Damping Oscillations of Microdroplets of a Droplet Cluster in an External Electric Field

The forces acting on levitating water microdroplets in the structure of a droplet cluster (A.A. Fedorets, 2004) have been analyzed. It is found that microdroplets in the cluster should undergo low-frequency vertical damping oscillations near the equilibrium position. Oscillations occur also when switching on an external electric field, which affects the oscillation frequency and equilibrium position. Microdroplets are shown to lose their stability in the critical range of parameters. The electric fields are calculated in the simplest experimental scheme for simulating the effect of electric field on a droplet cluster.     

Electrohydrodynamic behaviors of droplet under a uniform direct current electric field

The electrohydrodynamic behaviors and evolution processes of silicone oil droplet in castor oil under uniform direct current(DC)electric field are visually observed based on a high-speed microscopic platform.Subsequently,the effects of different working conditions,such as electric field strength,droplet size,etc.,on droplet behaviors are roundly discussed.It can be found that there are four droplet behavior modes,including Taylor deformation,typical oblique rotation,periodic oscillation,and fracture,which change with the increase of electric field strength.It is also demonstrated that the degree of flat ellipse deformation gets larger under a stronger electric field.Moreover,both of the stronger electric field and smaller droplet size lead to an increase in the rotation angle of the droplet.     

Numerical study on transient response of droplet deformation in a steady electric field

The transient response of droplet deformation in a steady electric field is investigated by the numerical simulation and the motion of interface is captured by level-set method. The numerical scheme is validated and found to be in good agreement with classic analytical solutions. The effects of electric field intensity, interfacial tension, oil viscosity and droplet size on the transient deformation process are systematically discussed. The numerical results show that electric field intensity can accelerate the deformation of the droplet, while interfacial tension and oil viscosity damp it. Furthermore, the relation between electric capillary number and dimensionless deformation time is obtained.     

VELOCITY AND TEMPERATURE FIELDS IN A PLANE POISEUILLE FLOW WITH VOLUME HEATING: MEASUREMENTS UNDER ELECTRIC FIELD

This study concerns the measurement of the velocity and temperature fields in a laminar flow within a vertical channel of rectangular cross section. The fluid is heated in volume by direct electric conduction induced by an alternating electric field. Particular attention is paid to the procedure for temperature measurement under an electric field. The feasibility of such a measurement technique is shown, and specific aspects of the mixed convection regime are analyzed. Flow and temperature fields are also calculated by solving numerically the complete Navier-Stokes and energy equations for comparison with the experimental results. Differences between the experimental and numerical results are discussed, and the underlying physical mechanisms are analyzed.     

Combustion of ethanol fuel droplet in vertical direct current electric field

Flame is affected by an external electric field because it contains ions and electrons related to chemical reactions. On the other hand, the movement of ions and electrons affects the external electric field due to their charge. This paper reports the combustion experiments of ethanol droplets in vertical electric field with variable distance electrodes apparatus in order to discuss the change of the external electric field due to the existence of flame. From a one-dimensional steady-state analysis, if the electric field is changed spatially, its effect on combustion behavior is aligned with V²/L³ and not V/L, where V is the applied voltage between electrodes, and L is distance between the electrodes. The droplet is burned between the two horizontal parallel electrodes. The flame deformation and the electric current are characterized by various electrode distances, and respectively, applied voltages. The vertical electric field induces a body force downwards on the flame. The flame deforms downward in the electric field because the electric body force counters the natural buoyancy. The relation between the applied voltage and electrode distance is investigated when the flame becomes vertically symmetrical and the results show that the deformation is the function of V/L^1.5. This indicates that the change in the electric field should be considered to discuss the effect of an external electric field on combustion behavior. The experimental results are rearranged using εV²/L³ where ε is electric permittivity of air because its unit is N/m³ and it considered to be the representative electric body force. Although its application is limited, qualitatively it can help to explain the experimental results of a droplet combustion. In addition, the degree of electron attachment to neutral molecules is discussed to interpret our experimental results.     

Trapping single molecules by dielectrophoresis

We have trapped single protein molecules of R-phycoerythrin in an aqueous solution by an alternating electric field. A radio frequency voltage is applied to sharp nanoelectrodes and hence produces a strong electric field gradient. The resulting dielectrophoretic forces attract freely diffusing protein molecules. Trapping takes place at the electrode tips. Switching off the field immediately releases the molecules. The electric field distribution is computed, and from this the dielectrophoretic response of the molecules is calculated using a standard polarization model. The resulting forces are compared to the impact of Brownian motion. Finally, we discuss the experimental observations on the basis of the model calculations.     

A new electrostatic field measurement method: The coherent-notch field mill

Conventional electric field mill designs are not able to measure quasi-static atmospheric electric fields in the vicinity of high-voltage power systems. The electric fields generated by such power systems produce severe perturbations to the quasi-static background electric field. Filtering of these perturbations by the field mill circuitry seems to be easy because of their periodically alternating character. However, the operation of a field mill involves an effective signal filtering method, as presented in this paper. Because field mill operation gives rise to amplitude modulation of the incident electric field under measurement, a new technique based on coherent demodulation and notch filtering is proposed. Simulation and experimental results validate the excellent performance of the proposed method for measuring sudden step functions in electric field in presence of a severely perturbed ambient.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.     

Frequency-dependent deformation of liquid crystal droplets in an external electric field

Nematic droplets suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. To keep the system at a constant nematic/isotropic volume ratio with constant droplet size, we carefully kept the temperature in the isotropic/nematic coexistence region, which was broadened by adding small amounts of a non-mesogenic liquid. Whereas the nematic droplets remained spherical at low (in the order of 10 Hz) and high frequencies (in the order of 1 kHz), at intermediate frequencies we observed a marked flattening of the droplets in the plane perpendicular to the applied field. Droplet deformation occurred both in liquid crystals (LCs) with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to finite conductivity.     

Generation of high harmonics by an alternating electric field in superlattices based on graphene

The conductivity of a superlattice based on graphene in an alternating electric field with a vector of intensity directed along the axis of the superlattice is described. The electronic system is described in terms of the kinetic Boltzmann equation in approximation of a constant time of relaxation. Nonlinear responses to the applied harmonic field are calculated and analyzed.     

Helical instability of a semiconductor plasma in silicon wafers at 77 K

This paper reports an analysis of the experimental results on how the threshold electric field intensity required to excited helical instability of a semiconductor plasma in wafers of p-silicon depends on the magnitude of magnetic induction. Also cited are data on the dependence of the threshold frequency on electric field intensity. The variations of the amplitude of the alternating current, caused by the development of helical instability, with electric field intensity and magnetic induction well above the instability excitation threshold are examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 54–60, August, 1991.     

Harmonic Fields in a Non Uniform Plasma

The magnitudes of the harmonic fields which are generated when an alternating electric field is applied to a plasma are calculated. The plasma has a density gradient and is immersed in a uniform static magnetic field. The harmonics are very strongly excited near the upper hybrid frequency. A fluid theory is employed to describe the highly non linear behavior near resonance and a kinetic theory is used to find the effect of a finite temperature. It is found that kinetic effects are important if the radius of gyration is comparable in size to the scale length.     

Dynamics of critical fluctuations in a binary mixture of limited miscibility under a strong electric field

Experimental investigations of relaxation after switching off the strong electric field in a nitrobenzene-dodecane mixture are presented. Studies were conducted for mixtures of critical and noncritical concentrations using the time-resolved nonlinear dielectric effect. The decays obtained can be portrayed by means of the stretched exponential function with the value of the exponent in agreement with the dynamic droplet model predictions. It has been shown that experimental decays exhibit a universal scaling behavior. The relaxation time (scaling factor) shows a power behavior with the exponent y approximately 1.2 for the critical mixture and y-->1 for the noncritical one. These values are much smaller than theoretically predicted y=1.8-1.9. Based on the assumption that a strong electric field induces in the mixture a quasinematic structure with semiclassical critical properties, a quantitative explanation of this difference is proposed.     

Abnormal polarization enhancement effects of P(VDF-TrFE) films during fatigue process

The effects of alternating electric field on the fatigue behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymer films were investigated. The value of the remanent polarization (Pr) reached a maximum with the increase of cycle number of alternating electric filed, and then decreased as observed from the curve of polarization vs. switching cycles. It was found that the maximum point is associated with the frequency of alternating electric field. Dual effects, i.e., polarization enhancement and degradation, were supposed to coexist during the process of fatigue. A model considering the two effects was proposed to describe the fatigue behaviors, and the simulated data fit well with the experimental data.     

Modelling of the effect of DC and AC electric fields on the stability of a lifted diffusion methane/air flame

The application of an external electric field is known to improve flame stability significantly. Until now, few studies have proposed modelling approaches for combustion in the presence of an externally applied voltage. In these numerical studies, the negative ions are overlooked, and only the displacement of positive ions and electrons under the effect of a direct electric field was examined. In the present paper, a simplified mathematical model including negative ions is proposed based on a kinetic mechanism featuring 39 ionic reactions and 5 charged species. This mechanism is first evaluated by comparison of a monodimensional premixed flame with the available experimental data. Then it is used to analyse the stabilisation mechanism of a diffusion lifted flame in the presence of direct or alternating electric fields. It was concluded that the role of negative ions is crucial, and they are not to be neglected. Moreover, the simulations have shown that the magnitude of the flame stabilisation improvement depends, mainly, on the intensity and polarity of the applied voltage. If the applied voltage is alternating, its frequency is also found to influence the extent of the flame stabilisation improvement.     

Vacuum breakdown on metal surfaces

The unipolar arc model is described. Experimental proof that unipolar arcing represents a discharge form which easily leads to explosive plasma formation is provided. Using a laser-produced plasma, it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of 0.5 MV/cm. The experimental results show that cathode spots are formed by unipolar arching. The localized buildup of plasma above an electron-emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of a unipolar arc provides explosive plasma formation     

Electromagnetic emission of a strongly charged oscillating droplet

Analytical expressions for electric field in the vicinity of an oscillating strongly charged droplet of nonviscous conducting liquid and intensity of electromagnetic radiation are derived in the linear approximation with respect to perturbation amplitude of the droplet surface. Order-of-magnitude estimations of the radiation intensity are presented. The intensity of electromagnetic radiation of a ball lightning that can be simulated using a charged droplet is not related to the surface oscillations.     

Dissipative solitons in carbon nanotubes

The theoretical possibility of the existence of dissipative solitons in an array of carbon nanotubes when they are subjected to external uniform high-frequency electric field is discussed. An external alternating field is used for energy pumping of the electron subsystem, while a finite relaxation time leads to energy dissipation. The generation of a periodic sequence of electromagnetic pulses is revealed.     

测量密立根油滴的平衡电压

通过分析密立根油滴的受力情况和运动状态,推导出仪器显示电压不等于油滴平衡电压时油滴的运动规律. 当判断仪器显示电压是平衡电压时需要等待一段时间让油滴静止不动,这段等待时间的长短难有统一的标准. 而用在密立根油滴受电场力的作用下,测量油滴运动确定距离的时间计算平衡电压.     

电场下液滴合并自弹跳行为的分子动力学研究

在工程上通常利用滴状冷凝提高冷凝换热效率、进而强化传热.而当冷凝液滴发生合并自弹跳时,冷凝换热系数是传统滴状冷凝的1.3至1.5倍,因此液滴合并自弹跳现象对冷凝传热强化的贡献是非常大的.一些宏观实验和理论研究表明,加入外电场能进一步促进冷凝液滴合并自弹跳的频率和高度,但在纳米尺度下是否仍遵守这一规律还未可知,因此本文使...