bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigation of the Electrokinetic Flow in Microchannels With Internal Electrodes
Authors: Gizewski Carsten , Ehrhard Peter
Published: July 2013

In microfluidic devices, electrokinetic phenomena can be engaged to manipulate liquids, particles, or cells. In general, electrokinetic phenomena are related to the existence of electrical double layers (EDL), which are present e.g. where solid walls are in contact with electrolytes. Due to surface charges of the solid, ions of opposite charge are attracted and lead to a thin electrically non-neutral zone — the EDL. The application of an electrical field, consequently, leads to forces onto the fluid inside the EDL, capable of driving a flow within the microchannel. This particular flow is termed electroosmotic. If further the electrodes are placed inside the microchannel, due to small distances, strong inhomogeneous electrical fields can be induced, which in turn cause complex flow fields. This is true despite low applied electrode potentials of a few Volts. Numerical simulations of various authors have shown the potential for pumping and mixing of such complex flows in microchannels with internal electrodes.

We focus on the electroosmotic flow within rectangular microchannels of 100 × 200 μm cross section. In each microchannel eight pairs of electrodes are placed onto the top and bottom glass covers, whereas the spatial offset of the electrodes is chosen differently in each of the microchannels. Due to the limited optical access through the glass covers, only the two velocity components which are tangential to the glass cover (i.e. the x– and y–components) are accessible by means of the micro–particle–image velocimetry (μPIV). The expected electroosmotic flow, however, features vortices which cannot be recognized within this x–y velocity plane. Therefore, a number of two–dimensional, two–component velocity fields is measured at different heights of the microchannel. From an integration of the continuity equation, subsequently, the third velocity component in the z–direction, which is normal to the glass cover, can be determined at reasonable accuracy. A further difficulty arises from the fact that the microparticles, used for μPIV, are not electrically neutral. Hence, in addition to drag forces from the electroosmotic flow, they experience electrophoretic forces. Hence, the particle movement appears to be a superposition of electroosmotic and electrophoretic effects.
Molded Collagen Microchannels for the Study of Cancer Cell Invasion
Authors: Kraning-Rush Casey M. , Carey Shawn P. , Reinhart-King Cynthia A.
Published: July 2013

Metastasis is the cause of 90% of cancer-related deaths and yet the precise mechanism of metastasis is poorly understood[1]. To metastasize, cells break free from the primary tumor, migrate through the surrounding tissue, and enter the vascular system to move to a secondary site. To migrate through the stroma, cell can both degrade the tissue and use physical force to move the tissue from its path. However, the relative roles of matrix degradation and cellular force are not well-understood. Previous work has shown that as cell move through the matrix, they create channels that other cells can then use to more easily escape from the primary tumor [2, 3]. To investigate the mechanisms by which metastatic cells move through 3D matrices, we fabricated microchannels from collagen that simulate the paths that are made and used by cells during metastasis. Here, we will present our method for molding channels in compliant collagen gels to investigate cell migration. The channels dimensions approximate the size of a cell and permit cell migration without the need for matrix degradation. We demonstrate that the channels cause persistent, unidirectional cell migration that is significantly faster than the migration observed in 3D matrices without channels. These channels provide a unique platform to probe 3D cellular migration and permit the dissection of the relative roles of matrix degradation and force generation in facilitating cell invasion.
Thin-Film Heat-Flux Microsensor for Heat-Transfer Measurement in Micro-Heat Exchangers/Microreactors
Authors: Ammar Houssein , Hamadi David , Garnier Bertrand , Ould El Moctar Ahmed , Peerhossaini Hassan , Monti Fabrice , Willaime Herve
Published: July 2013

Heat-transfer analysis in microfluidic devices is of great importance in applications such as micro-heat exchangers and microreactors. This work reports on improvements in temperature measurement techniques, which can be the source of large errors due to their intrusiveness and the unreliability of conventional thermal sensors. Gold thin films were deposited on a borosilicate substrate to realize a 2D heat flux sensor for heat-transfer measurement along the main flow within microchannels. Two applications are shown, one related to micro-heat exchangers and the other to microreactors. For the micro-heat exchanger, the effect of length scale on heat transfer in a straight microchannel was investigated and the validity of macroscale correlations for convective heat transfer was checked for deionized water flowing in microchannels of heights 12 to 52 μm. For the microreactor, the reaction enthalpy of an acid–base reaction measured using the new heat-flux sensor had only a 5% discrepancy from the standard value, showing the efficiency of the new thin-film device.
Mixing, Reaction and Heat Transfer in a Pulsatile Flow Microreactor: Infrared Measurements
Authors: Ammar Houssein , Garnier Bertrand , Ould El Moctar Ahmed , Peerhossaini Hassan
Published: July 2013

Single-phase microreactors and micro-heat-exchangers have been widely studied over the last decade. Greater safety, better temperature control and hence better reaction products are provided by the high surface-to-volume ratios and compactness of microscale devices, making them more attractive than conventional systems for future industrial applications. Since the flow in microfluidic devices is predominantly laminar long mixing channels or complex geometries are needed for molecular diffusion and completion of the reaction. Hoping to remove this drawback, we demonstrate experimentally the merits of flow pulsation in enhancing mixing efficiency in a simple T channel (50 μm high, 500 μm wide and 40 mm long). For this purpose, a U-shaped PDMS microchannel is enclosed by a 0.1 mm glass plate coated with an opaque layer that constitutes the fourth wall of the microchannel. We investigate experimentally the effect of flow pulsation on mixing of an acid-base neutralization reaction. The mixing of HCl and NaOH reactants, both of 0.8 mol/L concentration, in a T-shaped microchannel is assured by time-pulsed flows at small Reynolds numbers. The effect of mixing is observed at the location along the microchannel at which the temperature reaches its maximum value. For this purpose, infrared thermography, a non-intrusive temperature measurement technique with spatial resolution of a few tens of microns, is used. The mixing efficiency is shown to depend strongly on the ratio between the peak amplitude and the mean flow rate (between 0.4 and 1 mL/h). Saturation is observed for values of this ratio greater than 2.5. Mixing also appears to be enhanced for frequencies of the periodic inlet flows increasing from 1.25 Hz to 5 Hz, i.e. for increasing Strouhal number.
Design of Tree-Shaped Microchannel Networks Submitted to Simultaneous Pressure Driven and Electro-Osmotic Flows
Authors: Barrot Christine , Colin Stéphane
Published: July 2013

Microchannel networks can be efficiently used for several applications. For example, they can be the main elements of micro chemical reactors or micro heat exchangers for cooling electronic chips. In such networks, the flow of liquid can be generated either by a pressure difference, by electro-osmosis or by both of them. The design of the network can be optimized in order to deliver a maximum flowrate. In this paper, an analytical study of a pressure driven and electro-osmotic flow in tree-shaped microchannel network is developed. The network is built with a series of rectangular microchannels with high aspect ratio. Each bifurcation connects a parent microchannel to a couple of twin child microchannels. The objective of this work is to determine the geometrical configuration which offers the highest flowrate. The efficiency of the tree-shaped network is compared to the efficiency of a series of parallel microchannels, for the same inlet and outlet values of electric potential and pressure and for the same network volume. Focusing on one bifurcation, the influence of the thickness of the electrical double layer is discussed. The optimal geometric dimensions, such as the ratio of the child over parent microchannel widths and the ratio of the parent over total microchannel lengths, are calculated. The influence of the number of bifurcations is also analyzed and optimal design rules are proposed.
Sensitivity Analysis of Mass Transport Properties of Gas Diffusion Layers of Polymer Electrolyte Membrane Fuel Cells
Authors: Tahseen Siddiq Husain , Milani Abbas S. , Hoorfar Mina
Published: July 2013

A prodigious amount of research has been conducted to study and measure the different properties associated with the mass transfer phenomenon inside the GDL like diffusivity, permeability, porosity, thickness. However, the functional relationship of these parameters with the limiting current; the effect of the individual factor over others; and the interaction, correlation and interdependence of these factors have been subject of little work. Using the experimental data presented in the literature, this paper presents a methodology developed based on a regression model to predict the limiting current from GDL properties. Statistical techniques like factorial design and response surface methodology are used to perform the sensitivity analysis of relevant parameters. The emphasis will be on factor screening to identify efficiently the parameters with most dominant effects. The results obtained will then be used to select the most effective GDL (among those characterized) based on multi criteria decision making tools like weighting sum method (WSM) and TOPSIS with entropy weight. The final goal is to elucidate the impact of each property, and hence to identify the most important parameters that need to be studied and characterized to engineer a better GDL with enhanced water management capabilities.
Flow Pattern, Void Fraction and Pressure Drop During Adiabatic Two-Phase Gas-Liquid Flow in Vertical Micro-Channel
Authors: Saisorn Sira , Wongwises Somchai
Published: July 2013

The experimental investigation is performed to study two-phase flow pattern, void fraction and pressure drop characteristics in a vertical micro-channel. The test section is a fused silica tube with a diameter of 0.53 mm and a length of 320 mm. Air and water are used as working fluid which is introduced to the test section in vertical upward direction. The test runs are done at superficial velocities of gas and liquid ranging respectively from 0.375 to 21.187 m/s and 0.004 to 2.436 m/s. Stereozoom microscope mounted together with camera are employed to conduct flow visualization from which slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed. Based on image analysis, void fraction data are obtained and found to be linear relationship with volumetric quality. The frictional pressure drop is relatively high when the formation of churn flow is established. Besides, the two-phase frictional multiplier is found to be strongly dependent on both mass flux and flow pattern.
An Experimental Study on Heat Transfer Performance of Iron Oxide Based Ferrofluids
Authors: Kurtoglu Evrim , Kaya Alihan , Yagci Acar Havva Funda , Kosar Ali
Published: July 2013

Nanofluids are colloidal compounds, where the solid phase material is composed of nano sized particles, and the liquid phase can potentially be any fluid but aqueous media are common. As a common nanofluid type, ferrofluids are formed by holding solid nanoparticles in suspension by weak intermolecular forces and may be produced from materials with different magnetic properties. Magnetite is one of the materials used for its natural ferromagnetic properties. Heat transfer performance of ferrofluids is one of the crucial properties among many others that should be analyzed and considered for their wide range of applications. For this purpose, experiments were conducted in order to characterize heat transfer properties of ironoxide based ferrofluids flowing through a microchannel. Promising results were obtained from this study, which are suggesting the use of ferrofluids for heat transfer applications can be advantageous.
Kinetic Analysis on Nanoparticle Condensation by Molecular Dynamics
Authors: Suh Donguk , Yasuoka Kenji
Published: July 2013

Condensation on a cubic seed particle was simulated by classical molecular dynamics. Seed size and supersaturation ratio of the system were the factors that were examined in order to observe the effects of the dimension of seeds and thermodynamic conditions. Two stages of nucleation were observed in the phenomenon, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the nucleation rate and growth rate were each calculated by the Yasuoka-Matsumoto method. As seed size increased the growth rate decreased, but there was no clear seed influence on the homogeneous nucleation characteristics. Besides the classical nucleation theory, cluster formation free energy and kinetic analysis were conducted. The free energy in the exponential term of the classical nucleation theory and that obtained from the cluster formation free energy showed different characteristics.
An Integrated Platform for Single Molecule Free Solution Hydrodynamic Separation Using Yoctomoles of DNA and Picoliter Samples
Authors: Liu Kelvin J. , Rane Tushar D. , Zhang Yi , Beh Cyrus W. , Friedrich Sarah M. , Shin Dong Jin , Wang Tza-Huei
Published: July 2013

We report a microfluidic platform for sizing and separation of DNA called PicoSep that is among the most sensitive to date, requiring only yoctomoles of DNA (10−24 moles) and picoliters of sample (<10 pL). The cohesive integration of cylindrical illumination confocal spectroscopy based (CICS) single molecule counting with free solution hydrodynamic separation increases detection sensitivity >103-fold over traditional laser induced fluorescence (LIF) and vastly improves quantification accuracy. Separation is performed via single molecule free solution hydrodynamic separation (SML-FSHS). SML-FSHS relies on the wall exclusion mechanism to separate DNA as it is driven down buffer filled microchannel. High separation efficiency (plate number = 105 – 106) and high sizing resolution (37 bp – 2.1 kbp) are obtained across a wide dynamic range of DNA (100 bp – 27 kbp) in a single separation. Quantitative accuracy up to the limits imposed by molecular shot noise is achieved, and a limit of detection <20 molecules is demonstrated. Through the development of PicoSep, we have significantly reduced the cost and complexity of single molecule instrumentation while achieving sizing and quantification performance that surpasses capillary electrophoresis (CE). Unlike CE, no viscous sieving matrices, high voltage power supplies, or capillary wall coatings are necessary, making devices inexpensive, simple to fabricate, and easy to incorporate into lab on a chip systems.
Evaluation of Lattice Boltzmann Method for Reaction-Diffusion Process in a Porous SOFC Anode Microstructure
Authors: Paradis Hedvig , Sundén Bengt
Published: July 2013

In the microscale structure of a porous electrode, the transport processes are among the least understood areas of SOFC. The purpose of this study is to evaluate the Lattice Boltzmann Method (LBM) for a porous microscopic media and investigate mass transfer processes with electrochemical reactions by LBM at a mesoscopic and microscopic level. Part of the anode structure of an SOFC for two components is evaluated qualitatively for two different geometry configurations of the porous media. The reaction-diffusion equation has been implemented in the particle distribution function used in LBM. The LBM code in this study is written in the programs MATLAB and Palabos.

It has here been shown that LBM can be effectively used at a mesoscopic level ranging down to a microscopic level and proven to effectively take care of the interaction between the particles and the walls of the porous media. LBM can also handle the implementation of reaction rates where these can be locally specified or as a general source term. It is concluded that LBM can be valuable for evaluating the risk of local harming spots within the porous structure to reduce these interaction sites. In future studies, the information gained from the microscale modeling can be coupled to a macroscale CFD model and help in development of a smooth structure for interaction of the reforming reaction and the electrochemical reaction rates. This can in turn improve the cell performance.
Heat Transfer Enhancement During Pool Boiling of Water Over Horizontal and Vertical Tubes With Micro Structured Surfaces
Authors: Mehta Jeet S. , Kandlikar Satish G.
Published: July 2013

Pool boiling is a stable and an efficient method for transferring large quantities of heat. This mode of heat transfer is used in a wide range of applications, including steam generation in boilers, petrochemical, pharmaceutical, cryogenic and many other industrial processes. It also holds promise for cooling of microelectronic devices, such as lasers, microprocessors and others. The objective of this work is to investigate the heat transfer augmentation due to an array of micro structured surfaces over a circular tube. The effects of horizontal and vertical orientation of the tubular test section on heat transfer enhancement are also studied. The bubble nucleation, growth and interactions over the micro structured surfaces are analyzed using high speed cameras to understand the bubble dynamics.
Micro Particle Image Velocimetry and Numerical Investigation of Micro Couette Blood Flow
Authors: Mehri R. , Mavriplis C. , Fenech M.
Published: July 2013

The purpose of the work presented this paper is to design a model to study experimentally and numerically a micro-Couette blood flow to obtain a constant and controlled shear rate that is a suitable environment for analysis of Red Blood Cell (RBC) aggregation. Due to the simplicity of the flow conditions, aggregate size can be related to the constant shear rate applied. This Couette flow is created by the motion of a second fluid that entrains the blood. The experimental work is coupled with 3D numerical simulations performed using a research computational fluid dynamics solver, Nek5000, based on the spectral element method, while the experiments are conducted using a micro-particle image velocimetry system. Two models of microchannels, with different dimensions, 150 × 33μm and 170 × 64μm, are fabricated in the laboratory using standard photolithography methods. The design of the channel is based on several parameters determined by the simulations. A Newtonian model is tested numerically and experimentally. Blood is then tested experimentally to be compared to the simulation results. We find that using a velocity ratio of 4 between the two Newtonian fluids, we create a flow where one third of the channel thickness is filled with the fluid destined to be blood. In the blood experiments, the velocity profile in this layer is approximately linear, resulting in the desired controlled conditions for the study of RBC aggregation.
Dynamic Wetting on Moving Surfaces: A Molecular Dynamics Study
Authors: Ritos Konstantinos , Dongari Nishanth , Zhang Yonghao , Reese Jason M.
Published: July 2013

We report molecular dynamics (MD) simulations of the dynamic wetting of nanoscale droplets on moving surfaces. The dynamic water contact angle and contact angle hysteresis are measured as a function of capillary number on smooth silicon and graphite surfaces. The hydrogen bonding and density profile variations are also reported, and the width of the water depletion layer is evaluated for droplets on three different static surfaces: silicon, graphite and a fictitious super-hydrophobic surface. Our results show that molecular displacements at the contact line are mostly influenced by interactions with the solid surface, while the viscous dissipation effects induced through the movement of surfaces are found to be negligible, especially for hydrophobic surfaces. This finding is in contrast with the wetting dynamics of macroscale droplets, which show significant dependence on the capillary number. This study may yield new insight into surface-wettability characteristics of nano droplets, in particular, developing new boundary conditions for continuum solvers for liquid flows in micro- and nanoscale devices.
Pool Boiling Heat Transfer Over Microchannel Surfaces With Ethanol at Atmospheric Pressure
Authors: Kalani Ankit , Kandlikar Satish G.
Published: July 2013

The growing trend in miniaturization has brought forth boiling as an important research topic for augmentation of heat transfer in electronic cooling application. Pool boiling has the ability to dissipate a large quantity of heat while maintaining a small temperature difference. The heat dissipation capacity of pool boiling is further augmented through the use of enhanced microchannel surfaces. The objective of this work is to investigate the pool boiling performance of various microchannel structures using ethanol as the fluid medium. Ethanol has higher heat of vaporization compared to refrigerants and lower saturation temperature compared to water. Hence it has the potential to be used as an alternative cooling medium. This investigation will focus on experimentally studying the effect of enhanced surface structures on pool boiling of ethanol at atmospheric pressure.
Biotemplated Nanostructured Surfaces for Enhanced Phase Change Heat Transfer
Authors: King S. M. , Rahman Md. M. , Krick A. K. , Branco L. D. , Olceroglu E. , McCarthy M.
Published: July 2013

The fabrication and characterization of biotemplated nanostructured coatings based on the Tobacco mosaic virus for enhanced phase-change heat transfer is reported. A simple room temperature nanofabrication process, using the self-assembly and mineralization of the Tobacco mosaic virus (TMV), has been implemented to create superhydrophilic surfaces. Using this technique, a variety of structured surfaces have been fabricated and characterized showing enhanced surface wettability and heat transfer characteristics. High-speed images of droplet impact evaporation on flat and hierarchical samples have been recorded, showing increased wetting and evaporation for the nanostructured surfaces. The addition of nanostructures increases the heat transfer rate by more than a factor of three as compared to the flat surfaces, and hierarchical surfaces exhibit heat transfer rates more than an order of magnitude larger than flat non-structured surfaces. Additionally, an increase in Leidenfrost temperature of 100°C as compared to flat samples has been recorded. TMV nanostructures were also assembled onto the walls of heated minichannels, promoting continuous bubble detachment as well as reduced slug formation and instabilities during flow boiling. While bare minichannel exhibits nearly complete dry-out, the nanostructured channels maintain annular flow at similar loadings. This work demonstrates the feasibility of enhancing phase-change heat transfer using TMV structured coatings.
Creeping Flow Through Microchannels With Integrated Micro-Pillars
Authors: Tamayol Ali , Gunda Naga S. K. , Akbari Mohsen , Mitra Sushanta K. , Bahrami Majid
Published: July 2013

Pressure drop through micro-pillar-integrated mini/microchannels is studied experimentally and analytically. Following our previous studies, the low aspect ratio micropillars embedded in a microchannel are modeled as a porous medium sandwiched between channel walls. The pressure drop is expressed as a function of the salient geometrical parameters such as channel dimension, diameter and spacing between the adjacent cylinders as well as their arrangement. To verify the developed model, several silicon/glass samples with and without integrated pillars are fabricated using the deep reacting ion etching (DRIE) technique. Pressure drop measurements are performed over a range of water flow rates ranging from 0.1 ml/min to 0.5 ml/min. The proposed model is successfully verified with the present experimental data. A parametric study is performed by employing the proposed model, which shows that the flow resistance has a reverse relationship with the micro-pillar diameter and the mini/microchannel porosity. In addition, staggered arrangements have a significantly lower flow resistance than squared arrays of pillars especially in dense structures.
Ten Stage Knudsen Gas Pump
Authors: Miles Stephanie D. , McNamara Shamus , Pharas Kunal
Published: July 2013

A Knudsen gas pump is driven by the thermal transpiration effect. In this study, a Knudsen gas pump was fabricated which is powered by using a thermoelectric module. By adding current to the thermoelectric module, each module’s ceramic plate is heated/cooled causing a temperature gradient when positioned properly across the nano-porous membranes. Thermal transpiration occurs, causing a pressure gradient across the channels of the membrane which then induces a gas flow across the membrane. During the study, each single pump was initially tested for pressure performance by powering the thermoelectric at 6 and 15 Watts. Loaded and unloaded flow rates were also tested for a single pump. Five and 10 pumps were then connected into series and the pressure performance was tested at 6 and 15 Watts. The flow rate under loaded and unloaded conditions was also tested. It has been demonstrated that the 10 stage series Knudsen pump creates a significantly larger pressure than a 5 stage series or a single stage Knudsen pump.
Harnessing Chaotic Flow Effects in Microscale Natural Convection to Achieve Accelerated Biochemistry
Authors: Ugaz Victor M.
Published: July 2013

The lack of rapid, affordable, and easy to use medical diagnostic technologies is a critical issue confronting global public health. A major challenge to these efforts lies in the design of instrumentation used to perform a key step in the analysis. This step, the polymerase chain reaction (PCR), involves a sequence of thermally activated biochemical processes that selectively replicate well-defined sub regions within a longer DNA strand. Although PCR is generally considered to be a mature technology from a biochemical standpoint, many limitations are still imposed by the highly inefficient design of conventional PCR thermocycling hardware that is slow, expensive, and consumes considerable electrical power to repeatedly heat and cool the reagent mixture. Here we describe an alternative thermocycling approach that has the potential to addresses these needs by harnessing thermally driven natural convection to perform rapid DNA amplification via the PCR. A buoyancy driven instability is induced within a confined volume of fluid by imposing a spatial temperature gradient. Under the right conditions (fluid properties, geometry, temperature gradient, etc.) a stable circulatory flow pattern can be established that will repeatedly transport PCR reagents through temperature zones associated with each stage of the reaction. The inherently simple design (similar in principle to a lava lamp) and minimal electrical power consumption make this approach well-suited for use in portable applications. We also describe our computational and experimental studies of the flow fields established within convective thermocycling reactors, revealing a rich complexity not found in most steady laminar flows. These complexities arise because, under the thermal conditions associated with PCR, the nature of the buoyancy driven instabilities that initiate and sustain motion make it necessary to operate in a transition regime associated with the onset of convective turbulence. These unique characteristics can be harnessed to guide the design of new devices capable of generating optimal conditions for ultra-rapid PCR replication.
Simulation of Constant Pressure and Flow Rate Through Mini-Channels Inserted Into Distribution Systems (WDS)
Authors: Banna Muinul H. , Najjaran Homayoun , Sadiq Rehan , Rodriguez Manuel J. , Imran Syed A. , Hoorfar Mina
Published: July 2013

The miniaturised online sensors that were developed in the laboratories were for atmospheric pressure and steady state flow, but in the water distribution network neither the pressure nor the flow is steady. Many of the state of the art drinking water quality monitoring sensors can be operated well below the drinking Water Distribution System (WDS) pressure. Moreover, each of the sensors requires different flow rates. This paper discusses simulation and design of an affordable constant flow and constant outlet pressure system and shows an easy way to provide different flow rates for different sensors. The other criterion which should be met is the flow rate of the water bled (leakage) from WDS which must also be low. To meet the above criteria a 2-D model was developed to represent the constant pressure constant flow system for online water quality monitoring (WQM) sensors. Different configuration of the system is considered and the optimum design includes 1.044 m/s flow velocity which is low enough for the flow to be steady.
Evaporation-Induced Cassie Droplets on Superhydrophilic Microstructured Surfaces
Authors: Adera Solomon , Raj Rishi , Enright Ryan , Wang Evelyn N.
Published: July 2013

A droplet deposited on a rough, lyophilic surface satisfying the imbibition condition, results in complete wetting. However, in this work, we demonstrate that this behavior can be altered by superheating the substrate such that droplets can reside in a non-wetting Cassie state due to evaporation. Photolithography and deep reactive ion etching were used to fabricate a well-defined silicon micropillar array with diameter, height, and center-to-center spacings of 5.3, 21.7 and 27.5 μm, respectively. Water droplets placed on this microstructured surface at room temperature demonstrated superhydrophilic behavior with liquid filling the voids between pillars resulting in a vanishing contact angle. However, when the microstructured surface was superheated above a critical value, the superhydrophilicity was lost and non-wetting Cassie droplets were formed. The superheat required to deposit a Cassie droplet (>75°C) was found to be significantly higher than that required to sustain an already deposited Cassie droplet (<35°C). Interestingly, the superheat required to sustain a Cassie droplet after the initial deposition was found to decrease with the square of the droplet radius. These observations where an inherently superhydrophilic structured surface turns into superhydrophobic at nominal superheats has implications for phase change based heat transfer applications where the loss of contact between the substrate and the heat transfer fluid can be detrimental to the device performance.
Simulations of Forced Convection Heat Transfer of Nanofluids in Parallel-Plate Microchannels Using Dissipative Particle Dynamics
Authors: Yamada Toru , Asako Yutaka , Hong Chungpyo , Faghri Mohammad
Published: July 2013

The effect of nanoparticle volume fraction on hydrodynamic and thermally fully developed forced convection heat transfer in parallel-plate channels was numerically studied under constant wall temperature boundary condition using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be two-dimensional with periodic boundary condition in the flow direction and no-slip boundary conditions at top and bottom walls of the channel. The results were shown via fully developed velocity and temperature distributions across the channel cross-section. Nusselt numbers are calculated from the gradient of temperature at the wall and compared with the analytical solutions. The effect of nanoparticle volume fraction on Nusselt number was discussed.
Microfluidics Based Impinger for Air Sampling
Authors: Song Minghao , Sun Hongwei
Published: July 2013

Capturing particles from the air into the liquid is critical for the development of analysis systems for bio-particles such as virus, toxins and spores. We report a miniaturized airborne particle sampling (collection) device that relies on a bubbling process formed in a small chamber where air (to be sampled) is flown through a microchannel array into the liquid in the chamber. The airborne particles trapped in the tiny bubbles (diameter: 80 microns) are diffused into liquid and captured into the liquid and analyzed. The whole device is fabricated on a glass slide using soft-lithography method with Polydimethylsiloxane (PDMS) as the structural material. To prevent the air leakage at the connections, a special sealing process depending on PDMS only without the use of any glue was successfully developed. Hydrophobicity of channel surface was found to be critical for generating continuous and stable bubble lines in the bubbling process. The collection efficiency is measured by collecting polystyrene latex particles (diameter: ∼1 micron) on polycarbonate membrane filters at the inlet and outlet of the device. It was found that a collection efficiency of 90% from the microfluidics based impinger is achieved, which is much higher than that of conventional impinger device. Furthermore, a collection time of 10 minutes is needed for this device compared to a few hours for a conventional impinger.
Numerical Study of the Impact of Manifold and Microchannel Geometry on Flow Uniformity and Pressure Characteristics Within a Parallel Microchannel Array
Authors: Mohammadi Mahshid , Sharp Kendra , Jovanovic Goran , Paul Brian
Published: July 2013

Flow uniformity between individual channels within a microchannel array can be a significant factor affecting the performance of laminated-sheet chemical microreactors and micro-heat exchangers. Small dead volume and low residence time through the manifold are also typically favorable in these applications. In the specific application where mass transfer is desired between membrane-separated layers of microchannels, the use of shallow microchannels is important for achieving effective diffusion but negatively impacts the flow distribution among the channels. However, by implementing a thoughtful manifold design the manifold volume can be minimized while maintaining flow uniformity. In this study, numerical modeling is used to quantitatively investigate the impact of the geometry of the manifold and the structural parameters of the microchannels on desired uniformity and pressure drop within the low-Reynolds number system. The CFD tool COMSOL is used for the simulations.
Experimental Study on the Effect of Synthetic Jets on Flow Boiling Heat Transfer in Microchannels
Authors: Fang Ruixian , Khan Jamil
Published: July 2013

Two-phase flow instabilities in micro-channel exhibit pressure and temperature fluctuations with different frequencies and amplitudes. An active way to suppress the dynamic instabilities in the boiling micro-channels is to introduce synthetic jets into the channel fluid. Thus the bubbles can be condensed before they clog the channel and expand upstream causing flow reversal.

The present work experimentally investigated the effect of synthetic jets on the suppression of flow boiling instabilities exhibited in a micro-channel heat sink. The heat sink is consisted of five parallel rectangular microchannels measured 500 μm wide, 500 μm deep each. An array of synthetic jets was placed right above the micro-channels with each channel corresponds to 8 jet orifices. The strength and frequency of the jets are controlled by changing the driving voltage and frequency of the piezoelectric driven synthetic jet actuator. Tests were performed with synthetic jets operating at 80 Hz and 150 Hz respectively. It is found that the bubbles were effectively condensed inside the jet cavity. The boiling flow reversals were notably delayed by the synthetic jets. Meanwhile, the pressure fluctuation amplitudes were substantially reduced. Test results were analyzed and discussed in detail.
A Standard Method to Determine Loss Coefficients of Conduit Components Based on the Second Law of Thermodynamics
Authors: Schmandt Bastian , Herwig Heinz
Published: July 2013

Losses in conduit components of a pipe system can be accounted for by using component specific loss coefficients K. Especially in mini- and micro-systems an exact knowledge of these loss coefficients (which in laminar flow strongly depend on the Reynolds number) is important. Limited space will generally lead to a high loss-contribution of single components compared to the contribution of the straight channels. The determination of K-values of single components based on a numerical simulation using the Second Law Analysis (SLA) has turned out to be a very attractive method. The simulation of the flow field shows the distribution of losses and upstream and downstream lengths of impact (Lu, Ld) where the otherwise fully developed flow is affected by the component. The numerical SLA-Method is introduced as a standard method, illustrated and validated with highly accurate measurements in a 90 deg bend with a square cross section. The local entropy generation rates based on the numerical simulation of the flow field are computed and carefully interpreted. Component specific values of K, Lu are Ld are collected in a table and illustrated by plots of the entropy generation rate distribution along the bend’s centerline. Validation is achieved with experimental results from a test facility exclusively built for this purpose: Laminar flow in a 90 deg bend is induced by a controlled gear pump with polydimethylsiloxanes of different viscosities as working fluids.
On the Kinetics of Formation of Hollow Poly(Lactic Acid) Microcapsules Fabricated From Microbubble Templates
Authors: Molino Jay J. , Daiguji Hirofumi , Takemura Fumio
Published: July 2013

Biodegradable hollow poly-lactic acid (PLA) microcapsules were synthesized by directly adsorbing the polymer to N2 (air) microbubbles using the bubble template method. In this method microbubbles nucleate inside droplets, made of a solution of PLA in dichloromethane, as a result of solvent diffusion into a continuous phase of either water or a PVA aqueous solution. Subsequently PLA adsorbs to the microbubble surface, covers it and then the microbubble covered with PLA is spontaneously released from the drople’s interior yielding to hollow PLA microcapsules. For special fabrication conditions, it was found that the final capsules were uniform in size. It was found that the high uniformity is directly correlated to the type of release. This was classified into either single or multiple bubble release. When the predominant type of release is single bubble release, the resultant microcapsules were uniform. In this study we also aim to elucidate the conditions required to attain single bubble release. It is believed that this type of release is attained when the energy barrier at the liquid-liquid interface is lowered, thus microcapsules can be smoothly released. From experiments, it was understood that the use of PVA, a low molecular weight PLA and an initially low PLA concentration is the requirement in order to attain single bubble release. Furthermore, based on rheological and the surface tension measurements at the liquid-liquid interface and inner flow circulation inside the droplet we proposed two plausible mechanisms for single bubble release: (a) mechanism based on the repulsive force between two adjacent microbubbles and (b) mechanism based on the imbalance of interfacial tensions around a bubble.

SEM images revealed that microcapsules obtained from ‘multiple bubble’ type can display a non-spherical cores. In addition, these capsules can have briged shells or bridged cores (share cores). Microcapsule bridging can happen inside the droplet (when cores are shared) or outside (when only the shell is bridged). Increasing the initial concentration of PLA or using a PLA of high molecular weight yields to this phenomenon. The zeta potential of these capsules at pH = 7.0 was above 20mV, and thus the colloidal suspension of microcapsules in water was stable.
Transitional and Turbulent Convective Heat Transfer of Compressible Gas Flows Through Microtubes
Authors: Yang Yahui , Hong Chungpyo , Morini Gian Luca , Asako Yutaka , Lorenzini Marco , Brandner Juergen J.
Published: July 2013

This paper presents the results of experimental and numerical investigation of forced convection of gas flows through stainless steel microtubes having inner diameters of 750 μm, 510 μm and 170 μm. The study covers both transitional and turbulent flow regimes (3000<Re<12000). In these regimes the flow is highly compressible, inducing conversion from thermal energy to kinetic energy inside microtubes. Moreover, reverse energy conversion takes place immediately after the fluid is vented to the outlet chamber where the measurement of fluid outlet temperature is performed. In this work the effects of fluid compressibility on the forced convection at microscale is quantitatively discussed by combining experimental data with numerical predictions. It is evidenced that compressibility effects can distinctively enhance convective heat transfer in terms of Nusselt number. This enhancement turns out to be more pronounced for microtubes with smaller inner diameter even at medium Reynolds numbers. In order to explore in-depth the heat transfer mechanism, the system is numerically simulated adopting the Arbitrary-Lagrangian-Eulerian (ALE) method and the Lam-Bremhorst Low-Reynolds number turbulence model to evaluate eddy viscosity coefficient and turbulence energy. The crossing of the numerical data, which provide the local value of pressure and temperature, with the experimental ones helps to explain the physical sense of the experimental results. In addition, the convective heat transfer coefficients obtained in the present work are compared with both classical correlations validated for conventional pipes and the correlations proposed for gas flows through microtubes.
Numerical Simulations of Hydrodynamics and Heat Transfer in Wavy Falling Liquid Films on Vertical and Inclined Walls
Authors: Yu Hongyi , Gambaryan-Roisman Tatiana , Stephan Peter
Published: July 2013

The flow of thin falling liquid films is unstable to long-wave disturbances. The flow instability leads to development of waves at the liquid-gas interface. The wave patterns depend on the properties of the liquid, the Reynolds number, the plate inclination angle, and the distance from the film inlet. The effect of the waves on heat and mass transfer in falling liquid films is a subject of ongoing scientific discussion.

In this work numerical investigation of the wave dynamics has been performed using a modified Volume of Fluid (VOF) method for tracking the free surface. The surface tension is described using the Continuum Surface Force (CSF) model. At low disturbance frequency solitary waves of large amplitude are developed, which are preceded by low-amplitude capillary waves. At high disturbance frequency low amplitude sinusoidal waves are developed. The wave parameters (peak height, length, propagation speed) are computed from the simulation results and compared with available experimental correlations in a wide range of parameters. The effects of the disturbance frequency and the plane inclination angle on the wave dynamics have been studied. The interaction of waves initiated by simultaneous disturbances of two different frequencies has been investigated.

The heat transfer in the wavy film has been simulated for constant wall temperature boundary condition. The effect of the Prandtl number and the disturbance frequency on the local and global heat transfer parameters has been investigated. It has been shown that the influence of waves on heat transfer is significant for large Prandtl numbers in a specific range of disturbance frequencies.
Numerical Simulation of Thermo-Pneumatic Effects for Gas-Liquid Flows in Heated Microchannels
Authors: Prigent Guillaume , Duluc Marie-Christine , Le Quéré Patrick
Published: July 2013

Numerical simulations of an air bubble flowing in a micro-channel filled with liquid water and heated on the walls are presented. Due to the small channel size, pressure changes may be induced by a heat supply on the walls. The aim is to quantify these pressure changes. A numerical model, allowing for the description of an incompressible liquid and a compressible gas bubble is presented. Air in the bubble is modeled as a perfect gas, under the low Mach approximation while the liquid is modeled as an incompressible fluid. The Navier Stokes equations and the energy equation are solved. The interface description is performed combining front tracking techniques and a Heaviside step function. A first validation test-case is presented. Then, various heating conditions are examined including uniform and non uniform heating conditions, constant and time dependent heating conditions. It is shown that pressure oscillations in the bubble may be induced by an appropriate thermal actuation.
Aggregation Inducing Abnormal Viscosity Increase in Graphite Nanofluids
Authors: Wong T. F. , Crivoi A. , Duan Fei
Published: July 2013

The effect of aggregation on the viscosity was experimentally investigated in the graphite water-based nanofluids. The shear thinning non-Newtonian behavior is observed in the effective viscosity measurement with the shear rate in the nanofluids. On the basis of the best fitting of the experimental data, the viscosities at zero shear rate and at infinite shear rate are determined. As a function of the particle volume concentration and holding period of the nanofluids after preparation, the effective viscosity the increases. The maximum enhancement of the viscosity at infinite rate of shear is over 24 times higher in the 4 vol % 3-day nanofluids compared with the base fluid. A scanning electron microscope was applied to reveal the morphology of aggregated nanoparticles in the nanofluids. The large and irregular aggregation of the particles is found if the nanofluids were kept for three days. Experimental results indicate that the abnormal effective viscosity increase of nanofluids is related to the nanoparticle aggregation in their base fluids.
PIV Measurements of Air Flow Velocity Profile in Microchannels
Authors: Doucet Daniel J. , Henning James C. , Sastry Sudeep , Abramson Alexis R. , Kadambi Jaikrishnan R.
Published: July 2013

The knowledge of the flow field in the microchannels of the ionic wind pump and other microscale heat removal devices for electronic components is important. The understanding of the flow field will lead to more effective and improved designs. Non-intrusive particle image velocimetry (PIV) utilizing microscopic objective lens is utilized to obtain the velocity field in microchannels of scales similar to those encountered in such devices. Microchannels with dimensions ranging from 0.8 mm to 2 mm are used, along with an open-channel design. The flow rate of air is held constant for the duration of each experiment and olive oil droplets (0.5 to 1.0 micrometer diameter) are used as the seed particles. Computational fluid dynamics (CFD) models are used to replicate each test. ANSYS Fluent code is used for the computational investigations. The CFD flow fields are compared with the PIV results for validation purposes. Velocity profile information is obtained at three locations along each channel: 20%, 50%, and 80% of the channel length. Experiments are conducted on three 2 mm long channels. The cross-sections (depth and width) are 1 mm × 2 mm, 1 mm × 1 mm and 0.8 mm × 2 mm. The CFD flow fields are compared with the PIV results for validation purposes, with relative errors between CFD and PIV typically between 2% and 10%. The agreement between the PIV data and computational results validate this method.