In the present work, a chip embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed. Embedded current collectors are expected to increase the number of contact points with the porous electrodes and thereby reduce the overall contact resistance and ohmic losses of the cell. The micromachining based thin film process is compatible with the overall cell fabrication scheme, which is based on soft lithography, and does not require a substantial modification of the original cell design. The flow-through microfluidic fuel cell architecture with porous carbon paper electrodes is used as a proof-of-concept in this study. Our preliminary study shows that the peak power density of the cell with the current collector is increased by 79% at 300 μL min −1 compared to an otherwise identical cell without current collector, indicating that the contact resistance is significantly reduced. Electrochemical impedance spectroscopy analysis is carried out to estimate the overall cell ohmic resistance and confirms a 32% reduction using the current collectors.

This work presents numerical study on the feasibility of implementing a newly introduced electromagnetic pump for mixing fluids in microscales. It also introduces an interesting energization scheme that can be used to adjust the mixing length until obtaining complete mixing. The electromagnetic gentle pump was proposed for biomedical applications. It consists of an annular fluidic channel, set of solenoids, two permanent magnets and a cover. The pumping concept depends on controlling the rotation of the two hard magnets placed in an annular channel in opposing polarity under the influence of an electronically controlled moving electromagnetic field. This concept is currently under development for microfluidic applications using polymer micromachining. Results have been visualized through plotting mass fraction contours along the channel length. Results are motivating and showed higher mass fraction of H 2 O at low channel widths.

Spark Assisted Chemical Engraving (SACE) is an interesting technology for micro-machining several types of non-conductive materials like glass, quartz, polymers and some ceramics. The process takes place in an electrochemical cell with two electrodes immersed in an electrolyte. The electrolytic solution is typically sodium hydroxide (30%wt NaOH) or potassium hydroxide (KOH). The cathode is used as tool and the anode as counter-electrode. When the applied voltage is higher than a critical value (typically around 30V, depending on the electrolyte and tool-electrode geometry) bubbles grow so dense on the electrode surface that they coalesce into a gas film. Electrical discharges occur between the electrode and the electrolyte. Machining begins consequently if the electrode is placed close enough to the surface to be machined (typically 25μm for glass). In the present paper, the forces exerted on the tool-electrode during constant velocity feed drilling is investigated experimentally and a model is proposed. The setup is composed of a machine head mounted on XYZ precision linear stage holding the tool-electrode. The machining head further incorporates a force sensor which is able to monitor, during drilling operation, the force exerted on the tool-electrode based on the zero displacement measurement principle.

Numerical simulations and experiments are used to evaluate the flow and mixing characteristics of a proposed convective 3-D T-type micromixer. The study presents a parametric study and performance optimization of this micromixer based on the variation of its geometry. To investigate the effect of design and operation parameters on the device performance, a systematic design and optimization methodology is applied; it combines Computational Fluid Dynamics (CFD) with an optimization strategy that integrates Design of Experiments (DOE), Surrogate modeling (SM) and Multi-Objective Genetic Algorithm (MOGA) techniques. The degree of mixing and the pressure loss in the mixing channel are the performance criteria to identify optimum designs at different Reynolds numbers (Re). The convective flow generated in the 3-D T-type micromixer drastically enhances mixing at Re > 100 by making the two fluids to roll up along the mixing channel. The resulting optimum designs are fabricated on polymethylmethacrylate (PMMA) by CNC micromachining. Experiments are carried out to visualize the streams of de-ionized water and aqueous fluorescein solution, by which the extent of mixing is determined, based on the standard deviation of fluorescein intensities on cross-section images. This study applies a systematic procedure for evaluation and optimization of a proposed 3-D T-mixer which has a configuration of channels that promote convective mixing since the two fluids come into contact. The methodology applied can also be used to efficiently modify and customize current micromixers.

Laminar flow limits the mixing performance and heat transfer rates that occurs within microdevices. Synthetic jets in the microscale could disrupt laminar flow and improve the performance of such devices. In this paper a synthetic microjet integrated in a microchannel was designed and fabricated using micromachining techniques. The channel flow was driven by a syringe pump at a rate of 1.39μL/s and the device was actuated using a piezoceramic disc at a frequency of 600Hz. Flow fields were measured phase locked to the actuation cycle using the MicroPIV technique in the mid plane of the jet. The resultant fields revealed a jet with a largest velocity of 2.3m/s. The average velocity during expulsion was estimated to be 0.73m/s using a comparison to the oscillatory solution to flow in a square duct. Measurements at different phases in the cycle revealed a jet strong enough to impinge on the opposing wall and the growth and decay of a pair of vortices formed at the edge of the orifice. It was also shown that the synthetic jet significantly altered the flow patterns showing promising signs for enhancing mixing and heat transfer in microchannels.

Microfluidics is often presented for applications where only microliters sample volumes are available. But the benefits of microchannels do not reduce to a low consumption of fluids. From a physical and mechanical point of view, microfluidics can offer high shear rates combined with low Reynolds number and low viscous heating. It becomes possible to explore high shear rheology on a lab-on-chip. We have micromachined microviscometers to study the rheological properties of nanofluids under very high shear rates conditions. Nanofluids are fluid suspensions of solid nanoparticles. Recent experiments have indicated an anomalous increase in thermal conductivity of these suspensions. But less attention has been payed to the rheological properties of nanofluids. The few results concerning the viscosity of nanofluids exhibit scattered values higher than those of fluid suspensions of microparticles, because of a higher rate of collisions due to Brownian motion and shearing motion which enhance aggregation. These experiments were performed with commercially available rheometers over a limited range of shear rates. Our viscometers on chip are silicon — Pyrex microchannels (H ≈ 10 – 20 μ m) equipped with local pressure drop sensors. Nanofluids under test were ethanol-based SiO 2 nanoparticles. For particle sizes from 20 nm to 190 nm, and solid volume fractions from 1.4% to 7%, a newtonian behaviour has been observed up to 5.10 4 s −1 . High shear rheology is the only way to reach high Peclet number values with nanoparticles in a laminar flow. It was possible to cover a wide range of Peclet number and to have Pe > 1 with diameter in the tens of nanometers range. Our results have demonstrated that an apparent solid volume fraction φ a > φ, due to aggregation, was responsible of the increment of viscosity. More important was the demonstration that the shape of the clusters could be modified and that the ratio φ a /φ could be lowered by a very high shear rate. Very high shearing rates in microchannels appear to be a way for nanofluids to converge to a well-defined value of viscosities.

Plastics are the choice materials for fabrication of many microfluidic devices as mass production methods such as the injection moulding and hot embossing processes can be used for disposable polymer device fabrications. However, fabrication of mould inserts with micro features is a very critical step towards successful moulding of polymer micro components. Due to technical limitations many micro features cannot be fabricated directly onto metal mould inserts. Silicon based micro machining technologies, on the other hand, are well developed and are capable of producing very fine micro structures. In the study, silicon insert moulding method is developed for fabrication of polymeric biochips. Although it has been reported to use silicon dies for hot embossing applications, careful consideration of mould design is required to use fragile silicon inserts in moulding applications where higher mould clamping forces are encountered. Silicon inserts instead of traditional metal inserts, are successfully manufactured and used for polymer biochip moulding. It has been found that the silicon inserts can last for tens of shots if properly installed in an injection mould.

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

A new surface micromachining technique for Poly dimethylsiloxane (PDMS) microchannels has been developed to address the leakage problem affecting traditional PDMS microchannel process with embedded tall structures or sudden topographical transitions. Bulk micromachined PDMS cannot be conformally bonded with the surfaces having tall topological features such as thick film electrodes, porous reactor beds and other structural features. Surface micromachining technique with PDMS as structural material and photoresist as a sacrificial material allows the creation of PDMS microchannels on substrates with significant topography. Adhesion of the structural layer with the substrate was characterized for different prepolymer ratios using standard tensile test and 1:3 (Curing agent: base) combination was found to be the best with maximum adhesion strength of 7.5 MPa. The effectiveness of this technique is demonstrated by the fabrication of microchannels with embedded 6μm thick Silver electrodes. The microchannels were leak proof and conformal contact between the PDMS and electrode was confirmed through SEM. The release time for microchannels was reduced to 1 min irrespective of the length of the microchannel. The extension of this technique for fabrication of multi layered microchannel structure was demonstrated through a microfluidic valve. The valve closure occurred at 6.37 kPa.

Bead and cell suspension based flow-through assays are popular for high throughput biological analysis. Several technologies incorporate a tagging scheme with beads to enable multiplexing. Modern flow-through systems such as flow cytometers and cell sorters are large, bulky and expensive; consequently, much research has been performed using microfluidics to miniaturize these systems. However, several problems remain with these systems, notably it remains difficult to perform manipulations on the beads (or cells), and in the case of multiplexed systems, it remains difficult to read the tags quickly. In this paper, we present a micromachined micro-carrier, referred to as a ‘micropallet’, designed to move through a microfluidic device, which helps to solve several of these problems. Micropallets are small carrier structures, micromachined out of plastic or other materials, that are used to carry attached biological or chemical samples through a microfluidic system (e.g., DNA, RNA, proteins, antibodies, adherent cells, organisms). Similar to conventional factory pallets that carry a product through an automated manufacturing line, micropallets are engineered to carry their cargo through a micro-scale system. Thus micropallets may contain shapes, structures and materials designed to interact with and work in a microfluidic system, such as for docking, sorting, manipulation and readout. Additionally, micropallets may include bar codes or other markings, and be engineered to optimally suit the cargo they carry (for example, a micropallet might contain 3-D structures and treated sections for cells, molecules or organisms to attach). Results are presented for the use of micropallets in cell assays, DNA assays and antibody assays. Micropallets may be designed to carry a sample through a microfluidic system or for use in a static assay system, enabling versatile customisation of the micropallets and flow system for design of a programmable system that interacts with the micropallets for detection, control and manipulation.

Investigation of wet anisotropic etching of silicon microchannels on (100) silicon is presented with the emphasis to simultaneously fabricate bifurcated silicon channels in <100> and <110> directions. Silicon crystal planes that are playing major role of microchannels sidewalls are characterized for their roughness and etch rates. Beside the standardly smooth {111} crystal planes, microroughness of {110} sidewalls is of particular interest. It is shown that by implementing TMAH-Triton etchant, the roughness of {110} sidewalls was significantly improved over the standard KOH-IPA or TMAH-IPA etching system without affecting the neighboring crystal plane sidewalls. Agitation of the etching solution is found to have significant influence on both, the etch rate and surface roughness. Optimal etching conditions were determined to obtain smooth microchannel sidewalls in two major directions. Furthermore, reduced convex corner underetching was obtained compared to other etching system. Two viable solutions for the connecting through holes or vias are presented by using single or double mask layer photolithography prior to performing the etching step.

In this silicon bulk micromachining project we are evaluating new approaches to develop well-controlled 3-D microstructures for microfluidics and diffractive elements. Several approaches have been fabricated and reported using rectangular structures on (0 0 1) and (0 1 1) silicon wafers. In this work, n-type, 2–5 ohm-cm, two inch, 300 μ-thick, high index (1 1 3), (1 1 4), ( 5 5 12), (0 1 1) silicon wafers were used, which were produced from [1 1 2] ingots. Most micromachined structures compatible in Microsystems are fabricated on (0 0 1) and (0 1 1) wafers and rarely using high index wafers. Several sophisticated microstructures maybe fulfilled by using this kind of substrates with different crystallographic orientations. In our experimental work, the etching pattern was designed including several polygon-like structures to analyze the resulting morphology and crystallographic planes developed during a long-time etching. One of them, an array of 10 squared structures is arranged as follows: the first square is aligned parallel/perpendicular to the (0 1 1) main flat and then the next squares were slanted ranging from 5° till 45° in 5° steps. We have developed an experimental set using different KOH-H2O dilution at 60°C. All the experimental procedure was developed at the same time on (0 0 1) and (0 1 1) silicon wafers, which were used as control samples. Our etching mask containing several polygon-like structures was transferred parallel to the main (0 1 1) flat of the low and high index wafers. We are reporting the etching of several structures, watching the roughness evolution of the walls and bottom surfaces, with the main purpose of developing microstructures for microfluidics and integrated optics applications. In this way we have observed a couple of interesting structures whose very smooth walls could be used to develop a very simple fabrication procedure of blazed diffraction gratings for infrared applications.

Microchannel devices with channel widths in the range from one micron to several hundred microns have become increasingly important structures for heat transfer applications. This paper will examine several classes of fabrication technologies that are employed in the field. Established technologies, such as bulk and surface micromachining, high aspect ratio machining, and conventional machining have been perfected over the last several decades and are the principal methods of creating microchannel structures. Synthesizing technologies, such as wafer bonding and micromolding techniques, allow these primary structures to be assembled together into working devices or enable high volume, low cost manufacturing using microstructured masters. Established and synthesizing technologies have reached a high level of performance and are generally the subject of refinement efforts rather than innovative investigations. Active research into new materials, processes, and fabrication strategies are found in the class of emerging technologies. This paper will briefly review the dimensional spectrum of microchannels and survey the established and synthesizing technologies. Then an exploration of emerging technologies will be made. The topic of rapid prototyping will be given particular emphasis.

The present work investigates experimentally the two-phase flow instability of convective boiling heat transfer in a system of two parallel microchannels at a given low mass flux and several different heating surface temperatures. The test section employed in this work is fabricated by silicon bulk micro machining and anodic bounding processes. Both channels are triangular with the channel-width of the topside of 100 i`m and hydraulic diameter of 51.7 i`m. The topside width of the central island between two channels is 20 i`m. The forced flow of de-ionized water is provided by a syringe pump. The two-phase flow visualization under boiling conditions is made possible by a high-speed digital video camera. The temperatures of inlet and outlet reservoirs, the inlet pressure as well as the pressure drop during the experiments are recorded and analyzed with flow visualization. The results of the study demonstrate clearly that two-phase flow instabilities with significant oscillations in two-phase flow properties can be developed in the system of double microchannels at high heating surface temperatures due to channel-to-channel interactions. Flow visualization confirms the presence of flow reversal during large amplitude oscillations. Such two-phase flow instabilities involving flow reversal should be of great concern for the design of a micro evaporator.

The knowledge of the fundamental aspects of hydrodynamics at microscales is an exciting challenge. Some authors have published conflicting results concerning the friction and the thermal exchange coefficients, the transition to a turbulent flow regime (Qu et al. 2000, Mala and Di 1999, Papautsky et al. 1999). Some explanations, based on surface effects, have been proposed, but microeffects, if they are, are probably hidden by experimental artefacts. We aim at performing local measurements of pressure drops in monophasic microstreams. Precedent works (Baviere et al. 2003) have shown that a great care has to be taken with the intrepretation of anomalous or unexpected results, and that the metrological set up of these experiments is crucial. We have performed and tested cupro-nickel strain gauges micromachined on different sorts of silicon nitride membranes. The design of the gauges obeys an electrical Wheatstone bridge configuration. The experimental signals are in good agreement with the expected electromechanical response of the bridge. The sensitivity ranges from 50 to 100 μV/V/bar with a thermal drift below 0.011%.°C −1 . Such sensors have been integrated along smooth and rough silicon microchannels with hydraulic diameter of 15 μm, and no deviation from the laminar regime has been observed with such local pressure sensors. The micromachining of these devices is described and the first local pressure drops measurements performed with deionized water of low electrical resistivity are presented and discussed.

In microchannel flow, gas-liquid interface behavior is important for many applications, e.g. micro-reactors, micro heat pipes to name only two of them. Microfluidic channels shape are generally rectangular or triangular with associated solid corner. Those corners are interesting for they drastically increase capillary effects of wetting liquids compared to smooth solid boundaries. We study capillarity driven gas–liquid flows in a flat triangular channel. A channel of triangular cross section was micro-machined in a polymeric material and covered by a transparent plexiglas plate. The wetting fluid is injected at a controlled flow rate and the interface motion along the corners is recorded with a CCD video camera. A simple lubrication theory predicts the temporal evolution of the liquid-gas interface. A good agreement is found between those predictions and experimental results. the theory also predicts when the invasion of the channel bulk occurs. The dynamics of the bulk meniscus is discussed. The results suggest that the bulk meniscus dynamics is affected by the growth of the liquid fingers that develop along the edges of the channel.

Boiling heat transfer and corresponding two-phase flow phenomena are of significant interest for the design of a compact evaporator. The present work investigates experimentally, using a high-speed digital CCD camera, the two-phase flow phenomena for boiling in a silicon-based, two parallel trapezoid microchannels, which were prepared by the combination of silicon bulk micro machining and Pyrex-silicon wafer bonding. Onset of nucleate boiling, bubbly flow, slug flow, and partial dry out slug flow are typically observed along the flow direction. The appearance of the partial dryout slug flow may degrade the nucleate boiling heat transfer in the microchannel. At a low flow rate, reversed vapor flow is observed. In such a flow pattern, liquid droplets are formed intermittently on the inner wall of top Pyrex glass due to vapor condensation. Moreover, the reversed vapor flow usually accompanies with large magnitude two-phase flow oscillations.

Cell based biosensors offer the capability for quickly detecting chemical and biological agents with high sensitivity in a wide spectrum. Membrane excitability in cells plays a key role in modulating the electrical activity due to chemical agents. However, the complexity of these signals makes the interpretation of the cellular response to a chemical agent rather difficult. It is possible to determine a frequency spectrum also known as the signature pattern vector (SPV) for a given chemical agent through analysis of the power spectrum of the cell signal. I will describe a system for the measurement of extracellular potentials from live cells isolated onto micromachined planar microelectrode arrays. Fast Fourier and wavelet transformation techniques are used to extract information related to the frequency of firing and response times from the extracellular potential. Quantitative dose response curves and response times are obtained using local time domain characterization techniques.

We study gas-liquid flows in a channel having a large aspect ratio at small capillary number. A channel of sinusoidal cross section was micro-machined in a polymeric material and covered by a transparent Plexiglas plate. The non-wetting gas (air) is injected at a controlled flow rate while the interface motion is recorded with a CCD video camera. From the convergence of asymptotic, numerical and experimental analysis, we found simple dependences for the finger width and total curvature as a function of channel aspect ratio. These results provide simple and general expressions for the pressure drop needed to over-come capillary forces and push the air finger inside the channel.

The present study investigates experimentally the evolution of two-phase flow pattern in converging and diverging, silicon-based microchannels with mean hydraulic diameter of 128 μm with CO 2 bubbles produced by chemical reactions of sulfuric acid (H 2 SO 4 ) and sodium bicarbonate (NaHCO 3 ). The microchannels are prepared by bulk micromachining and anodic bonding. Three different concentrations of 0.2, 0.5 and 0.8 mol/L of each reactant at the inlet before mixing and 10 different flow rates from 1.67×10 −9 m 3 /s to 16.7×10 −9 m 3 /s are studied. Flow visualization is made possible by using a high-speed digital camera. It is found that the acceleration effect in a converging microchannel tends to diminish chemical reactions and no bubbles are formed at low concentrations and high flow rates, while large spherical bubbles are generated in the regions near the inlet and slug flow is formed in the regions near the exit for high concentrations and low flow rates. On the other hand, the deceleration effect in a diverging microchannel tends to promote chemical reactions, especially in the regions near the exit, and many bubbles are produced in those regions. Slug flow with large bubble slugs tends to appear in most parts of the diverging microchannel for low concentrations. Merger of bubbles appears frequently in the diverging microchannel. The results demonstrate the strong effects of concentration and acceleration or deceleration on the evolution of two-phase flow pattern in a converging or diverging microchannel.