Showing papers in "Microfluidics and Nanofluidics in 2007"
TL;DR: To understand the opportunities and limitations of EWD microfluidics, this paper looks at the development of lab-on-chip applications in a hierarchical approach.
Abstract: The suitability of electrowetting-on-dielectric (EWD) microfluidics for true lab-on-a-chip applications is discussed. The wide diversity in biomedical applications can be parsed into manageable components and assembled into architecture that requires the advantages of being programmable, reconfigurable, and reusable. This capability opens the possibility of handling all of the protocols that a given laboratory application or a class of applications would require. And, it provides a path toward realizing the true lab-on-a-chip. However, this capability can only be realized with a complete set of elemental fluidic components that support all of the required fluidic operations. Architectural choices are described along with the realization of various biomedical fluidic functions implemented in on-chip electrowetting operations. The current status of this EWD toolkit is discussed. However, the question remains: which applications can be performed on a digital microfluidic platform? And, are there other advantages offered by electrowetting technology, such as the programming of different fluidic functions on a common platform (reconfigurability)? To understand the opportunities and limitations of EWD microfluidics, this paper looks at the development of lab-on-chip applications in a hierarchical approach. Diverse applications in biotechnology, for example, will serve as the basis for the requirements for electrowetting devices. These applications drive a set of biomedical fluidic functions required to perform an application, such as cell lysing, molecular separation, or analysis. In turn, each fluidic function encompasses a set of elemental operations, such as transport, mixing, or dispensing. These elemental operations are performed on an elemental set of components, such as electrode arrays, separation columns, or reservoirs. Examples of the incorporation of these principles in complex biomedical applications are described.
TL;DR: In this article, a review of micro-mixing schemes based on DC and AC electrokinetics, including electrowetting-on-dielectric (EWOD), dielectrophoresis (DEP), and electroosmosis (EO), is presented.
Abstract: The applications of electrokinetics in the development of microfluidic devices have been widely attractive in the past decade. Electrokinetic devices generally require no external mechanical moving parts and can be made portable by replacing the power supply by small battery. Therefore, electrokinetic-based microfluidic systems can serve as a viable tool in creating a lab-on-a-chip (LOC) or micro-total analysis system (μTAS) for use in biological and chemical assays. Mixing of analytes and reagents is a critical step in realizing lab-on-a-chip. This step is difficult due to the low Reynolds numbers flows in microscale devices. Hence, various schemes to enhance micro-mixing have been proposed in the past years. This review reports recent developments in the micro-mixing schemes based on DC and AC electrokinetics, including electrowetting-on-dielectric (EWOD), dielectrophoresis (DEP), and electroosmosis (EO). These electrokinetic-based mixing approaches are generally categorized as either active or passive in nature. Active mixers either use time-dependent (AC or DC field switching) or time-independent (DC field) external electric fields to achieve mixing, while passive mixers achieve mixing in DC fields simply by virtue of their geometric topology and surface properties, or electrokinetic instability flows. Typically, chaotic mixing can be achieved in some ways and is helpful to mixing under large Peclet number regimes. The overview given in this article provides a potential user or researcher of electrokinetic-based technology to select the most favorable mixing scheme for applications in the field of micro-total analysis systems.
TL;DR: In this article, three different microfluidic trifurcation geometries have been designed and compared for their droplet fusion efficiencies, and the fusion of up to six droplets has been observed in these devices.
Abstract: Microfluidic flow is geometrically mediated at a trifurcating junction allowing periodically formed, equally spaced out emulsion droplets to redistribute and fuse consistently. This is achieved by controlling the ratio between the droplet transport time across the trifurcating junction and the drainage time of the fluid volume separating the droplets t r/t d. Three different microfluidic trifurcation geometries have been designed and compared for their droplet fusion efficiencies. Fusion of up to six droplets has been observed in these devices. The fusion of two droplets occurs when t r/t d is equal to 1.25 and the number of fused droplets increases with t r/t d. When the junction length (d) is 216 μm fusion of 2–6 six droplets are possible however when the junction length is increased to 360 μm fusion of only two droplets is observed.
TL;DR: In this paper, a facile microfluidic device is used for droplet generation and droplet-based micro-reactor by adjusting the flow rates without bringing reagents into prior contact.
Abstract: Droplet generation and droplet-based microreactor are realized by a facile microfluidic device. The relative concentrations of reactants could be well controlled by adjusting the flow rates without bringing reagents into prior contact; this new method shows considerable advantages for the control and rapid mixing of reagents with no dispersion.
TL;DR: In this article, a simple model is proposed to predict the friction factor and Reynolds product fRe for slip flow in most non-circular microchannels and the developed model has an accuracy of 10% for most common duct shapes.
Abstract: Microscale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS) technology. When the mean free path of the gas is comparable to the channel’s characteristic dimension, the continuum assumption is no longer valid and a velocity slip may occur at the duct walls. Non-circular cross sections are common channel shapes that can be produced by microfabrication. The non-circular microchannels have extensive practical applications in MEMS. Slip flow in non-circular microchannels has been examined and a simple model is proposed to predict the friction factor and Reynolds product fRe for slip flow in most non-circular microchannels. Through the selection of a characteristic length scale, the square root of cross-sectional area, the effect of duct shape has been minimized. The developed model has an accuracy of 10% for most common duct shapes. The developed model may be used to predict mass flow rate and pressure distribution of slip flow in non-circular microchannels.
TL;DR: In this paper, a two-temperature continuous-flow polymerase chain reaction (PCR) polymer chip has been constructed that takes advantage of droplet technology to avoid sample contamination and adsorption at the surface.
Abstract: A two-temperature continuous-flow polymerase chain reaction (PCR) polymer chip has been constructed that takes advantage of droplet technology to avoid sample contamination and adsorption at the surface. Samples contained in aqueous droplets are continuously moved by an oil carrier-fluid through various temperature zones, introducing the possibility of real-time quantitative PCR. In the present paper, we investigate many of the factors affecting droplet-based PCR chip design, including thermal mass, flow rate, and thermal resistance. The study focuses particularly on the fluid and substrate temperature distribution within the PCR chip and the droplet residence times in critical temperature zones. The simulations demonstrate that the flow rate strongly affects the temperature field within the carrier-fluid. Above a critical flow rate, the carrier-fluid fails to achieve the required temperatures for DNA amplification. In addition, the thermal resistances of the different layers in the chip are shown to have a major impact on the temperature profile in the channel.
TL;DR: In this paper, a controlled drug delivery system potential for in vitro injection of diabetics is presented, which incorporates some integrated circuit units and microelectromechanical system devices, such as micropump, microneedle array and microsensor.
Abstract: We present the design of a new controlled drug delivery system potential for in vitro injection of diabetics. The system incorporates some integrated circuit units and microelectromechanical system devices, such as micropump, microneedle array and microsensor. Its goal is to achieve safer and more effective drug delivery. Moreover, a valveless micropump excited by the piezoelectric actuator is designed for the drug delivery system, and a simple fabrication process is proposed. A dynamic model is developed for the valveless micropump based upon the mass conservation. To characterize the micropump, a complete electro-solid-fluid coupling model, including the diffuser/nozzle element and the piezoelectric actuator, is built using the ANSYS software. The simulation results show that the performance of micropump is in direct proportion to the stroke volume of the pump membrane and there is an optimal thickness of the piezoelectric membrane under the 500 V/mm electric field. Based on this simulation model, the effects of several important parameters such as excitation voltage, excitation frequency, pump membrane dimension, piezoelectric membrane dimension and mechanical properties on the characteristics of valveless micropump have been investigated.
TL;DR: It is shown that the hydrostatic pressure and self-movement of motile sperm can be used to solve separating, aligning and orienting sperm in the microchannel.
Abstract: In vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are the most commonly used assisted reproductive technologies to overcome male infertility problems. One of the obstacles of IVF and ICSI procedures is separating motile sperm from non-motile sperm to select the most competent sperm population from any given sperm sample. In addition, orientation and separation of the head from the tail is another obstacle for ICSI. Using the self-movement of sperm against flow direction, motile and non-motile sperm can be separated with an inexpensive polymeric microfluidic system. In this paper, we describe the development of a microfluidic system obtained through low-cost fabrication processes. We report experimental results of sperm sorting using hydrostatic pressure of three different species: bull, mouse, and human. The movement of cells in these channels was observed under a microscope and recorded with a digital camera. It is shown that the hydrostatic pressure and self-movement of motile sperm can be used to solve separating, aligning and orienting sperm in the microchannel.
TL;DR: In this paper, the tangential momentum accommodation coefficient (TMAC) of isothermal steady flows for various gases (nitrogen, argon, helium, and fused silica) was derived from experiments.
Abstract: Experimental investigations of isothermal steady flows for various gases have been carried out in a silica micro tube. This study is focused on the mass flow rate measurements of these flows in slip regime using a suitable powerful platform. First we analyse, for each gas, the pertinence of a first or second order continuum treatment; then we deduce from experiments, using the appropriate treatment, the tangential momentum accommodation coefficient (TMAC) of each gas. The TMAC obtained for the various pairs of gas (nitrogen, argon, helium)/surface (fused silica) exclude a full diffuse reflection.
TL;DR: In this paper, the authors investigated the effect of flow rate on droplet formation in a co-flowing microfluidic device and found that the droplet size is either independent of or strongly dependent on the flow rate ratio.
Abstract: Flow rate effect on droplet formation in a co-flowing microfluidic device is investigated numerically. Transition conditions are discovered that the droplet size is either approximately independent of or strongly dependent on the flow rate ratio. This phenomenon is explained by the relation between strain rate and droplet diameter. Regions of four drop patterns are demarcated and conditions that give polydisperse drops are described, which is helpful to assure the accuracy and efficiency in droplet production.
TL;DR: In this paper, a hybrid molecular dynamics (MD)-continuum simulation with the principle of crude constrained Lagrangian dynamics for data exchange between continuum and MD regions is performed to resolve the Couette and Poiseuille flows.
Abstract: The present study deals with multiscale simulation of the fluid flows in nano/mesoscale channels. A hybrid molecular dynamics (MD)-continuum simulation with the principle of crude constrained Lagrangian dynamics for data exchange between continuum and MD regions is performed to resolve the Couette and Poiseuille flows. Unlike the smaller channel heights, H < 50σ (σ is the molecular length scale, σ ≈ 0.34 nm for liquid Ar), considered in the previous works, this study deals with nano/mesoscale channels with height falling into the range of 44σ ≤ H ≤ 400σ, i.e., O(10)–O(102) nm. The major concerns are: (1) to alleviate statistic fluctuations so as to improve convergence characteristics of the hybrid simulation—a novel treatment for evaluation of force exerted on individual particle is proposed and its effectiveness is demonstrated; (2) to explore the appropriate sizes of the pure MD region and the overlap region for hybrid MD-continuum simulations—the results disclosed that, the pure MD region of at least 12σ and the overlap region of the height 10σ have to be used in this class of hybrid MD-continuum simulations; and (3) to investigate the influences of channel height on the predictions of the flow field and the slip length—a slip length correlation is formulated and the effects of channel size on the flow field and the slip length are discussed.
TL;DR: In this paper, the authors used particle image velocimetry (PIV) to study the internal circulation of aqueous plugs in two-phase flow within 762-μm internal diameter FEP Teflon tubing with FC-40 as the segmenting fluid.
Abstract: The use of two-phase flow in lab-on-chip devices, where chemical and biological reagents are enclosed within plugs separated from each other by an immiscible fluid, offers significant advantages for the development of devices with high throughput of individual heterogeneous samples. Lab-on-chip devices designed to perform the polymerase chain reaction (PCR) are a prime example of such developments. The internal circulation within the plugs used to transport the reagents affects the efficiency of the chemical reaction within the plug, due to the degree of mixing induced on the reagents by the flow regime. It has been hypothesised in the literature that all plug flows produce internal circulation. This work demonstrates experimentally that this is false. The particle image velocimetry (PIV) technique offers a powerful non-intrusive tool to study such flow fields. This paper presents micro-PIV experiments carried out to study the internal circulation of aqueous plugs in two phase flow within 762 μm internal diameter FEP Teflon tubing with FC-40 as the segmenting fluid. Experiments have been performed and the results are presented for plugs ranging in length from 1 to 13 mm with a bulk mean flow velocity ranging from 0.3 to 50 mm/s. The results demonstrate for the first time that circulation within the plugs is not always present and requires fluidic design considerations to ensure their generation.
TL;DR: In this article, a low-cost micro-Coulter counter fabricated from biocompatible materials (polydimethylsiloxane, glass and gold) and incorporating hydrodynamic focusing is presented.
Abstract: We report on the fabrication and characterization of a low-cost micro-Coulter counter fabricated from biocompatible materials (poly-dimethylsiloxane, glass and gold) and incorporating hydrodynamic focusing. The developed micro-Coulter counter offers a low-cost alternative to equivalent existing devices and, thanks to the hydrodynamic focusing, provides high versatility, being able to probe particles with a wide range of sizes within a single device. The device has been successfully tested for counting 20 latex micro beads in suspension.
TL;DR: In this paper, the electrostatic force on a microdroplet transported via electrowetting on dielectric (EWOD) was examined and the consequences of these theoretically and numerically obtained results for design and fabrication of EWOD devices were considered.
Abstract: This paper examines the electrostatic force on a microdroplet transported via electrowetting on dielectric (EWOD). In contrast with previous publications, this article details the force distribution on the advancing and receding fluid faces, in addition to presenting simple algebraic formulae for the net force in terms of system parameters. Dependence of the force distribution and its integral on system geometry, droplet location, and material properties is described. The consequences of these theoretically and numerically obtained results for design and fabrication of EWOD devices are considered.
TL;DR: In this article, a new portable microfluidic platform, "lab-on-a-display", that manipulates polystyrene beads with optoelectronic tweezers on a liquid crystal display (LCD) is presented.
Abstract: This paper reports a new portable microfluidic platform, “lab-on-a-display,” that microparticles are manipulated by optoelectronic tweezers (OET) on a liquid crystal display (LCD). The OET has been constructed by assembling a ground layer, a liquid chamber, and a photoconductive layer. Without lens or optical alignments, the LCD image directly forms virtual electrodes on the photoconductive layer for dielectrophoretic manipulation. The lab-on-a-display was first realized by a conventional monochromatic LCD module and a light source brighter than 5,000 lux. It was successfully applied to the programmable manipulation of 45 μm polystyrene beads; more than 100 particles were transported with an optical image-driven control, following the moving edge of the image at every moment. The effects of bead size and bias voltage on the manipulation speed were also investigated. Due to the portability and compatibility for disposable applications, this new platform has potential for programmable particle manipulation or chip-based bioprocessing including cell separation and bead-based analysis.
TL;DR: In this paper, the authors report the experimental results on kinematics and deformation of ferrofluid droplets driven by planar coils, which can be controlled and manipulated by an external magnetic field.
Abstract: This paper reports the experimental results on kinematics and deformation of ferrofluid droplets driven by planar coils. Ferrofluid droplets act as liquid magnets, which can be controlled and manipulated by an external magnetic field. In our experiments, the magnetic field was generated by two pairs of planar coils, which were fabricated on a double-sided printed circuit board. The first pair of coils constrains the ferrofluid droplet to a one-dimensional motion. The second pair generates the magnetic gradient needed for the droplet motion. The direction of the motion can be controlled by changing the sign of the gradient or of the driving current. Kinematic characteristics of the droplet such as the velocity–position diagram and the aspect ratio of the droplet are investigated. The analysis and discussion are based on the different parameters such as the droplet size, the viscosity of the surrounding medium, and the driving current. This simple actuation concept would allow the implementation of lab-on-a-chip platforms based on ferrofluid droplets.
TL;DR: In this article, superparamagnetic beads suspended in a fluid are used for accelerated transportation in lab-on-a-chip bio-assays, and the induced behavior of single beads and ordered chains is analyzed and compared to a theoretical model.
Abstract: Magnetic actuation principles using superparamagnetic beads suspended in a fluid are studied in this paper An experimental setup containing a sub-microliter fluid volume surrounded by four miniaturized electromagnets was designed and fabricated On the basis of optical velocity measurements, the induced behavior of single beads and ordered chains was analyzed and compared to a theoretical model This research can be used to develop new techniques for accelerated transportation in lab-on-a-chip bio-assays
TL;DR: In this paper, a microfluidic system for separation of microparticles based on the use of dielectrophoretic barriers is presented, which are constructed by aligning two layers of microelectrode structure face-to-face on the top and bottom sides of the microchannel.
Abstract: This paper presents a microfluidic system for separation of microparticles based on the use of dielectrophoretic barriers, which are constructed by aligning two layers of microelectrode structure face-to-face on the top and bottom sides of the microchannel. The energized barriers tend to prevent the particles in the flow from passing through. However, particles may penetrate the barriers if a sufficiently high flow rate is used. The flow velocity at which the particles begin to penetrate the barrier is defined as threshold velocity. Different particles are of different threshold velocities so that they can be separated. In this paper, the electrodes are configured with open ends and aligned with a certain angle to the direction of the flow. Polystyrene microbeads of different sizes (i.e., 9.6 and 16 μm in diameter) are studied in the tests. Under the experimental conditions, two particle trajectories are observed: the 9.6 μm beads penetrate the barriers and move straightly toward the fluidic outlet, while the 16 μm beads snake their way along the electrode edges at a relatively low speed. The two subpopulations of particles are separated into spatial distance of ∼10 mm within tens of seconds. The system presents a rapid and dynamic separation process within a continuous flow.
TL;DR: In this article, the integration of in-situ particle concentrators on microcantilevers is described, where only a thin metal layer is required to generate microfluidic convection of particles from solution bulk onto microcantevers surfaces, greatly enriching local particle counts and enhancing sensitivity.
Abstract: Microcantilevers are finding wide applications in detecting biochemical agents. However, their usage has been limited to highly concentrated samples to ensure sufficient deposition of agents onto cantilevers. A pre-concentration or enrichment step will expand their application range to more dilute, practical samples and real-time detection. This paper reports the integration of in-situ particle concentrators on microcantilevers. Only a thin metal layer on microcantilevers is required to generate microfluidic convection of particles from solution bulk onto microcantilever surfaces, greatly enriching local particle counts and enhancing sensitivity of the system. A working prototype is presented in the paper. Preliminary experiments concentrating latex particles were conducted and the particle concentration effect has been experimentally verified using AFM probes as microcantilevers. As ACEO concentrator has no dependence on particle properties, the method is expected to be applicable to bio-particles collection.
TL;DR: In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel and a new concept termed as effective pumping period is introduced to characterize the reservoir size effect.
Abstract: In electrokinetically driven microfluidic applications, reservoirs are indispensable and have finite sizes. During operation processes, as the liquid level in reservoirs keeps changing as time elapses, a backpressure is generated. Thus, the flow in microfluidic channels actually exhibits a combination of the electroosmotic flow and the time-dependent induced backpressure-driven flow. In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel. Important parameters that describe the effect of finite reservoir size on flow characteristics are discussed. A new concept termed as “effective pumping period” is introduced to characterize the reservoir size effect. The proposed model identifies the mechanisms of the finite-reservoir size effects and is verified by experiment using the micro-PIV technique. The results reported in this study can be used for facilitating the design of microfluidic devices.
TL;DR: This work presents an integrated and reusable PCR–CE glass microfluidic chip capable of multi-chamber PCR and sequential CE, with emphasis on a unique chip reusability approach to avoid CXC.
Abstract: Clinical diagnostics and genomic research often require performing numerous genetic tests. While microfluidic devices provide a low-cost alternative to such demands, integrated microfluidic devices are fabricated using expensive technology not always affordable for single use. However, carryover cross-contamination (CXC) concerns (i.e. either false positive or false negative PCR data) in PCR chips prevent reuse, defying much of the advantages of miniaturized systems developed using expensive MEMS processing. In this work, we present an integrated and reusable PCR–CE glass microfluidic chip capable of multi-chamber PCR and sequential CE, with emphasis on a unique chip reusability approach to avoid CXC. For reliable PCR, the surface of the chamber is re-configured from its virgin hydrophilic (CA 110°) by silanization. To then extend this silanization method as a chip reusability technique, the silanization coating is ‘stripped and re-silanized’ (SRS) to create a fresh coating prior to each successive PCR run. Experimental confirmation of the effectiveness of SRS method in avoiding the CXC is demonstrated using plasmid DNA and HIV-1 infected DNA samples. We also present passive plug microvalves incorporated in the chip to enable fluid/vapor retention during the PCR and controlled fluid flow from the PCR chamber to the CE section for further analysis.
TL;DR: In this paper, an experimental campaign was carried out studying laminar and turbulent heat transfer in uniformly heated smooth glass and rough stainless steel microtubes from 0.5mm down to 0.12mm.
Abstract: An experimental campaign was carried out studying laminar and turbulent heat transfer in uniformly heated smooth glass and rough stainless steel microtubes from 0.5 mm down to 0.12 mm. Heat transfer in turbulent regime proved to be coherent—within experimental accuracy—with the classic Gnielinski correlation for the Nusselt number. For the laminar case, an anomalous drop in Nusselt number for decreasing Reynolds number was observed in the smooth glass tubes. As the stainless steel tubes manifested relatively normal diabatic behaviour in this regime (apart from the evident influence of the thermal development region that increases heat transfer above the thermally fully developed value), the explanation of this unexpected diminution of the Nusselt number must be sought in the dispersion of heat, put in externally through the thin film deposited on the glass tube outer surface, to peripheral attachments to the test section. This distorts the measured energy balance of the experiment, especially as the convective force of the fluid diminishes, resulting in lower Nusselt numbers at lower Reynolds numbers.
TL;DR: In this paper, a volume-of-fluid approach was used to simulate bubble formation in a microfluidic flow focusing device, and the results of the simulation showed good agreement with previous experimental results.
Abstract: Bubble formation in a microfluidic flow-focusing device is simulated using the volume-of-fluid approach to achieve a complete solution of the Navier–Stokes equations for both the gas and liquid phases. The results of the simulation show good agreement with previous experimental results. A detailed examination of the predicted pressure and velocity profiles from the simulation also provide further validation for the conclusions drawn previously with experimental results. The simulation results show the existence of two distinct modes of bubble formation. Simulations of systems an order of magnitude smaller than those investigated experimentally indicate that such reduced systems sizes are a viable approach that would result in much smaller bubble sizes.
TL;DR: In this article, a multiphysics model which simultaneously takes into account the hydrodynamics, thermal and mass transfer (convection, diffusion and chemical reaction) is proposed, and the set of partial differential equations resulting from the model is solved with the help of the finite elements method either in a 2D or a 3D approach.
Abstract: This paper investigates the modeling of styrene free radical polymerization in two different types of microreactor. A multiphysics model which simultaneously takes into account the hydrodynamics, thermal and mass transfer (convection, diffusion and chemical reaction) is proposed. The set of partial differential equations resulting from the model is solved with the help of the finite elements method either in a 2D or a 3D approach. The different modeled microreactors are on one hand an interdigital multilamination microreactor with a large focusing section, and on the other hand a simple T-junction followed by a straight tube with three different radii. The results are expressed in terms of reactor temperature, polydispersity index, number-average degree of polymerization and monomer conversion for different values of the chemical species diffusion coefficient. It was found that the 2D approach gives the same results as the 3D approach but allows to dramatically reduce the computing time. Despite the heat released by the polymerization reaction, it was found that the thermal transfer in such microfluidic devices is high enough to ensure isothermal conditions. Concerning the polydispersity index, the range of diffusion coefficients over which the polydispersity index can be maintained close to the theoretical value for ideal conditions increases as the tube reactor radius decreases. The interdigital multilamination microreactor was found to act as a tubular reactor of 0.78 mm ID but with a shorter length. This underlines that the use of microfluidic devices can lead to a better control of polymerization reactions.
TL;DR: In this article, a two-dimensional numerical investigation into the mixing of magnetic microparticles with bio-cells in a chaotic micromixer is carried out by using a multiphysics finite element analysis package.
Abstract: A two-dimensional numerical investigation into the mixing of magnetic microparticles with bio-cells in a chaotic micromixer is carried out by using a multiphysics finite element analysis package. Fluid and magnetic problems are simulated in steady-state and time-dependent modes, respectively. Intensity of segregation is utilized as the main index to examine the efficiency of the mixer. Trajectories of the particles are used in order to detect chaos in their motion and quantify its extent. Moreover, probability of the collision between particles and target bio-cells is examined as a supplemental index to study the effects of driving parameters on the mixing process. Simulation results reveal that while in some ranges of operating conditions all indices are in good agreement, there are some ranges where they appear to predict contradicting results which is discussed in details. It is found that optimum operating conditions for the system is obtained when the Strouhal number is less than 0.6, which corresponds to the efficiency of about 85% in a mixing length of 500 μm (The mixer design described here is patent pending).
TL;DR: In this paper, a simple external in-line valve for use in microfluidic devices constructed of elastomer such as polydimethylsiloxane (PDMS) is described.
Abstract: A simple, external in-line valve for use in microfluidic devices constructed of elastomer such as polydimethylsiloxane (PDMS) is described. The actuation of the valve is based on the principle that flexible polymer walls of a liquid channel can be pressed together by the aid of a permanent magnet and a small metal bar. In the presence of a small NdFeB magnet lying below the channel of interest, the metal bar is pulled downward simultaneously pushing the thin layer of PDMS down thereby closing the channel stopping any flow of fluid. The operation of the valve is dependent on the thickness of the PDMS layer, the height of the channel, the gap between the chip and the magnet and the strength of the magnet. The microfluidic channels are completely closed to fluid flows commonly used in microfluidic applications. The valve allows for fabrication of a “thin chip” that allows for detection of chromophoric species within the microchannel via an external fiber optics detection system. C18-Modified reverse phase silica particles are packed into the microchannel using a temporary taper created by the magnetic valve and separations using both pressure and electrochromatographic driven methods is detailed.
Abstract: In this work, we propose a novel carbon nanofiber (CNF) emitter for electrospray ionization (ESI)–mass spectrometry (MS) applications The proposed emitter comprises an array of CNFs around the orifice of a microscale capillary The electrospray ionization process is simulated using a CFD code based on Taylor–Melcher leaky-dielectric formulations for solving the electrohydrodynamics and volume-of-fluid (VOF) method for tracking the interface The code is validated for a conventional multiple electrospray emitter and then applied to simulate the CNF emitter model The modeling results show that under steady state condition, individual cone-jets are established around each of the CNFs resulting in an array of electrosprays The approach being taken to fabricate the CNF emitter is briefly discussed Effects of geometrical parameters including aspect ratio of CNFs, total number of CNFs and distribution pattern of the CNFs on the electrospray performance are studied The influence of operating parameters such as flow rate, potential difference and physical properties of the solvent on the electrospray behavior is thoroughly investigated The spray current, ‘onset’ potential and jet diameter are correlated with total number and distribution of CNFs and physical properties of the liquid The correlation results are compared with the available results in the literature Higher spray current and lower jet diameter indicate that the device can perform equivalent to nanospray emitters while using a micro-scale orifice This allows higher sample throughput and eliminates potential clogging problem inherent in nano-capillaries
TL;DR: In this article, the steady reduction rate of a reversible redox species at an embedded microband working electrode is monitored amperometrically, where all three electrodes, including counter and reference electrodes, are integrated on-chip for complete miniaturization of the sensor.
Abstract: We present a new electrochemical velocimetry approach with direct electrical output that is capable of complete device-level integration. The steady reduction rate of a reversible redox species at an embedded microband working electrode is monitored amperometrically. Only one working electrode of arbitrary width is required; all three electrodes, including counter and reference electrodes, are integrated on-chip for complete miniaturization of the sensor. Experimental results are complemented by a theoretical framework including a full 3D electrochemical model as well as empirical mass transfer correlations and scaling laws. When the sensor is operated in the convective/diffusive transport controlled mode, the output signal becomes a predictable function of velocity in two distinct regimes: (i) in the low velocity regime, the signal is directly proportional to flow rate, and (ii) in the high velocity regime, the signal scales as the cube root of the mean velocity. The proposed velocimetry technique is applicable to all practicable pressure-driven laminar flows in microchannels with known cross-sectional geometry.
TL;DR: In this article, the authors present a novel method of creating and using geometric asymmetries for AC electroosmotic pumping, which relies on grouping similar electrodes together in terms of applied voltage.
Abstract: In this paper we present a novel method of creating and using geometric asymmetries for AC electroosmotic pumping. The method relies on grouping similar electrodes together in terms of applied voltage, in order to create configurable asymmetries in periodic electrode arrays, which induce a net pumping AC electroosmotic velocity. Using a numerical model for a system designed by applying the described method, it is demonstrated that by varying the degree of asymmetry it is possible to control the direction of the pumping velocity at a given voltage by simple switching of the voltages on the electrodes.
TL;DR: In this article, a bubble-powered micropump which consists of a pair of nozzle-diffuser flow controller and a pumping chamber was fabricated and tested, and the volume flow rate was found to be dependent on the duty ratio and operation frequency.
Abstract: A bubble-powered micropump which consists of a pair of nozzle-diffuser flow controller and a pumping chamber was fabricated and tested in this study. The two-parallel micro line heaters were fabricated to be embedded in the silicon dioxide layer above a silicon wafer which serves as a base plate for the micropump. A pumping chamber, a pair of nozzle-diffuser unit and microchannels including the liquid inlet and outlet port were fabricated by etching through another silicon wafer. A glass wafer having two holes of inlet and outlet ports of liquid serve as upper plate of the pump. Sequential photographs of bubble nucleation, growth and collapse were visualized by CCD camera. The liquid flow through the nozzle during the period of bubble growth and slight back flow of liquid at the collapse period can be clearly seen. The volume flow rate was found to be dependent on the duty ratio and the operation frequency. The volume flow rate decreases as the duty ratio increases in the micropump with either circular or square pumping chamber.