Two-phase AC electrothermal fluidic pumping in a coplanar asymmetric electrode array

The ability to achieve fast fluid flow yet maintain a relatively low temperature rise is important for AC electrothermal (ACET) micropumping, especially in applications such as bioMEMS and lab-on-a-chip systems. In this paper, we propose a two-phase ACET fluidic micropump using a coplanar asymmetric electrode array. The proposed structure applies a two-phase AC voltage, i.e., voltage of phase 0°/180°, to the narrow electrodes while the wide electrodes are at ground potential. Numerical simulation demonstrates that this simple coplanar electrode configuration can achieve at least 25% faster fluid flow rates than using a single AC signal. By selecting certain design parameters, a two-phase ACET structure can achieve up to 50% faster fluid flow rates than a corresponding single-phase structure. The simple two-phase AC signal sources are easily produced by using inverter buffers, which is a considerable improvement compared to the multi-phase AC signals required by other electrokinetic micropumping methods, such as traveling wave structures.     

Optimization of planar interdigitated microelectrode array for biofluid transport by AC electrothermal effect

Point-of-care (POC) diagnostics is one of the most important applications for microfluidic research. However, the development of microfluidic POC devices needs to overcome great obstacles to reach market. One challenge is to find a chip-scale pumping strategy that is of low cost, small size, and light weight. Because of their simple implementation, electrokinetic techniques have been extensively investigated as a promising candidate for realizing disposable pumps, with the majority of research effort focusing on direct current and alternating current (AC) electroosmosis. As POC applications often need to handle conductive biofluids with medium to high salt content, AC electrothermal (ACET) effect has been investigated recently for pumping of biofluids, albeit with less than desirable pumping performance. In order to achieve effective on-chip ACET micropumps, this paper presents one of the first efforts in optimizing ACET micropump design utilizing planar interdigitated electrodes. The effects of electrode dimensions on pumping rate were numerically studied using COMSOL Multiphysics and MATLAB, and an optimal ratio of electrode geometry was found for various pumping scenarios. The optimal geometry ratio was tested to be valid over a wide range of electrode characteristic lengths, AC signals, and fluid ionic strengths. Experimental validation of the simulation results was also conducted, and higher flow velocities over prior reports were consistently demonstrated by optimized electrode arrays.     

Numerical investigation of AC electrokinetic virus trapping inside high ionic strength media

A numerical investigation of the mechanism by which viral particles suspended in physiologically relevant (i.e., high ionic strength) media can be electrokinetically sampled on a surface is presented. Specifically, sampling of virus from a droplet is taking place by means of a high frequency non-uniform electric field, generated by energized planar quadrupolar microelectrodes deposited on an oxidized silicon chip. The numerical simulations are based on experimental conditions applied in our previous work with vesicular stomatitis virus. A 3D computer model is used to yield the spatial profiles of electric field intensity, temperature, and fluid velocity inside the droplet, as well as the force balance on the virus. The results suggest that rapid virus sampling can be achieved by the synergistic action of dielectrophoresis and electrothermal fluid flow. Specifically, electrothermal fluid flow can be used to transport the virus from the bulk of a sample to the surface, where dielectrophoretic forces, which become significant only at very small length scales away from the surface, can cause its stable capture.     

Implicit treatment of molar mass equations in secondary oil migration

The hydrocarbon migration can be described by a coupled set of partial differential equations describing the dynamics of the temperature, component flow, pressure and velocity. A sequential solution procedure where the component flow is solved explicitly, gives severe restrictions on the time step given by the CFL condition. In this paper an implicit solution procedure is given and results from numerical tests are presented. The results are compared with the explicit solutions. As expected the implicit algorithm allows for substantially larger time steps. Received: 31 January 2001 / Accepted: 30 September 2001     

A numerical study of an electrothermal vortex enhanced micromixer

Temperature gradients aroused from the Joule heating in a non-uniform electrical field can induce inhomogeneities of electric conductivity and permittivity of the electrolyte, thus causing an electrothermal force that generates flow motion. A 2D numerical investigation of a micromixer, utilizing electrothermal effect to enhance its mixing efficiency, is proposed in this paper. Results for temperature and velocity distributions, as well as sample concentration distribution are obtained for an electrolyte solution in a microchannel with different pairs of electrodes under AC potentials with various frequencies. Numerical solutions were first carried out for one pair of electrodes, with a length of 10 μm separated by a gap of 10 μm, on one side wall of a microchannel having a length of 200 μm and a height of 50 μm. It is found that the electrothermal flow effect, in the frequency range for which Coulomb force is predominant, induces vortex motion near the electrodes, thus stirring the flow streams and enhancing its mixing efficiency. If more than one pair of electrodes is located on the opposite walls of the microchannel, the mixing efficiency depends on the AC potential applied pattern and the electrodes arrangement pattern. The distance between two pairs of electrodes on two opposite walls is then optimized numerically. Sample mixing efficiencies, using KCl solutions as the working fluid in microchannels with different number of electrodes pairs at optimal electrodes arrangement pattern, are also investigated. If root mean squared voltages of 10 V in an AC frequency range of 0.1–10 MHz are imposed on 16 pairs of electrodes separated at an optimal distance, the numerical results show that a mixing efficiency of 98% can be achieved at the end of the microchannel having a length of 700 μm and a height of 50 μm at Re = 0.01 Pe _C = 100, and Pe _T = 0.07. However, the mixing efficiency decreases sharply at a frequency higher than 10 MHz owing to the drastically decrease in the Coulomb force.     

Thermal and flow analysis of a magneto-hydrodynamic micropump

A study of transient fully developed laminar flow and temperature distribution in a magnetohydrodynamic (MHD) micropump is presented. The micropump is driven using the Lorentz force which is induced as a result of interaction between an applied electric field and a perpendicular magnetic field. The governing equations are solved analytically by an eigenfunction expansion method, and numerically by a finite-difference (ADI) method. The numerical and analytical results are found to be in good agreement with each other. The effect of different parameters on the transient velocity and temperature, such as aspect ratio, Hartman number, Prandtl number, and Eckert number is studied. The results obtained showed that controlling the flow and the temperature can be achieved by controlling the potential difference, the magnetic flux, and by a good choice of the electrical conductivity.     

Three-dimensional finite element method for the filling simulation of injection molding

With the development of molding techniques, molded parts have more complex and larger geometry with nonuniform thickness. In this case, the velocity and the variation of parameters in the gapwise direction are considerable and cannot be neglected. A three-dimensional (3D) simulation model can predict the filling process more accurately than a 2.5D model based on the Hele–Shaw approximation. This paper gives a mathematical model and numeric method based on 3D model to perform more accurate simulations of a fully flow. The model employs an equal-order velocity–pressure interpolation method. The relation between velocity and pressure is obtained from the discretized momentum equations in order to derive the pressure equation. A 3D control volume scheme is used to track the flow front. During calculating the temperature field, the influence of convection items in three directions is considered. The software based on this 3D model can calculate the pressure field, velocity field and temperature field in filling process. The validity of the model has been tested through the analysis of the flow in cavities.     

Analysis of Hybrid Electrothermomechanical Microactuators With Integrated Electrothermal and Electrostatic Actuation

The goal of this paper is to integrate electrothermal and electrostatic actuations in microelectromechanical systems (MEMS). We look at cases where these two types of actuation are intimately coupled and argue that such integrated electrothermomechanical (ETM) microactuators have more advantages than pure electrothermal or electrostatic devices. We further propose a framework to model hybrid ETM actuation to get a consistent solution for the coupled mechanical, thermal, and electrical fields in the steady state. Employing a Lagrangian approach, the inhomogeneous current conduction equation is used to describe the electric potential, while the thermal and displacement fields are obtained by solving the nonlinear heat conduction equation and by performing a large deformation mechanical analysis, respectively. To preserve numerical accuracy and reduce computational time, we also incorporate a boundary integral formulation to describe the electric potential in the medium surrounding the actuator. We show through the example of a hybrid double-beam actuator that ETM actuation results in low-voltage low-power operation that could be used for switching applications in MEMS. We also extend the same device toward bidirectional actuation and demonstrate how it may be used to overcome common problems like stiction that occur in MEMS switches.     

A nodal analysis model for the out-of-plane beamshape electrothermal microactuator

This paper introduces a nodal analysis model for the out-of-plane beamshaped electrothermal microactuators. The electrothermal microactuator is traditionally simulated with finite element method (FEM) due to the complex coupling of the electrical, thermal and mechanical problem. This complex problem is classified into two parts in this paper: the coupling electrothermal problem and the coupling thermomechanical problem. By utilizing the characteristic of the temperature distribution, the nodal analysis model for the coupling electrothermal behavior of the electrothermal microactuator is built. The model scale is much smaller than the finite element model, which results in less computational consumption. Then the coupling thermomechanical behavior, such as the shift of the heat flow from the bottom of each part of the actuator to the substrate due to its out-of-plane movement, is modeled to make the model more reasonable. Many other effects which remarkably influence the behavior of the actuator are also taken into account. This model is verified by available experimental results, and achieves an agreement.     

Flow analysis of a ribbed helix lip seal with consideration of fluid–structure interaction

Ribbed helix lip seals for rotating shafts have been widely used to retain oil and exclude contaminants in many applications throughout the industry. The objective of this study is to better understand the basic flow behavior associated with the pumping process of a ribbed helix lip seal. The theoretical model consists of a flow analysis of the lubricating film of the hydraulic fluid in conjunction with a stress analysis of the lip seal distortion. The complicated mechanical interaction between the oil flow and rubber deformation was simulated using a coupled fluid–structure approach implemented in a commercial computational fluid dynamics (CFD) code ESI-CFD, ACE+®. The flow characteristics and rubber deformation around a ribbed helix lip seal were fully resolved in a pumping-rate test environment, where both air and oil sides were filled with oil initially. The three-dimensional pressure field solved by the model via the coupled flow-stress analysis was compared with the predictions obtained from the model via the nondeformable rubber assumption to elucidate the significant effect of the fluid–structure interaction on accurate simulation of the oil pumping behavior. In the rotating speed ranging from 1000 to 6000 rpm, both measured and calculated pumping rates increase with the shaft speed for a ribbed helix lip seal. As compared to the baseline case, calculations with considering the fluid–structure interaction at higher rotary speeds can result in thicker oil films, and in turn produce greater pumping rates.     

平面内运动热执行器的节点模型

本文提出了一种平面运动热执行器的热-电-机械耦合节点模型,此模型不仅可以仿真热执行器中随温度变化的热导率、电阻率和热膨胀率效应,而且可以仿真热执行器机械域的几何非线性效应,从而使用于仿真热执行器的行为的节点法模型实用化.ANSYS有限元仿真的结果证明此模型的不但精度较高,而且计算效率很高.     

Thermal and flow analysis of two-dimensional fully developed flow in an AC magneto-hydrodynamic micropump

The effect of fluctuating Lorentz force on the Ac magnetohydrodynamic micropump is studied. A two-dimensional transient fully developed laminar flow and temperature distribution are modeled. The governing Navier–Stokes and energy equations are solved numerically by a finite-difference (ADI) method. The effect of different parameters on the transient and steady flow velocity and temperature, such as aspect ratio, Hartman number, Prandtl number, and Eckert number is studied. The results obtained showed that controlling the flow and the temperature can be achieved by controlling the potential difference, the magnetic flux, and by a good choice of the electrical conductivity. The effect of Stanton number and phase angle is also included, and it is found that at high frequency, the pulsed volume is small which yield a continuous flow instead of pulsating flow, and the magnitude and direction of the flow can be controlled by the phase shift between the electrical and magnetic fields.     

A New Multiple-relaxation-time Lattice Boltzmann Method for Natural Convection

This article is devoted to the study of multiple-relaxation-time (MRT) lattice Boltzmann method with eight-by-eight collision matrix for natural convection flow. In the velocity space, eight speed directions are used and the corresponding incompressible multiple-relaxation-time model with force term is presented. D2Q4 model is for temperature field. The coupled double distribution functions (DDF) overcome artificial compressible effect corresponding to the standard MRT model. The simulations of natural convection flows with Pr=0.71 for air and Ra=10³–10⁹ are carried out and excellent agreements are obtained to demonstrate the numerical accuracy and stability of the proposed model.     

超声速旋转火箭弹气动特性仿真和分析

采用CFD方法,基于剪切应力输运（Shear Stress Transport,SST）湍流模型,求解大长细比卷弧翼火箭弹在超声速情况下的气动力和气动热问题．对火箭弹流场进行数值计算,与实验数据进行对比．采用薄壁模型模拟结构耦合传热,计算在一定海拔和旋转情况下火箭弹的气动加热,并与不旋转的情况进行对比．计算结果表明该数值方法能较好地计算气动力因数和气动热分布．在特定的低转速和海拔情况下的火箭弹温度分布比不旋转的稍微大一点,在旋转情况下的火箭弹尾部截面压力分布不对称,尾部流线更加紊乱;弹头和尾翼前缘温度较高,应当在火箭弹设计中予以考虑．     

Microfluidic flow reversal at low frequency by AC electrothermal effect

This paper describes a flow reversal phenomenon for fluids with moderate conductivity. Fluids with conductivities of 2 × 10⁻⁴ S/m, 0.02 S/m and up to 0.1 S/m were experimented at frequencies ranging from 1 to 110 kHz. Flow reversal was observed only at ~1 kHz and 5.3 V _rms for σ = 0.02 S/m, and our analysis indicates that AC electrothermal effect could be responsible. Analysis of the system impedance and simulation of power consumption show that the distribution of electric power consumption is dependent on conductivity and AC frequency. At low frequencies, possibly more electric power is consumed at surface/electrolyte interface rather than within the fluid, which consequently changes the location of temperature maximum and the directions of temperature gradients. The direction of AC electrothermal force is reoriented, causing the flow reversal. Numerical simulation is also performed and agrees within the experiments.     

聚合物熔体三维等温流动的罚有限元分析

本文针对幂律流体和非线性粘弹性PTT流体，采用罚有限元法，形成了求解聚合物熔体三维等温流动速度场和粘弹性应力场的有限元模型．对非线性粘弹性PTT流体，为了降低模拟计算对计算机硬件的要求，并使模拟计算更加稳定，采用了去耦算法，包括拟体力方法和动量方程的椭圆类方程转化方法等。文中还给出了总体有限元方程组的形成和求解过程。     

An electrothermal functional model of the microwave FET suitable for nonlinear simulation

This article introduces a CAD-oriented functional model of the microwave FET depending on temperature as an additional state variable in addition to the usual gate and drain voltages. The model is fully conservative and non-quasistatic, and thus considerably more advanced than other currently available empirical models. A novel parameter extraction strategy allows the simultaneous determination of the voltage and temperature dependence of the constitutive relations, making use of a set of DC measurements and bias-dependent scattering matrices obtained at a single ambient temperature. The model can be coupled to a harmonic-balance algorithm for the efficient electrothermal simulation of temperature-dependent nonlinear microwave circuits, which the authors discussed in a previous article.     

Modeling steady axis-symmetric thermal plasma flow of air by a parallelized magneto-hydrodynamic flow solver

This paper discusses a parallelized magneto-hydrodynamic flow solver for modeling axis-symmetric thermal plasma flow using Cartesian grid system and taking the induced electrical and magnetic effects into account, where the magneto-hydrodynamic equations, including the continuity equation, momentum equations, energy equation, current continuity equation and turbulence transport equations are solved by a finite volume discretization in a segregated manner. The thermal plasma flow of a 476 mm long, transferred well-type plasma torch operating with air is simulated for two power conditions, i.e. I = 432 A and 901 A, to demonstrate the capability of proposed numerical model to analyze the heat and mass transfer characteristics of axis-symmetric thermal plasma flow, where the location of cathode is determined by fixing the measured voltage drop between two electrodes. The numerical calculation suggests that the high-power case can deliver an axial velocity of 400 m/s and 15,000 K in temperature at the center of torch outlet, where a strong jetting vortex is expected emitting from the torch body. The low-power case is predicted with a longer electric arc than that of the high-power one, which clearly results in a large high-temperature region between the gas inlet and cathode and unfavourable to reduce the cathode erosion and to increase thermal efficiency.     

Finite element-based analysis of shunted piezoelectric structures for vibration damping

Piezoelectric patches shunted with passive electrical networks can be attached to a host structure for reduction of structural vibrations. This approach is frequently called “shunted piezo damping” and has the advantage of guaranteed stability and low complexity in implementation. For numerical treatment of such structures, a finite element modelling methodology is presented that incorporates both the piezoelectric coupling effects of the patches and the electrical dynamics of the connected passive electrical circuits. It allows direct computation of the achieved modal damping ratios as a major design criterion of interest. The damping ratios are determined from the eigenvalue problem corresponding to the coupled model containing piezoelectric structure and passive electrical circuit. The model includes local stiffening and mass effects as a result of the attached patches and, therefore, enables accurate prediction of the natural frequencies and corresponding modal damping ratios. This becomes crucial for choosing the patch thickness to achieve optimal modal damping for a given host structure. Additionally, structures with complex geometry or spatially varying material properties can easily be handled. Furthermore, the use of a finite element formulation for the coupled model of piezoelectric patches and a host structure facilitates design modifications and systematic investigations of parameter dependencies. In this paper, the impact of parameters of the passive electrical network on modal damping ratios as well as the variation of the patch thickness are studied. An application of this modelling method is realized by commercial software packages by importing fully coupled ANSYS^© – models in MATLAB^©. Afterwards, modal truncation is applied, the dynamic equations of the passive electrical network are integrated into the piezoelectric model and eigenvalue problems are solved to extract the increase in modal damping ratios. The numerical results are verified by experiments.     

Numerical investigation of polymer rheology in electrohydrodynamic structuring on geometrical dielectric (ESGD) process

Electrohydrodynamic structuring on geometrical dielectric (ESGD) is an interesting approach to fabricate micro-/nanostructures from a rheological polymer which uses an electric field to produce the dielectrowetting force on the three-phase contact line, electrostatic force at the air–polymer interface and electrical attraction between the electrode pair to promote the fluidic polymer deformation. In this paper, the dynamic evolution of polymer rheology is investigated to understand the ESGD process from the viewpoints of the morphology of the air–polymer interface, polymer height, contact angle and the polymeric velocity, which is performed via a proposed numerical model that couples electric field, flow field and level set formulation accompanied with a verification by comparing the numerical simulations with the experimental tests. Furthermore, the driving force is disassembled to study the role of each one, by which the components of driving force can be listed in a sequence from strong to weak: dielectrowetting force, electrostatic force and electrical attraction. Finally, the influence of process parameters on the deformations, consisting of voltage, polymer thickness, dielectric permittivity and thickness of the dielectric layer coated on the template, is studied from the point of view of dielectrowetting force and electrostatic force, which provides a possible approach to enhance the driving force for a high aspect ratio.