bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer Characteristics of Gaseous Flows in Micro-Channels With Constant Wall Temperature: Cooled From Walls
Authors: Hong Chungpyo , Asako Yutaka
Published: September 2008

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in micro-channels with CWT (constant wall temperature) whose temperature is lower than the inlet temperature. The combined effect of viscous dissipation and compressibility is also investigated. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The stagnation temperature is fixed at 300K and the computations were done for the wall temperature of 250K, 280K, and 290K. The bulk temperature based on the static temperature and the total temperature are compared with those of the heated case and also compared with those of the incompressible flow in a conventional sized channel. The identical heat transfer coefficients are obtained for both heated and cooled cases of the incompressible flow. However, in the case of the gaseous flow in micro-channels, different heat transfer coefficients are obtained for each heated and cooled case. A correlation for the prediction of the heat transfer rate of the gaseous flow in the micro-channel is proposed.
Complex Three-Dimensional Fluidic Reservoirs to Control Beads/Living Cells Contacts
Authors: Monneret Serge , Belloni Federico , Soppera Olivier
Published: September 2008

In this paper, we combine holographic multiple optical tweezers with a three-dimensional microfluidic system to create a versatile microlaboratory. In order to determine cells local and/or temporal response to stimuli, and therefore draw their map of sensitivity, one convenient way is to apply antigen-covered latex beads in order to bind to plasma membrane, by means of optical tweezers. Using multiple optical traps could improve the efficiency of the measurements, but also their versatility. Therefore, we have developed a complete system based on holographic optical tweezers to realise multiple-point interactions between beads and cells with control of the stimulation places, timing, and durations. As we plan to use our system to study biological events in the hour timescale, we have to keep beads and cells separated, in order to prevent unwanted beads to circulate freely in the sample and bind to the target cell during the experiment. We then introduced microstereolithography as a 3D micro-manufacturing approach to the rapid prototyping of three-dimensional fluidic microchambers of complex shapes inside the sample, comprising wells, channels and walls to inject beads locally and keep them separated from cells in our assays. We demonstrated the possibility for microSL to easily and rapidly (typically one hour) fabricate small and three-dimensional observation chambers with customized design of the flow channels, including fluidic reservoirs of typically 500–1500 μm diameter, 5–12 mm height, in order to facilitate manual filling. Several shapes of reservoirs designed to keep beads and cells separated in liquid samples have been realized and successfully tested. Some of them included up to 3 reservoirs, in order to allow co-distribution of different types of beads. Each reservoir typically contained 2–10 μl of solution holding the beads, with a horizontal outlet of 100–200 μm in diameter which allows beads to deposit locally on the microscope cover glass placed under the reservoir outlet. Limited extension of beads under the outlet on the glass has been confirmed, and the ability of the polymeric structures to confine beads in a restricted area has been demonstrated. In the following we present examples of manipulations by multiple holographic optical tweezers consisting at first in extracting several beads from such an area by going through an aperture designed in the structure, making them travel to the target cell, and finally depositing on its outer membrane.
Molecular Dynamics for Near Surface Flows in Nano Liquid and Micro Gas Systems
Authors: Macpherson Graham B. , Reese Jason M.
Published: September 2008

Conventional fluid mechanics (Navier–Stokes equations with linear constitutive relations) is, on the whole, applicable for simulating very small scale liquid and gas systems. This changes (for simple fluids) only in the vicinity of solid surfaces (approximately 5 molecular diameters for liquids, or one mean free path for gases) or under very high temperature or velocity gradients. It is shown that typical experimental conditions in practical systems do not give rise to gradients of this magnitude. Therefore, only surface effects cause significant deviation from results expected by conventional fluid mechanics. In micro and nano systems, however, large surface area to volume ratio means that the detail of boundary conditions and near surface dynamics can dominate the flow characteristics. In this paper, the use of non–equilibrium molecular dynamics (NEMD) to study these fluid mechanics problems in an engineering simulation context is discussed. The extent of systems that can be studied by NEMD, given current computational capabilities, is demonstrated. Methods for reducing computational cost, such as hybridisation with continuum based fluid mechanics and extracting information from a small representative systems are also discussed. Non–equilibrium surface effects in gas micro systems may also been studied using NEMD. These occur at boundaries in the form of discontinuities (velocity slip and temperature jump) and within approximately one mean free path of a surface, in the form of a Knudsen layer. The distributions of molecular velocities, free path between collisions and time spent in collision have been calculated for an unbounded equilibrium fluid. The influence of a solid surface on the state of a fluid or flow can be investigated by measuring how these fundamental properties are affected.
Micro-Flow-Through Synthesis of Metal Nanoparticles in the Presence of DNA
Authors: Recknagel Ch. , Wagner J. , Albert J. , Ko¨hler J. M.
Published: September 2008

The interaction of DNA with metal nanoparticles is of interest due to the application of nanoparticles as labels in biomolecular diagnostics and for the construction of nanoelectronic devices by molecular nanotechnology. The experimental arrangement was based on static micro mixers made lithographically by Si/glass technology. Four educt components were used in the experiments: (1) HAuCl4-solution, (2) ascorbic acid solution, (3) buffer and (4) DNA solution. The colloidal product solutions were characterized by spectrophotometry, differential centrifugal sedimentation (DCS) and SEM. The rate of nanoparticle formation as well as the optical properties of the product solution and the size and aggregation tendency were influenced by the conditions in the micro flow-through process. The addition of DNA leads always to a decrease in the reaction rate. Simultaneously, smaller particles became stabilized by DNA and aggregation was partially suppressed by the biomolecular effector. The effect was reflected by the optical properties as well as by the centrifuge data and SEM images. In the absence of DNA and at higher concentrations of DNA some larger aggregates of nanoparticles were found beside single nanoparticles. Particular types of aggregates appeared at a low concentration of DNA (0.046 μg/μl). At these conditions, network like structures were observed, whereas more compact aggregate structures arose at higher DNA contents. DNA affects the formation of gold nanoparticles during the reaction in a micro flow-through system assemblies.
Gas Flow in Hot Porous Materials: The Solar Air Receiver and Spin-Off Applications
Authors: Fend Thomas , Hoffschmidt Bernhard , Reutter Oliver , Sauerhering Jo¨rg , Pitz-Paal Robert
Published: September 2008

This article presents an overview on research results from various projects, which deal with one common problem: gas flow in hot porous materials. First, the solar air receiver, which converts concentrated solar radiation into heat in an air circuit, is described as far as the basic principle and the materials employed are concerned. Then, results from experiments in concentrated solar radiation are presented. Materials employed in these applications are extruded ceramic materials as well as metal and ceramic foams with pore sizes on the milli- and micrometer scale. As it turned out, the material properties significantly influence the efficiency of the solar air receiver. It is shown, that under specific conditions flow instability occurs, which may lead to a thermal overload of the material. Measures to avoid these overloads are proposed. Two approaches how to predict gas flow theoretically are reported. Additionally, it is shown, how material quantities such as pressure drop characteristics influence the flow behaviour and the temperature distribution inside the material. Finally, before a conclusion is given, two further applications, which have been dealt with because similar phenomena occur, are reported: an advanced cross flow particle filter and a gas turbine cooling system.
Bounds on Two-Phase Frictional Pressure Gradient in Minichannels and Microchannels
Authors: Awad M. M. , Muzychka Y. S.
Published: September 2008

Simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. The lower bound is based on Ali et al. correlation for laminar-laminar flow. This correlation is based on modification of simplified stratified flow model derived from the theoretical approach of Taitel and Dukler for the case of two-phase flow in a narrow channel. The upper bound is based on Chisholm correlation for laminar-laminar flow. The model is verified using published experimental data of two-phase frictional pressure gradient in circular and non-circular shapes. The published data include different working fluids such as air-water mixture and nitrogen-water mixture, and different channel diameters. The bounds models are also presented in a dimensionless form as two-phase frictional multiplier (φl and φg) versus Lockhart-Martinelli parameter (X) for different working fluids such as air-water mixture and nitrogen-water mixture. It is shown that the published data can be well bounded.
Experimental Study on Flow Boiling Heat Transfer in an Extremely Small Tube
Authors: Ohta Haruhiko , Inoue Koichi , Shimada Yuichiro
Published: September 2008

Flow boiling heat transfer in a single small tube is investigated by using FC72 as a working fluid. The heat transfer coefficients are measured in the ranges of heat flux 2–24kW/m2 and mass velocity 100–400kg/m2s under the condition of near atmospheric pressure. Test tube, made of stainless steel, has an inner diameter of 0.51mm and a heated length of 200mm. The tube is located horizontally in a vacuum chamber to reduce the heat loss and to minimize the time to obtain data regarded as that of steady state. In the single-phase region, heat transfer coefficients due to forced convection are in good agreement with the values from the conventional theories. In the saturated region, measured heat transfer characteristics are quite different depending on whether the test liquid is deaerated or not deaerated before the experiments. By using deaerated liquid, three different heat transfer regimes are observed: In the first regime, the heat transfer is dominated by nucleate boiling in low vapor quality, and the heat transfer is deteriorated or enhanced depending on the channel confinement and heat flux. In the second regime, the heat transfer is dominated by two-phase forced convection in moderate quality as is well known for the tubes of normal size. In the third regime, the heat transfer is dominated again by two-phase forced convection, but is deteriorated in high quality. One or two regimes can disappear or become unclear depending on the conditions of flow and heating. The effects of vapor quality and mass velocity on the heat transfer characteristics due to two-phase forced convection in the moderate vapor quality are clarified in the experimental ranges tested. And a reason for the gradual heat transfer deterioration observed in high quality is discussed based on the liquid-vapor behaviors inherent in small diameter tubes.
A Continuous Flow Polymerase Chain Reactor for DNA Expression Analysis
Authors: Sirr Noel , Ciobanu Doina , Grimes Ronan , Davies Mark
Published: September 2008

The polymerase chain reaction (PCR) has revolutionised molecular biology, and is at the forefront of many current efforts to document and understand human genetic diversity. Recent years has seen a move towards incorporating the PCR technique into a micro Total Analysis System (μTAS) thus exploiting its full potential. Micro scale PCR design offers the opportunity to integrate all functional steps of DNA expression analysis into a miniaturised device allowing for high throughput and reduced analysis times, reduced sample volume requirements and cost efficiency. Consequently, it is desirable to replace the traditional stationary or well based thermal cyclers with continuous flow designs. A continuous flow polymerase chain reaction device consisting of a cylindrical heating core, which is segmented axially into three symmetric heating zones providing the denaturating, annealing and extension phases of the polymerase chain reaction, and a flow through capillary tube which is wound helically around the core has been fabricated and shown to consistently amplify target plasmid DNA samples. At the inlet to the device, PCR samples are segmented into droplets and entrained in an immiscible carrier fluid to eliminate cross contamination between PCR samples. This approach also prevents degradation of the micro-reactor droplets from inhibitory effects posed by the high surface to volume ratios associated with the device. The droplet train is then cycled through the capillary tube with each complete revolution constituting one cycle of the PCR reaction. The results reported in this paper include, initial validation of the spiral cycler design in comparison to an existing commercial PCR platform, and subsequent optimisation of the reaction time and its effects on the devices performance. The spiral thermal cycler has demonstrated successful PCR amplification at the nano scale with stable trains of 30–35nl droplet volumes being processed in an amplification time of 32 minutes. At this level the device offers the potential to process approximately 3500 such droplets in of order one hour.
Characteristics of Hydrodynamically Focused Streams for Use in Microscale Particle Image Velocimetry (Micro-PIV)
Authors: Domagalski Piotr M. , Mielnik Michal M. , Lunde Ingrid , Sætran Lars R.
Published: September 2008

This paper presents the characteristics of hydrodynamically focused streams to be used in Microscale Particle Image Velocimetry (micro-PIV). The investigation was done by means of Confocal Laser Scanning Microscopy (CLSM) in channels of cross sections 260 × 200 μm and 1040 × 800 μm, within the downstream velocities range from 0,1 to 2,5 cm/s. The formulation of a curvature of the sheet, its dependence on the side stream ratio and overall downstream velocity is discussed qualitatively and quantitatively. The results show that the curvature is highly dependent on the velocities of the system. Sheet characteristics such as curvature and observed sheet width variation become amplified with higher velocities. This leads to conclusion that hydrodynamic focusing is promising as a selective seeding technique for use in micro-PIV up to velocities of few cm/s. However, the fact that this is the upper velocity range in microfluidics, renders the SeS-PIV technique as a very suitable tool for complicated flows visualization.
Periodic Interface Destabilization in High-Speed Gas-Liquid Flows at the Capillary Scale
Authors: Herescu Alexandru , Allen Jeffrey S.
Published: September 2008

Recent research efforts have illustrated the importance of capillarity on the behavior of two-phase flow (gas-liquid) in low Bond number systems; that is, systems where capillary forces are important relative to gravitational forces. Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. High speed microscopy experiments visualizing air-water flow through a 500 micrometer square glass capillary, 10 cm long were conducted. The flow rates are significant with velocities of 6.2 m/s for the air and 0.2 m/s for the water. A unique annular flow with periodic destabilization of the gas-liquid interface has been observed. Standing waves develop on the liquid film and grow into annular lobes typical of that observed in low speed two-phase flow in capillary tubes. Atypical is the interface destabilization phenomena. The leading face of the lobe will decelerate and suddenly become normal to the wall of the square capillary while the trailing face of the lobe will remain gently sloped back into the annular liquid film. The transition between the leading and trailing faces acquires a sharp edge having a exceptionally large curvature. The entire structure then rapidly collapses and produces travelling waves which propagate upstream and downstream along the annular liquid film. The entire sequence of events takes approximately a half millisecond. This destabilization phenomenon is regular and periodic. Visualization of the destabilization from the high speed microscopy setup and preliminary analysis are presented.
Modeling Low Knudsen Number Transition Flows Using a Computationally Efficient Continuum-Based Methodology
Authors: Mizzi Simon , Emerson David R. , Barber Robert W. , Reese Jason M.
Published: September 2008

This paper presents a new technique that combines Grad’s 13-moment equations (G13) with a phenomenological approach. The combination of these approaches and the proposed solution technique manages to capture important non-equilibrium phenomena that start to appear in the early transition-flow regime. In contrast to the fully-coupled 13-moment equations, a significant advantage of the present solution technique is that it does not require extra boundary conditions. The solution method is similar in form to the Maxwellian iteration used in the kinetic theory of gases. In our approach, Grad’s equations for viscous stress and heat flux are used as constitutive relations for the conservation equations instead of being solved as equations of transport. This novel technique manages to capture non-equilibrium effects and its relative computational cost is low in comparison to other methods such as fully-coupled solutions involving many moments or discrete methods. In this study, the proposed numerical procedure is applied to a planar Couette flow and the results are compared to predictions obtained from the direct simulation Monte Carlo method. In the transition regime, this test case highlights the presence of normal viscous stresses and tangential heat fluxes that arise from non-equilibrium phenomena. These effects cannot be captured by the Navier-Stokes-Fourier constitutive equations or phenomenological modifications thereof. Moreover, simply using the G13 equations, along with the decoupled solution method, does not capture the nonlinearities occurring in the proximity of a solid wall. However, combining phenomenological scaling functions and slip boundary conditions with the G13 equations provides a better representation of these important non-equilibrium phenomena but at a relatively low computational cost.
Optimization of Metallic Multi-Microchannel Array Evaporators
Authors: Henning Torsten , Brandner Juergen J. , Schubert Klaus , Lorenzini Marco , Morini Gian Luca
Published: September 2008

Pulsation and maldistribution effects (hot spot formation) with characteristic frequencies below 100 Hz occurring in electrically powered microchannel array heat exchangers used as evaporators for water were investigated primarily with the aid of visualisation techniques. Pulsation at subaudio frequencies was found to be dominated by the consequences of boiling in the inlet plenum, creating large vapour bubbles that intermittently entered the microchannel array, pushing liquid water at velocities too high to achieve complete evaporation. A new design minimising the residence time in the inlet and comprising an intermediate void was found to produce a two phase mixture that could be evaporated in an array of 68 microchannels, each 200 μm wide, 100 μm deep, and 20mm long, at a mass flux of 60 kgm−2 s−1 at an average surface temperature of 220°C. The redesign led to a change in characteristic flow patterns in the microchannel arrays from plug and slug flow to film flow and drops moving along the walls of the microchannels, as evident from high speed (103 fps) video sequences. By means of infrared thermography of the surface of a metallic device comprising four layers of microchannel arrays, maldistribution between the microchannel array layers leading to hot spot formation was observed when the device was operated in constant power mode. The formation of these hot spots could be avoided by the use of simple temperature control electronics operating at characteristic frequencies below 10−1 Hz.
Selective Adsorption of Solvents in a Multiscale Device
Authors: Henning Torsten , Brandner Juergen J. , Eichhorn Lutz , Schubert Klaus , Schreiber Martin , Gu¨ngerich Martin , Gu¨nther Hermann , Klar Peter J. , Rebbin Vivian , Fro¨ba Michael
Published: September 2008

We have incorporated microspheres, 50 μm to 80 μm in diameter, of periodic mesoporous organosilica (inner surfaces up to 1000 m2/g and pore sizes in the nanometre range) with two types of organic functionalities (benzene and ethane bridges, respectively) inside microstructured channels (each 200 μm wide and 100 μm deep) and, exemplarily, monitored by Raman microscopy that the temperature characteristics of the adsorption-desorption equilibria of benzene and ethanol vary significantly with the type of organic functionality of the microspheres and the pore morphology. The integration of this class of nanostructured material into devices by means of microchannels is a promising novel approach to, among others, substance separation in analytics, micro process engineering, and micro chemistry.
Microscale Heat Transfer of Confined Miniature Jets
Authors: Glynn Colin , Robinson Anthony J. , Murray Darina B. , Lupton Thomas L.
Published: September 2008

An experimental study of the microscale heat transfer characteristics of a 1.22 mm confined miniature jet is presented for both air and water. The experimental apparatus utilises infrared thermography to measure the temperature profiles on the underside of a heated thin-foil upon which the jet is impinging. The local heat transfer coefficient calculations account for lateral conduction within the foil, which, in contrast to conventional practice, has been shown to be non-negligible. The tests are carried out over a jet Reynolds number range of 5000–20000 and non-dimensional jet to target spacing (H/d) range of 2–5. Results are presented in terms of local heat transfer coefficients. It has been determined that (i) lateral heat conduction effects are significant for the air jet measurements but are negligible for the water jet results, and (ii) the two fluids display hugely different heat transfer coefficient profiles for ostensibly the same non-dimensional flow conditions.
Gas Flow Through Microscale Orifice Plates
Authors: O’Hare Lynne , Scanlon Thomas J. , Reese Jason M.
Published: September 2008

In this paper, the phenomenological “wall function” method of Lockerby, Reese and Gallis [1] is used to capture important non-equilibrium aspects of gas microflows. The approach is a constitutive scaling method that captures slip at solid walls and the Knudsen layer, a near-wall region where non-linear constitutive behaviour is observed. In this paper the wall function approach is applied to model gas flow through microscale orifice plates, an industrially relevant application. Two key test geometries are investigated, flow through a constriction in a microchannel and flow through a microscale venturi in a microchannel. A range of incompressible, isothermal flows are analysed with the OpenFOAM computational fluid dynamics (CFD) code [10], and numerical results are validated against available experimental data [12]. Following successful verification of the model, the increasing impact of non-equilibrium effects on flow through the test geometries at higher Knudsen numbers is shown. The integration of the constitutive scaling approach with mainstream CFD is shown to be a flexible technique, well-suited to engineering design applications.
Hydrodynamic Scaling Approaches for the Biomimetic Design of Microfluidic Channels
Authors: Emerson David R. , Cies´licki Krzysztof , Barber Robert W.
Published: September 2008

It is now possible to fabricate networks of micro-channels that mimic the bifurcating structures found in biological vasculatures. The optimum ratio between the diameters of the parent and daughter branches in biological vascular systems can be described by Murray’s law. If the network consists of symmetric bifurcations, an important consequence of Murray’s law is that the tangential shear stress at the wall will remain constant throughout successive generations. This important hydrodynamic concept can be used to provide the basis for developing simple but elegant biomimetic design rules that can be applied to microfluidic networks with arbitrary cross-sectional geometries. The present paper shows how biomimetic networks of constant-depth rectangular and trapezoidal channels can be fabricated using standard photolithographic techniques. The design rule employed is based on the mean shear stress and is used to predict and control the stress distribution within the hierarchical network. The validation of the biomimetic design rule is obtained through a comprehensive series of computational fluid dynamic studies. A range of alternative biomimetic and hydrodynamic scaling principles are also discussed, including power and Reynolds number, that enable different parameters to be controlled in each successive generation.
Adiabatic Two-Phase Pressure Drop of Refrigerants in Small Channels
Authors: Field Brandon S. , Hrnjak Pega
Published: September 2008

The adiabatic pressure drop of two-phase refrigerant flow in small channels has been investigated. A rectangular channel with dh = 148.0 μm has been tested with four refrigerants: R134a, R410A, Propane (R290) and Ammonia (R717). This data has been combined with data taken from five different channels, with dh varying from 70 μm to 305 μm, of R134a. The measured pressure drops are compared to many published separated-flow and homogeneous pressure drop models. A new correlation for C, the Chisholm parameter, has been developed based on Reynolds number of the vapor phase (which contains the majority of the kinetic energy) and the dimensionless grouping ψ — a dimensionless ratio of viscous to surface tension effects taken from the analysis of capillary flow performed by Sou and Griffith (1964). This allows the new correlation to account for the varying fluid properties (including surface tension) that are found in the different refrigerants. The new correlation takes flow regime into account by means of a Weber number based flow transition criteria, following the flow map of Akbar et al. [10].
Frictional Pressure Drop and Two-Phase Flow Pattern of Gas-Liquid Two-Phase Flow in Circular and Rectangular Minichannels
Authors: Ohtake Hiroyasu , Ohtaki Hideyasu , Koizumi Yasuo
Published: September 2008

The frictional pressure drops of gas-liquid two-phase flow in mini-pipes and mini-rectangular channels were investigated experimentally. The following test channels were used in the present experiments: commercial stainless-steel circular tubes with 0.6, 0.5 and 0.25 mm in inner diameter, FEP circular tube of 0.4 mm in inner diameter and rectangular channels, made of Acrylic resin, with 0.39 × 20.4 mm, 0.21 × 9.75 mm, 0.26 × 4.28 mm and 0.18 × 1.87 mm in height and width, respectively. The pressure drops of straight pipe, sudden enlargement and sudden contraction of gas-liquid two-phase flow in mini-pipes were measured for the test mini-channels. The pressure drops of rectangular minichannel were also measured. Experimental result showed that measured two-phase friction multipliers agreed well with a conventional Lockhart-Martinelli correlation for circular tubes and Mishima-Hibiki’s correlation for rectangular channels. Observed flow patterns by visualization were bubbly, slug, churn, ring and annular flow; the flow patterns in the present experiments were reproduced well by Baker’s flow pattern map.
Magnetic Micromanipulation of a Single Magnetic Microsphere in a Microchannel
Authors: Sinha Ashok , Ganguly Ranjan , Puri Ishwar K.
Published: September 2008

Magnetic microspheres are well known for their ability to provide high surface-to-volume ratio mobile reaction surfaces for chemical and biochemical reactions. Their use in microfluidic devices opens up novel avenues for uses in ‘lab-on-a-chip’ applications, e.g., as magnetic tweezers. Cantilevers and optical tweezers are widely used for micromanipulating cells or biomolecules in order to measure their mechanical properties, or for biosensor applications. However, they do not allow for ease of rotary motion and can sometimes damage the handled material. We present herein a system of magnetic tweezers that uses functionalized magnetic microspheres as mobile substrates for biological and biochemical reactions and offers better manipulation of the cells or organic molecules. The predominant transport issue for these magnetic tweezers is the precise magnetic manipulations of the microbeads so that the chemical/biological reactions at the bead surface are controlled. The best way to obtain unambiguous information about the behavior of particles is to begin with the study of a single isolated particle in a microchannel flow. We have conducted a fundamental study to manipulate an isolated magnetic microparticle using the concept of ‘action-at-a-distance’. An external magnetic field is used to direct and steer the particle across a microchannel. Such a study is directly pertinent to practical applications where usually a small number of such microspheres are utilized, such as DNA sequencing and separation, cell manipulation and separation, exploration of complex biomolecules by specific binding enabling folding and stretching, etc. Numerical simulation of the microchannel flow and particle manipulation is performed using a finite-volume transient CFD code and Lagrangian tracking of magnetic microspheres in the flow under an imposed magnetic field gradient. Experimental validation of the numerical results is then performed. The effects of varying viscosity and flowrate using two different particle sizes are investigated. Parametric study is performed to tune the external magnetic field so as to obtain a desired particle trajectory. Finally, the proof of concept demonstration of the magnetic tweezing is reported. We conclude that magnetic tweezers are viable and can be fabricated as part of a biocompatible setup that could become a suitable alternative to the other available micromanipulators.
Modeling of Laser-Induced Material Removal During Fabrication of Micro- and Nanochannels
Authors: Ajaev Vladimir S. , Willis David A.
Published: September 2008

Laser melting of thin metal films on a substrate is an essential step in a recently proposed technique for the fabrication of micro- and nanochannels of cross-sectional sizes varying from several hundred nanometers and a few micrometers. The metal film is electrolessly deposited on a glass substrate and then irradiated by a tightly focused laser beam. The molten material is removed from the hot region by the combined action of thermocapillary forces and evaporation. The film can then be used in subsequent manufacture steps such as wet etching or contact photolithography. We propose a mathematical model that describes laser-induced material removal from thin metal films. Our approach incorporates several important physical effects such as liquid flow in the melt, evaporation, thermocapillarity, as well as the re-solidification of the material due to heat losses into the substrate.
A Experimental Study of Condensation Heat Transfer and Pressure Drop in a Single High Aspect Ratio Micro-Channel for Refrigerant R134a
Authors: Chowdhury Sourav , Al-Hajri Ebrahim , Dessiatoun Serguei , Shooshtari Amir , Ohadi Michael
Published: September 2008

Only recently, experimental data is available in open literature in condensation of various refrigerants in small hydraulic diameter microchannels. The phenomenon of two-phase flow and heat transfer mechanism in small diameter microchannels (< 1 mm) may be different than that in conventional tube sizes due to increasing dominance of several influencing parameters like surface tension, viscosity etc. This paper presents an on-going experimental study of condensation heat transfer and pressure drop of refrigerant R134a is a single high aspect ratio rectangular microchannel of hydraulic diameter 0.7 mm and aspect ratio 7:1. This data will help explore the condensation phenomenon in microchannels that is necessary in the design and development of small-scale heat exchangers and other compact cooling systems. The inlet vapor qualities between 20% and 80% and mass fluxes of 130 and 200 kg/m2s have been studied at present. The microchannel outlet conditions are maintained at close to thermodynamic saturated liquid state through a careful experimental procedure. A unique process for fabrication of the microchannel involving milling and electroplating steps has been adopted to maintain the channel geometry close to design values. Measurement instruments are well-calibrated to ensure low system energy balance error, uncertainty and good repeatability of test data. The trends of data recorded are comparable to that found in recent literature on similar dimension tubes.
High-Throughput, Long-Term Imaging of Salmonella Infecting Macrophages in a Micro-Environmental System
Authors: Chao Shih-Hui , Strovas Tim J. , Wang Ting-She M. , Jones-Isaac Kendan A. , Fink Susan L. , Cookson Brad T. , Meldrum Deirdre R.
Published: September 2008

Real-time single cell analysis is necessary to understand dynamic cellular functions in time and space. Such analyses require the simultaneous measurement of multiple variables in real-time, due to heterogeneity in cellular populations. We report the application of using a micro-environmental chamber on an automatic laser scanning confocal microscope to observe murine macrophage cells in incubation conditions for more than 18 hours. The motorized stage of the microscope was programmed to scan through pre-defined monitoring locations to increase the observation throughput. The acquired images were post-processed to extract the information of each cell. In contrast to current single-cell technologies, such as fluorescence-activated cell sorter (FACS) based systems, the reported architecture records the history of the physiological responses of individual cells.
Experimental and Numerical Analysis of High Heat Transfer Phenomenon in Minichannel Gaseous Cooling
Authors: Hara Kazuo , Furukawa Masato , Akihiro Naoki
Published: September 2008

The authors have reported that minichannel flow system had high heat transfer coefficient, the reason of which was investigated experimentally and numerically for single and array minichannels combined with impingement flow system. The diameter D of the channel was 1.27 mm and length to diameter ratio L/D was 5. The minichannel array was so called shower head which was constructed by 19 minichannels located at the apex of equilateral triangle, the side length S of which was 4 mm. Single stage block was used to investigate the heat transfer without impinging flow system. Two stage blocks were used to compose an impingement heat exchanger system with an impingement distance of H. H/D ranged from 1.97 to 7.87. A comparison of heat transfer performance was made between minichannel flow system and impingement jet using the single and two stage heat transfer experimental data. It was found that heat transfer performance of the minichannel was equivalent to that of impingement jet. The mechanism of high heat transfer was studied numerically by the Reynolds averaged Navier-Stokes equation and k-ω turbulence model. The limiting streamline pattern was correlated well to the surface heat flux distribution. The high heat transfer in the single minichannel was achieved by suppressing the development of boundary layer under strong pressure gradient near the channel inlet and by the formation of large recirculating flow system in the downstream plenum of the minichannel exit. These heat transfer mechanisms became dominant when the channel size fallen into the regime of minichannel. For the array of 19 minichannels, the high heat transfer around the channel inlet was also observed clearly in the target plate of the impingement jet where minichannels of second stage were bored to exhaust the fluid of impingement jet.
Velocity Measurement of Gas Flow Using Micro PIV Technique in Polymer Electrolyte Fuel Cell
Authors: Sugii Y. , Okamoto K.
Published: September 2008

This paper describes a novel velocity measurement technique of the gas flow in micro scale in order to investigate the flow field. In order to visualize the micro gas flow, a fluorescent oil particle as a trace particle suitable for a gas flow was developed. A fluorescent oil particle generator was developed to optimize its diameter, high fluorescent intensity, no-adsorption onto channel surface, density, particle concentration, non-evaporating, electrically neutral, non-toxic, stability with water and no-effect for efficiency of electronic generation. The technique was applied to a flow in a transparent model of PEFC under non-operation initially, which consisted of straight channels of 1.0 mm width and 0.5 mm depth. The velocity distribution of the gas flow successfully measured. The number of vectors was 1516 and the maximum was 0.8 m/sec around the center of the channel at Reynolds number 26. The profile without the near wall region was similar to the theoretical profile obtained using Poiseuille’s profile with assumption of non-slip condition at the wall and steady flow. The velocity in the near wall region was slightly large compared with theoretical values. The technique is useful for investigation of flow field in a PEFC with chemical reaction.
Numerical Investigation of Forced Convection From Small Rectangular Coiled Pipes
Authors: Conte Ibrahima , Yang Zhen , Peng Xiaofeng
Published: September 2008

Investigations are done to numerically analyze forced convection from rectangular coiled pipes using commercial CFD software Fluent 6.0. The problems considered were three-dimensional laminar flow with water inside and outside the pipe. Calculations are done for two rectangular coiled pipes of different values of pitch (distance between two adjacent coils centers). The shift of location of the water flow maximum velocity was significantly observed in the case of smaller pitch. It is expected that the higher pitch value reduces the effects of the torsion. The shift of the flow maximum velocity area allows the fluid to be mixed well inside the pipe. For the outside flow, the flow behavior and the heat transfer should be improved by increasing the outside inlet velocity for the case of small pitch.
Microfluidic Cell Optimization for Polymer Membrane Fabrication
Authors: Shapiro E. , Drikakis D. , Gargiuli J. , Vadgama P.
Published: September 2008

Dual-fluid laminar flow in microchannels can be utilised through microfabrication to create polymer membranes at the interface between aqueous and organic solutions. In order to enable smooth membrane growth it is necessary not only to maintain a stable interface between the aqueous and organic phase, but also to minimise near-wall stresses, which affect membrane attachment at the initial stages of membrane formation. The characteristics of the dual-fluid flow in the entrance region of the micro-channel can be significantly affected by the geometry of the inlet and flow rates involved. We present a numerical study of the effects of the inlet geometry on the flow development and near-wall stresses in xylene/water flows, which represent the initial stages of nylon 6,6 membrane formation on the interface between an aqueous solution of hexamethylenediamine and adipoyl chloride solution in xylene. The shape of the inlets considered here varies from a T-inlet (90 degrees inlet angles) to an M-inlet (0 degrees inlet angles). We show that although higher flow rates are needed in order to contain reagents to the narrow region near the interface, the increase of the flow rate leads to significant increase of the shear stresses with the maximum values being obtained in the entrance region thus preventing membrane attachment. CFD validation against experimental data for rhodamine diffusion broadening in a microfluidic is also presented.
Homogeneous Nucleation of Vapor at Preferred Sites During Rapid Transient Heating of Liquid in Micropassages
Authors: Carey Van P. , Padilla Jorge , Gan Yu
Published: September 2008

Rapid heating of a liquid at the wall of a micropassage may produce homogenous nucleation of vapor in the liquid in contact with the surface. In such circumstances, nucleation is generally expected to be most likely to occur in the hottest liquid closest to the surface. It is known, however, that in many cases the liquid molecules closest to the surface will experience long-range attractive forces to molecules in the solid, with the result that the equation of state for the liquid near the surface will differ from that for the bulk liquid. In micro- and nanopassages, this wall-affected region may be a significant fraction of the passage interior volume. Recent investigations of wall force effects on the liquid indicate that these forces increase the spinodal temperature in the near-surface region. The results of these previous investigations suggest that for heated surfaces with nanoscale roughness, protrusion of bulk fluid into crevices in the surface may make them preferred sites for homogeneous nucleation during rapid heating. A detailed model analysis of the heat transfer in a model conical crevice is developed and used to explore the plausibility and apparent mechanisms of preferred site homogeneous nucleation. The analysis predicts that protrusion of bulk liquid into a conical cavity does, under some conditions, make the cavity a preferred site for the first occurrence of homogenous nucleation. The analysis is used to examine the range of conditions under which a crevice will be a preferred site. The implications of this increase for nucleation near a solid surface during rapid heating are also explored for circumstances similar to those for bubble nucleation adjacent to heaters in microheater reservoirs in inkjet printer heads.
Rotor Effect to a Micro-Stirring Mixer Operated by Optical Force
Authors: Dinh Thien X. , Ogami Yoshifumi
Published: September 2008

The enhance mixing by a four-paddle rotor placed in the junction of a three-dimensional Y-shape micro-mixer, whose dimensions are miniaturized of order tens micrometers, was investigated by numerical simulation. The working conditions of the mixer are: low Reynolds (Re∼0.001), high Schmidt (Sc∼10E5) numbers, rotation speed of the rotor ranging from 100–800 RPM (round per minute). Flow visualization shows that the rotor created circular, helical flows in the region closed to the rotor and transversal velocity in the mixing channel. However, the latest decrease by a power law and is confined in downstream distance defined by the rotor radius scale, and produces effective diffusion process. The two priors build a considerable uniform concentration distribution compared with the without rotor case. Therefore, the effective mixing process by the rotor was observed in the junction and a small distance from the rotor in the mixing channel. However, the mixing improvement in the later region is not significant. Out of this region, the mixing behavior is the same as solely diffusion process based on the diffusion coefficient of the reagent. The process occurring during mixing of fluid in the junction bases on stretching by the motion of the rotor and on folding by the rotor shape. However, the folding mechanism seems to be disappeared at high RPM and has inconsiderable role at low RPM. The mixing efficiency on the cross sections located just after the rotor and located where the rotor has no further effect depends logarithmically on RPM. Overall view, the mixing efficiency of the rotor archives 60–90%. The fluid torque exercising on the rotor is well self-similar to the frequency of the paddle tip interval by a power law. The results of this study can be used for efficient design a micro-mixer.
A Study of Flows Over Microgrooves for Different Flow Pattern
Authors: Chen Q. , Amano R. S.
Published: September 2008

During condensation of R134a the flow patterns inside two three-dimensional (3-D) micro-fin tubes with different fin geometries were investigated. The flow patterns and their transitions were visibly observed and recorded. The experimental findings revealed the following results: a comparison of the condensation flow patterns in the 3-D micro-fin tubes with those in smooth tubes revealed no qualitative differences. The mist flow and the mist-annular flow that appeared in the smooth tube entrance region were not observed in 3-D micro-fin tubes. In the maps of the Mandhane flow regime and the Soliman flow regime, the area of annular flow region of the 3-D micro-fin tube extends towards lower Fr number range in the Soliman map and smaller vapor velocity range in the Mandhane map when compared with that of a smooth tube. The criterion of the flow regime transition between the annular flow and the wavy flow decreases from Fr = 7 to Fr = 2 in the Soliman flow regime map. However, no significant effect on the criterion for the plug flow transition was observed. The experimental data points of plug flow in the 3-D micro-fin tubes were also obtained in the same regime of smooth tube in Mandhane flow regime map. The Soliman flow regime map indicates the criterion for plug flow transition to be Fr = 0.4.
Single-Phase Liquid Heat Transfer in Plain and Enhanced Microchannels
Authors: Steinke Mark E. , Kandlikar Satish G.
Published: September 2008

The development of advanced microchannel heat exchangers and microfluidic devices is dependent upon the understanding of the fundamental heat transfer processes that occur in these systems. There have been great advancements in our understanding of the heat transfer and fluid flow mechanisms that occur in microchannels. There is several research areas in microchannel heat transfer that so promise for such applications as microprocessor cooling. An enhanced microchannel heat exchanger (EMCHX) that uses single-phase liquid flows has been developed. This EMCHX uses flow obstructions to create a continually developing flow condition and the enhancement in heat transfer associated with that flow regime. A silicon substrate is chosen to create off-set strip fins in the microchannel flow field. Experimental verification of this new method shows excellent improvement in heat transfer over plain or traditional microchannels with straight, continuous walls. However, careful attention must be paid to the added pressure drop that is created by adding these obstructions. A new microchannel parameter called pumping power flux is developed to aid in the comparison between plain and enhanced microchannels. The pumping power flux is used in conjunction with the heat flux to calculate a coefficient of performance to demonstrate the heat transfer enhancement. The enhanced microchannels provide a much higher COP for the same flow conditions. Therefore, the improved heat transfer provided outweighs the added pressure drop caused by the enhanced microchannels.