Showing papers in "Journal of Micromechanics and Microengineering in 2005"
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TL;DR: In this paper, the authors report the progress on the recent development of micromixers and present different types and designs of active and passive MCMs, as well as the operation points of the MCMs.
Abstract: This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.
1,651 citations
TL;DR: In this article, the effect of microstructure on microcantilever bending stiffness was examined using an isotropic Hooke's law constitutive relationship, compared to a model based upon a micropolar elasticity constitutive model.
Abstract: This work examines the effect of microstructure upon microcantilever bending stiffness. An existing beam theory model, based upon an isotropic Hooke's law constitutive relationship, is compared to a model based upon a micropolar elasticity constitutive model. The micropolar approach introduces a bending stiffness relation which is a function of any two independent elastic constants of the Hooke's law model (e.g., the elastic modulus and the Poisson's ratio), and an additional material constant (called γ). A consequence of the additional material constant is the prediction of an increased bending stiffness as the cantilever thickness decreases—a stiffening due to the material microstructure which becomes measurable at micron-order thicknesses. Polypropylene microcantilevers, which have a non-homogeneous microstructure due to their semi-crystalline nature, were fabricated via injection molding. A nanoindenter was used to measure their stiffness. The nanoindenter-determined stiffness values, which include the effect of the additional micropolar material constant, are compared to stiffness values obtained from beam theory. The nanoindenter stiffness values are seen to be at least four times larger than the beam theory stiffness predictions. This stiffening effect has relevance in future MEMS applications which employ materials with non-homogeneous microstructures instead of the conventional MEMS materials (e.g., silicon, silicon nitride), which have a very uniform microstructure.
741 citations
TL;DR: In this paper, the performance of microelectromechanical systems (MEMS) actuators and sensors as a function of operating principle is compared with each other and with equivalent macroscopic devices.
Abstract: This paper presents an exercise in comparing the performance of microelectromechanical systems (MEMS) actuators and sensors as a function of operating principle. Data have been obtained from the literature for the mechanical performance characteristics of actuators, force sensors and displacement sensors. On-chip and off-chip actuators and sensors are each sub-grouped into families, classes and members according to their principle of operation. The performance of MEMS sharing common operating principles is compared with each other and with equivalent macroscopic devices. The data are used to construct performance maps showing the capability of existing actuators and sensors in terms of maximum force and displacement capability, resolution and frequency. These can also be used as a preliminary design tool, as shown in a case study on the design of an on-chip tensile test machine for materials in thin-film form.
395 citations
TL;DR: In this article, a disc-type electrode was introduced to reduce the taper of the machining depth and 3D micro structures including a hemisphere with 60 µm diameter were fabricated by electrochemical milling.
Abstract: In this paper, electrochemical machining (ECM) for fabricating micro structures is presented. By applying ultra short pulses, dissolution of a workpiece can be restricted to the region very close to an electrode. Using this method, 3D micro structures were machined on stainless steel. Good surface quality of the structures was obtained in the low concentration electrolyte, 0.1 M H2SO4. In ECM, when the machining depth increases, structures taper. To reduce the taper, a disc-type electrode is introduced. By electrochemical milling, various 3D micro structures including a hemisphere with 60 µm diameter were fabricated.
234 citations
TL;DR: In this paper, the authors present an analysis and simulations for the dynamics of electrically actuated microbeams under secondary resonance excitations, and they show that, once the subharmonic resonance is activated, all frequency response curves reach pull-in, regardless of the magnitude of the ac forcing.
Abstract: We present an analysis and simulations for the dynamics of electrically actuated microbeams under secondary resonance excitations. The presented model and methodology enable simulation of the transient and steady-state dynamics of microbeams undergoing small or large motions. The microbeams are excited by a dc electrostatic force and an ac harmonic force with a frequency tuned near twice their fundamental natural frequencies (subharmonic excitation of order one-half) or half their fundamental natural frequencies (superharmonic excitation of order two). In the case of superharmonic excitation, we present results showing the effect of varying the dc bias, the damping and the ac excitation amplitude on the frequency–response curves. In the case of subharmonic excitation, we show that, once the subharmonic resonance is activated, all frequency–response curves reach pull-in, regardless of the magnitude of the ac forcing. We conclude that the quality factor has a limited influence on the frequency response in this case. This result and the fact that the frequency–response curves have very steep passband-to-stopband transitions make the combination of a dc voltage and a subharmonic excitation of order one-half a promising candidate for designing improved high-sensitive RF MEMS filters. For both excitation methods, we show that the dynamic pull-in instability can occur at an electric load much lower than a purely dc voltage and of the same order of magnitude as that in the case of primary-resonance excitation.
215 citations
TL;DR: In this paper, a simple computationally efficient closed-form model has been developed to determine the pull-in voltage of a cantilever beam actuated by electrostatic force, which is based on a linearized uniform approximate model of the nonlinear electrostatic pressure and the load deflection model.
Abstract: A simple computationally efficient closed-form model has been developed to determine the pull-in voltage of a cantilever beam actuated by electrostatic force. The approach is based on a linearized uniform approximate model of the nonlinear electrostatic pressure and the load deflection model of a cantilever beam under uniform pressure. The linearized electrostatic pressure includes the electrostatic pressure due to the fringing field capacitances and has been derived from Meijs and Fokkema's highly accurate empirical expression for the capacitance of a VLSI on-chip interconnect. The model has been verified by comparing the results with published experimentally verified 3D finite element analysis results and also with results from similar closed-form models. The new model can evaluate the pull-in voltage for a cantilever beam with a maximum deviation of ±2% from the finite element analysis results for wide beams, and a maximum deviation of ±1% for narrow beams (extreme fringing field).
200 citations
TL;DR: In this paper, a continuum simulation of polymer flow during nano-imprint lithography (NIL) is presented, and three parameters can predict polymer deformation mode: cavity width to polymer thickness ratio, polymer supply ratio and capillary number.
Abstract: This paper presents continuum simulations of polymer flow during nanoimprint lithography (NIL). The simulations capture the underlying physics of polymer flow from the nanometer to millimeter length scale and examine geometry and thermophysical process quantities affecting cavity filling. Variations in embossing tool geometry and polymer film thickness during viscous flow distinguish different flow driving mechanisms. Three parameters can predict polymer deformation mode: cavity width to polymer thickness ratio, polymer supply ratio and capillary number. The ratio of cavity width to initial polymer film thickness determines vertically or laterally dominant deformation. The ratio of indenter width to residual film thickness measures polymer supply beneath the indenter which determines Stokes or squeeze flow. The local geometry ratios can predict a fill time based on laminar flow between plates, Stokes flow, or squeeze flow. A characteristic NIL capillary number based on geometry-dependent fill time distinguishes between capillary- or viscous-driven flows. The three parameters predict filling modes observed in published studies of NIL deformation over nanometer to millimeter length scales. The work seeks to establish process design rules for NIL and to provide tools for the rational design of NIL master templates, resist polymers and process parameters.
195 citations
TL;DR: In this article, a nonlinear mechanism manifesting a voltage threshold is responsible for both contact angle saturation and the observed clamping of the electromechanical force on liquid volumes on a substrate.
Abstract: Electrowetting on dielectric-coated electrodes involves two independently observable phenomena: (i) the well-known voltage dependence of the apparent contact angle and (ii) a central electromechanical force that can be exploited to move and manipulate small liquid volumes on a substrate. The electromechanical force does not depend upon field-mediated changes of the contact angle; it is operative even if the liquid surface is constrained. Forces on liquid volumes may be determined using capacitance or the Maxwell stress tensor with no reference made to liquid surface profiles. According to this interpretation, a nonlinear mechanism manifesting a voltage threshold is responsible for both contact angle saturation and the observed clamping of the electromechanical force.
192 citations
TL;DR: In this article, the authors discuss grating-based and interferometer-based mini- and micro-spectrometers, shows performances already reported, the trends, the potential, the limitations and approaches to obtain a sufficient optical performance, in terms of spectral resolution and throughput, for serving the majority of applications.
Abstract: Miniaturized free-field based optical microspectrometers have huge potential for application in industry, science, medicine, agriculture and biology. State-of-the-art is the micro-assembly of micromachined optical components on a mini-bench and the trend is towards fully integrated optical microsystems. Complete silicon IC compatible MEMS-based opto-electrical microsystems on a single chip may offer huge cost benefits in these potentially high-volume applications. On-chip integration does, however, impose limitations. The required process compatibility and limited choice of acceptable materials does not necessarily give optimum optical performance. Also, the dimensional downscaling is not generally an optical advantage. This overview discusses grating-based and interferometer-based mini- and microspectrometers, shows performances already reported, the trends, the potential, the limitations and approaches to obtain a sufficient optical performance, in terms of spectral resolution and throughput, for serving the majority of applications.
190 citations
TL;DR: In this paper, a fiber-optic Fabry-Perot interferometric pressure sensor was developed for medical use, which was used to detect changes in the internal pressure of the heart and aorta.
Abstract: We have developed a fiber-optic Fabry–Perot interferometric pressure sensor of 125 µm in diameter and a detection system for medical use. A Fabry–Perot cavity is formed at an optical fiber end. A deformation of the diaphragm of the Fabry–Perot cavity induced by pressure varies the cavity length. White light interferometry is used to avoid error and noise caused by bending of the optical fiber and fluctuation of the light source. The reflection light of the sensor cavity is detected by a commercial high-speed spectrometer. A pressure change has been detected by using the developed sensor system. Animal experiments using a goat have been carried out and dynamic pressure changes in the internal pressure of heart and aorta have been successfully monitored.
188 citations
TL;DR: In this article, a PDMS-membrane micropump with two one-way ball check-valves for lab-on-a-chip and microfluidic applications is presented.
Abstract: In this paper, we present a low-cost, PDMS-membrane micropump with two one-way ball check-valves for lab-on-a-chip and microfluidic applications. The micropump consists of two functional PDMS layers, one holding the ball check-valves and an actuating chamber, and the other covering the chamber and holding a miniature permanent magnet on top for actuation. An additional PDMS layer is used to cover the top magnet, and thereby encapsulate the entire device. A simple approach was used to assemble a high-performance ball check-valve using a micropipette and heat shrink tubing. The micropump can be driven by an external magnetic force provided by another permanent magnet or an integrated coil. In the first driving scheme, a small dc motor (6 mm in diameter and 15 mm in length) with a neodymium–iron–boron permanent magnet embedded in its shaft was used to actuate the membrane-mounted magnet. This driving method yielded a large pumping rate with very low power consumption. A maximum pumping rate of 774 µL min−1 for deionized water was achieved at the input power of 13 mW, the highest pumping rate reported in the literature for micropumps at such power consumptions. Alternatively, we actuated the micropump with a 10-turn planar coil fabricated on a PC board. This method resulted in a higher pumping rate of 1 mL min−1 for deionized water. Although more integratable and compact, the planar microcoil driving technique has a much higher power consumption.
TL;DR: In this article, a performance comparison between open-loop and closed-loop control strategies for micro electromechanical (MEMS) devices is presented, based on experimental results obtained using both open-and closedloop strategies and to address the comparative issues of driving and control for MEMS devices.
Abstract: From a controls point of view, micro electromechanical systems (MEMS) can be driven in an open-loop and closed-loop fashion. Commonly, these devices are driven open-loop by applying simple input signals. If these input signals become more complex by being derived from the system dynamics, we call such control techniques pre-shaped open-loop driving. The ultimate step for improving precision and speed of response is the introduction of feedback, e.g. closed-loop control. Unlike macro mechanical systems, where the implementation of the feedback is relatively simple, in the MEMS case the feedback design is quite problematic, due to the limited availability of sensor data, the presence of sensor dynamics and noise, and the typically fast actuator dynamics. Furthermore, a performance comparison between open-loop and closed-loop control strategies has not been properly explored for MEMS devices. The purpose of this paper is to present experimental results obtained using both open- and closed-loop strategies and to address the comparative issues of driving and control for MEMS devices. An optical MEMS switching device is used for this study. Based on these experimental results, as well as computer simulations, we point out advantages and disadvantages of the different control strategies, address the problems that distinguish MEMS driving systems from their macro counterparts, and discuss criteria to choose a suitable control driving strategy.
TL;DR: In this paper, a fully automated microfluidic system for the DNA amplification process by integrating an electroosmotic pump, an active micromixer and an on-chip temperature control system is presented.
Abstract: The present paper reports a fully automated microfluidic system for the DNA amplification process by integrating an electroosmotic pump, an active micromixer and an on-chip temperature control system. In this DNA amplification process, the cell lysis is initially performed in a micro cell lysis reactor. Extracted DNA samples, primers and reagents are then driven electroosmotically into a mixing region where they are mixed by the active micromixer. The homogeneous mixture is then thermally cycled in a micro-PCR (polymerase chain reaction) chamber to perform DNA amplification. Experimental results show that the proposed device can successfully automate the sample pretreatment operation for DNA amplification, thereby delivering significant time and effort savings. The new microfluidic system, which facilitates cell lysis, sample driving/mixing and DNA amplification, could provide a significant contribution to ongoing efforts to miniaturize bio-analysis systems by utilizing a simple fabrication process and cheap materials.
TL;DR: In this article, the wind-receptor hairs of crickets are used to detect the movement of the membranes of suspended SixNy membranes, and capacitance versus voltage, frequency dependence and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays.
Abstract: This paper presents the modelling, design, fabrication and characterization of flow sensors based on the wind-receptor hairs of crickets. Cricket sensory hairs are highly sensitive to drag-forces exerted on the hair shaft. Artificial sensory hairs have been realized in SU-8 on suspended SixNy membranes. The movement of the membranes is detected capacitively. Capacitance versus voltage, frequency dependence and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays, showing the viability of the concept.
TL;DR: In this article, the authors report solutions to the issues of profile control, microloading effect and suppression of the sidewall roughness of submicrometer trenches by modifying the regular conditions of the Bosch process that is often employed in the inductively coupled plasma (ICP) deep reactive ion etching (DRIE) system.
Abstract: This paper reports solutions to the issues of profile control, microloading effect and suppression of the sidewall roughness of submicrometer trenches by modifying the regular conditions of the Bosch process that is often employed in the inductively coupled plasma (ICP) deep reactive ion etching (DRIE) system. Additionally, under the modified processing conditions, a high efficient antireflection structure can be fabricated.
TL;DR: In this article, a droplet-based micro oscillating-flow PCR chip was designed and fabricated by the silicon microfabrication technique and three different temperature zones, which were stable at denaturation, extension and annealing temperatures, were integrated with a single, simple and straight microchannel to form the chip's basic functional structure.
Abstract: Polymerase chain reactions (PCR), thermally activated chemical reactions which are widely used for nucleic acid amplification, have recently received much attention in microelectromechanical systems and micro total analysis systems because a wide variety of DNA/RNA molecules can be enriched by PCR for further analyses. In the present work, a droplet-based micro oscillating-flow PCR chip was designed and fabricated by the silicon microfabrication technique. Three different temperature zones, which were stable at denaturation, extension and annealing temperatures and isolated from each other by a thin-wall linkage, were integrated with a single, simple and straight microchannel to form the chip’s basic functional structure. The PCR mixture was injected into the chip as a single droplet and flowed through the three temperature zones in the main microchannel in an oscillating manner to achieve the temperature maintenance and transitions. The chip’s thermal performance was theoretically analyzed and numerically simulated. The results indicated that the time needed for the temperature of the droplet to change to the target value is less than 1 s, and the root mean square error of temperature is less than 0.2 ◦ C. A droplet of 1 µ lP CR mixture with standard HPV (Human Papilloma Virus)-DNA sample inside was amplified by the present chip and the results were analyzed by slab gel electrophoresis with separation of DNA markers in parallel. The electrophoresis results demonstrated that the micro oscillating-flow PCR chip successfully amplified the HPV-DNA, with a processing time of about 15 min which is significantly reduced compared to that for the conventional PCR instrument. (Some figures in this article are in colour only in the electronic version)
TL;DR: In this paper, the authors report the characterization of a microfluidic packaging technique involving the use of press-fit interconnects to micro fluidic channels molded in PDMS.
Abstract: This paper reports the characterization of a microfluidic packaging technique involving the use of press-fit interconnects to microfluidic channels molded in PDMS. This packaging technique is implemented by, first, coring a small hole in the PDMS to access molded or buried microchannels using a modified 20 gauge needle; and second, inserting an unmodified needle into the hole to create a direct connection to the microchannel that requires no bonding or molding. The needles can then easily be removed and reinserted multiple times since the seal is created purely by the compression of the PDMS around the needle. The luer fitting on the needles can easily be connected to standard fluid fittings. The quality of the interconnects is correlated with observations of the PDMS after coring. Methods of coring examined include pushing straight through and twisting the coring tool by hand or by machine. These comparisons demonstrated that all methods can produce viable interconnects; however, machine coring was the most consistent. The interconnects were characterized mechanically primarily by measuring their leak resistance under pressure. Leak tests were performed on interconnects (1) fabricated using different methods, (2) experiencing rotation or bending and (3) fabricated at various linear densities. Static pressure testing revealed that interconnect pressure limits varied from 100 kPa to over 700 kPa depending on the fabrication method. Suggestions are presented on how the technique could be modified to reach much higher pressures. Interconnect flexibility testing demonstrated a minimum of 30° of bending and a maximum of 60° before failure depending on the direction rotated. Density testing showed that PDMS was strong enough to allow at least six interconnects on a 1 cm linear channel.
TL;DR: In this article, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally, and the model developed in part I formed the basis for the parameters studied experimentally in part II.
Abstract: In this two-part paper, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally. The electromechanical coupling coefficient is crucial to the performance of piezoelectric energy conversion devices. A membrane-type geometry is chosen for the study. In part I a one-dimensional model is developed for a membrane composed of two layers, a passive elastic material and a piezoelectric material. The lumped-parameter model is then used to explore the effect of design and process parameters, such as residual stress, substrate thickness, piezoelectric thickness and electrode coverage, on the electromechanical coupling coefficient. The model shows that the residual stress has the most substantial effect on the electromechanical coupling coefficient. For a given substrate material and thickness an optimum piezoelectric thickness can be found to achieve the maximum coupling coefficient. The substrate stiffness affects the magnitude of the maximum coupling coefficient that can be obtained. Electrode coverage was found to be important to electromechanical coupling. The model predicts an optimum electrode coverage of 42% of the membrane area. The model developed in part I formed the basis for the parameters studied experimentally in part II.
TL;DR: In this article, the influence of several coating parameters on the thickness and uniformity of the photoresist layer is investigated, and a model describing the dependence of the thickness on the major parameters is presented.
Abstract: In this paper, a new method of photoresist coating, direct spray coating, is studied. This method is especially suited to coat high topography surfaces for some special applications in microelectromechanical systems, radio frequency components and packaging. The most suitable photoresist type and coating process are found. The influence of several coating parameters on the thickness and uniformity of the photoresist layer is investigated. A model describing the dependence of the thickness on the major parameters is presented. Very promising results are obtained using spray coating for the fabrication of several three-dimensional structures.
TL;DR: In this paper, a new solder bonding method for the wafer level packaging of MEMS devices was reported, where the electroplated magnetic film was heated using induction heating causing the solder to reflow.
Abstract: This paper reports a new solder bonding method for the wafer level packaging of MEMS devices. Electroplated magnetic film was heated using induction heating causing the solder to reflow. The experiment results show that it took less than 1 min to complete the bonding process. In addition, the MEMS devices experienced a temperature of only 110 °C during bonding, thus thin film materials would not be damaged. Moreover, the bond strength between silicon and silicon wafer was higher than 18 MPa. The step height of the feed-through wire (acting as the electrical feed-through of the bonded region) is sealed by the electroplated film. Thus, the flatness and roughness of the electroplated surface are recovered by the solder reflow, and the package for preventing water leakage can be achieved. The integration of the surface micromachined devices with the proposed packaging techniques was demonstrated.
TL;DR: In this article, the elastic and failure mechanical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) MEMS structures were investigated via in situ direct and local displacement measurements by a method that integrates atomic force microscopy (AFM) with digital image correlation (DIC).
Abstract: The elastic and failure mechanical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) MEMS structures were investigated via in situ direct and local displacement measurements by a method that integrates atomic force microscopy (AFM) with digital image correlation (DIC). On-chip MEMS-scale specimens were tested via a custom-designed apparatus that was integrated with an AFM to conduct in situ uniaxial tension tests. Specimens 10 µm and 50 µm wide and of 1.5 µm average thickness were used to measure the elastic properties while 340 µm wide tension specimens with a central elliptical perforation resulting in a stress concentration factor of 27 were tested to investigate local effects on material strength. The Young's modulus, Poisson's ratio and tensile strength were measured as 759 ± 22 GPa, 0.17 ± 0.03 and 7.3 ± 1.2 GPa, respectively. In an effort to understand the effect of local defects and assess the true material strength, the local failure stress at sharp central elliptical notches with a stress concentration factor of 27 was measured to be 11.4 ± 0.8 GPa. The AFM/DIC method provided for the first time local displacement fields in the vicinity of microscale perforations and these displacement fields were in accordance with those predicted by linear elasticity.
TL;DR: In this article, the resonance frequency of the fundamental and four higher order modes of a silicon dioxide microcantilever is measured theoretically and experimentally, and an analytical solution to the eigenmodes of a multi-layered cantilever and verify its validity by comparison to finite element analysis as well as the experimentally obtained results.
Abstract: The resonance frequency of the fundamental and four higher order modes of a silicon dioxide microcantilever is measured. The effect on these modes of depositing a 400 nm gold coating is investigated theoretically and experimentally. We derive an analytical solution to the eigenmodes of a multi-layered cantilever and verify its validity by comparison to finite-element analysis as well as the experimentally obtained results. The temperature and pressure dependence of the resonance frequencies is investigated experimentally and found to be in good agreement with theoretical models. An experimentally obtained value for the temperature dependence of Young's modulus of elasticity for thermally grown SiO2 is presented.
TL;DR: In this paper, the authors model and analyze the deflections and motions of a shaped microbeam in a capacitive-based MEMS device and derive analytical expressions approximating the range of travel and pull-in voltage.
Abstract: We model and analyze the deflections and motions of a shaped microbeam in a capacitive-based MEMS device. The model accounts for the system nonlinearities including mid-plane stretching and electrostatic forcing. The differential quadrature method (DQM) is used to discretize the microbeam partial differential equation. It is shown that the use of 11 grid points in the DQM is sufficient to capture the response of the device. It is also observed that, unlike the shooting methods, DQM does not face the problems of system differential equations stiffness and solution sensitivity to the initial guess. The static response to a dc voltage is first determined to investigate the influence of varying the geometric parameters of the device on the range of travel and pull-in voltage. Analytical expressions approximating the range of travel and pull-in voltage, as functions of the capacitor gap size and microbeam width and thickness, are derived. Symmetric and asymmetric spatial distributions of these parameters are considered. For symmetric distribution, an increase (decrease) in the beam width and/or thickness at the middle with respect to those at the endpoints results in an increase (decrease) in the pull-in voltage and a decrease (increase) in the range of travel. An increase (decrease) in the gap size at the middle with respect to those at the endpoints results in an increase (decrease) in the pull-in voltage and an insignificant effect on the range of travel. The dynamic response of the microbeam to a dc voltage is also determined for various distributions of the microbeam width and thickness and the gap size. It is shown that decreasing the microbeam thickness at the middle is the most effective method to reduce the pull-in time.
TL;DR: In this article, a 3D vortex micromixer for micro-total-analysis-systems (μTAS) applications was proposed, which utilizes self-rotation effects to mix fluids in a circular chamber at low Reynolds numbers.
Abstract: This paper proposes a novel three-dimensional (3D) vortex micromixer for micro-total-analysis-systems (μTAS) applications which utilizes self-rotation effects to mix fluids in a circular chamber at low Reynolds numbers (Re). The microfluidic mixer is fabricated in a three-layer glass structure for delivering fluid samples in parallel. The fluids are driven into the circular mixing chamber by means of hydrodynamic pumps from two fluid inlet ports. The two inlet channels divide into eight individual channels tangent to a 3D circular chamber for the purpose of mixing. Numerical simulation of the microfluidic dynamics is employed to predict the self-rotation phenomenon and to estimate the mixing performance under various Reynolds number conditions. Experimental flow visualization by mixing dye samples is performed in order to verify the numerical simulation results. A good agreement is found to exist between the two sets of results. The numerical results indicate that the mixing performance can be as high as 90% within a mixing chamber of 1 mm diameter when the Reynolds number is Re = 4. Additionally, the results confirm that self-rotation in the circular mixer enhances the mixing performance significantly, even at low Reynolds numbers. The novel micromixing method presented in this study provides a simple solution to mixing problems in the lab-chip system.
TL;DR: In this article, an almost linear relationship between the Q-factor and the resonance frequency of the uncoated cantilevers is observed, implying that a higher sensitivity can be attained by actuation at higher order resonant modes.
Abstract: The resonance frequency and the Q-factor of the fundamental and higher order flexural modes of silicon dioxide microcantilevers have been characterized at different pressures and for different thicknesses of gold coating. We present the experimental results and discuss the effect of the gold film on the performance and sensitivity of the cantilevers when used as mass sensors. An almost linear relationship between the Q-factor and the resonance frequency of the uncoated cantilevers is observed, implying that a higher sensitivity can be attained by actuation at higher order resonant modes. We also find that even a thin gold coating may reduce Q-factors by more than an order of magnitude.
TL;DR: In this paper, a simple design is presented for an electro-thermally actuated microfabricated gripper capable of providing a piezoresistive read-out of the gripper deflection, which can be used to measure the forces applied to the grabbed object.
Abstract: Microfabricated grippers and tweezers are promising tools for manipulation of micro- and nanoscale objects. As with ordinary macroscale grippers, the ability to sense the forces involved in grabbing would be advantageous for controlling the operation as well as for measuring the mechanical properties of the grabbed object. A simple design is presented for an electro-thermally actuated microfabricated gripper capable of providing a piezoresistive read-out of the gripper deflection, which can be used to measure the forces applied to the grabbed object. Measurements of actuation of test devices are presented and found to be in reasonable agreement with expected values. Finally, piezoresistive measurements of the gripper deflection are demonstrated.
TL;DR: In this paper, Triton-x-100 surfactant was used as an additive to 25% TMAH in anisotropic etching of {1 1 0} silicon passive mirror planes.
Abstract: Etching characteristics and properties of {1 1 0} silicon crystal planes used as 45° optical mirrors for deflecting optical beams from/to optical fibers were investigated. Fiber aligning grooves and passive mirror-like planes were realized by wet micromachining of (1 0 0) silicon in KOH–IPA and TMAH–IPA systems. Implementation of Triton-x-100 surfactant as an additive to 25% TMAH in anisotropic etching of {1 1 0} silicon passive mirror planes is reported and discussed. It was found that Triton-x-100 contents in the range of 10–200 ppm to the 25% TMAH–water etchant significantly increase the anisotropy mostly by decreasing the {1 1 0} etch rate and retaining the {1 0 0} etch rate. It is also shown that {1 1 0} surface roughness is substantially improved compared to two other etching systems. Furthermore, efficient convex corner underetching reduction is demonstrated. The results of optical characterization of passive mirrors with 632 nm incident light show reduced scattering of reflected optical beam due to improved microroughness for mirrors made by TMAH–Triton. For the reflection of the optical beam with 1.33 µm and 1.54 µm wavelengths, sputtered layer of gold is used as reflective coating on silicon mirrors thus increasing the reflected optical beam intensity by an additional 8%.
TL;DR: In this paper, a laser-induced backside wet etching (LIBWE) was used for the fabrication of microfluidic microtrenches from glass chips using computer drawing software and then automatically translated into computer numerical control motion.
Abstract: Glass is an excellent material for use as a microfluidic chip substrate because it has great chemical and thermal stability. This work describes a flexible platform for the rapid prototyping of microfluidic chips fabricated from glass. A debris-free laser direct-writing technology that requires no photomask generation is developed. A 266 nm laser with a high repetition rate is employed in laser-induced backside wet etching (LIBWE) for glass machining. A microfluidic pattern is designed using computer drawing software and then automatically translated into computer numerical control motion so that the microtrench is directly fabricated on the glass chip. The overall machining speed can be increased by increasing the repetition rate to ~6 kHz. Without a clean room facility or the highly corrosive acid, HF, the overall development time is within hours. Trenches with complex structures that are hard to fabricate by photolithography were easily produced by laser direct-writing. An integrated microreactor/concentrator is demonstrated. The crack-free and debris-free surface was characterized by SEM and a surface profiler. Various effective etching chemicals for the LIBWE process were investigated to understand the etching mechanism. The minimal laser power used for glass etching was approximately 20 mW for a 6 µm wide microtrench. Several new compounds have been demonstrated to be effective in ablation. The etch threshold is minimum and does not decrease further as the unit length absorbance increases above 8000 in acetone solution.
TL;DR: In this article, the authors demonstrate that the parametric excitation of a microstructure by periodic (ac) voltages may have a stabilizing effect and permits an increase of the steady component of the actuation voltage beyond the pull-in value.
Abstract: Electrostatically actuated microstructures are inherently nonlinear and can become unstable. Pull-in instability is encountered as a basic instability mechanism. We demonstrate that the parametric excitation of a microstructure by periodic (ac) voltages may have a stabilizing effect and permits an increase of the steady (dc) component of the actuation voltage beyond the pull-in value. An elastic string as well as a cantilever beam are considered in order to illustrate the influence of fast-scale excitation on the slow-scale behavior. The main conclusions about the stability are drawn using the simplest model of a parametrically excited system described by Mathieu and Hill's equations. Theoretical results are verified by numerical analysis of microstructure subject to nonlinear electrostatic forces and performed by using Galerkin decomposition with undamped linear modes as base functions. The parametric stabilization of a cantilever beam is demonstrated experimentally.
TL;DR: By controlling the ratio of nitrogen to oxygen in the source gas as used in the CVD method, the ultimate nitrogen, carbon and fluorine concentrations in the film can be controlled and hence the dielectric constant of the film so produced is controlled.
Abstract: This paper describes a simple method for the microfabrication of mechanically compliant, magnetically-responsive microstructures. These microstructures were fabricated in one step by using a ferromagnetic photoresist, which, in turn, was prepared by suspending nickel nanospheres in a negative photosensitive epoxy (SU8). The nominal diameter of the nickel nanospheres was 80–150 nm, that is, much smaller than the wavelength of the UV light (365 and 405 nm) used to expose the photoresist. Diffraction and scattering of light from the nanospheres allowed for full exposure of the photoresist, even after the incorporation of nanospheres at levels at which it became opaque. The ferromagnetic photoresist was cross-linked after exposure and development, and yielded a stable, compliant, ferromagnetic pattern. The paper characterizes the effect of the weight density of the nickel nanospheres on the transmittance of films made by this technique at wavelengths from 330 nm to 610 nm. It also describes a number of microstructures made with the photoresist: examples include lines, posts and meshes. As a demonstration, the procedure was applied to the microfabrication of a set of magnetically-actuated micromirrors. These micromirrors achieved large deflections: deflection at the tip of a 12 mm long, 250 µ mw ide and 70 µm thick cantilever of the ferromagnetic photoresist exceeded 1.4 mm, when actuated by a NbFeB permanent magnet with field strength ∼120 mT. The cantilever maintained its mechanical properties after cycling ∼10 6 times. S This article has online supplementary material