Oscillating flow in a 2-D diffuser

Separating oscillating flows in an internal, adverse pressure gradient geometry are studied experimentally. Simultaneous velocity and pressure measurements demonstrate that the minor losses associated with oscillating flow in an adverse pressure gradient geometry can be smaller or larger than those for steady flow. Separation is found to begin high in the diffuser and propagate downward. The flow is able to remain attached further into the diffuser with larger Reynolds numbers, small displacement amplitudes, and smaller diffuser angles. The extent of separation grows with L ₀/h. The minor losses grow with increasing displacement amplitude in the measured range 10 < L ₀/h < 40. Losses decrease with increasing Re _δ in the measured range of 380 < Re _δ < 740. It is found that the losses increase with increasing diffuser angle over the measured range of 12° < θ < 30°. The nondimensional acoustic power dissipation increases with Reynolds number in the measured range and decreases with displacement amplitude.     

On the Development of a Mesoscale Friction Tester

MEMS and NEMS devices typically have a large surface area to volume ratio. As a result, a major concern in the development of such devices is friction. Contact radii in MEMS and NEMS devices are expected to range from 10⁻⁸<a<10⁻⁵ m. This regime, which generally lies between the limits of single asperity and macroscopic contact, has yet to be explored because the apparati used to characterize friction at these limits do not operate in the range of forces appropriate to these length scales. A Mesoscale Friction Tester (MFT) with smooth probe tip radii from 50 nm to 50 μm and capable of applying forces ranging from 10 nN to l mN over contact radii from 10 nm to 10 μm has been developed to address this need. With carefully planned experiments, this device has the potential to help answer unresolved questions regarding friction mechanisms in the mesoscale range.     

A New Method for Measuring Displacements of Micro Devices by an Optical Encoding System

Micro electro mechanical systems (MEMS) frequently require displacement measurements with high accuracy, a high sampling rate, and a long sensing travel. However, the available methods for measuring displacements of micro devices are typically limited in terms of at least one of these figures of merit. In this paper, we present a novel implementation of an optical encoding method for measuring displacements of micro devices that provides good capabilities in all of these figures of merit. The optical encoding system combines a commercial reading apparatus with a custom-made metal grating that can be easily produced during MEMS fabrication. Experimental tests demonstrate the ability of the system to measure displacements with a resolution of 25 nm and sampling rate of 1 MHz, under a variety of displacement rate functions.     

<Emphasis Type="Italic">In situ</Emphasis> Characterization of Nanomechanical Behavior of Free-standing Nanostructures

The present paper reports the development of an in situ nanotensilometer that enables highly reliable mechanical tensile testing on individual micro-/nano-scale structures. The device features independent measurement of force and displacement histories in the specimen with nanoNewton force and sub-nanometer displacement resolutions, respectively. Moreover, the device is well suited for in situ testing of free-standing micro/nano-structures within a high resolution scanning electron microscope (SEM), which permits continuous high-resolution imaging of the specimen during straining. The device is comprised of two main parts: (a) a three-plate capacitive transducer that doubles up both as an actuator and a force sensor, and (b) a commercially available nanomanipulator that facilitates transportation and positioning of nanoscale structures with sub-nanometer precision. In order to conduct the mechanical tests the ends of the specimen are attached to the probe tips of the device using ion-beam induced deposition. The general capabilities and features of the nanotensilometer are illustrated by presenting results of nanomechanical tensile tests on electrospun polyaniline microfibers.     

<Emphasis Type="BoldItalic">In-Situ</Emphasis> Mechanical Testing for Characterizing Strain Localization During Deformation at Elevated Temperatures

An experimental methodology has been developed to characterize local strain heterogeneities in alloys via in-situ scanning electron microscope (SEM) based mechanical testing. Quantitative measurements of local strains as a function of grain orientation, morphology and neighborhood are crucial for mechanistic understanding and validation of crystal plasticity models. This study focuses on the technical challenges associated with performing creep tests at elevated temperatures ≤700°C in an SEM. Samples of nickel-based superalloy Rene 104 were used for this study, but the technique is applicable to testing of any metal samples at elevated temperature. Electron beam lithography was employed to produce a suitable surface speckle pattern of hafnium oxide to facilitate full field displacement measurements using a commercial software package. The speckle pattern proved to have good thermal stability and provided excellent contrast for image acquisition using secondary electron imaging at elevated temperature. The speckle pattern and microscope magnification were optimized to obtain the resolution necessary to discern strain localizations within grain interiors and along grain boundaries. Minimum strain resolution due to SEM image distortions was determined prior to tensile testing, and image integration methods were utilized to minimize imaging artifacts. Limitations due to the present specimen heating method and potential solutions to these limitations are also addressed.     

A review of MEMS-based microscale and nanoscale tensile and bending testing

Thin films at the micrometer and submicrometer scales exhibit mechanical properties that are different than those of bulk polycrystals. Industrial application of these materials requires accurate mechanical characterization. Also, a fundamental understanding of the deformation processes at smaller length scales is required to exploit the size and interface effects to develop new and technologically attractive materials. Specimen fabrication, small-scale force and displacement generation, and high resolution in the measurements are generic challenges in microscale and nanoscale mechanical testing. In this paper, we review small-scale materials testing techniques with special focus on the application of microelectromechanical systems (MEMS). Small size and high force and displacement resolution make MEMS suitable for small-scale mechanical testing. We discuss the development of tensile and bending testing techniques using MEMS, along with the experimental results on nanoscale aluminum specimens.     

THE SURFACE EFFECT ON THE TENSILE STRENGTH OF MICROMACHINED POLYSILICON FILMS FOR MEMS

In order to accomplish reliable mechanical design of MEMS, the influences of surface roughness and octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on the mechanical properties of micromachined polysilicon films for MEMS are investigated. Surface effect on the fracture properties of micromachined polysilicon films is evaluated with a new microtensile testing method using a magnet-coil force actuator. Statistical analysis of the surface roughness effects on the tensile strength predicated the surface roughness characterization of polysilicon films being tested and the direct relation of the mechanical properties with the surface roughness features. The fracture strength decreases with the increase of the surface roughness. The octadecyltrichlorosilane self-assembled monolayers coating leads to an increase of the average fracture strength up to 32.46%. Surface roughness and the hydrophobic properties of specimen when coated with OTS films are the two main factors influencing the tensile strength of micromachined polysilicon films for MEMS.     

Experimental Study of the Dynamic Flexural Strength of Concrete

In this paper, we analyze the increase in the dynamic flexural strength of concrete according to strain rate. A simple beam with center-point loading and a classical electro-mechanical testing machine are used to determine the static flexural strength. The dynamic measurements are conducted using a split Hopkinson pressure bar (SHPB) device in the same three-point bending configuration. The outer faces of the beams are instrumented with strain gauges to record the extreme tensile strains. Moreover, full-field displacement measurements are obtained using digital image correlation (DIC) on images recorded by a very high-speed camera. The strain gauge and DIC measurements are compared and used to determine the onset of failure and to evaluate the rate-related tensile strength. Several tests are performed at strain rates in the range from 1/s to 15/s. As expected, a significant increase in the flexural tensile strength with strain-rate is observed, which is consistent with results from the literature.     

Effect of Pressure on Fluid Damping in MEMS Torsional Resonators with Flow Ranging from Continuum to Molecular Regime

High quality factor of dynamic structures at micro and nano scale is exploited in various applications of micro electro-mechanical systems (MEMS) and nano electro-mechanical system. The quality factor of such devices can be very high in vacuum. However, when vacuum is not desirable or not possible, the tiny structures must vibrate in air or some other gas at pressure levels that may vary from atmospheric to low vacuum. The interaction of the surrounding fluid with the vibrating structure leads to dissipation, thus bringing down the quality factor. Depending on the ambient fluid pressure or the gap between the vibrating and the fixed structure, the fluid motion can range from continuum flow to molecular flow giving a wide range of dissipation. The relevant fluid flow characteristics are determined by the Knudsen number which is the ratio of the mean free path of the gas molecule to the characteristic flow length of the device. This number is very small for continuum flow and reasonably big for molecular flow. In this paper, we study the effect of fluid pressure on the quality factor by carrying out experiments on a MEMS device that consists of a double gimbaled torsional resonator. Such devices are commonly used in optical cross-connects and switches. We only vary fluid pressure to make the Knudsen number go through the entire range of continuum flow, slip flow, transition flow, and molecular flow. We experimentally determine the quality factor of the torsional resonator at different air pressures ranging from 760 Torr to 0.001 Torr. The variation of this pressure over six orders of magnitude ensures required rarefaction to range over all flow conditions. Finally, we get the variation of quality factor with pressure. The result indicates that the quality factor, Q, follows a power law, Q ∝P ^–r , with different values of the exponent r in different flow regimes. In the second part of the paper, we propose the use of effective viscosity for considering velocity slip conditions in solving Navier–Stokes equation numerically. This concept is validated with analytical results for a simple case and then compared with the experimental results presented in this paper. The study shows that the effective viscosity concept can be used effectively even for the molecular regime if the air-gap to length ratio is sufficiently small (h ₀/L<0.01). As this ratio increases, the range of validity decreases.     

Evaluation of the Material Properties of an OFHC Copper Film at High Strain Rates Using a Micro-Testing Machine

The material properties of an oxygen-free high thermal conductivity (OFHC) film with a thickness of 0.1 mm were evaluated at strain rates ranging from 10⁻³/s to 10³/s using a high-speed material micro-testing machine (HSMMTM). The high strain-rate material properties of thin films are important especially for an evaluation of the structural reliability of micro-formed parts and MEMS products. The high strain-rate material testing methods of thin films, however, have yet to be established to the point that the testing methods of larger specimens for electronics, auto-body, train, ship, and ocean structures are. For evaluation, a new type of HSMMTM was developed to conduct high-speed tensile tests of thin films. This machine is capable of testing at a sufficiently high tensile speed with an electromagnetic actuator, a novel gripping mechanism, and an accurate load measurement system. The OFHC copper film shows high strain-rate sensitivity in terms of the flow stress, fracture elongation, and strain hardening. These measures increase as the tensile strain rate increases. The rate-dependent material properties of an OFHC copper film are also compared with those of a bulk OFHC copper sheet with a thickness of 1 mm. The flow stress of an OFHC copper film is relatively low compared to that of a bulk OFHC copper sheet in the entire range of strain rates, while the fracture elongation of an OFHC copper film is much larger than that of a bulk OFHC copper sheet. A quantitative comparison would provide material data at high strain rates for the design and analysis of micro-appliances and different types of micro-equipment.     

Dynamic Analysis of Multi-Beam MEMS Structures for the Extraction of the Stress-Strain Response of Thin Films

Freestanding MEMS structures made of two long connected beams from different materials are fabricated and released in order to extract the stress-strain properties of thin films. The first material, named actuator, contains a high internal tensile stress component and, when released, pulls on the other beam. The strain in the beams is calculated based on the measurement of the displacement with respect to the reference configuration using scanning electron microscopy. The stress is estimated using two different methods. The first method, already reported, is based on the displacement of the actuator and the knowledge of its internal stress. The method which constitutes the novelty of the present study is based on the dynamic analysis of the multi-beam structures, and the determination of the stress value that corresponds to the measured resonance frequencies. The dynamic analysis is performed via two different methods: (i) the modified Rayleigh–Ritz technique and (ii) the Euler–Bernoulli beam dynamics. Results are provided for palladium thin films which deform plastically and for monocrystalline silicon thin films, exhibiting a purely elastic behavior. The results show the higher accuracy of the dynamic measurements for the estimation of the stress compared to the static method. The dynamic measurements also show that the Rayleigh–Ritz technique tends to give a higher bound for the resonance frequencies compared to the Euler–Bernoulli technique. This dynamic method extends the potential of this on-chip material testing technique which can also be adapted to stress controlled sensors applications.     

Design and fabrication of a micro thermal actuator for cellular grasping

The development of a novel polymer-based micro robotic gripper that can be actuated in a fluidic medium is presented in this paper. Our current work is to explore new materials and designs for thermal actuators to achieve micromanipulation of live biological cells. We used parylene C to encapsulate a metal heater, resulting in effectively a tri-layered thermal actuator. Parylene C is a bio-compatible dielectric polymer that can serve as a barrier to various gases and chemicals. Therefore, it is suitable to serve as a thermal/electrical/chemical isolation material for protecting the metal heater from exposing to an aqueous environment. We have demonstrated parylene actuators(2 mm&#215;100/μm&#215;0.5μm) to operate in an aqueous environment using 10 to 80 mW input power. The temperature of these actuators at full deflection was estimated to be～ 60℃, which is much lower than the typical requirement of &gt; 100℃ to actuate other conventional MEMS actuators. Danio rerio follicles in fluidic medium were captured successfully using these actuators. Moreover, these actuators were found to be responsive to moderate rise in environmental temperature, and hence, we could vary the fluidic medium temperature to actuate trimorphs on a chip without any input of electrical energy, i.e., raising the fluidic temperature from 23℃ to 60℃ could actuate the trimorphs to grasp follicles of～1mm size in diameter. At 60℃, the embryos inside the follicles were observed to be alive, i.e., they were still moving in the biological fluid isolated by the follicle membrane. The smallest follicles grasped were～500μm in diameter using 800μm&#215;130μm&#215;0.6μm actuators. The fabrication process, modeling, and optimization of the trimorph actuators are presented. Based on the successful operation of these polymer-based actuators, we are currently developing multifinger thermal microgrippers for cellular grasping and manipulation, which can potentially be hybridly integrated with circuits for computer control.     

The effect of chamber length and Reynolds number on jet precession

The jet axial velocity field exiting from a nozzle/chamber configuration with an expansion ratio of 5 is investigated using Stereo-PIV for a range of chamber lengths and Reynolds (Re) numbers. The jet can exit the chamber in axial jet (AJ) mode with the maximum velocity near the chamber axis or precessing jet (PJ) mode with the maximum velocity near the chamber wall and rotating or precessing about the chamber axis. Algorithms were developed to determine the jet mode from exit conditions and allow conditional averaging of the velocity field in PJ mode. The probability of the jet in PJ mode was found to be a strong function of chamber length, L/D and only a mild function of Re for Re > 10,000. High precession probability was found for chambers of length in the range 2 < L/D < 2.75 for all cases for Re > 10,000. An abrupt reduction in precession probability occurred for chamber lengths L/D~3. For increasing chamber lengths, an increase in precession probability was observed. The ratio of entrainment-into-the-chamber of surrounding fluid to jet exit fluid was found not to be a function of Re or jet mode (AJ or PJ) but only a function of L/D. A maximum ratio entrainment-into-the-chamber was observed to occur in the range 2 < L/D < 2.5. Conditionally averaged velocity profiles also showed the exiting jet to be a strong function of L/D and with only a mild effect of Re for all cases of Re > 10,000.     

A microsystem for the fracture characterization of polysilicon at the micro-scale

To characterize the effective fracture energy G_IC of polysilicon wafers at room temperature, an on-chip MEMS test structure has been designed and fabricated. The device can provide fatigue pre-cracking at the notch apex and subsequently impose a monotonical load up to failure. The proposed procedure combines the experimental data with outcomes of numerical simulations. A continuously monitored decrease in stiffness of the system is linked to the crack length and the effective fracture energy for the non-standard geometry of the testing device. An average value of G_IC = 12.0 ± 1.8 N m^?1 is found. These values are then used in numerical micro-scale fracture analyses taking into account the material heterogeneity due to the grain structure and reproducing the crack propagation process with a cohesive approach.     

Turbulent free convection in a vertical converging channel

This paper presents the results of experimental and numerical investigations of the problem of turbulent natural convection in a converging-plate vertical channel. The channel has two isothermally heated inclined walls and two adiabatic vertical side walls. The parameters involved in this study are the channel geometry represented by the channel width at exit, the inclination of the heated walls and the temperature difference between the heated walls and the ambient. The investigation covered modified Rayleigh numbers up to 10⁸ in the computational study and up to 9.3 × 10⁶ in the experimental work. The experimental measurements focused on the velocity field and were carried out using a PIV system and included measurements of the mean velocity profiles as well as the root-mean-square velocity and shear stress profiles. The experiments were conducted for an inclination angle of 30°, a gap width of 10 mm and two temperature differences (∆T=25.4°C and 49.8°C). The velocity profiles in the lower part of the channel indicated the presence of two distinct layers. The first layer is adjacent to the heated plate and driven by buoyancy forces while the second layer extends from the point of maximum velocity to the channel center plane and driven mainly by shear forces. The velocity profile at the upper portion of the channel has shown the merging of the two boundary layers growing over the two heated walls. The measured values of the Reynolds shear stress and root mean square of the horizontal and vertical velocity fluctuation components have reached their maximum near the wall while having smaller values in the core region. The computational results have shown that the average Nusselt number increases approximately linearly with the increase of the modified Rayleigh number when plotted on log–log scale. The variation of the local Nusselt number indicated infinite values at the channel inlet (leading edge effect) and high values at the channel exit (trailing edge effect). For a fixed value of the top channel opening, the increase of the inclination angle tended to reduce flow velocity at the inlet section while changing the flow structure near the heated plates in such a way to create boundary-layer type flow. The maximum value of the average Nusselt number occurs when θ = 0 and decreases with the increase of the inclination angle. On the other hand, the increase of the channel width at exit for the same inclination angle caused a monotonic increase in the flow velocity at the channel inlet.     

Scalar dispersion in the near wake of a simplified model car

Experimental results on tracer gas diffusion within the near wake of a simplified model car (Ahmed model with a rear slant angle of 25°) are presented. Pollutant emission is simulated using heated air injected through a small pipe at one side of the model base. Fine cold wire thermometry is used to measure instantaneous temperature excess in the near wake. Characteristics of the temperature field over the Reynolds number range (1.3×10⁴<Re _L<7×10⁴) show strong differences as a result of transition in the wake at a critical Reynolds number Re _Lc=2.7×10⁴.     

Experimental study of compressible boundary layer on a cone at angles of attack

Experimental study was conducted for boundarylayers on a sharp 5° half-angle cone of 400mm length at angles of attack. The model was tested in the T-326 hypersonic wind tunnel （ITAM） at freestream Mach number M = 5.95. Mean and fluctuation wall characteristics of the boundary layer are measured at 0°, 2°, 3° and 4° angles of attack for different stagnation pressures. Pulsation measurements are carried out by means of ALTP sensor. Pressure and temperature distributions along the model are obtained, and transition beginning and end locations have been found. Boundary layer stabilization with the increase of angle of attack and the decrease of stagnation pressure is observed. High frequency pulsations inherent to hypersonic boundary layer （second mode） have been detected.     

Numerical study of mixed convection and conduction in a 2-D square ventilated cavity with an inlet at the vertical glazing wall and outlet at the top surface

A steady state numerical study of combined laminar mixed convection and conduction heat transfer in a ventilated square cavity is presented. The air inlet gap is located at the bottom of a vertical glazing wall and air exits the cavity via a gap located at the top surface. Three locations for the opening at the top surface: left (case a), center (case b) and right side (case c) are considered. All the remaining surfaces are considered adiabatic. The mass, momentum and energy conservation equations were solved using the finite volume method for different Rayleigh numbers in the interval of 10⁴ < Ra < 10⁶ and Reynolds number in the interval of 100 < Re < 700. Temperature, flow field, and heat transfer rates are analyzed. The effect of the interaction between ambient conditions outside the glazing and the air inlet gap at the bottom for different air outlet gap positions at the top surface modifies the flow structure and temperature distribution of the air inside the cavity. The Nusselt number as a function of the Reynolds number was determined for the three cases. It was found that configuration for case (a) removes a higher amount of heat entering the cavity compared to cases (b) and (c). This is due to the short distance between the main stream and the glass wall surface. Thus, the forced airflow entering the cavity is assisted by the buoyancy forces, and most of the cavity remains at the inlet flow temperature, which should be appropriate for warm climates. These results may provide useful information about the heat transfer and fluid flow for future studies.     

Development of high-temperature microsample testing

Microsample tensile testing has been established as a means of evaluating the room temperature mechanical properties of specimens with gage sections that are tens to hundreds of microns thick and several hundred microns wide. The desire to characterize the mechanical response of materials at elevated temperatures has motivated the development of high-temperature microsample testing that is reported here. The design of specially insulated grips allows the microsamples to be resistively heated using approximately 2 V DC and currents ranging between 2 to 6 A. An optical pyrometer with nominal spot size of 290 μm and 12 μm diameter type K thermocouples was employed to measure and verify the temperature of the microsamples. The ability of the pyrometer to accurately measure temperature on microsamples of different thicknesses and with slightly different emissivities was established over a temperature range from 400°C to 1100°C. The temperature gradient along the length and thickness of the microsample was measured, and the temperature difference measured in the gage section used for strain measurements was found to be less than 6.5°C. Examples of elevated temperature tensile and creep tests are presented.     

MEMS惯性器件敏感电容结构相关温度漂移特性仿真

基于MEMS器件的微型惯导系统的精度和MEMS惯性器件的全温稳定性具有很高的相关性.MEMS结构相关的温度漂移主要来自材料之间的热失配应力,工艺引入的应力,以及封装应力等.而相关应力在MEMS结构中的分布以及所造成的应变又和MEMS结构具有一定相关性.通过ANSYS有限元分析软件建立了多种MEMS惯性器件常用梁-质量块结构的FEM模型,具体包括悬臂梁结构、双端固支梁结构、L形梁结构、对角支撑梁结构.通过热-力耦合仿真,研究了热失配应力在上述结构中的分布以及所产生的结构变形.对比分析了不同芯片粘胶形式,包括中心粘胶、三点粘胶、整片粘胶对上述MEMS结构引入的封装应力以及其全温(-40℃～60℃)温度漂移特性.此外,还分析研究了不同衬底厚度对MEMS结构封装应力的隔离效果.