluidics and biotechnological applications. Employing AC electric field gives frequency as an additional attribute to manipulate the particles or drops in such applications. In the present work we experimentally studied the dielectrophoretic behavior of a freely suspended drop in another fluid in the presence of AC electric field. The response of a conducting drop dispersed in a dielectric medium to the non-uniform AC electric field with up-down asymmetry is observed to be distinctly different than DC dielectrophoresis in the same fluid system. The effect of frequency of the applied sinusoidal field on the motion and shape evolution of a drop is thoroughly investigated. We also used analytical theory to predict the periodic dielectrophoretic motion of the drop between electrodes. The orientation of molecular dipoles, instantaneously upon application of the electric field, is a major contributor to the overall phase lag between the applied field and the drop's response in addition to the lag due to charge dynamics. The periodic motion and shape of a dielectrophoretically moving drop are found to be governed by different parameters in addition to the frequency of applied field.

Dynamic conductivity of ac–dc-driven graphene superlattice

The dynamic conductivity of graphene superlattice in the presence of ac electric field and dc electric field with longitudinal and transversal components with respect to superlattice axis was calculated. In the case of strong transversal component of dc field conductivity of graphene superlattice was shown to be such as if the electrons had got the effective mass. In the case of weak transversal component of dc field conductivity was shown to change its sign if the frequency of ac field was an integer multiple of half of Bloch frequency.

Manipulating ice crystallization of 0.9 wt. % NaCl aqueous solution by alternating current electric field

In this paper, we propose a physical method to manipulate the ice crystallization by applying an AC electric field during the liquid-solid transition of 0.9 wt. % NaCl aqueous solution. By using optical microscopic observation, we found that the ice grains are miniaturized in the presence of AC electric field during freezing process. Further study suggests that the grain size and the associated ice crystallization fraction are dependent on the field strength as well as frequency. And AC electric field can reduce the grain size and the crystallization fraction down to 35 μm and 71%, respectively, on the optimal condition (with field strength E = 100 kV/m, frequency f = 106 Hz). Moreover, the dielectric spectrum measurement of our sample close to transition temperature implies that such an optimal condition corresponds with a dielectric relaxation state, which might be the reason for the reduction of crystallized ice. Our finding may provide a potential way for cryoprotective application.

Electro-optics of plate-like silica particle suspension

We focused on square plate-like silica particle, i.e., H+-exchanged ilerite (H-ilerite). On the basis of the characteristics of ion concentration polarization and relaxation in asymmetric electrical double layer (EDL) around H-ilerite platelets, electro-optics of H-ilerite suspensions was characterized. The responses of H-ilerite suspension to electric field were assessed by impedance dispersion, microscopic observation and electro-optics optical transmittance of light with the wavelength of 550nm. As electric field is increased, the relaxation frequency of EDL polarization for H-ilerite systematically shifted from 480Hz (random orientation) to 126Hz (alignment to electric field). When electric field of 141Vrms/cm and 10kHz was switched on, transmittance decreased instantaneously. It was also observed that H-ilerite platelets were individually aligned to electric field in dilute suspension, followed by the formation of face-to-face aggregations of the platelets in concentrated suspension in the presence of AC electric field. When large aggregations were formed, the transmittance change in electro-optics was remarkably enhanced. When electric field was switched off, the transmittance showed fast and slow relaxations. The fast one corresponds to the rotary Brownian diffusion and becomes slow with increasing the volume fraction. The slow one can be related to the release of face-to-face aggregations.

Collapse of DNA in ac electric fields.

We report that double-stranded DNA collapses in the presence of ac electric fields at frequencies of a few hundred Hertz, and does not stretch as commonly assumed. In particular, we show that confinement-stretched DNA can collapse to about one quarter of its equilibrium length. We propose that this effect is based on finite relaxation times of the counterion cloud, and the subsequent partitioning of the molecule into mutually attractive units. We discuss alternative models of those attractive units.

Asymmetrical reverse vortex flow due to induced-charge electro-osmosis around carbon stacking structures

Broken symmetry of vortices due to induced-charge electro-osmosis (ICEO) around stacking structures is important for the generation of a large net flow in a microchannel. Following theoretical predictions in our previous study, we herein report experimental observations of asymmetrical reverse vortex flows around stacking structures of carbon posts with a large height (~110 μm) in water, prepared by the pyrolysis of a photoresist film in a reducing gas. Further, by the use of a coupled calculation method that considers boundary effects precisely, the experimental results, except for the problem of anomalous flow reversal, are successfully explained. That is, unlike previous predictions, the precise calculations here show that stacking structures accelerate a reverse flow rather than suppressing it for a microfluidic channel because of the deformation of electric fields near the stacking portions; these structures can also generate a large net flow theoretically in the direction opposite that of a previous prediction for a standard vortex flow. Furthermore, by solving the one-dimensional Poisson-Nernst-Plank (PNP) equations in the presence of ac electric fields, we find that the anomalous flow reversal occurs by the phase retardation between the induced diffuse charge and the tangential electric field. In addition, we successfully explain the nonlinearity of the flow velocity on the applied voltage by the PNP analysis. In the future, we expect to improve the pumping performance significantly by using stacking structures of conductive posts along with a low-cost process.

Phase Selection in Capillary Breakup in AC Electric Fields

We study the detachment of conductive aqueous drops in ambient oil from an electrode in the presence of ac electric fields. Making use of the electrowetting effect, we determine the charge of the detached sessile drops. Drops are found to be discharged at high ac frequency in line with earlier predictions. At low frequencies, we find a wide but unexpected charge distribution displaying a frequency-dependent nonzero minimum charge. This observation is explained in terms of the stabilization of capillary bridges in electric fields, which prevents the hydrodynamic pinch-off for certain phases of the ac field.

Electrokinetics in extremely bimodal suspensions

Prompted by the results obtained by Mantegazza et al. [Nature Physics 1 (2005) 103], where the electric birefringence of suspensions of elongated particles was strikingly affected by the presence of a sea of very small (size ratio lower than 10:1) colloidal spheres, we have undertaken an investigation of other electrokinetic phenomena in suspensions containing various relative concentrations of large (Teflon or polystyrene latex) and small (nanometer-sized silica spheres) colloids. We have determined the quantities that might be greatly affected by the size distribution of the particles, mainly in the presence of ac electric fields, since the response of the suspensions will show very characteristic relaxations, dominated in principle by the size of the particles. In this work, we report on measurements of the dielectric dispersion of mixed particles as a function of the concentration, ionic strength, and field frequency. The results indicate that the response is not just a simple combination of those obtained with suspensions of the individual particles, and in fact the presence of even small amounts of the small particles affects considerably the frequency response of the suspensions.

The effect of stationary and sweeping frequency AC electric fields on frost crystal removal on a cold plate

The effect of stationary and sweeping frequency AC electric fields on frost crystals growth and frost control/removal on a cold plate was studied for the first time in this paper. The main results of this study showed that the presence of AC electric fields can greatly affect both the frost crystals growth pattern and mass accumulation on cold surfaces. The ice surface electrical properties and basic electrostatics were used to explain the main findings in this paper. Up to 46% frost reduction was obtained when the electric field frequency spanned 370 Hz to 7.5 kHz while the applied voltage was 14.5 kV. Two different sets of environmental conditions were tested, which showed that the plate temperature placed an important effect on frost crystals growth under electric fields. An optimum application time of the AC electric fields was found based on least frost mass accumulation on the cold plate.

The Stability of Natural Convection in an Inclined Fluid Layer in the Presence of AC Electric Field

Linear stability theory is applied to the problem of the natural convection that occurs in an inclined fluid layer with uniformly distributed internal heat sources and subjected to the action of a normal ac electric field. It is assumed that the two plates between which the fluid layer is confined are maintained at constant and equal temperature. Both the dielectric constant and density of the fluid are assumed to be functions of temperature. The power series method is used to obtain the eigenvalue equation which is then solved numerically to obtain the stable and unstable solutions.

Drift instability in the presence of a ac electric field

The dispersion relation and the growth rate of the drift wave in an inhomogeneous plasma in the presence of ac electric field applied perpendicular to the impressed magnetic field, have been obtained by investigating the trajectories of the charged particles and the wave particle energy exchange method. The general dispersion relation for the electrostatic drift wave has been obtained. It is found that when a low frequency ac electric field less than the ion cyclotron frequency is applied in the presence of a weak magnetic field, then it significantly modifies and enhances the growth rate. Frequencies higher than the ion cyclotron frequency do not affect the growth rate for a weak applied electric field. At higher frequencies high order of electric fields are required to excite the wave. Frequencies higher than 500 Hz do not show any significant deviations in ω vs k⊥ diagram for the assumed space plasma parameters. Application of these results have been made for the ionospheric plasma. A dispersion relation has also been calculated for the laboratory plasma.