Application of astigmatism μ-PTV to analyze the vortex structure of AC electroosmotic flows

The three-dimensional (3D) flow due to AC electroosmotic (ACEO) forcing on an array of interdigitated symmetric electrodes in micro-channels is experimentally analyzed using astigmatism micro-particle tracking velocimetry (astigmatism μ-PTV). Upon application of the AC electric field with a frequency of 1,000 Hz and a voltage of 2 Volts peak–peak, the obtained 3D particle trajectories exhibit a vortical structure of ACEO flow above the electrodes. Two alternating time delays (0.03 and 0.37 s) were used to measure the flow field with a wide range of velocities, including error analysis. Presence and properties of the vortical flow were quantified. The steady nature and the quasi-2D character of the vortices can combine the results from a series of measurements into one dense data set. This facilitates accurate evaluation of the velocity field by data-processing methods. The primary circulation of the vortices due to ACEO forcing is given in terms of the spanwise component of vorticity. The outline of the vortex boundary is determined via the eigenvalues of the strain-rate tensor. Overall, astigmatism μ-PTV is proven to be a reliable tool for quantitative analysis of ACEO flow.     

Validated numerical analysis of vortical structures in 3D AC electro-osmotic flows

This paper presented an experimental validation of a numerical study on the vortical structures in AC electro-osmotic (ACEO) flows. First, the 3D velocity field of ACEO vortices above the symmetric electrodes was experimentally investigated using astigmatism microparticle tracking velocimetry. The experimentally obtained velocities were used to validate an extended nonlinear Gouy–Chapman–Stern model accounting for the surface conduction effect. A qualitative agreement between the simulations and experiments was found for the velocity field when changing AC voltage (from 0.5 to 2 V) and the frequency (from 50 to 3,000 Hz). However, the predicted magnitude of the velocity profiles was much higher than the experimentally obtained ones, except in some cases at low frequency. For frequencies higher than 200 Hz, a correction factor was introduced to make the numerical results quantitatively comparable to the experimental ones. In addition, the primary circulation, given in terms of the spanwise component of vorticity, was numerically and experimentally analyzed as function of frequency and amplitude of the AC voltage. The outline of the vortex boundary was determined via the eigenvalues of the strain-rate tensor estimated from the velocity field. It revealed that the experimental circulation was frequency dependent, tending to zero at both low and high frequency and the maximum changing from around 600 Hz for 1 V to 300 Hz for 2 V. The variation in the predicted vortex circulation as function of frequency and voltage, after using the above correction factor, was in good correspondence with the experiments. These results yield first insights into the characteristics of 3D ACEO flows and the ability of current numerical models to adequately describe them.     

A three-phased circular electrode array for electro-osmotic microfluidic pumping

A circular micro-electrode array with three phases is designed and prototyped using PolyMUMPs process for AC electro-osmotic flow pumping. Finite element model of the micro electrode array has been developed using COMSOL Multiphysics^™. Performance of the electrode array is simulated and a double peak velocity phenomenon is found for this design, which is confirmed by experimental testing. Using ethanol as testing medium, the two time-averaged peak flow velocities are approximate 320 μm/s at 7 Hz and 850 μm/s at 100 Hz. It is found that the simulated and experimental results agree well.     

Novel systems for configurable AC electroosmotic pumping

In this paper we present a novel method of creating and using geometric asymmetries for AC electroosmotic pumping. The method relies on grouping similar electrodes together in terms of applied voltage, in order to create configurable asymmetries in periodic electrode arrays, which induce a net pumping AC electroosmotic velocity. Using a numerical model for a system designed by applying the described method, it is demonstrated that by varying the degree of asymmetry it is possible to control the direction of the pumping velocity at a given voltage by simple switching of the voltages on the electrodes.     

Performance improvement of an AC electroosmotic micropump by hydrophobic surface modification

This paper describes the improvement of bi-directional micropump velocity by deposition of a hydrophobic nanocomposite monolayer. A polymer base nanocomposite coating consisting of a homogeneous mixture of silicon nanoparticles in polydimethylsiloxane (PDMS) is used to improve the hydrophobicity of the micropump surfaces. For hydrophobic nature of PDMS and the monolayer coating with nanoscale surface roughness, the hydrophilic surface of a biased AC electroosmotic micropump will transform to a hydrophobic surface. In our previous research the applied AC voltage, frequency, channel dimension, and electrode width were optimized (Islam and Reyna, Electrophoresis 33(7), 2012). Based on the prior results obtained for the biased AC electroosmotic micropump, the pumping velocity was 300 micron/s in 100-μm channel thickness for applied voltage of 4.4 V at 1 kHz frequency. Here in this work, improvement of the micropump velocity is investigated through a surface modification process. The highest velocity of 450 micron/s is observed by modifying the surface characteristics. This paper will also discuss the synthesis process and characteristics of the polymer base nanocomposite monolayer. In addition to hydrophobicity improvement, adding a thin nanocomposite monolayer will physically separate the electrodes from the pumping liquid, thus eliminating their reaction, which is usually observed due to the application of voltage. As a result, higher voltages can be applied to the electrodes and higher pumping rates are achievable.     

Design of a relaying electroosmosis pump driven by low-voltage DC

Electroosmosis pumps (EOPs) have been widely used for manipulating small amounts of reagents for chemical and biological analysis. Traditionally, a high-voltage DC has to be applied in order to achieve the required flow rate. One alternative is to use low AC voltage. Here we propose another solution, which, instead of using a high-voltage DC or low AC voltage, adds a low-voltage DC to an array of electrodes. This design of EOP is called a relaying EOP or cascade EOP. In this study, we intend to push the limit of the low-voltage further down to 2 V by patterning a dense electrode array in a straight microchannel. Two patterns of interdigitated electrodes, symmetric with equal size electrodes and asymmetric with unequal size electrodes, are proposed. Simulations are performed to optimize the distribution and geometrical parameters of the electrode array in order to achieve the maximum flow rate. The proposed low-voltage DC electroosmosis pump shows an advantage in integrating EOP into portable Lab-on-a-chip devices. In addition, the low-voltage DC EOP shows a good promise for in vivo biomedical applications such as drug delivery.     

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.     

Improved particle concentration by cascade AC electroosmotic flow

The importance of electrokinetics in microfluidic technology has been growing owing to its versatility and simplicity in fabrication, implementation, and handling. Alternating-current electroosmosis (ACEO), which is the motion of fluid due to the ion movement by an interaction between AC electric field and an electrical double layer on the electrode surface, has a potential for a particle concentration method to detecti rare samples flowing in a microchannel. This study investigates an improved ACEO-based particle concentration by cascade electrokinetic approach. Flow field induced by ACEO and accumulation behavior of particles were parametrically measured to discuss the concentrating mechanism. The accumulation of particles by ACEO can be explained by a balance between the attenuating electroosmotic flow to transport particles and the inherent diffusive motion of the particles, which is hindered due to the near-wall location. Although a parallel double-gap electrode geometry enables particles to be collected at the center of electrode very sharply, it has scattering zones with accumulated particles at sidewalls of the channel. This drawback can be overcome by applying sheath flow or introducing cascade electrode pattern upstream of the focusing zone. As a result, total concentration efficiency was 98.4 % for all the particles flowing in the cascade device. The resultant concentrated particles exist on the electrode surface within 5 μm, and three-dimensional concentration of particle with the concentration factor as large as 700 is possible using a monolithic channel, co-planar electrode, and sheathless solution feeding. This cascade electrokinetic method provides a new and effective preconcentrator for ultra-sensitive detection of rare samples.     

Experiments on opto-electrically generated microfluidic vortices

Strong microfluidic vortices are generated when a near-infrared (1,064 nm) laser beam is focused within a microchannel and an alternating current (AC) electric field is simultaneously applied. The electric field is generated from a parallel-plate, indium tin oxide (ITO) electrodes separated by 50 μm. We present the first μ-PIV analysis of the flow structure of such vortices. The vortices exhibit a sink-type behavior in the plane normal to the electric field and the flow speeds are characterized as a function of the electric field strength and biasing AC signal frequency. At a constant AC frequency of 100 kHz, the fluid velocity increases as the square of the electric field strength. At constant electric field strength fluid velocity does not change appreciably in the 30–50 kHz range and it decreases at larger frequencies (>1 MHz) until at approximately 5 MHz when Brownian motion dominates the movement of the 300 nm μ-PIV tracer particles. Presence of strongly focused laser beams in an interdigitated-electrode configuration can also lead to strong microfluidic vortices. When the center of the illumination is focused in the middle of an electrode strip, particles experiencing negative dielectrophoresis are carried towards the illumination and aggregate in this area.     

A three phase serpentine micro electrode array for AC electroosmotic flow pumping

A new three-phase electrode array with a serpentine electrode is designed and prototyped using PolyMUMPs process for micro flow pumping. Numerical model of the micropump has been developed using COMSOL Multiphysics™. Experimental testing is conducted and time-averaged flow velocities from testing and simulation agree well. Peak time-averaged flow velocity of 270 μm/s is achieved at 30 Hz using ethanol.     

Configurable AC electroosmotic generated in-plane microvortices and pumping flow in microchannels

This paper proposes the patterned AC electroosmotic flows (AC-EOF) by simply grouping discrete electrodes together to form various electrode configurations for generation of in-plane microvortices with clockwise/counter-clockwise rotation, and pumping flow in a microchannel. The rotational direction of in-plane microvortices and pumping flow direction can be controlled using the same electrode pattern by simple switching of the voltages on the electrodes. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices and pumping flow. The rotational strength of microvortices and flow rates of pumping flows were found to increase with the increase of the applied voltages, and an optimal value was achieved at an appropriately applied frequency. Moreover, the dependency of the applied voltages, frequencies, and the heights of the measured planes on the rotational strength of in-plane microvortices between the interdigitated and discrete electrode configurations were examined. The discrepancy in electrode geometry results in a small performance reduction, whereas it can be compensated for the ability of switching the rotational direction of in-plane microvortices using the same device. The configurable in-plane microvortices and pumping flow in microchannels provide the potential for micromixing applications and for the integration into a lab-on-a-chip system.     

Numerical simulation of AC electrothermal micropump using a fully coupled model

The classic model by Ramos et al. for numerical simulation of alternating current electrothermal (ACET) flow is a decoupled model based on an electrothermal force derived using a linear perturbation method, which is not appropriate for the applications, where Joule heating is large and the effect of temperature rise on material properties cannot be neglected. An electrically–thermally–hydrodynamically coupled (fully coupled) ACET flow model considering variable electrical and thermophysical properties of the fluids with temperature was developed. The model solves AC electrical equations and is based on a more general electrostatic force expression. Comparisons with the classic decoupled model were conducted through the numerical simulations of an ACET micropump with asymmetric electrode pairs. It was found that when temperature rise is small the fully coupled model has the same results with the classic model, and the difference between the two models becomes larger and larger with the increasing temperature. The classic decoupled model underestimates the maximum temperature rise and pumping velocity, since it cannot consider the increase in electrical conductivity and the decrease in viscosity with temperature. The critical frequencies where the lowest velocity occurs or pumping direction reverses are shifted to higher frequencies with the increasing voltage according to the fully coupled model, while are kept unchanged according to the classic model.     

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.     

The effect of electrode height on the performance of travelling-wave electroosmotic micropumps

Arrays of microelectrodes for pumping electrolytes in microchannels are currently under research. In this letter we demonstrate theoretically an enhancement in the performance of arrays of microelectrodes subjected to travelling wave potentials. Net fluid velocity can increase up to 2.7 times when the electrodes are raised above the channel bottom, proving that the viscous friction on the walls between electrodes is a key factor to account for in the design of microelectrode arrays. Finally, we perform a comparison with previous results for the asymmetric step electrode array. It is found that the maximum velocity for the travelling wave array is roughly twice that for the asymmetric step array.     

交流电渗微泵流速影响因素的研究

影响交流电渗微泵流速的因素很多,如电极参数、驱动信号参数、溶液的导电率等。通过分析非对称电极交流电渗驱动理论,揭示了交流电渗微泵非对称电极表面流速的计算方法。在分析电极表面流速计算方法的基础上,探讨了以上参数对交流电渗微泵流速的影响规律,这对设计非对称电极交流电渗微泵、驱动信号参数的选择及溶液性质的影响具有指导意义。     

Stable SOI micromachined electrostatic AC voltage reference

We have designed and manufactured a micromachined moving plate capacitor to be used as an AC voltage reference in electrical metrology. The reference is based on the characteristic AC current–voltage curve of the component having a maximum, the value of which ideally depends only on the geometry of the component and material properties of single crystalline silicon. The electrode surface stability is essential in this application and hence a new fabrication process has been developed to metallize both surfaces of an electrostatically actuated micromachined structure. The stability of the AC reference voltage at a frequency of 100 kHz and an RMS voltage value 6.4 V was measured to be ±60 ppm over 14 h.     

Short focal length tunable liquid crystal lenticular lens array based on fringe field effect

A liquid crystal (LC) lenticular lens array based on fringe field effect is proposed. The gradient refractive index (GRIN) profile can be generated in the LC layer because of the fringe field between the strip‐shaped electrodes and the bottom electrode. The proposed LC lenticular lens array possesses ideal lens‐like phase profile and shortest focal length (1.199 mm) when the driving voltage is 5.4 V. The focal length can be tuned with millisecond response time by changing the driving voltage of the proposed LC lenticular lens array. The rise time τ_rise and decay time τ_decay of the proposed LC lenticular lens array are 162 and 94 ms, respectively.     

Design, simulation and fabrication of piezoelectric micro generators for aero acoustic applications

Energy harvesters based on acoustic vibration sources can generate electrical power through piezoelectric transduction. Significant miniaturization of electro mechanical devices using MEMS fabrication technology has encouraged the creation of portable, miniature energy harvesters. A niche application is aero acoustics, where wasted, high dB and high frequency sound generated by aircrafts are transformed into useful energy. Having self-powered, miniature acoustic sensors allows noise detection monitoring systems to be self-sustaining. This paper illustrates an Aluminium doped Zinc Oxide (AZO) cantilever beam on stainless steel substrate with a top copper electrode. Design and finite element modelling of the design was conducted using Coventorware™. The AZO piezoelectric thin film was RF-sputtered on the stainless steel substrate. Characterizations were performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to evaluate the piezoelectric qualities and surface morphology, respectively. Experimental measurements indicate approximately 345.4 mV AC output voltage (open circuit voltage) is produced at vibration frequencies of 30 kHz. This is in accordance with the Coventorware™ simulation results. This measured power level is sufficient to power a miniature wireless acoustic sensor nodes to monitor noise generated by aircrafts.     

Design, fabrication and characterization of a high fill-factor micromirror array for wavelength selective switch applications

This paper presents the design, fabrication and characterization of a high fill-factor micromirror array in application of wavelength selective switch (WSS). The micromirror array consists of 52 independent micromirrors. Each micromirror is composed of a cantilever-type micromirror plate (800 μm × 120 μm) with a bumper and an eight-terraced bottom electrode with a limiting plane. A cantilever beam is designed to obtain the rotation angle of micromirror plate and achieve a high fill-factor for the micromirror array. Meanwhile, the bumper and limiting plane are used to prevent the damage possibly caused by the pull-in effect or some vibration instance. An eight-terraced electrode is utilized for reducing the driving voltage. The micromirror array with a high fill-factor in excess of 97% has been successfully achieved using the bulk micromachining technologies. The measured static and dynamic characteristics show that the micromirror can achieve a maximal rotation angle of 0.87° with a Direct Current (DC) driving voltage of 156 V. The turn-on responding time is 0.57 ms, and the turn-off responding time is 4.36 ms. Furthermore micromirror plate can be easily released from the pull-in state without damaged due to the novel bumper design. The switching function between the two output ports of a WSS optical system has also been demonstrated.     

交流电沉积实现微纳器件定向连接研究

使用交流电化学方法定向制备了预制微电极之间的电连接。通过调节交流电压与偏置直流电压幅值可以控制电连接的生长方向。如果施加的交流电压幅值高于生成电连接的电压阈值,并且该值恒定时,频率越小,浓度越大,电连接晶体越粗壮;而当电解液浓度与频率恒定时,电压幅值变化对形貌影响较小。有限元仿真模拟进一步说明：当施加的交流电压高于阈值时,电极处于交流电渗流上游。电极扩散层厚度增加将诱导电极之间电连接晶体的生长。在交流电压上叠加直流偏置电压时,电连接晶体从偏置电压相对较负的一端向另一端生长。伏安特性测试结果表明：该连接具有优异的接触特性。