A three-phased circular electrode array for electro-osmotic microfluidic pumping

A circular micro-electrode array with three phases is designed and prototyped using PolyMUMPs process for AC electro-osmotic flow pumping. Finite element model of the micro electrode array has been developed using COMSOL Multiphysics^™. Performance of the electrode array is simulated and a double peak velocity phenomenon is found for this design, which is confirmed by experimental testing. Using ethanol as testing medium, the two time-averaged peak flow velocities are approximate 320 μm/s at 7 Hz and 850 μm/s at 100 Hz. It is found that the simulated and experimental results agree well.     

High-current density DC magenetohydrodynamics micropump with bubble isolation and release system

One of the major challenges for integrated Lab-on-a-Chip (LOC) systems is the precise control of fluid flow in a micro-flow cell. Magnetohydrodynamics (MHD) micropumps which contain no moving parts and capable of generating a continuous flow in any ionic fluid offer an ideal solution for biological applications. MHD micropumping has been demonstrated by using both AC and direct current (DC) currents by a number of researchers with varying degrees of success. However, current MHD designs based on DC do not meet the flow rate requirements for fully automated LOC applications (>100 μl/min). In this research, we introduce a novel DC-based MHD micropump which effectively increases flow rate by limiting the effects of electrolysis generated bubbles at the electrode–electrolyte interface through isolation and a mechanism for their release. Gas bubbles, particularly, hydrogen generated by high current density at the electrodes are the main culprit in low experimental flow rate compared with theoretical values. These tiny bubbles coalesce in the flow channel thus obstructing the flow of fluid. Since hydrolysis is inevitable with DC excitation, compartmentalized electrode channels with bubble isolating and coalescence retarding mechanisms and bubble release systems are implemented to prevent the coalescence of these bubbles and minimize their effects on flow rate. In this novel design called bubble isolation and release system, flow rate of up to 325 μl/min is achieved using 1 M NaCl solution in DC mode with potentials of 5 V and current density of about 5,000 A/m² for a main channel of 800 μm × 800 μm cross-section and 6.4 mm length.     

Numerical analysis of high frequency pulsating flows through a diffuser-nozzle element in valveless acoustic micropumps

The concept of a valveless acoustic micropump was investigated. Two-dimensional, time-varying, axisymmetric, incompressible viscous flows through a planar diffuser-nozzle element were analyzed for applications in valveless acoustic micropumps. The diffuser divergence half-angles (θ), and the maximum pressure amplitudes (P) were independently varied. The inflow was periodic and the excitation frequency (f) was varied over the range 10 kHz ≤  f ≤  30 kHz. The net time-averaged volume flux and the rectification capability of the diffuser were found as functions of θ, f, and P. The phase difference between pressure and velocity waveforms, the life time and the size of large scale flow recirculation regions inside the microdiffuser, and energy losses were found to be strongly frequency dependent.     

High-throughput electrokinetic bioparticle focusing based on a travelling-wave dielectrophoretic field

This study presents a sheathless and portable microfluidic chip that is capable of high-throughput focusing bioparticles based on 3D travelling-wave dielectrophoresis (twDEP). High-throughput focusing is achieved by sustaining a centralized twDEP field normal to the continuous through-flow direction. Two twDEP electrode arrays are formed from upper and lower walls of the microchannel and extend to its center, which induce twDEP forces to provide the lateral displacements in two directions for focusing the bioparticles. Bioparticles can be focused to the center of the microchannel effectively by twDEP conveyance when the characteristic time due to twDEP conveying in the y direction is shorter than the residence time of the particles within twDEP electrode array. Red blood cells can be effectively focused into a narrow particle stream (~10 μm) below a critical flow rate of 10 μl/min (linear flow velocity ~5 mm/s), when under a voltage of 14 V_p–p at a frequency of 500 kHz is applied. Approximately 90% focusing efficiency for red blood cells can be achieved within two 6-mm-long electrode arrays when the flow rate is below 12 μl/min. Blood cells and Candida cells were also focused and sorted successfully based on their different twDEP mobilities. Compared to conventional 3D-paired DEP focusing, velocity is enhanced nearly four folds of magnitude. 3D twDEP provides the lateral displacements of particles and long residence time for migrating particles in a high-speed continuous flow, which breaks through the limitation of many electrokinetic cell manipulation techniques.     

Low-voltage electroosmotic pumping using porous anodic alumina membranes

This study demonstrated electroosmotic pumping with high flow rate per unit area at a rather low applied voltage by using alumina nano-porous membrane. The platinum mesh electrode is perpendicular to, and has direct contact with the nano-channel inlet for proving uniform electric field and for reducing the electric voltage drop in the reservoir. The measured flow rate versus electrolyte (KCl) concentration reveals two distinct characteristics. First, the flow rate is usually high at low concentrations (10⁻⁵ to 10⁻⁷ M) in which a maximum value occurs. Second, a remarkable drop of flow rate is seen when the concentration surpasses 10⁻⁴ M. The maximum flow rate achieved from this study is 0.09 mL/min V cm² and the energy transfer efficiency is 0.43% at an operation voltage of 20 V. The mesh electrodes with 33 wire spacing are capable of providing an uniform electric field, the nano-porous membrane with a low electrolyte concentration provides the environment for strong overlapping of electric double layer, in association with the thin alumina membrane, leading to a high flow rate at a rather low applied voltage (20–80 V). The flow rate is comparable to the existing results whereas the corresponding operation voltage of this study is about one to two orders lower than most of the existing results.     

Numerical study of enhancing the mixing effect in microchannels via transverse electroosmotic flow by placing electrodes on top and bottom of the channel

This paper reports the enhancement of the mixing effect via the transverse electroosmotic flow by using a 3D microelectrode system, which is structured by aligning two layers of electrodes face-to-face placed on both the top and bottom sides of the channel. The fluid was stretched and folded due to the transverse electroosmotic flow generated by applying an electric field on the electrodes. In this paper, two type of electrode designs (a parallel type electrode design and squarewave type electrode design) were chosen and six design patterns with different combinations of these two types of electrode designs were investigated by using the numerical method. The mixing effect at different design patterns was investigated via comparing the flow structure, mixing mechanism, Poincaré map, and the index of mixing performance. An optimum pattern was obtained when squarewave type electrodes were placed on both top and bottom of the channel. A minimum mixing length of 1.6 mm is required for the optimum pattern when the flow velocity is 1.5 mm/s, and the amplitude of the applied electric potential is 1.2 Volts. The effects of the geometric size and flow rate for the optimum pattern are discussed.     

Particle streaming and separation using dielectrophoresis through discrete periodic microelectrode array

This study presents a particle manipulation and separation technique based on dielectrophoresis principle by employing an array of isosceles triangular microelectrodes on the bottom plate and a continuous electrode on the top plate. These electrodes generate non-uniform electric fields transversely across the microchannel. The particles within the flowing fluid experience a dielectrophoretic force perpendicular to the fluid flow direction due to the non-uniform electric fields. The isosceles triangular microelectrodes were designed to continuously exert a small dielectrophoretic force on the particles. Particles experiencing a larger dielectrophoretic force would move further in the perpendicular direction to the fluid flow as they traveled past each microelectrode. Polystyrene microspheres were used as the model particles, with particles of ∅20 μm employed for studying the basic characteristics of this technique. Particle separation was subsequently demonstrated on ∅10 and ∅15 μm microspheres. Using an applied sinusoidal voltage of 20 V_pp and frequency of 1 MHz, a mean separation distance of 0.765 mm between them was achieved at a flow rate of 3 μl/min (~1 mm/s), an important consideration for high throughput separation capability in a micro-scale technology device. This unique isosceles triangular microelectrodes design allows heterogeneous particle populations to be separated into multiple streams in a single continuous operation.     

AC electrokinetic pumping on symmetric electrode arrays

AC electro-osmotic (ACEO) pumping is experimentally demonstrated on a symmetric gold electrode array. Using asymmetric connection of electrodes to the applied AC voltage, spatial asymmetry along the array is created, which produces unidirectional flow of electrolyte. An aqueous solution of 100 μM KCl is selected as the pumping fluid. The liquid velocity obtained as a function of voltage and frequency is compared to that generated using travelling-wave electroosmosis (TWEO) with the same electrode array. The expected velocities from the linear electrokinetic models of ACEO and TWEO are computed numerically. The comparison shows that TWEO generates greater velocity amplitudes and the streamlines are smoother than those generated by ACEO.     

Numerical modeling of interaction of a current with a circular cylinder near a rigid bed

The numerical modeling of 2D turbulent flow around a smooth horizontal circular cylinder near a rigid bed with gap ratio G/D = 0.3 at Reynolds number Re_D = 9500 is investigated. Ansys^® 10.0-FLOTRAN program package is used to solve the governing equations by FEM, and the performance of the standard k ? ε, standard k ? ω, and SST turbulence models are examined. A sensitivity study for the three turbulence models is carried out on three computational meshes with different densities near the cylinder surface. The computational velocity fields and the Strouhal numbers from the present simulations are compared with those obtained from the PIV measurement. It is found that the time-averaged velocity field of the flow in the proximity of the cylinder is closely affected by the mesh resolution near the cylinder surface, and the mesh refinement in radial direction improves the results of present simulations. The shedding of vortices in the cylinder wake is not predicted by k ? ε model on all the three meshes. The results for the time-averaged velocity field show that the numerical modeling using either of k ? ω and SST turbulence models on the finest mesh used on the cylinder surface is reasonably successful.     

Use of electrochemical microsensors for hydrodynamics study in crossing microchannels

The study of microfluidic systems is an important research challenge related to the design of microdevices for chemical processes. The understanding of physical phenomena, such as flow behaviour and heat and mass transfer performance is needed in order to develop these microsystems for industrial applications as mixers, reactors or heat exchangers. This work aims at characterizing two flow pattern behaviours, by using an electrochemical method, in a microdevice composed of crossing microchannels. A nonintrusive electrodiffusion method involving an electrochemical reaction of active species on an electrode flush-mounted into a wall is used to investigate wall shear stress. The measured limiting diffusion current is related to the wall shear rate in the vicinity of the electrode. The experimental cell consists of two crossing microchannels intersecting at right angle. Two channels sections are investigated, respectively 500 and 833 μm in hydraulic diameter. In each case, the influence of the crossing on the flow behaviour and on the mixing performance are characterized locally by using microelectrodes implemented at several positions on the wall of the channels located after the crossing. The experimental results are analyzed and a comparison with the results of CFD simulations using Fluent is performed.     

A micropump based on water potential difference in plants

In land plants, water vapor diffuses into the air through the stomata. The loss of water vapor creates a water potential difference between the leaf and the soil, which draws the water upward. Quantitatively, the water potential difference is 1–2 MPa which can support a water column of 100–200 m. Here we present the design and operation of a biomimetic micropump. The micropump is mainly composed of a 48-μm thick metal screen plate with a group of 102-μm diameter micropores and an agarose gel sheet with nanopores of 100 nm diameter. The micropores in the screen plate imitate the stomata to regulate the flow rate of the micropump. The agarose gel sheet is used to imitate the mesophyll cells around the stomata. The lost of water from the nanopores in the gel sheet can generate a water potential difference (more than 30 kPa) which can drive solution flow in a microfluidic chip. Results have shown that a precise flow rate of 4–8 nl/min can be obtained by using this micropump, and its ultra-high flow rate is 113–126 nl/min. The advantages of this biomimetic micropump include adjustable flow rate, simple structure and low fabrication cost. It can be used as a “plug and play” fluid-driven unit in microfluidic chips without any external power sources or equipments.     

Optimal aerodynamic design of airfoils in unsteady viscous flows

A continuous adjoint formulation is used to determine optimal airfoil shapes in unsteady viscous flows at Re = 1 × 10⁴. The Reynolds number is based on the free-stream speed and the chord length of the airfoil. A finite element method based on streamline-upwind Petrov/Galerkin (SUPG) and pressure-stabilized Petrov/Galerkin (PSPG) stabilizations is used to solve both the flow and adjoint equations. The airfoil is parametrized via a Non-Uniform Rational B-Splines (NURBS) curve. Three different objective functions are used to obtain optimal shapes: maximize lift, minimize drag and minimize ratio of drag to lift. The objective functions are formulated on the basis of time-averaged aerodynamic coefficients. The three objective functions result in diverse airfoil geometries. The resulting airfoils are thin, with the largest thickness to chord ratio being only 5.4%. The shapes obtained are further investigated for their aerodynamic performance. Maximization of time-averaged lift leads to an airfoil that produces more than six times more lift compared to the NACA 0012 airfoil. The excess lift is a consequence of the large peak and extended region of high suction on the upper surface and high pressure on the lower surface. Minimization of drag results in an airfoil with a sharp leading edge. The flow remains attached for close to 70% of the chord length. Minimization of the ratio of drag to lift results in an airfoil with a shallow dimple on the upper surface. It leads to a fairly large value of the time-averaged ratio of lift to drag (～ 17.8). The high value is mostly achieved by a 447% increase in lift and 16% reduction in drag, compared to a NACA 0012 airfoil. Imposition of volume constraint, for the cases studied, is found to result in airfoils that have lower aerodynamic performance.     

Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk

A polymeric membrane ion-selective electrode for determination of melamine is described in this paper. It is based on a molecularly imprinted polymer (MIP) for selective recognition, which can be synthesized by using melamine as a template molecule, methacrylic acid as a functional monomer and ethylene glycol dimethacrylate as a cross-linking agent. The membrane electrode shows near-Nernstian response (54 mV/decade) to the protonated melamine over the concentration range of 5.0 × 10⁻⁶ to 1.0 × 10⁻² mol L⁻¹. The electrode exhibits a short response time of 16 s and can be stable for more than 2 months. Combined with flow analysis system, the potentiometric sensor has been successfully applied to the determination of melamine in milk samples. Interference from high concentrations of ions co-existing in milk samples such as K⁺ and Na⁺ can be effectively eliminated by on-line introduction of anion- and cation-exchanger tandem columns placed upstream, while melamine existing as neutral molecules in milk of pH 6.7 can flow through the ion-exchanger columns and be measured downstream by the proposed electrode in an acetate buffer solution of pH 3.7.     

Equivalent electrical network for performance characterization of piezoelectric peristaltic micropump

Utilizing an electronic–hydraulic analogy, this study develops an equivalent electrical network of a piezoelectric peristaltic micropump which has not been modeled the whole system operation completely by computational fluid dynamics (CFD) or equivalent electrical network so far due to its excessive complicated structure. The validity of the proposed model is verified by comparing the simulation results obtained using the SPICE (simulation program with integrated circuit emphasis) software package for flow rate spectrum and its maximum state of a typical micropump with the experimental observations for two working fluids, namely DI water and blood. The simulation results predict a maximum flow rate frequency and flow rate of 280 Hz and 43.23 μL/min, respectively, for water, and 210 Hz and 24.12 μL/min for blood. The corresponding experimental results are found to be 300 Hz and 41.58 μL/min for water and 250 Hz and 23.75 μL/min for blood. The relatively poorer agreement between the two sets of results when using blood as the working fluid is thought to be the result of the non-Newtonian nature of blood, which induces a more complex, non-linear flow behavior within the micropump. Having validated the proposed model, the equivalent network is used to perform a systematic analysis of the correlation between the principal micropump design parameters and operating conditions and the micropump performance. The results confirm the validity of the equivalent electrical network model as the first microfluidic modeling tool for optimizing the design of peristaltic micropumps and for predicting their performance.     

Enhanced electrophoresis separation of non-electroactive amino acids on poly(dimethylsiloxane) microchip coupled with direct electrochemical detection on a copper electrode

Electrochemical determination of amino acids on a Cu electrode was established as an attractive scheme for non-electroactive amino acids after microchip electrophoresis separation. Five amino acids (arginine, proline, histidine, valine, and serine) achieved efficient separation within 60 s on a titanium dioxide nanoparticles (TiO₂ NPs) coated poly(dimethylsiloxane) (PDMS) microchip, and then successfully detected on a Cu electrode in end-channel detection mode. In the slightly basic borate medium, anodic currents occur for amino acids due to their ability to form Cu(II) complexes and thereby enhance the electrochemical dissolution of Cu electrode substrate. The increase of the anodic current measured is proportional to the amino acid concentration added to the solution, and therefore, enables direct detection of non-electroactive amino acids on the Cu electrode. The detection limits (S/N = 3) for arginine, proline, histidine, valine, and serine were measured to be 7, 6, 5, 6, and 5 μM, respectively, with the linear ranges all from 25 to 500 μM. In addition, compared with the native PDMS microchip, resolutions and separation efficiencies of amino acids on the modified microchip were considerably enhanced with the theoretical plate numbers of 8.9 × 10³, 6.6 × 10⁴, 4.8 × 10⁴, 5.6 × 10⁴, and 4.4 × 10⁴ plates m⁻¹, respectively. The proposed Cu electrode response demonstrated good reproducibility and stability, with no apparent loss of response for periods as long as 4 weeks.     

Distribution-Free Connectivity Testing for Sparse Graphs

We consider distribution-free property-testing of graph connectivity. In this setting of property testing, the distance between functions is measured with respect to a fixed but unknown distribution D on the domain, and the testing algorithm has an oracle access to random sampling from the domain according to this distribution D. This notion of distribution-free testing was previously defined, and testers were shown for very few properties. However, no distribution-free property testing algorithm was known for any graph property. We present the first distribution-free testing algorithms for one of the central properties in this area—graph connectivity (specifically, the problem is mainly interesting in the case of sparse graphs). We introduce three testing models for sparse graphs:

Integrated SWCNT sensors in micro-wind tunnel for air-flow shear-stress measurement

We have developed SWCNT sensors for air-flow shear-stress measurement inside a polymethylmethacrylate (PMMA) “micro-wind tunnel” chip. An array of sensors is fabricated by using dielectrophoretic (DEP) technique to manipulate bundled single-walled carbon nanotubes (SWCNTs) across the gold microelectrodes on a PMMA substrate. The sensors are then integrated in a PMMA micro-wind tunnel, which is fabricated by SU-8 molding/hot-embossing technique. Since the sensors detect air flow by thermal transfer principle, we have first examined the I–V characteristics of the sensors and confirmed that self-heating effect occurs when the input voltage is above ~1 V. We then performed the flow sensing experiment on the sensors using constant temperature (CT) configuration with input power of ~230 μW. The voltage output of the sensors increases with the increasing flow rate in the micro-wind tunnel and the detectable volumetric flow is in the order of 1 × 10⁻⁵m³/s. We also found that the activation power of the sensors has a linear relation with 1/3 exponential power of the shear stress which is similar to conventional hot-wire and polysilicon types of convection-based shear-stress sensors. Moreover, measurements of sensors with different overheat ratios were compared, and results showed that sensor is more sensitive to the flow with a higher overheat ratio.     

Separation of beta-human chorionic gonadotropin from fibrinogen using a MEMS size exclusion chromatography column

This article reports a MEMS (Micro-Electro-Mechanical-Systems)-based column separator designed for potential integration with portable medical point-of-care testing (POCT) devices. The MEMS column uses size exclusion chromatography (SEC) to pre-separate raw samples by size, and is made of polydimethylsiloxane (PDMS) fabricated on a glass slide. The MEMS SEC column separates 300 ng/mL of beta-human chorionic gonadotropin (β-hCG), a cancer biomarker, from a fibrinogen-rich solution (20 μg/mL) in 10 min, showing 2.12 resolution and 0.036 mm plate height through fluorescent detection. Results are further verified by β-hCG and anti-β-hCG antibody conjugate using surface plasmon resonance (SPR). The collected β-hCG-rich eluent at 8 min shows 11 mDeg of angle shift. The fluorescent detection and SPR results demonstrate the complete discrimination of β-hCG from fibrinogen using the MEMS SEC column, and illustrate its viability for integrating a sample preparation stage in POCT devices to assist cancer screening and prognosis.     

Pressure drop of gas–liquid Taylor flow in round micro-capillaries for low to intermediate Reynolds numbers

In this paper, a model is presented that describes the pressure drop of gas–liquid Taylor flow in round capillaries with a channel diameter typically less than 1 mm. The analysis of Bretherton (J Fluid Mech 10:166–188, 1961) for the pressure drop over a single gas bubble for vanishing liquid film thickness is extended to include a non-negligible liquid film thickness using the analysis of Aussillous and Quéré (Phys Fluids 12(10):2367–2371, 2000). This result is combined with the Hagen–Poiseuille equation for liquid flow using a mass balance-based Taylor flow model previously developed by the authors (Warnier et al. in Chem Eng J 135S:S153–S158, 2007). The model presented in this paper includes the effect of the liquid slug length on the pressure drop similar to the model of Kreutzer et al. (AIChE J 51(9):2428–2440, 2005). Additionally, the gas bubble velocity is taken into account, thereby increasing the accuracy of the pressure drop predictions compared to those of the model of Kreutzer et al. Experimental data were obtained for nitrogen–water Taylor flow in a round glass channel with an inner diameter of 250 μm. The capillary number Ca _gl varied between 2.3 × 10⁻³ and 8.8 × 10⁻³ and the Reynolds number Re _gl varied between 41 and 159. The presented model describes the experimental results with an accuracy of ±4% of the measured values.     

Micro-electroforming metallic bipolar electrodes for mini-DMFC stacks

This paper describes the development of metallic bipolar plate fabrication using micro-electroforming process for mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels using a photomask and exposure process. Micro-fluidic channels mold with 300 μm thick and 500 μm wide were firstly fabricated in a negative photoresist onto a stainless steel plate. Copper micro-electroforming was used to replicate the micro-fluidic channels mold. Following by sputtering silver (Ag) with 1.2 μm thick, the metallic bipolar plates were completed. The silver layer is used for corrosive resistance. The completed mini-DMFC stack is a 3.5 × 3.5 cm² fuel cell stack including a 1.5 × 1.5 cm² MEA (membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks using the completed metallic bipolar plates. All test results showed the metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power density is 9.3 mW/cm² and current density is 100 mA/cm² when using 8 vol.% methanol as fuel and operated at temperature 30°C. The output power result is similar to other reports by using conventional graphite bipolar plates. However, conventional graphite bipolar plates have certain difficulty to be machined to such micro-fluidic channels. The proposed micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC stacks for further portable 3C applications.