Coping with electrode polarization for development of DC-Driven electrical impedance tomography

An electrical impedance tomography (EIT) system design is proposed for imaging of phase distribution in gas-water two-phase flow from boundary measurement of electrical potentials in response to direct current (DC) injection. DC injection simplifies substantially the system design, but introduces problems due to polarization of injection electrodes. Electrode polarization means charge accumulation on the electrode-water interface causing a drift in the interfacial potential difference. The polarization problems are coped with by using dedicated electrodes for injection and potential measurement, and using a current source unaffected by the polarization of current-carrying electrodes (CCEs). Furthermore, the polarization of CCEs is controlled, to lessen the possible influence on the sensing electrodes (SEs), by using a short (milliseconds in width) pulse for injection with a charge balanced injection strategy. The impact of electrode polarization and the effectiveness of countermeasures introduced in the present design are discussed through comparisons of measured boundary potentials and of images reconstructed for a simple object simulating large bubbles in water.     

Characteristics of cooling water fouling in a heat exchange system

This study investigated the efficiency of the physical water treatment method in preventing and controlling fouling accumulation on heat transfer surfaces in a laboratory heat exchange system with tap and artificial water. To investigate the fouling characteristics, an experimental test facility with a plate type heat exchange system was newly built, where cooling and hot water moved in opposite directions forming a counter-flow heat exchanger. The obtained fouling resistances were used to analyze the effects of the physical water treatment on fouling mitigation. Furthermore, the surface tension and pH values of water were also measured. This study compared the fouling characteristics of cooling water in the heat exchange system with and without the mitigation methods for various inlet velocities. In the presence of the electrode devices with a velocity of 0.5m/s, the fouling resistance was reduced by 79% compared to that in the absence of electrode devices.     

Cross-sectional capacitance measurement of particle concentration in a microchannel with multi-layered electrodes

In the present study, cross-sectional capacitance is measured in a microchannel with 12 multi-layered electrodes. The cross-sectional capacitances of microparticles of three different diameters are measured upstream and downstream. Based on these measurements, the particle concentration in terms of particle volume fraction is calculated. The particle volume fractions at three configuration areas of electrode pairs, namely, at ones containing adjacent, horizontal, and vertical pairs, are considered for ease of understanding of the particle concentration for different electrode pairs. The particle volume fraction shows an increment of 1.0–9.3% for electrode pairs near the channel walls, but decreased by 0.4–0.96% for center electrode pairs from the upstream cross-section to the downstream cross-section. Moving from the inlets to the outlet, the particles drift away from the center of the microchannel and approach the channel walls and the number of particles that migrate toward the channel walls increases clearly in the outlet area.     

An experimental study on the AC electroosmotic flow around a pair of electrodes in a microchannel

This paper presents an experimental study on the AC electroosmotic flow in a microchannel having a pair of rectangular electrodes on the bottom wall with narrow gap. The microchannel was made of PDMS (Polydimethylsiloxane) and the electrodes of ITO (Indium Tin Oxide). The electrodes were arranged such that the electric field is mainly perpendicular to the channel’s longitudinal direction, thus creating a transversal secondary flow. The primary flow was driven by a pressure force through the fluid-level difference on both reservoirs of the channel. To measure the velocity distributions around the electrodes, we used a micro-PTV (particle tracking velocimetry) technique. We find that on the surface of the electrodes the flow velocity caused by the AC electroosmosis is directed from the electrode edge toward the side wall of the channel, and the maximum crosswise velocity occurs at the frequency 120Hz. A smooth profile of the crosswise velocity component along a vertical line was successfully obtained from the present experimental technique, and it shows a flow reversal due to the mass conservation principle.     

Investigation of water electrical parameters as a function of measurement frequency using cylindrical capacitive sensors

In this study electrical properties of different water liquids at frequency range of 100–2000 Hz are investigated by using the short invasive and non-invasive cylindrical capacitive sensors (CCSs). Operation of the capacitance measurement module for such probes is based on the auto balancing bridge method. Comparison of the measured capacitances and measured resistances for different water liquids shows decrease by increasing the frequency. In another study the dielectric constant of distilled water, mineral water, tap water and salt water are measured. The effects of the frequency on the resistivity, permittivity and conductance of the different water liquids are also investigated.     

Application of dual-plane ERT system and cross-correlation technique to measure gas–liquid flows in vertical upward pipe

The progress of process tomography provides a new method for two-phase flow measurement. The dual-plane electrical resistance tomography (ERT) is combined with the correlation measurement technique to carry out the two-phase flow measurement in which the continuous phase is conductive liquid. The method of the estimation of void fraction and the disperse phase velocity by extracting the eigenvalue of the dual-plane ERT boundary measured data is presented. This method is applied to the transient flow-rate measurement of the air–water two-phase flow in vertical pipe. The information of disperse phase void fraction and distribution variation with time change can be considered adequately, and the estimated value of disperse phase void fraction and velocity can be gained fairly accurately in this method, which provides the data for the calculation of the transient flow-rate. The experiment results indicate that this kind of measurement method is valid when the distance between the upstream and downstream measured cross section is short enough.     

Measurement of velocity profiles in multiphase flow using a multi-electrode electromagnetic flow meter

This paper describes an electromagnetic flow meter for velocity profile measurement in single phase and multiphase flows with non-uniform axial velocity profiles. A Helmholtz coil is used to produce a near-uniform magnetic field orthogonal to both the flow direction and the plane of an electrode array mounted on the internal surface of a non-conducting pipe wall. Induced voltages acquired from the electrode array are related to the flow velocity distribution via variables known as ‘weight values’ which are calculated using finite element software. Matrix inversion is used to calculate the velocity distribution in the flow cross section from the induced voltages measured at the electrode array. This paper presents simulations and experimental results including, firstly the effects of the velocity profile on the electrical potential distribution, secondly the induced voltage distribution at the electrode pair locations, and thirdly the reconstructed velocity profile calculated using the weight values and the matrix inversion method mentioned above. The flow pipe cross-section is divided into a number of pixels and, in the simulations, the mean flow velocity in each of the pixels in single phase flow is calculated from the measured induced voltages. Reference velocity profiles that have been investigated in the simulations include a uniform velocity profile and a linear velocity profile. The results show good agreement between the reconstructed and reference velocity profiles. Experimental results are also presented for the reconstructed velocity profile of the continuous water phase in an inclined solids-in-water multiphase flow for which the axial water velocity distribution is highly non-uniform. The results presented in this paper are most relevant to flows in which variations in the axial flow velocity occur principally in a single direction.     

Micro wire-mesh sensor for two-phase flow measurement in a rectangular narrow channel

A micro wire-mesh sensor (μWMS) based on an electrical conductivity measurement between electrodes installed on the walls has been developed for gas–liquid two-phase flow measurements in a narrow rectangular channel. This measuring method applies a principle of conventional wire-mesh tomography, which can measure the instantaneous void fraction distributions in the cross-section of the relatively large flow channel. In two-phase flow measurement using μWMS the void fraction distributions in the narrow channel were obtained by the measured conductivities between electrodes arranged on each wall. Therefore, the gas phase structures and the bubble behaviors can be investigated in the flow channel with narrow gap. In the present paper, a μWMS for the air–water flow between parallel flat plates with a gap of 3 mm was developed and simultaneous measurements with a high speed video camera were conducted to compare the measured results in bubbly flow.     

Airborne particles detection and sizing at single particle level by a novel electrical current pulse sensor

A novel method for detecting and sizing airborne particles at single particle level is presented in this paper. When a particle hits a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated in the measurement system. The number of the signal pulse represents the number of particles in the sample. To determine the effect of the particle size on the magnitude of the signal, the correlation between the magnitude of the signal and the size of particle was experimentally investigated. The results show that the magnitude of the measured signal is linearly proportional to the square of particle’s diameter. Such a correlation can be used to evaluate particle size from the measured signal. The airborne particle detection method presented in this paper can be used for counting and sizing airborne particles at single particle level.     

Toner Display Based on Movement of Tribo-electrically Charged Particles

The mechanism of toner display based on an electrical movement of black and white charged particles has been investigated. Two kinds of particles of black and white charged in the different electric polarity are enclosed in two ITO transparent electrodes using an insulating spacer. The particle movement is controlled by the external electric field applied between two transparent electrodes. The black toner is adhered on the electrode by an electrostatic force across the CTL to display a black image. The toners can be put back to the counter electrode by applying a reverse electric field, and a white image is formed. The black and white solid images are displayed by the switch of polarity of applied voltage in toner display cell. The polarity of charge and the value of charge to mass ratio of two particles were measured by observation of the particle separation on the surface-type electrodes and using q/m meter, respectively.     

Ultra-flat coplanar electrodes for controlled electrical contact of molecular films

Reliable measurement of electrical charge transport in molecular layers is a delicate task that requires establishing contacts with electrodes without perturbing the molecular structure of the film. We show how this can be achieved by means of novel device consisting of ultra-flat electrodes separated by insulating material to support the molecular film. We show the fabrication process of these electrodes using a replica technique where gold electrodes are embedded in a silicon oxide film deposited on the angstrom-level flat surface of a silicon wafer. Importantly, the co-planarity of the electrode and oxide areas of the substrate was in the sub-nanometer range. We illustrate the capabilities of the system by mapping the distribution of electrical transport pathways in molecular thin films of self-assembled oligothiophene derivatives using conductive atomic force microscopy. In comparison with traditional bottom contact non-coplanar electrodes, the films deposited on our electrodes exhibited contact resistances lower by a factor of 40 than that of the similar but non-coplanar electrodes.     

电容耦合非接触电极及心电信号获取

电极与皮肤间接触所导致的不适感,是穿戴式心电信号测量系统实际应用中的常见问题。设计了一种非接触心电信号测量系统。采用印刷电路板制作的测量电极,借助电容耦合测量位移电流的方式获取心电信号。采用反接二极管提供测量所需的高阻值偏置电阻,结合高输入阻抗仪表放大器,制作了测量电极信号提取电路。测量系统由两个测量电极与一个直接与测量电路地相连的参考电极组成。选择金属铝板、导电纤维和导电橡胶作为参考电极,实验研究了共模干扰抑制性能与参考电极接触阻抗之间的量化关系。将主元分析与奇异谱分析相结合,提出了一种心电信号处理算法。实验结果表明,该系统可在棉质线衣外侧有效获得满意的心电信号。     

基于三维速度场构建的微流量测量方法研究

采用微流体粒子图像测速仪(microscale particle image velocimetry,micro-PIV)对200μm宽、60μm深的长直通道三维速度场进行了非接触定量可视化测量,并在此基础上计算了通道内的微流量。实验采用二维分层速度场测量方法,将通道沿物镜景深方向划分为11个流体层,通过高精度的位移平台实现流体跨层粒子图像采集。分别针对64×64像素和32×32像素2种判读域,采用micro-PIV系综相关算法对流体层二维速度场进行分析,获得三维全场速度分布,在此基础上利用截面速度离散积分原理计算出截面微流量。实验结果表明,基于微流体粒子图像测速仪的三维速度场分析能够实现对微通道流量的精确测量。对于64×64像素判读域,输入流量在2.481～5.788μL/min范围的测量结果精度较高,最大相对误差为3.87%;对于32×32像素判读域,输入流量在3.307～8.269μL/min范围内均有较高测量精度,最大相对误差为3.69%,表明采用32×32像素判读域的流量测量精度总体上优于64×64像素判读域。     

Permittivity measurements using adjustable microscale electrode gaps between millimeter-sized spherical electrodes

This paper presents a technique for measuring the electrical permittivity of liquids and gases using millimeter-sized spherical electrodes with adjustable microscale separation. This technique eliminates the need for wet calibration by using the precise adjustment of electrode separation to remove the inherent errors of parasitic capacitance and electrode polarization. The spherical electrode geometry also eliminates the need for precise parallel adjustment of electrode separation, and enables small-volume, small-electrode-gap measurements where the applied electric field is constrained in a region of well-defined geometry. By further leveraging the fact that spherical electrodes can be obtained with extremely high diametrical accuracy, absolute permittivity measurement accuracies within 1% of the established values has been demonstrated. Finally, the apparatus also enables the creation of nanometer electrode gaps between macroscopic electrodes with precisely controlled separation, which can be used to study the electrical properties of liquids in highly confined states. The electrode gaps created in this manner can be adjusted from 20 nm to 50 microm, in increments of 0.25 nm.     

Study on electrodynamic sensor of multi-modality system for multiphase flow measurement

Accurate measurement of multiphase flows, including gas/solids, gas/liquid, and liquid/liquid flows, is still challenging. In principle, electrical capacitance tomography (ECT) can be used to measure the concentration of solids in a gas/solids flow and the liquid (e.g., oil) fraction in a gas/liquid flow, if the liquid is non-conductive. Electrical resistance tomography (ERT) can be used to measure a gas/liquid flow, if the liquid is conductive. It has been attempted to use a dual-modality ECT/ERT system to measure both the concentration profile and the velocity profile by pixel-based cross correlation. However, this approach is not realistic because of the dynamic characteristics and the complexity of multiphase flows and the difficulties in determining the velocities by cross correlation. In this paper, the issues with dual modality ECT/ERT and the difficulties with pixel-based cross correlation will be discussed. A new adaptive multi-modality (ECT, ERT and electro-dynamic) sensor, which can be used to measure a gas/solids or gas/liquid flow, will be described. Especially, some details of the electrodynamic sensor of multi-modality system such as sensing electrodes optimum design, electrostatic charge amplifier, and signal processing will be discussed. Initial experimental results will be given.     

LBM simulation on mixing enhancement by the effect of heterogeneous zeta-potential in a microchannel

In this study, we use the lattice Boltzmann method to perform 2D and 3D numerical simulations to investigate the electrokinetic effect on the fluid flow and mixing in a rectangular microchannel. Zeta-potential serves as the main factor representing the electrokinetic effect. The numerical simulations in particular aim at evaluation of the mixing performance of two new designs of electrode distribution in the microchannel. One uses the periodically distributed rectangular electrodes on lateral walls of the microchannel, for which 2D and 3D simulations were implemented with time-periodic application of the electric potential on the electrodes. In another model, a periodic array of trapezoidal electrodes with a constant electric potential on them was attached on the bottom walls, for which 3D simulation was carried out. Through the parametric studies for both designs, it was shown that there exists an optimum parameter value leading to the best mixing performance.     

基于循环脱毒技术的高浓度消毒液在线监控系统

次氯酸克拉克电极在高浓度次氯酸的环境中容易中毒,失去检测稳定性,因此,该系统利用自来水进行间断性清洗,在电极深度中毒之前将次氯酸从电极电解液中清除,达到在检测过程中循环退毒的效果,从而维持测试的稳定性.实验显示,在300 ppm(1 ppm =10(-6))的次氯酸环境中进行为期60s的浓度测试,通过180s的自来水退毒就可有效达到退毒要求.在电极稳定工作的基础上,基于系统硬件开发出相应算法,以弥补退毒过程采样频率过低而给消毒系统稳定所带来的影响.现场测试结果表明:该系统可以达到使用要求,维持消毒池次氯酸浓度在±5 ppm的范围区间.     

A dynamic measurement system for evaluating dry bio-potential surface electrodes

In this paper, a dynamic measurement system (DMS) is presented for assessing performance of wearable and dry bio-potential surface electrodes (DBSEs) in air, especially motion artifacts. A pair of perforated membranes is assembled on the two ends of the barrel which contains electrolyte to simulate surface of the skin. The pressure of electrode/membrane can be controlled and measured, one of electrodes can move on membrane by the motion controller. Experimental results show maximum coefficient of variation (CV) of electrical impedance spectra (EIS) and dynamic open circuit potential variation (DOCP, so-called motion artifacts) of gold electrode pair are, respectively, 4.3% and 7.2% in continuous measurements, 12.6% and 22% in discontinuous measurements, the pressure and the length of motion track between electrode and membrane are strongly linear correlation with DOCP variations. EIS and DOCP of five types of electrodes were measured on DMS and their parameters of equivalent circuit (EC) of EIS and DOCP variation show larger is equivalent capacitance in electrode/electrolyte interface, less is DOCP variation. DOCP variations of two batches of electrode performed respectively on DMS and on skin have approximate 0.64 of Pearson correlation coefficient. In general, the DMS provides a powerful tool to investigate the mechanism of DOCP variation and evaluates the static or dynamic performance of electrodes. However, parameters of EC model by fitting EIS can predict the motion artifact and help the design of electrode and its integrated wearable garments.     

Bubble size measurement using wire-mesh sensors

A wire-mesh sensor with a time resolution of 1.2 kHz was used to measure bubble size distributions in a gas-liquid flow. It is designed for a pipe of 51.2 mm diameter and consists of two electrode grids with 16 electrodes each, put in the flow direction behind each other. The local instantaneous electrical conductivity is directly measured between all pairs of crossing wires, a tomographic image reconstruction is not necessary. The resulting 16 × 16 sensitive points are equally distributed over the cross section. This resolution is sufficient to detect individual bubbles, which are imaged in several successive frames during their transition through the measuring plane. To investigate the influence on bubbles, a model of the sensor was tested in a transparent channel with a rectangular cross section of 50 × 50 mm at liquid velocities between 0 and 0.8 m/s. A comparison with high-speed video observations has shown that the sensor causes a significant fragmentation of the bubbles. Nevertheless, the measured signals still represent the structure of the two-phase flow before it is disturbed by the sensor. Bubble sizes can therefore be determined by integrating local instantaneous gas fractions over an area of the measuring points occupied by the bubble. Bubble size distributions are obtained by analysing large assemblies of bubbles. The method was applied to study the formation of slug flow along a vertical tube. The bubble size distributions obtained show the effect of coalescence as well as bubble fragmentation.     

基于氧化石墨烯的阻变存储器制备

阻变存储器是一种新型非易失性存储器,其在外加电场作用下可实现高阻态和低阻态之间的切换。存储器电极材料和活性层材料的选择及相互作用是实现器件阻变特性的主要因素。石墨烯是具有优良导电性和高延展性的二维材料,通过激光加工还原氧化石墨烯是高效获取石墨烯的极佳方法。传统存储器的制备过程复杂,不利于大规模加工制造。以金属金（Au）和还原氧化石墨烯（rGO）作为电极,氧化石墨烯（GO）作为阻变层进行器件制备,很好地实现了存储器的阻变功能。简单高效的制备方式为大规模、高集成化生产阻变存储器提供了参考。