bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport of Ions and Molecules in Nanofluidic Devices
Authors: Karnik Rohit , Duan Chuanhua , Castelino Kenneth , Fan Rong , Yang Peidong , Majumdar Arun
Published: June 2009

Interesting transport phenomena arise when fluids are confined to nanoscale dimensions in the range of 1–100 nm. We examine three distinct effects that influence ionic and molecular transport as the size of fluidic channels is decreased to the nanoscale. First, the length scale of electrostatic interactions in aqueous solutions becomes comparable to nanochannel size and the number of surface charges becomes comparable to the number of ions in the channel. Second, the size of the channel becomes comparable to the size of biomolecules such as proteins and DNA. Third, large surface area-to-volume ratios result in rapid rates of surface reactions and can dramatically affect transport of molecules through the channel. These phenomena enable us to control transport of ions and molecules in unique ways that are not possible in larger channels. Electrostatic interactions enable local control of ionic concentrations and transport inside nanochannels through field effect in a nanofluidic transistor, which is analogous to the metal-oxide-semiconductor field effect transistor. Furthermore, by controlling surface charge in nanochannels, it is possible to create a nanofluidic diode that rectifies ionic transport through the channel. Biological binding events result in partial blockage of the channel, and can thus be sensed by a decrease in nanochannel conductance. At low ionic concentrations, the effect of biomolecular charge is dominant and it can lead to an increase in conductance. Surface reactions can also be used to control transport of molecules though the channel due to the large surface area-to-volume ratios. Rapid surface reactions enable a new technique of diffusion-limited patterning (DLP), which is useful for patterning of biomolecules and surface charge in nanochannels. These examples illustrate how electrostatic interactions, biomolecular size, and surface reactions can be used for controlling ionic and molecular transport through nanochannels. These phenomena may be useful for operations such as analyte focusing, pH and ionic concentration control, and biosensing in micro- and nanofluidic devices.
Detection of Advancing Edge and Existing Length of Precursor Film Ahead Macroscopic Contact Line of Droplet Spreading on Solid Substrate
Authors: Ueno Ichiro
Published: June 2009

The author introduces a series of experimental studies on a simple but complex wetting process; a droplet spreads on a solid substrate. The spreading droplet on the solid substrate is accompanied with the movement of a visible boundary line so-called ‘macroscopic contact line.’ Existing studies have indicated there exits a thin liquid film known as ‘precursor film’ ahead the macroscopic contact line of the droplet. The present author’s group has dedicated their special effort to detect the advancing edge of the precursor film by applying a convectional laser interferometry and a high-speed camera, and to evaluate the spreading rate of the precursor film.
Low-Reynolds-Number Instability of the Laminar Flow Between Wavy Walls
Authors: Kowalewski Tomasz A. , Szumbarski Jacek , Blonski Slawomir
Published: June 2009

Instability of a viscous incompressible flow in a channel with wavy walls is investigated theoretically, numerically and experimentally. Linear stability analysis shows that appropriately chosen wall waviness leads to flow destabilization at surprisingly low Reynolds numbers. The unstable mode of disturbances forms a vortex array, which travels downstream. The remarkable feature is that the most destabilizing waviness does not introduce any additional flow resistance. The outcome of the stability analysis are consistent with the result of direct numerical simulation obtained using CFD finite volume package FLUENT (Ansys Inc.). Preliminary experimental data gained for a channel with appropriately corrugated wall seem to confirm these predictions.
Flow Boiling Heat Transfer in a Silicon Microchannel Heat Sink Using Liquid Crystal Thermography
Authors: Megahed Ayman , Hassan Ibrahim , Ahmad Tariq
Published: June 2009

The present study focuses on the experimental investigation of boiling heat transfer characteristics and pressure drop in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 295 μm, width of 254 μm, and a length of 16 mm. Un-encapsulated Thermochromic liquid Crystals (TLC) are used in the present work to enable nonintrusive and high spatial resolution temperature measurements. This measuring technique is used to provide accurate full and local surface-temperature and heat transfer coefficient measurements. Experiments are carried out for mass velocities ranging between 290 to 457 kg/m2.s and heat fluxes from 6.04 to 13.06 W/cm2 using FC-72 as the working fluid. Experimental results show that the pressure drop increases as the exit quality and the flow rate increase. High values of heat transfer coefficient can be obtained at low exit quality (xe < 0.2). However, the heat transfer coefficient decreases sharply and remains almost constant as the quality increases for an exit quality higher than 0.2.
Simulation of Microfluidic Flow Using Ferrofluids
Authors: Gunde Akshay C. , Mitra Sushanta K.
Published: June 2009

Present day microfluidics widely uses electrokinetic effects like eletrosmosis and electrophoresis to achieve flow control. These methods require extensive micromachining processes. Also, the fabrication of valves and valve-seats is difficult, which frequently leads to leakages and eventual breakdown of the system. This paper introduces the use of ferrofluids as an alternative for flow control in microchannels. Numerical simulation of flow through a microchannel using a ferrofluid in the presence of an external magnetic field is performed by coupling the flow and magnetic phenomena. An additional term calculated from the ferrofluid magnetization equations, is introduced in the Navier-Stokes equations to account for the magnetic force. The maximum velocity in a magnetically driven flow is shown to be a linear function of magnitude of magnetization of the permanent magnet. Further, the insertion of micron-size magnetic particles (referred here as magnetic plugs) in the flow field has been discussed. These plugs can be used to provide appropriate barriers to the flow by controlling their movement externally. Using the combination of ferrofluid and magnetic plugs, flow control can be achieved by the variation of external magnetic field alone.
Couette Flow With Heat Transfer in the Whole Range of the Knudsen Number
Authors: Misdanitis Serafeim , Valougeorgis Dimitris
Published: June 2009

The steady-state one-dimensional flow of a gas between two parallel plates moving with relative velocity and maintained at different temperatures is examined on the basis of the nonlinear Bhatnagar-Gross-Krook (BGK) model equation, subject to Maxwell diffuse boundary conditions. The computational scheme is based on the discrete velocity method. Results are provided for the bulk quantities of the flow in the whole range of the Knudsen number from the free molecular through the transition and slip regimes all the way up to the hydrodynamic limit. Several interesting issues related to the combined momentum and heat transfer effects are examined including the presence of a heat flux parallel to the plates, induced by the shear flow. This is a cross effect, which is absent in the Navier-Stokes-Fourier regime. The validity of the results is confirmed in several ways including the recovery of the analytical solutions at the free molecular limit.
Micro Oscillation Heat Pipe Fabricated on Silicon Wafer
Authors: Kohno Masamichi , Nishizono Takashi , Onaka Yasunori , Hidaka Sumitomo , Takahashi Koji , Takata Yasuyuki
Published: June 2009

Heat transport characteristics of micro oscillation heat pipe have been investigated. A single winding flow path consists of 28 turns microchannels fabricated on a silicon wafer the size of which was 31mm×27mm. We used heat pipe with non-uniformed cross section. Equivalent diameters of channels were 0.19 and 0.10mm. Test fluid was R141b and liquid fractions were 0, 75, 85%. It was found that steady pulsating flow occurred by increasing the number of turns and the frequency of vibration has an effect on heat transfer performance.
Rheology of Nanofluids in Microchannels
Authors: Ayela Fre´de´ric , Tillement Olivier , Chevalier Julien
Published: June 2009

Microfluidics is often presented for applications where only microliters sample volumes are available. But the benefits of microchannels do not reduce to a low consumption of fluids. From a physical and mechanical point of view, microfluidics can offer high shear rates combined with low Reynolds number and low viscous heating. It becomes possible to explore high shear rheology on a lab-on-chip. We have micromachined microviscometers to study the rheological properties of nanofluids under very high shear rates conditions. Nanofluids are fluid suspensions of solid nanoparticles. Recent experiments have indicated an anomalous increase in thermal conductivity of these suspensions. But less attention has been payed to the rheological properties of nanofluids. The few results concerning the viscosity of nanofluids exhibit scattered values higher than those of fluid suspensions of microparticles, because of a higher rate of collisions due to Brownian motion and shearing motion which enhance aggregation. These experiments were performed with commercially available rheometers over a limited range of shear rates. Our viscometers on chip are silicon — Pyrex microchannels (H ≈ 10 – 20 μm) equipped with local pressure drop sensors. Nanofluids under test were ethanol-based SiO2 nanoparticles. For particle sizes from 20 nm to 190 nm, and solid volume fractions from 1.4% to 7%, a newtonian behaviour has been observed up to 5.104 s−1. High shear rheology is the only way to reach high Peclet number values with nanoparticles in a laminar flow. It was possible to cover a wide range of Peclet number and to have Pe > 1 with diameter in the tens of nanometers range. Our results have demonstrated that an apparent solid volume fraction φa > φ, due to aggregation, was responsible of the increment of viscosity. More important was the demonstration that the shape of the clusters could be modified and that the ratio φa/φ could be lowered by a very high shear rate. Very high shearing rates in microchannels appear to be a way for nanofluids to converge to a well-defined value of viscosities.
The Capillary Rise of Liquids in a Metallic Weave: Evaporation Effects
Authors: Fries N. , Odic K. , Conrath M. , Dreyer M.
Published: June 2009

Many applications rely on the capillary rise of liquids in porous media. In spacecrafts for example, propellant management devices (PMDs) are used to provide gas free delivery of propellant during all acceleration conditions of the flight. Many PMDs use a metallic weave which, when wetted by the propellant, prevents gas from entering below a critical bubble point pressure. If volatile or cryogenic liquids are used refilling of the weave becomes important as it could dry out and become inoperative. We study the role of evaporation for the capillary rise of different liquids in a dry Dutch Twilled Weave (DTW 200×1400). The Lucas and Washburn model is extended to include evaporation and gravity effects. By comparing the experimental results with the enhanced wicking model we find good qualitative agreement. It is also noted that evaporation may have a major impact on the height gained due to capillary action.
On the Performance of Miniature Centrifugal Fans With Varying Blade Cord Length
Authors: Walsh Patrick A. , Walsh Edmond , Grimes Ronan
Published: June 2009

This paper focuses on optimizing the blade design of miniature centrifugal flow fans for application in processor cooling solutions of portable power electronics. The design parameter varied is the blade chord length and the resulting fan performance is gauged by examining flow rate, pressure rise and power consumption characteristics. The former two of these are measured using a BS848 fan characterization rig and the latter by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15mm, 24mm and 30mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number; chord length to diameter of fan ratio; flow coefficient; pressure coefficient and power coefficient. When these characteristic parameters are plotted against Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data appears to be identical to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans and provides guidelines for designers to aid in selecting the optimum fan design for a specific application.
Raman-Spectroscopy for Measuring Chemical Reactions in Micro Reactors
Authors: Ewinger Angela , Rinke Guenter , Kerschbaum Sigrid , Rinke Monika , Schubert Klaus
Published: June 2009

Micro heat exchangers, micro mixers and micro reactors have gained importance in chemical, pharmaceutical and life sciences applications. Due to the large surface to volume ratio these devices provide efficient mass and heat transfer. This results in greater selectivity and higher yield for chemical reactions. The Institute for Micro Process Engineering is working on the development, manufacturing, and testing of micro channel devices mainly manufactured of stainless steel, where channel widths and depths lie in the range of 0.2 mm. In order to obtain a better understanding of the physical and chemical processes within such components and to optimize these devices it is necessary to get a look into these micro channels during a mixing process or a chemical reaction. For this purpose laser Raman spectroscopy can be applied. This method is very selective for individual chemical compounds and allows a spatial resolution better than 0.01 mm. Figure 1 shows the experimental setup. The light of an air cooled cw argon ion laser is focused by a microscope objective into a micro channel, measuring the Raman bands over its cross section at several distances from the mixing point. A spectrograph with a CCD-array detects the Raman light, which consists of lines that are characteristic for the chemical compounds flowing through the micro channels and can therefore be used to calculate their concentrations.
Traceable Flowrate Measurements for Microfluidic Systems
Authors: Graham Emmelyn M. , Glen Norman
Published: June 2009

The rapid development and uptake of microfluidic technologies operating at ultra-low flowrates has propelled the demand for new flow sensors and flow measurement capabilities. This has opened discussions on requirements for traceable flowrate measurements to ensure accuracy and consistency in emerging ultra-low flowrate technologies. This paper presents the findings of a wide-ranging consultation on the academic and industrial requirements within the UK for traceable measurements of ultra-low flowrates employed within microfluidic systems. Areas covered include device calibration, level of accuracy required in flow measurements, applicability of CFD and the view of National Measurement Institutes.
Velocity Measurements in Microscopic Two-Phase Flows by Means of Micro PIV
Authors: Miessner Ulrich , Lindken Ralph , Westerweel Jerry
Published: June 2009

This article examines the velocity distributions of microscopic liquid-liquid two-phase flows by means of micro particle image velocimetry (micro-PIV). Aqueous droplets are dispersed into an oil bulk at the T-junction of a micro fluidic Polydimethylsiloxane (PDMS) device. The channel geometry is rectangular (H: 100μm, W: 100μm). The flow is pressure driven. Tracer particles (D: 0.5–1.2μm) are added to either phase, enabling simultaneous measurements in both phases. However, the use of immiscible liquids causes optical disturbances due to a difference in refractive indices of the two liquids and due to a curved interface geometry. Particle images are thus imaged in a distorted field of view. The results of a PIV analysis will be inaccurate in scaling as well as in location of the velocity vectors — depending on the mismatch of the refractive index. We present a basic analysis on the effect of mismatched refractive indices on the precision of the velocity measurements. The estimation is based on Snell’s law and the simplified geometry of a spherical droplet. Furthermore, we propose a method to match not only the index of refraction accurately but also to leave one additional degree of freedom to set an additional property of the liquid-liquid system, e.g. viscosity ratio or density ratio. The latter ensures that properties of the modified liquid-liquid system are close to those of the non-modified two-phase system. The findings of this study are part of the design of a Lab-on-a-Chip device. It performs a DNA analysis in an online quality control application. The miniaturization of a two-phase flow combines the benefits of confined sample compartments (i.e. droplets) with the easy-to-control process parameters of a miniaturized device (e.g. temperature, pressure). Thus band broadening of the sample by Taylor-Aris dispersion is avoided and the processes can be set accurately.
Numerical Simulation of the Interplay of Electrical Double Layers, Electrode Reactions, and Pressure-Driven Flows in Microchannels
Authors: Wa¨lter Bettina , Ehrhard Peter
Published: June 2009

We investigate the influence of flow field and electrode reactions onto an electrical double layer (EDL), which is located in the immediate vicinity of the walls of a rectangular microchannel. The precise knowledge of the EDL appears to be important for many technical applications in microchannels of small width, since the electrokinetic effects, as electroosmosis or electrophoresis, in such cases depend on the detailed charge distribution. The mathematical model for the numerical treatment relies on a first–principle description of the EDL and the electrical forces caused by the electrical field between internal electrodes. Hence, the so–called Debye–Hu¨ckel approximation is avoided. The governing system of equations consists of a Poisson equation for the electrical potential, the Navier–Stokes equations for the flow field, species transport equations, based on the Nernst–Planck equation, and a model for the electrode reactions, based on the Butler–Volmer equation. The simulations are time–dependent and two–dimensional (plane) in nature and employ a finite–volume method. It is discussed, e.g., how the thickness of the EDL expands at the stagnation point of a forced flow, as the velocity (or Reynolds number) is increased. Furthermore, the effect of electrode reactions on the ionic strength and, hence, on the EDL and the electroosmotic flow, are discussed.
Boiling Heat Transfer and Flow Patterns in Narrow Channels
Authors: Cortina Di´az Marta , Boye Gunar , Schmidt Ju¨rgen
Published: June 2009

The experimental results concerning heat transfer characteristics and flow pattern visualization in narrow annular gaps will be presented. On this account, experiments were carried out using two different test sections. An outer heated concentric annulus allowed the calculation of the heat transfer coefficients through the measurement of the outer wall temperatures using infrared thermography. On the other hand, an inner heated annulus with an outer glass tube, having the same gap size, 500 μm, was employed for flow observations using a high speed camera. By these means, both measurement methods can be combined in order to correlate the trends of the heat transfer coefficients as function of vapor quality with the observed boiling regimes.
Micro-Structured Evaporators for Laboratory Applications and Mobile Power Generation
Authors: Kolb Gunther , Tiemann David , Schu¨rer Jochen , Hessel Volker
Published: June 2009

Different microstructured evaporator concepts are presented, namely a small scale laboratory evaporator driven by electrical energy, a hot gas driven 5 kW evaporator and an evaporator for methanol steam supply to a catalytic start-up burner of a 500 W DMFC system.
Bio Inspired Particle Enhanced Capillary Heat Exchanger
Authors: Hassanipour Fatemeh , Lage Jose´
Published: June 2009

This study proposes a new cooling concept using encapsulated phase-change material particles in mini-channels. This novel method is inspired by the gas exchange process in the lung capillaries. An important characteristic of capillary blood flow is that the red blood cells fit very snugly into the capillary opening. Hence, it is conjectured that using particles with diameter similar to the channel diameter, in a manner similar to red blood cells in lung capillaries, is likely to enhance the heat transfer coefficient, even under laminar flow. Preliminary tests are performed with encapsulated Octadecan paraffin (C18H38) in a low-conductivity thin melamine shell, flowing through a test module. The effect of flow rate on the heat transfer coefficient and also the effect of using particles on enhancement of Nusselt number has been measured. Temperature distribution on the chip has also been investigated under various particle concentrations, heat fluxes and Reynolds numbers.
Effect of Charging Amount on Evaporative Heat Transfer of CO2 in a Multi-Channel Micro Tube
Authors: Jeong Siyoung , Park Dongho , Kim Dae-Hwan , Min Byung-Jun
Published: June 2009

Carbon dioxide has gained a great interest as a promising alternative natural refrigerant due to good thermodynamic characteristics. One of the most important factors in applying micro tube to carbon dioxide refrigeration system is refrigerant distribution. In this paper, evaporating heat transfer characteristics were experimentally investigated relating the charge of carbon dioxide in a multi-channel micro tube. The heat transfer coefficient and the pressure drop of the 6-channel aluminum tube were measured at various charges. It was found that the dryout quality and heat transfer coefficient were heavily dependent on the maldistribution of carbon dioxide. If the charge of CO2 was not enough, maldistribution in the multichannel micro tube became serious and a sudden drop of heat transfer coefficient was observed. These phenomena were conspicuous at high mass flux of CO2. Also, the pressure drop was greater if the CO2 was not properly distributed.
Flow and Heat Transfer in 0.0196mm Quartz Microtube
Authors: Liu Zhigang , Guan Ning , Takei Masahiro
Published: June 2009

The flow and the heat transfer characteristics in quartz microtube with inner diameter of 0.0196mm are investigated experimentally. By measuring the pressure drop between the inlet and outlet of microtube with different working fluid and the average temperature of microtube wall heated by steam, the corresponding friction factors and Nusselt number are obtained. The experimental results show the friction factors in microtubes exceeds the prediction of Hagen-Poiseulle due to the predominance of the electrical double layer effect (EDL effect) when the working fluid is distilled water, and the experimental Nusselt number is less than the classical laminar at Reynolds number<500 due to the effects of the variation of the thermophysical properties with temperature and the conjugate heat transfer. However, the effect of viscous dissipation results in temperature rise of the working fluid, due to which the friction factors are less than the prediction of Hagen-Poiseulle with the working fluid of tetrachloromethane. Using nitrogen gas as the working fluid, the rarefaction effect still brings on that the friction factor is less than the classical laminar solution, though gas density and viscosity increases resulted from the stagnant velocity between gas and inner wall and the viscous heating lead to the increase of friction factor.
Oxygen Diffusion Characteristics of Gas Diffusion Layers With Moisture
Authors: Iwasaki Daigo , Utaka Yoshio , Tasaki Yutaka , Wang Shixue
Published: June 2009

The mass transfer characteristics of the gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) are closely related to the performance. In this study, the oxygen diffusivity of paper and cloth type porous media, which are generally used as GDLs, were measured with respect to liquid water content, using experimental apparatus consisting of an oxygen sensor based on a galvanic battery. Paper type porous media, both non treated and hydrophilic treated, and the cloth type porous media with non treated surface were used as GDL specimens. The porosity of both specimens was almost the same, but the representative pore diameter of the cloth type GDL was approximately three times larger than that of paper type GDL. Two methods were utilized to impregnate liquid water into the porous GDL media to realize different water distributions in the specimens at the initial state; vacuum impregnation and moist air condensation impregnation. The oxygen diffusivities of the specimens were measured to clarify the influence of the two impregnation methods on the oxygen diffusion characteristics. Moreover, the relation between the measurement of oxygen diffusivity and the visualization of the liquid water distribution by using Neutron Radiography [Tasaki et al. (2007)] was investigated for the paper and cloth type GDLs. The oxygen diffusivity in the paper type porous media decreased precipitously with increasing water saturation by the vacuum impregnation method, whereas the diffusivity decrease was relatively small when impregnated by the moist air condensation method. For the cloth type porous media with weaving threads, oxygen diffusion characteristics were independent of the water impregnation method. Thus, the porous medium’s microstructure plays an important role in determining diffusion characteristics, especially in the presence of liquid water.
Molecular Dynamics Simulation for a Sonoluminescing Micron Bubble With Mixed Boundary Conditions
Authors: Kim Ki Young , Lim Chansoo , Kwak Ho-Young
Published: June 2009

Sonoluminescence is light emission associated with the catastrophic collapse of bubble oscillation under ultrasound. Molecular dynamics (MD) simulation of collapsing bubbles with micro and nano size equilibrium radii was performed and the results were compared with the theoretical calculations. With proper value of the thermal accommodation coefficients at the bubble wall, the MD simulation results for the bubble wall velocity, the gas temperature and pressure around the collapse point for the sonoluminescing bubbles are in acceptable agreement with theoretical predictions. Our study also revealed that the collapsing process of a sonoluminescing gas bubble proceeds almost adiabatically.
A Pneumatic Micro Peristaltic Pump With Slanted Membranes
Authors: Sheen Jyh-Jong , Su Shang-Chian
Published: June 2009

We designed and fabricated a bi-directional micro pneumatic peristaltic pump driven by slanted membranes. Unlike most micro peristaltic pumps with discrete polydimethylsiloxane (PDMS) flat membranes, ours has two opposite slanted PDMS membranes tandem along the fluid channel. Since the thickness of the slanted membrane is not uniform, it will produce continuous and asymmetric deflections under increasing pneumatic pressure. The pumping action is achieved by using one membrane as the driving membrane and the other as a stop valve. The flow direction can be controlled by changing phase difference of two driving square waves. Experimental results show that our peristaltic pump achieves much higher maximum flow rate than that of a micro peristaltic pump with three flat membranes of similar size.
Controlled Production of Monodispersed Silica Microspheres Using a Double Step-Emulsification Device
Authors: Chokkalingam Venkatachalam , Weidenhof Boris , Maier Wilhelm F. , Herminghaus Stephan , Seemann Ralf
Published: June 2009

We explore droplet based microfluidics to perform chemical reactions within microfluidic channels. By dispensing the different chemicals in droplets and subsequently merging the droplets containing different chemicals, the reactive mixture never gets in contact with the walls of the surrounding microfluidic channel. Using this approach we can realize chemical reactions for gels or precipitates, which are neither possible in single phase microfluidics, nor in droplet based microfluidics if the chemicals are mixed prior to dispersing the droplets. We explore this explicitly for the production of porous silica particles from sol-gel chemistry. All processing steps ranging from droplet production, synchronization of the droplets containing the different chemicals, combining the droplets, mixing and post processing are discussed and optimized for the particular demands.
Application of Cross-Sectional Impedance Measurement Technique to Micro Channel
Authors: Takei Masahiro , Choi Je-Eun
Published: June 2009

Fluid flow in microchannels has emerged as an important area of research due to its various applications in medical and biomedical industries, computer chips, and chemical separations. To fabricate microchannel devices, a thorough understanding of the diffusion reactions on the inside of the microchannel from the viewpoint of multiphase flow dynamics is needed. Electrical impedance tomography (EIT) is a recently developed imaging technique in which conductivity or permittivity images of a subject can be rapidly collected with channels of external electrodes. Usually EIT is visualized into a cross-sectional image representing the distribution of density. Therefore, the application of EIT to microchannels is considered in this paper. This research is a first step to developing EIT for microchannel nanoparticles concentrations in a microchannel with 60 electrodes and measured using the cross-sectional impedance measurement technique. Using the EIT system in a microchannel, a particle distribution image can be obtained in a magnetic fluid. Based on the reconstructed image, it is easy to estimate the particle diffusion behaviors in a fluid flow.
Machine Vision Based Measurement of Dynamic Contact Angle in Microchannel Flows
Authors: Heiskanen Valtteri , Kallio Pasi , Marjanen Kalle
Published: June 2009

When characterizing flows in miniaturized channels, determination of the dynamic contact angle is important. By measuring the dynamic contact angle, the flow properties of the flowing liquid and the effect of material properties on the flow can be characterized. A machine vision based system to measure the contact angle of a front or rear meniscus of a moving liquid plug is described in this paper. In this research, transparent MABS-based flow channel structures, sealed with adhesive tapes are used. The transparency of the channels enables image based monitoring and measurements of flow variables, including the dynamic contact angle. It is shown that the dynamic angle can be measured from a liquid flow in a channel using the image based measurement system. The image processing algorithm has been developed in a MATLAB environment. Imaging is done using a CCD camera (Firewire connection) and illumination is created using a custom made ring light.
Roughness Effect on Pressure Drop for Electroosmotic (EO) Flow in Microtubes
Authors: Shokouhmand Hossein , Aghvami Maziar , Moghadami Mostafa , Babazadeh Hamed
Published: June 2009

This paper presents a theoretical model of the roughness effect on friction factor and pressure drop of fully developed, laminar flow in microtubes by considering the effect of the electrical double layer. The EDL potential distribution is calculated using the Poisson–Boltzmann equation and then the velocity profile is obtained by solving the fluid momentum equation with a body force term. The wall roughness in microtubes is modeled by utilizing a Gaussian, isotropic distribution. It is found that the effect of roughness is to increase the friction factor and pressure drop of the electroosmotic flow in microtubes.
Water Removal From Hydrophilic Fuel Cell Channels
Authors: Caulk D. A.
Published: June 2009

This paper describes an approximate method for analyzing two-phase flow of gas and liquid water in fuel cell channels whose surfaces are sufficiently hydrophilic for liquid water to wick spontaneously into the channel corners. This analysis is used to address the important question of whether the gas flow at typical stoichiometries in such channels is sufficient to remove all the liquid water generated in a Proton Exchange Membrane (PEM) fuel cell. Since fuel channels are usually much narrower than they are long, it is possible to adopt the usual approximations of lubrication theory and decompose the general solution for the liquid motion into two parts: (1) that driven by the channel pressure gradient, and (2) that driven by surface shear stress from the faster moving gas. When both parts of the solution are combined with the mass balance equations, it is possible to derive a pair of partial differential equations for the water depth and gas flow rate that depend on distance down the channel and time. Steady solutions of these equations are explored to determine the amount of liquid water that accumulates in the channel over a broad range of fuel cell operating conditions.
Simulation of Transport Phenomena and Interfacial Dynamics During Czochralski Growth of Oxide Crystals
Authors: Banerjee Jyotirmay , Muralidhar K.
Published: June 2009

The quality of crystal grown in a Czochralski apparatus depends on a well organized flow and thermal field in the vicinity of the melt-crystal interface. A mathematical model that explores transport phenomena and the formation, evaluation and dynamics of the interface in a Czochralski apparatus is presented. The numerical formulation allows for the consideration of conduction in the crystal, conduction and convection in the melt and the gas phase, surface tension gradients, bulk and surface radiation and crystal rotation. Low temperature experiments using liquid crystal thermography have been conducted for validating the numerical codes. The control parameters for the Czochralski crystal growth process are crystal rotation, crucible wall and the enclosure wall temperature. Numerical simulations are carried out to establish the influence of these control parameters on the quality of oxide crystals grown in a Czochralski apparatus. Rare earth garnet YAG is considered as representative oxide material for the purpose of modeling and numerical simulation. Quantities of interest are the shape of the melt-crystal interface and the variation of pull velocity for the growth of constant diameter crystal. Steady state simulations carried out for the full Czochralski domain reveals the possibility of superheating of the crystal beyond its melting point, thus leading to the grown crystal returning to the melt. Similarly, the possibility of subcooling of the melt below the melting point of YAG at locations away from the crystal edge is indicated. The quasi-steady simulation of the growth process establishes the need for simultaneous control of crystal rotation and the crucible and enclosure wall temperature, along with the pull velocity for growth of high quality constant diameter YAG crystals.
A Numerical Study of Bubbly Flow in a Rectangular Microchannel
Authors: Ji C. Y. , Yan Y. Y.
Published: June 2009

In the present study, two phase bubbly flow phenomenon in a two-dimensional rectangular channel (system) is studied numerically by employing the VOF (Volume of Fluid) method. Hydraulic diameter of the channel is of 0.2 mm and the Bond number, a value usually used to distinguish the regimes of microchannel and minichannel, is of 0.005, far less than 0.05. Air is injected through the inlet into the microchannel which is filled with water to form a bubble. The bubble developed from a hemisphere and reshape with the increase of air from the inlet. During the flow, the volume of the bubble increases and the bubble moves along the microchannel. Slug flow is simulated with single or multi bubbles inside the microchannel. Such two-phase flow at different Reynolds number and of different microchannel length is simulated to get according slug flow image in the microchannel. The dominance of different forces acting on the bubbles, such as the buoyancy force, surface tension, initial force, etc. are analyzed. Small multi-bubbles coalesce with each other after intermittently entering the microchannel if they stay in a short distance, or remain dispersed if they are apart far away. For the large slug bubbles, coalescence is difficult to occur. With a different wettability, the flow pattern displayed in the microchannel is different. For the hydrophilic solid wall, slug bubbles flow along the mainstream liquid, whereas for the hydrophobic wall, plug-type liquid appear in the microchannel with air entrained between them. This result is in accordance with experimental observation.
A Molecular Dynamics Simulation of Droplets Merging in Mist Flow of Flow Boiling Microchannel
Authors: Ji C. Y. , Yan Y. Y.
Published: June 2009

The present paper is concerned with a molecular dynamics simulation of the behaviour of droplets merging in mist flow of flow boiling in microchannel. Two identical droplets were assigned in one simulation system and the process of their merging is investigated. The droplets are assumed to be composed of Lennards-Jones type molecules. Periodic boundary conditions are applied in three coordinate directions of a three-dimensional system, where there exist two liquid droplets and their vapour. The two droplets merge when they come within the prescribed small distance. The evolution of the merging process is simulated and presented. The merging of two droplets apart from each other at different initial distances is tested and the possible larger (or critical) non-dimensional distance, in which droplet merging can occur, is discussed. The evolution of the merging process is simulated numerically by employing the molecular dynamics (MD) method. In the present modelling, the molecules near the boundary of one liquid droplet thermally fluctuate into the range of attraction of the other droplet, forming a bridge to connect the two droplets. A dumbbell shape is then formed and thereafter an elliptic merged droplet. Eventually a larger merged spherical droplet appears in the system and is in equilibrium with its vapour phase. More realistic simulation system will be established to further the present preliminary results for application in mist flow of flow boiling in microchannel.
Particles Separation by Oscillation in a Capillary Tube
Authors: Merlen Alain , Zoueshtiagh Farzam , Thomas Peter J.
Published: June 2009

The oscillatory motion of a fluid carrying micron-sized particles inside a capillary tube is investigated experimentally. It is found that initially uniformly distributed particles can segregate and accumulate to form regularly spaced micron-sized particle clusters. The wavelength of the micro clusters is compared to data for macro-scale sand-ripple patterns and found to obey the same universal scaling as these. A dimensional analysis is performed that confirms the universality of the experimentally observed scaling. The experimental data for the micro-ripple clusters further suggest the existence of a minimum particle length scale for which patterns can form and below which the Brownian motion associated with the molecules of the matrix fluid inhibits pattern formation.
Influence of Fluid Viscosity on the Residence Time Distribution of Micro Reactors Made of LTCC
Authors: Groß G. A. , Schneider S. , Schleif B. , Ko¨hler J. M.
Published: June 2009

The residence time distribution of LTCC microreactors was investigated depending on flow rates and fluid viscosities. A pulse trace experiment was used for monitoring the tracer signals before and behind the microreactors. The tracer signals were measured by use of micro flow-through photometers without disturbing the liquid flow. Therefore, the micro photometers were mounted directly onto FEP tubings. The residence time distribution (RTD) was determined by calculation of the dispersion model using the inlet and outlet tracer profiles. The RTD of a LTCC micro mixer and a LTCC plain meandered channel mixer were determined in the flow rate range between 50 μL/min and 3 000 μL/min using water and aqueous glycerol mixtures up to a glycerol content of 50%. Received data were compared with a PTFE tube (1 mm internal diameter) as reference. A complex relation of determined RTDs between the Reynolds number (Re) and the fluid viscosities was found. A significant non-monotonous effect of the fluid viscosity was observed. The RTD as well as the tailing behavior indicates clearly viscosity-dependent changes in the fluid regime and transport mechanisms.
Computational Study of Grooved Microchannel Enhancements
Authors: Solovitz Stephen A.
Published: June 2009

As electronics devices continue to increase in thermal dissipation, novel methods will be necessary for effective thermal management. Many macro-scale enhancement techniques have been developed to improve internal flow heat transfer, with a dimple feature being particularly promising due to its enhanced mixing with potentially little pressure penalty. However, because dimples may be difficult to fashion in microchannels, two-dimensional grooves are considered here as a similar alternate solution. Computational fluid dynamics methods are used to analyze the flow and thermal performance for a groove-enhanced microchannel, and the effectiveness is determined for a range of feature depths, diameters, and flow Reynolds numbers. By producing local impingement and flow redevelopment downstream of the groove, thermal enhancements on the order of 70% were achieved with pressure increases of only 30%. Further optimization of this concept should allow the selection of an appropriate application geometry, which can be studied experimentally to validate the concept.
A Novel Hydro Magnetic Micro-Pump and Flow Controller
Authors: Dehkordi Ehsan Alavi , Moghadam Mahdi Esmaily , Shafii Mohammad Behshad
Published: June 2009

In order to deal with the limitations of micro-pumps and micro-valves and meet the advantages of magnetic systems a novel plan is described here. The idea behind the plan is that magnetic particles, mixed and dispersed in a carrier liquid, can be accumulated and retained at specific sites to form pistons in a micro-tube using some external magnetic field sources along the tube. In other words, using some solenoids and switching them on and off, in a specific order and period, causes the desired external magnetic field variation through the tube. Changing the period and the mode of activation and deactivation of the solenoids, which are called switching time and switching mode, respectively, flow can either be pumped or controlled. It is to note that, if it is required, ferro-magnetic particles can be extracted and recharged to the flow. In this research a pressure driven setup has been fabricated to make the above idea feasible to execute. The effect of the working fluid, switching time, and different concentrations on the flow rate were investigated. The experimental results corresponded to an optimum switching mode for nickel particle of diameter less than 10 microns for a constant pressure head. In order to obtain both pumping and valving characteristics of the setup, switching time was varied from 0.01 s to 5.0 s. The graphs obtained from the experiments show that best pumping performance of the setup occurred at an optimum switching time and switching mode. In addition, concentration was an important factor that affected both pumping and valving characteristics of the setup. Also, due to the differences that exist between properties of water and ethanol, changing the working fluid to ethanol resulted in a different characteristic curve.
Deriving an Analytical Model for Hydro-Magnetic Micro Flow Controller
Authors: Esmaily Moghadam Mahdi , Shafii Mohammad Behshad
Published: June 2009

Fluid control, namely pumping and valving, is a critical factor in the performance of micro-fluidic systems. In recent years a variety of micro-fluidic systems are developed for the purpose of miniaturizing fluid handling, and chemical analysis to develop Lab On a Chip (LOC) technology. The mentioned facts resulted in design and fabrication of a novel hydro-magnetic flow controller. The idea behind this device is that magnetic particles, mixed and dispersed in a carrier liquid, can be accumulated in the form of a piston. Depending upon dragging speed of these pistons, which itself is a function of switching time, this device can be used to either increase (pumping) or decrease (valving) the flow rate. The valving characteristic of the setup, which occurs at higher switching times, was concurrent with regular forming of pistons in micro-tube. Experimental results in this part show a meaningful trend for the flow rate changes versus effective parameters of the flow. Considering this fact, lead us to propose a mathematical (analytical) model, which is a function of concerning parameters. Pressure head difference, concentration, material of particles, switching time, working fluid and, switching mode, depending on their complexity, have been introduced into the mathematical model, completely theoretically or semi-experimentally. The equations were derived based on the recognition of the leakage flow through the formed pistons and the pumped flow after each switching. The model was validated by the experimental results for nickel particles of less than 10μ in diameter and 0.5 gNi/100ccH2O concentration in water for a defined pressure head in a pressure driven flow setup.
Two-Phase Flow Modeling in Microchannels and Minichannels
Authors: Awad M. M. , Muzychka Y. S.
Published: June 2009

In the present paper, three different methods for two-phase flow modeling in microchannels and minichannels are presented. They are effective property models for homogeneous two-phase flows, an asymptotic modeling approach for separated two-phase flow, and bounds on two-phase frictional pressure gradient. In the first method, new definitions for two-phase viscosity are proposed using a one dimensional transport analogy between thermal conductivity of porous media and viscosity in two-phase flow. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. In the second method, a simple semi-theoretical method for calculating two-phase frictional pressure gradient using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the final method, simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. In all cases, the proposed modeling approaches are validated using the published experimental data.