Micro-machining by mechanical machine tool, or micro mechanical machining, is an emerging fabrication technology that broadens applicable material ranges including metals and plastics. One of the problems in this micro machining, however, is relatively large run-out which is comparable to the diameter of tools. A series of experiments were conducted to identify Total Indicator Reading (TIR) and dynamic run-out of micro-machining on aluminum workpieces. Results presented in this paper indicate the ratio of run-out to tool diameter ranged between 2.2% and 10%. To compensate for the run-out in the design stage of micro features, a Java-based Design for Manufacturing (DFM) tool was developed. This tool can assist designers to design micro channels accurately considering the effect of run-out. Furthermore, the feasibility of micro-injection molding was examined with micro-machined channels. Micro-molded parts were made from ABS plastic showing negligible shrinkage from the molding process.

This paper discusses several different approaches to increase the energy efficiency and decrease the environmental impact of space-conditioning systems. The use of microchannel components and hydronic coupling is presented as a method to drastically reduce the size and refrigerant inventories of the refrigerant-carrying components of vapor-compression heat pumps. Design aspects of heat pumps using carbon dioxide, a natural refrigerant with minimal environmental impact, are discussed, and novel component geometries that offer compactness are presented. The advantages of absorption heat pumps using waste heat and natural gas are discussed, and innovative component designs are presented. It is believed that these innovations will hasten the commercialization of these environmentally benign alternatives to CFC- and HCFC-based vapor-compression systems. The environmental benefits of waste heat-driven absorption chillers are quantified in terms of the energy savings, greenhouse gas emission reductions, and installed electric power reductions. Ground coupling of these heat pumps is also discussed, with specific examples of the performance improvement over similar air-coupled heat pumps.

Experimental data on heat transfer and pressure drop during near-critical evaporation of carbon dioxide (CO 2 ) in a microchannel aluminium tube are presented. The test tube had 25 circular ports/channels with 0.79 mm inner diameter. The CO 2 -side heat transfer coefficient was determined by a modified Wilson plot technique. Measured data were compared to correlations available in the literature. For evaporating heat transfer and pressure drop, common correlations did not correspond well to the test data. Under some conditions, the evaporating heat transfer coefficient dropped significantly as the vapour fraction increased. This critical heat flux effect was most pronounced for high heat and mass flux, and high evaporation temperature. Different system designs able to reduce the possible negative effects of critical heat flux are discussed. Negative effects of the phenomenon may be highly reduced if the effect is taken into account when designing the components and the system.

Novel air-coupled and hydronically coupled heat pumps using microchannel components were investigated in this study. The air-coupled system uses microchannel tube, multilouver fin heat exchangers as the evaporator and condenser. In the hydronically coupled heat pump, refrigerant in the evaporator as well as the condenser transfers heat to an intermediate fluid such as an ethyleneglycol solution. The glycol loops are connected to the indoor/outdoor air through liquid-air heat exchangers. Models to simulate cycle thermodynamics, and single- and two-phase heat transfer in the components were developed to design these systems for cooling and heating mode operation. The components were optimized to develop the most compact systems that would satisfy system performance requirements. These systems were also compared with a conventional round-tube, plate-fin heat pump, which was designed using a commercially available simulation tool. Results from this study show that indoor and outdoor units of air-coupled microchannel systems can be packaged in only one-half and one-third the space required for a conventional system. Even more compact refrigerant heat exchangers are required in the hydronically coupled system, because of the high heat transfer coefficients for these liquid-coupled heat exchangers, and the counterflow orientation. The hydronic coupling offers flexibility in system location, and is well suited for integrated space-conditioning and water heating systems. Both air-coupled and hydronically coupled systems result in significant reductions in refrigerant inventories compared to round-tube systems. The refrigerant charge of the microchannel air-coupled system is 20% less than that of the round-tube heat pump. For the hydronically coupled system, the refrigerant charge is only 10% of the charge in the round-tube heat pump.

Cooling of electronic components is often limited by space availability and power consumption. Capillary-pumped loops (CPL) are utilized to achieve a coolant circulation via self-activated capillarity (Faghri, 1995). However, CPL is extremely unstable due to a nonlinear feedback among capillarity, viscous force, and heat transfer. Conventional refrigeration theories, which usually allow a larger pressure jump due to an external pumping, failed to explain the CPL cycle. The dynamics of a flow through a CPL cooling cycle is investigated with a particular attention to the flow in the micro-channels. Full numerical approaches, by which multi-component multi-phase flows are solved, tend to obscure the engineers from identifying outstanding design parameters. Instead, one here adopts several simplified semi-analyitic approaches, namely, models based on single-phase flows with discrete zones of heating and cooling. The analytical and numerical results could explain some general tendencies in the dynamical features like dry-out and flooding. However, the problem still requires closer and more realistic modeling.

Turbulent transport and deposition of microscopic particles in commonly-used large, and micro-channels were investigated. The objective was to examine the suitability of the Reynolds-averaged Navier-Stokes (RANS)-type turbulence models, along with a Monte-Carlo Lagrangian particle tracking method that accounts for the stochastic particle-turbulent eddy interactions, for the modeling of aerosol transport when a multitude of forces act on the particles. The computer program KIVA-3 (Amsden et al., 1993) was modified and enhanced by adding several turbulence models, and including appropriate models for the effect of the following mechanisms on particle motion: drag, gravity, thermophoresis, Brownian dispersion, and shear-induced (Saffman) lift force. The effect of Brownian motion was modeled by including a random, white noise force term in the particle equation of motion. Parametric simulations were performed, leading to the following main observations. For the transport and deposition of microscopic particles, in addition to the turbulent dispersion, several other dispersion mechanisms were important. The k-ε and Reynolds-stress transport models provided similar predictions for sub-micron particles, but differed significantly for larger particles. The model provided physically consistent results with correct trends in all the simulations. The methodology, however, appears to be expensive in terms of computations, and further work is needed for streamlining the numerical solution methods and physical models.

Experimental investigation of a heat sink for electronics cooling is performed. The objective is to keep the operating temperature at a relatively low level of about 323–333K, while reducing the undesired temperature variation in both the streamwise and transverse directions. The experimental study is based on systematic temperature, flow and pressure measurements, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250μm. According to the objectives of the present study, Vertrel XF is chosen as the working fluid. Experiments on flow boiling of Vertrel XF in the microchannel heat sink are performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the two-phase results to a single-phase water flow.

An experimental set-up for pressure driven liquid flow through microchannels have been designed and tested. The flow rate is determined by tracking the free liquid surface in a precision bore hole using a laser distance meter. Measurements of the flow rate through silicon microchannels with a height of less than 0.9 μm show good results for Newtonian fluids (silicon oil, ethanol) at flow rates as low as 0.2 nl/s. The experimental results are also in very good agreement with predictions based on laminar channel flow using no-slip boundary conditions, indicating that standard macroscopic assumptions are still valid for these fluids under these conditions. However, experiments with aqueous solutions show anomalies in the form of unexpectedly low flow rates and time dependent variations. Possible explanations to these observations are discussed.

Extensive development of biomedical and chemical analytic microdevices involves microscale fluid flows. Merging of fluid streams is expected to be a key feature in such devices. An integrated microsystem consisting of merging microchannels and distributed pressure microsensors has been designed and characterized to study this phenomenon on a microscale. The two narrow, uniform and identical channels merged smoothly into a wide, straight and uniform channel downstream of a splitter plate. All of the devices were fabricated using standard micromachining techniques. Mass flow rates and pressure distributions were measured for single-phase gas flow in order to characterize the device. The experimental results indicated that the flow developed when both inlets were connected together to the gas source could be modeled as gas flow through a straight and uniform microchannel. The flow through a single branch while the other was blocked, however, could be modeled as gas flow through a pair of microchannels in series. Flow visualizations of two-phase flows have been conducted when driving liquid and gas through the inlet channels. Several instability modes of the gas/liquid interface have been observed as a function of the pressure difference between the two streams at the merging location.

We present the design, fabrication, and characterization of a micromixer that uses an electrokinetic instability (EKI) as the stirring mechanism. We have observed that microchannel liquid flows subjected to sinusoidally alternating electric fields exhibit, under certain electric field strengths and geometries, a flow instability capable of rapidly stirring micro- and nanoliter volume solutions [1]. We have performed a fluorescein dilution experiment to observe the rapid dispersion of dyed and undyed buffers within the mixing chamber as a result of EKI stirring. The flow field was imaged using an inverted, epifluorescent microscope fitted with a 12 bit, cooled CCD camera. We have initially characterized the mixer performance by computing ensemble-averaged probability density functions (PDF’s) and power spectra of the instantaneous, spatial fluorescence intensity fields. This novel stirring mechanism has the potential for significantly improving the functionality of microfluidic, bioanalytical devices.

This paper investigated experimentally effects of the temperature-dependent property on the laminar flow characteristics in the micro-channel, where water was used as a working fluid. A rectangular straight micro-channel was fabricated with the dimension of 57 μm (H) × 200 μm (W) × 48050 μm (L), in which the resistance temperature detectors (RTDs) were integrated to measure precise temperatures of the fluid directly on the inside-surface of the channel wall. A micro-heater was also installed at the outlet of the channel to generate the heat flux. We measured pressure drop by increasing mass flow rate and the applied heating power. At the same time, micro-Particle Image Velocimetry (micro-PIV) [1] measured the detailed velocity fields along the microchannel, where the wall temperature varied. Based on the pressure drop and Micro-PIV measurement, it was determined that the variation of the fluid property along the microchannel has an effect significantly on flow resistance but not considerably on the velocity profile. Also, it was observed that flow resistance and velocity field shows a good agreement with those estimated in the macro laminar theory under our experimental conditions.

A micro heat sink comprising 10 microchannels integrated with a local heater and a temperature microsensor array has been fabricated on a silicon wafer using standard micromachining techniques. A glass wafer was anodically bonded to the silicon wafer in order to cap the trapezoidal microchannels, about 14μm in depth and 120μm in average width. DI water was pressurized through the heat sink serving as the working fluid. Boiling curves of device temperature, at a few locations along the centerline, were measured as a function of the input power. In contrast to previously reported results, the boiling plateau associated with latent heat of phase change from liquid to vapor was detected. The transparent glass ceiling allowed the visualizations of flow phenomena dominated by surface effects. The classical bubble dynamics of bubble formation, growth, and detachment was observed at relatively low input power. However, this mode was completely suppressed at moderate power levels. Further increase of the input power resulted in a clear separation between the upstream vapor and the downstream liquid. The average location of the vapor/liquid interface shifted downstream with the input power, and near the critical heat flux condition the interface was located at the channel outlet. Thus, the added heat resulted in increased quality of the two-phase flow rather than increasing the mixture temperature.

In recent years, many micromachined polymerase chain reaction (PCR) chips have been reported. These miniaturized PCR chips have great advantages such as the significant reduction in reagent costs and vastly reduced reaction times over the conventional PCR reactions. In this paper a micro analysis system that will allow submicro-liter scale, continuous-flow PCR to be conducted in a glass chip has been presented. This glass chip is fabricated through thermally bonding two pyrex 7740 glass wafers. One pyrex wafer is etched to form the 20-cycle microchannel with 80 microns wide and 30 microns deep. Another pyrex wafer is thermally bonded to the etched wafer to produce closed a continuous microchannel for PCR. The total length of the microchannel is 0.5 m. This PCR chip has a significant reagent reduction with a volume of less than 1 micro-liter. With 1 micro-liter reagent we obtain a total reaction time of 0.5 min to 3 min depending on various flow rates. This analytical chip is fabricated using standard micromachining techniques with a total size of 56 mm × 24 mm × 1 mm. The advantages of this chip include very small quantities of reagent needed, rapid thermal cycling, standard micromachining processes and batch fabrication which result in a significant cost reduction.

We present an analysis and an experimental study of field amplified sample stacking. The analysis consists of a one dimensional, unsteady electromigration model of the concentration fields in sample stacking. The model predicts that the sample concentration develops as a non-linear concentration wave with a peak concentration increasing with time that reaches a maximum value determined by ratio of electric fields in the sample and the buffer region. The experimental work is a preliminary study of the effects of electroosmotic flow on sample stacking for a single fluid-fluid interface within a glass microchannel. Using a simple cross-channel configuration a high-gradient initial step-function in the buffer concentration field was established which causes an unsteady sample concentration process. The experimental results qualitatively confirm the wave-like profile of the stacked analyte predicted by the analytical model. Experiments also provide evidence of internal pressure generation due to mis-matched electroosmotic velocities during stacking. The effect of electric field strength and sample-to-background buffer concentration ratio, γ, are also presented. Stacked sample concentration at γ = 4 is found to initially increase with increasing electric field but saturates at higher electric fields. Stacking rates at higher γ ratios indicate that concentration enhancement is quickly limited by dispersion due to internally generated pressure gradients.

The ability to control the rotational direction of tethered motile bacteria such as Escherichia coli offers an alternative to power micro-electro-mechanical system (MEMS) devices. This paper describes our effort in the development of a micro-fluidic system for small but precise liquid delivery that uses the motility of tethered cellular motors as the driving force for fluidic transport. Based on CFD simulation, micro channels of various cross-sectional areas are fabricated on a 5″ × 5″ × 0.9″ quartz wafer. Integration of the cellular motors is done by bonding the micro channel wafer to a glass slide, which consists of a single layer of cellular motors previously tethered on the slide. With this system, rotation of the cellular motors will be initiated and the resulting volumetric fluid flow in the micro channel will be measured.

This paper presents an experimental investigation on compressible gas flow through microchannels with a constant area, rectangular cross-section. The microchannels are etched into silicon wafers, capped with smooth glass, and have hydraulic diameters between 4 and 100 μm. All measurements were made in the laminar flow regime with Reynolds numbers ranging from 0.02 to 1000. Smooth channels were obtained by etching (100) silicon wafers with potassium hydroxide (KOH) solution. Rough channel surfaces were obtained by etching (110) silicon wafers with KOH. The investigation shows that the friction factor for both smooth and rough microchannels compares closely with continuum theory.

Present study investigates the roughness effect on laminar gas flow for microchannels ranging from 40 to 600 μm with various roughness heights (40–82 nm) by systematical experiments. The micro-channels are manufactured by micro-machining technology and KOH anisotropic etching is employed to achieve various roughness patterns. Experimental results shows that higher product levels of Reynolds number (Re h ) and friction factor (f) are obtained for microchannels of larger size and smaller relative roughness and friction factor f approaches to laminar flow theory value f 0 for very smooth channel but the ratio of (f/f 0 ) decreases as the surface roughness increases.

Microfluidics plays a major role in the development of many innovative research activities aimed at the development of miniaturized devices and systems, and new applications related to microscale handling of fluids. As the field of microfluidics continues to grow, it is becoming increasingly important to understand the mechanisms and fundamental differences involved in microscale fluid flow. This paper presents a summary of the experimental research efforts in the area of microscale single-phase internal fluid flow and discusses issues associated with investigating microscale flows. While the currently available experimental data indicate the presence of microscale phenomena, they do not unequivocally identify the effects. There is a clear need for additional experimental investigations.

This work addresses the controversy of slip of a hydrophilic fluid over a hydrophobic boundary by applying micron resolution particle image velocimetry (μ-PIV) to measure the flow profiles of water flowing through 30 × 300 micron channels that have a clean surface (hydrophilic) and channels coated with hydrophobic molecules, OTS (octadecyltrichlorosilane). The results for hydrophilic liquids flowing over hydrophilic boundaries are consistent with the no-slip boundary condition. However, the results show that the no-slip boundary condition is not necessarily valid for hydrophilic liquids flowing over hydrophobic boundaries. Thus, modeling fluid flow at the micro-scale with the assumption of no-slip may or may not be valid, but will depend on the interactions between the fluid and the surface properties of the wall.

This paper presents global and point-wise comparisons of experimental measurements and numerical simulation results of electroosmotically driven flows in grooved micro-channels. The micro-channels are made by photolithography using poly-dimethyl-siloxane (PDMS), a type of silicon product, which is transparent, permeable to a variety of liquids and vapors. Experiments are performed using a micro-scale particle image velocimetry (μ-PIV) system developed for measurement of flows in micro-channel geometries. Full field velocity distributions are measured in Eulerian viewpoint by tracking the fluorescence images. The numerical algorithm is based on the spectral element formulation of incompressible Navier-Stokes equations with electroosmotic forcing. The algorithm utilizes modal expansions in mixed quadrilateral and triangular meshes, enabling complex geometry discretizations with spectral accuracy. Comparisons of experimental and numerical results show good agreements, validating both numerical and experimental methodologies.