Microflow Devices Research Articles

Theoretical analysis of the frictional losses in magnetohydrodynamic microflows considering slippage

In this article, we study the frictional losses in magnetohydrodynamic (MHD) microflows by analyzing the Poiseuille number defined through the Darcy–Weisbach friction factor. We consider two-dimensional fully developed flow models characteristic of MHD micropumps including the Hartmann braking effect and the existence of slippage. Unlike the purely hydrodynamic case, in MHD flows the Poiseuille number depends not only on the aspect ratio but also on the physical properties of the fluid and the externally applied magnetic field. Three different combinations of boundary conditions (slip and no-slip) are investigated. Calculations show that the Poiseuille number is considerably reduced as the dimensionless slip length is increased, while it increases as Hartmann number does. The obtained results are consistent with previous models and are helpful for the design of magnetohydrodynamic microflow devices.

Electro-osmotic flow in rectangular microchannels: geometry optimization

Micro-flow devices have turned over the years from the subject of fundamental research to fully-fledged industrial applications. Although for some cases the transport phenomena in micro-devices can be handled satisfactorily by using the same approach as for their larger-size counterparts, micro-effects such as electro-osmotic flow (EOF) are typical of small scales and can be employed to circulate coolant through heat sinks by means of electro-osmotic pumping. In order to obey the constraints that different engineering applications impose, design criteria must be employed to optimize the performance of the devices according to the criteria chosen. In this work optimization of the cross-sectional area of a microchannel where EOF and heat transfer at the walls occur is employed to demonstrate how approaches based on the first or second law of thermodynamics may yield opposite results, which are strongly dependent on the peculiarities of EOF, i.e. the ratio of Joule heating to heat transfer at the walls.

Influence of Stefan blowing on nanofluid flow submerged in microorganisms with leading edge accretion or ablation

The forced convective boundary layer flow of viscous incompressible time-dependent fluid containing water-based nanofluids and gyrotactic microorganisms simultaneously, from a flat surface with leading edge accretion (or ablation), is theoretically investigated in the present study. In doing so, the governing conservation equations are rendered into a nonlinear system of ordinary differential equations by means of utilizing appropriate coordinates transformations. MAPLE symbolic software is employed to solve these equations, which are subjected to impose boundary conditions using the Runge–Kutta–Fehlberg fourth-fifth order numerical method. It is noteworthy that the results of the present study are in an excellent agreement with previous solutions available in literature. The effect of selected parameters on velocity, temperature, nanoparticle volume fraction and motile microorganism density function is then investigated. Tabular solutions are included for the skin friction, heat transfer rate, nano-particle mass transfer rate and microorganism transfer rate. Applications of the study arise in advanced micro-flow devices to bio-modified nanomaterials processing.

Laser-Induced Single Microdroplet Formation and Simultaneous Water-to-Single Microdroplet Extraction/Detection in Aqueous 1-Butanol Solutions

Abstract Focused 1064-nm laser beam irradiation to an aqueous 1-butanol (BuOH) solution (7.1–7.4 wt % in H2O) resulted in formation of a single picoliter-volume BuOH droplet. Since water (H2O) absorbs 1064-nm laser light, an aqueous BuOH solution at the laser beam focus is heated via photo-thermal effects and this leads to thermal phase separation of the solution, producing a single BuOH microdroplet. In the presence of a fluorescent dye (10−5–10−7 mol/dm3) in an aqueous BuOH solution, the dye was extracted from the surrounding water phase to the BuOH droplet produced by laser irradiation as demonstrated by in situ fluorescence and Raman microspectroscopies. The present laser-induced water-to-single microdroplet extraction/detection was also extended successfully to that under pressure-driven and electroosmotic flow conditions in microflow devices. In both cases, the single BuOH microdroplets produced by 1064-nm laser irradiation were optically trapped against flow of the solution. Under electroosmotic flow conditions, highly sensitive detection of a fluorescent Al3+-chelate complex injected to an electrophoresis capillary tube was also achieved successfully by single BuOH microdroplet formation and simultaneous extraction of the Al3+ chelate to the droplet by 1064-nm laser irradiation.

Rapid Particle Patterning in Surface Deposited Micro-Droplets of Low Ionic Content via Low-Voltage Electrochemistry and Electrokinetics.

Electrokinetic phenomena are a powerful tool used in various scientific and technological applications for the manipulation of aqueous solutions and the chemical entities within them. However, the use of DC-induced electrokinetics in miniaturized devices is highly limited. This is mainly due to unavoidable electrochemical reactions at the electrodes, which hinder successful manipulation. Here we present experimental evidence that on-chip DC manipulation of particles between closely positioned electrodes inside micro-droplets can be successfully achieved, and at low voltages. We show that such manipulation, which is considered practically impossible, can be used to rapidly concentrate and pattern particles in 2D shapes in inter-electrode locations. We show that this is made possible in low ion content dispersions, which enable low-voltage electrokinetics and an anomalous bubble-free water electrolysis. This phenomenon can serve as a powerful tool in both microflow devices and digital microfluidics for rapid pre-concentration and particle patterning.

From 'Lab & Light on a Chip' to Parallel Microflow Photochemistry

Continuous-flow microreactors offer major advantages for photochemical applications. This mini-review summarizes the technological development of microflow devices in the Applied and Green Photochemistry Group at James Cook University, and its associates, from fixed microchips for microscale synthesis to flexible multicapillary systems for parallel photochemistry. Whereas the enclosed microchip offered high space–time-yields, the open capillary-type reactor showed a greater potential for further modifications. Consequently, a 10-microcapillary reactor was constructed and used successfully for process optimization, reproducibility studies, scale-up, and library synthesis. To demonstrate the superiority of microflow photochemistry over conventional batch processes, the reactors were systematically evaluated using alcohol additions to furanones as model reactions. In all cases, the microreactor systems furnished faster conversions, improved product qualities, and higher yields. UVC-induced [2+2] cycloadditions of furanone with alkenes were exemplarily examined in a capillary reactor, thus proving the broad applicability of this reactor type.

Development of Polymeric Palladium‐Nanoparticle Membrane‐Installed Microflow Devices and their Application in Hydrodehalogenation

We have developed a variety of polymeric palladium-nanoparticle membrane-installed microflow devices. Three types of polymers were convoluted with palladium salts under laminar flow conditions in a microflow reactor to form polymeric palladium membranes at the laminar flow interface. These membranes were reduced with aqueous sodium formate or heat to create microflow devices that contain polymeric palladium-nanoparticle membranes. These microflow devices achieved instantaneous hydrodehalogenation of aryl chlorides, bromides, iodides, and triflates by 10-1000 ppm within a residence time of 2-8 s at 50-90 °C by using safe, nonexplosive, aqueous sodium formate to quantitatively afford the corresponding hydrodehalogenated products. Polychlorinated biphenyl (10-1000 ppm) and polybrominated biphenyl (1000 ppm) were completely decomposed under similar conditions, yielding biphenyl as a fungicidal compound.

ChemInform Abstract: Development of Polymeric Metal Catalysts via Molecular Convolution and of Catalytic Membrane‐Installed Microflow Devices

AbstractReview: about 30 refs.

Development of micro-flow devices by direct-milling on poly(methyl methacrylate) substrates with integrated optical detection

The development of micro-flow devices by direct-milling on poly(methyl methacrylate) (PMMA) substrates and incorporating the optic detector into the same structure is presented, evaluated and applied to the determination of chlorhexidine. Hydrodynamic characteristics of the developed devices were compared against those obtained from flow systems based on classic Teflon reactors. The optical detector integrated in the PMMA substrate is composed of a light emitting diode (λ = 627 nm) and light dependence resistance as light source and light detector, respectively. The performance of the optical detector was evaluated and optimized. The applicability of the fluidic device was assessed in the determination of chlorhexidine through its colorimetric reaction with bromocresol green. Micro-pumps were used as propulsion units. Results obtained in real sample analysis were in good agreement with those obtained by the reference procedure.

Applications of ferrofluids in Micro Electro Mechanical Systems (MEMS) and micropumps

The micro-pump is one of the most promising micro-flow devices. At micro-level electronically controlled pumping of any fluid by a mechanical pump is not so easy and reliable. In the realm of nano-tech materials, ferrofluids have unique properties in both liquids and solids and have potential applications for MEMS/NEMS devices. This paper presents two new types of concepts, a micro-flowmeter based on a micro-turbine made using MEMS technology and the other is a micro-pump based on ferrofluidic. actuation. In our first device an optical photovoltaic sensor has also been integrated with this device. and the, micro-turbine rotates with a Speed of 50000 rpm. We have fabricated a ferrofluid-based glass micro-pump of size 20 x 20 x 10 mm(3). in which micro actuation is electrically, controlled by NdFeB (N50) permanent magnets (diameter 5 x 3mm, B(r) = 1400 mT, coercive field H(c) = 840 kA/m) with a ferrofluid bearing. The device is able to pump the fluid at the rate of 10 mu L/actuation.

PDMS microchannel fabrication technique based on microwire-molding

Micro-flow channel is basic functional component of microfluidic chip, and every step-forward of its construction technique has been receiving concern all over the world. This article presents a notcomplicated but flexible method for fabrication of micro-flow channels. This method mainly utilizes the conventional molding capability of polydimethylsiloxane (PDMS) and widespread commercial microwires as templates. We have fabricated out some conventional types of microchannels with different topological shapes, as examples for the demonstration of this flexible fabrication route which was not dependent on the stringent demands of photolithographical or microelectromechanical system (MEMS) techniques. The smooth surface, high-intensity, and high flexibility of the wires made it possible to create many types of topological structures of the two-dimensional or three-dimensional microchannel or channel array. The geometric shape of the cross-section of thus forming microchannel in PDMS was the negative of that of embedded-in microwire, in high-fidelity if suitable measures were taken. Moreover, such a microchannel fabrication process can easily integrate the conductivity and low resistivity of the metal wire to create micro-flow devices that are suitable for the electromagnetic control of liquid or the temperature regulation in the microchannel. Furthermore some preliminary optical analysis was provided for the observation of thus forming rounded microchannel. Based on this molding strategy, we even made some prototypes for functional microflow application, such as microsolenoids chip and temperature control gadgets. And an experiment of forming a droplet in the cross channel further confirmed the feasibility and applicability of this flexible microchannel forming technique.

Critical view on “new results in micro-fluid mechanics”: an example

Experiments in micro-flow devices almost always show deviations compared to the corresponding situations in macro-systems. Often special “micro-effects” are proposed to explain these unexpected results. However, based on a nondimensional form of the problem formulation these “micro-effects” can be identified as scaling effects referred to a standard analysis in macro-dimensions. Thus many “unexpected results” can be explained. This is demonstrated for a specific example recently published in this journal.

Microflow devices and systems

Microflow devices including microvalves, micropumps and microflow sensors fabricated by micromachining are reviewed from the point of view of the actuating principle and structures. Integration of microflow control devices and microflow sensors allowed very precise control of small flow. High performance liquid dosing microsystems and sophisticated chemical analysing microsystems were demonstrated by the combination of microflow devices and microsensors. Applications of microflow devices and systems are also introduced.