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Showing papers in "Microfluidics and Nanofluidics in 2009"


Journal ArticleDOI
TL;DR: A review of techniques for sealing thermoplastic microfluidics can be found in this paper, where the authors discuss a number of practical issues surrounding these various bonding methods and discuss a set of unique challenges which must be addressed to achieve optimal sealing results.
Abstract: Thermoplastics are highly attractive substrate materials for microfluidic systems, with important benefits in the development of low cost disposable devices for a host of bioanalytical applications. While significant research activity has been directed towards the formation of microfluidic components in a wide range of thermoplastics, sealing of these components is required for the formation of enclosed microchannels and other microfluidic elements, and thus bonding remains a critical step in any thermoplastic microfabrication process. Unlike silicon and glass, the diverse material properties of thermoplastics opens the door to an extensive array of substrate bonding options, together with a set of unique challenges which must be addressed to achieve optimal sealing results. In this paper we review the range of techniques developed for sealing thermoplastic microfluidics and discuss a number of practical issues surrounding these various bonding methods.

618 citations


Journal ArticleDOI
TL;DR: Recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology are summarized and two main categories are classified: physical approach including physisorption of charged or amphiphilic polymers and copolymers, as well as chemical approach including self assembled monolayer and thick polymer coating.
Abstract: Fast advancements of microfabrication processes in past two decades have reached to a fairly matured stage that we can manufacture a wide range of microfluidic devices. At present, the main challenge is the control of nanoscale properties on the surface of lab-on-a-chip to satisfy the need for biomedical applications. For example, poly(dimethylsiloxane) (PDMS) is a commonly used material for microfluidic circuitry, yet the hydrophobic nature of PDMS surface suffers serious nonspecific protein adsorption. Thus the current major efforts are focused on surface molecular property treatments for satisfying specific needs in handling macro functional molecules. Reviewing surface modifications of all types of materials used in microfluidics will be too broad. This review will only summarize recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology and classify them into two main categories: (1) physical approach including physisorption of charged or amphiphilic polymers and copolymers, as well as (2) chemical approach including self assembled monolayer and thick polymer coating. Pros and cons of a collection of available yet fully exploited surface modification methods are briefly compared among subcategories.

495 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the main significant developments that have been achieved in different aspects of micro-injection molding of microfluidic devices, including device design, machine capabilities, mold manufacturing, material selection and process parameters.
Abstract: Microfluidic devices have several applications in different fields, such as chemistry, medicine and biotechnology. Many research activities are currently investigating the manufacturing of integrated microfluidic devices on a mass-production scale with relatively low costs. This is especially important for applications where disposable devices are used for medical analysis. Micromoulding of thermoplastic polymers is a developing process with great potential for producing low-cost microfluidic devices. Among different micromoulding techniques, micro-injection moulding is one of the most promising processes suitable for manufacturing polymeric disposable microfluidic devices. This review paper aims at presenting the main significant developments that have been achieved in different aspects of micro-injection moulding of microfluidic devices. Aspects covered include device design, machine capabilities, mould manufacturing, material selection and process parameters. Problems, challenges and potential areas for research are highlighted.

445 citations


Journal ArticleDOI
TL;DR: In this paper, a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration is described, which takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces.
Abstract: In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.

316 citations


Journal ArticleDOI
TL;DR: The recent progress on EOP fabrications and applications are reviewed in order to promote the awareness of EOPs to researchers interested in using micro- and nano-fluidic devices.
Abstract: Electroosmotic pumping is receiving increasing attention in recent years owing to the rapid development in micro total analytical systems. Compared with other micropumps, electroosmotic pumps (EOPs) offer a number of advantages such as creation of constant pulse-free flows and elimination of moving parts. The flow rates and pumping pressures of EOPs matches well with micro analysis systems. The common materials and fabrication technologies make it readily integrateable with lab-on-a-chip devices. This paper reviews the recent progress on EOP fabrications and applications in order to promote the awareness of EOPs to researchers interested in using micro- and nano-fluidic devices. The pros and cons of EOPs are also discussed, which helps these researchers in designing and constructing their micro platforms.

291 citations


Journal ArticleDOI
TL;DR: In this article, a fast and homogenized mixing device through the use of a bubble-based microfluidic structure was reported, which worked by trapping air bubbles within the pre-designed grooves on the sidewalls of the channel.
Abstract: Due to the low Reynolds number associated with microscale fluid flow, it is difficult to rapidly and homogenously mix two fluids. In this letter, we report a fast and homogenized mixing device through the use of a bubble-based microfluidic structure. This micromixing device worked by trapping air bubbles within the pre-designed grooves on the sidewalls of the channel. When acoustically driven, the membranes (liquid/air interfaces) of these trapped bubbles started to oscillate. The bubble oscillation resulted in a microstreaming phenomenon—strong pressure and velocity fluctuations in the bulk liquid, thus giving rise to fast and homogenized mixing of two side-by-side flowing fluids. The performance of the mixer was characterized by mixing deionized water and ink at different flow rates. The mixing time was measured to be as small as 120 ms.

238 citations


Journal ArticleDOI
TL;DR: In this paper, the state-of-the-art in surface-enhanced Raman scattering (SERS) based optical detection techniques with an application focus on cancer diagnostics is reviewed.
Abstract: In this paper, we review the state-of-the-art in surface-enhanced Raman scattering (SERS) based optical detection techniques with an application focus on cancer diagnostics. As we describe herein, SERS has several analytical, biological and engineering advantages over other methods including extremely high sensitivity, inherent molecular specificity of unlabeled targets, and narrow spectral bands. We review advances in both in vitro and in vivo applications of SERS and examine how technical issues with the technology are being addressed. A special technology focus is given to emerging optofluidic devices which aim to merge microfluidic and optical detection technologies into simple packages. We conclude with a brief discussion of some of the emerging challenges in the field and some of the approaches that are likely to enhance their application.

205 citations


Journal ArticleDOI
TL;DR: An overview of electrokinetic phenomena associated with particles and cells in microchannel systems is provided in this article, where the most important phenomena covered include electrophoresis, dielectrophoreis, and induced-charge electrokinetics.
Abstract: This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.

192 citations


Journal ArticleDOI
JinHo Song, R.D. Evans1, Yan-You Lin1, Bang-Ning Hsu1, Richard B. Fair1 
TL;DR: In this article, a hydrodynamic scaling model of droplet actuation in an electrowetting-on-dielectric (EWD) actuator is presented that takes into account the effects of contact angle hysteresis, drag from the filler fluid, and change in the actuation force while a droplet traverses a neighboring electrode.
Abstract: A hydrodynamic scaling model of droplet actuation in an electrowetting-on-dielectric (EWD) actuator is presented that takes into account the effects of contact angle hysteresis, drag from the filler fluid, drag from the solid walls, and change in the actuation force while a droplet traverses a neighboring electrode. Based on this model, the threshold voltage, VT, for droplet actuation is estimated as a function of the filler medium of a scaled device. It is shown that scaling models of droplet splitting and liquid dispensing all show a similar scaling dependence on [t/er(d/L)]1/2, where t is insulator thickness and d/L is the aspect ratio of the device. It is also determined that reliable operation of a EWD actuator is possible as long as the device is operated within the limits of the Lippmann–Young equation. The upper limit on applied voltage, Vsat, corresponds to contact-angle saturation. The minimum 3-electrode splitting voltages as a function of aspect ratio d/L < 1 for an oil medium are less than Vsat. However, for an air medium the minimum voltage for 3-electrode droplet splitting exceeds Vsat for d/L ≥ 0.4. EWD actuators were fabricated to operate with droplets down to 35pl. Reasonable scaling results were achieved.

157 citations


Journal ArticleDOI
TL;DR: This paper describes some of the fundamental thermal and non-thermal micro-flow sensors presented in the literature over the past 30 years or so and provides a high level overview of the MEMS field.
Abstract: Micro-electro-mechanical system (MEMS) devices integrate various mechanical elements, sensors, actuators, and electronics on a single silicon substrate in order to accomplish a multitude of different tasks in a diverse range of fields. The potential for device miniaturization made possible by MEMS micro-fabrication techniques has facilitated the development of many new applications, such as highly compact, non-invasive pressure sensors, accelerometers, gas sensors, etc. Besides their small physical footprint, such devices possess many other advantages compared to their macro-scale counterparts, including greater precision, lower power consumption, more rapid response, and the potential for low-cost batch production. One area in which MEMS technology has attracted particular attention is that of flow measurement. Broadly speaking, existing micro-flow sensors can be categorized as either thermal or non-thermal, depending upon their mode of operation. This paper commences by providing a high level overview of the MEMS field and then describes some of the fundamental thermal and non-thermal micro-flow sensors presented in the literature over the past 30 years or so.

149 citations


Journal ArticleDOI
TL;DR: The microfluidic resistive pulse sensor advances in sensitivity over the traditional Coulter counter by improving signal amplification and noise reduction techniques and the capacitance counter is useful in liquids of low electrical conductivity and in sensing the changes of cell contents.
Abstract: Microfluidic particle counters are important tools in biomedical diagnostic applications such as flow cytometry analysis. Major methods of counting particles in microfluidic devices are reviewed in this paper. The microfluidic resistive pulse sensor advances in sensitivity over the traditional Coulter counter by improving signal amplification and noise reduction techniques. Nanopore-based methods are used for single DNA molecule analysis and the capacitance counter is useful in liquids of low electrical conductivity and in sensing the changes of cell contents. Light-scattering and light-blocking counters are better for detecting larger particles or concentrated particles. Methods of using fluorescence detection have the capability for differentiating particles of similar sizes but different types that are labeled with different fluorescent dyes. The micro particle image velocimetry method has also been used for detecting and analyzing particles in a flow field. The general limitation of microfluidic particle counters is the low throughput which needs to be improved in the future. The integration of two or more existing microfluidic particle counting techniques is required for many practical on-chip applications.

Journal ArticleDOI
TL;DR: In this paper, a new sort of nanofluids phase change materials (PCMs) is developed by suspending small amount of TiO2 nanoparticles in saturated BaCl2 aqueous solution.
Abstract: A new sort of nanofluids phase change materials (PCMs) is developed by suspending small amount of TiO2 nanoparticles in saturated BaCl2 aqueous solution. The resulting nanofluids PCMs possess remarkably high thermal conductivities compared to the base material. Cool storage/supply experiments conducted in a small apparatus have shown the excellent phase change performance of the nanofluids PCMs. The cool storage/supply rate and the cool storage/supply capacity all increase greatly those that of BaCl2 aqueous solution without added nanoparticles. The higher thermal performances of nanofluids PCMs indicate that they have a potential for substituting conventional PCMs in cool storage applications.

Journal ArticleDOI
TL;DR: In this article, the authors revisited the problem of micro-channel compressible gas flows and showed that the axial diffusion of mass engendered by the density (pressure) gradient becomes increasingly significant with increased Knudsen number compared to the pressure driven convection.
Abstract: Despite the enormous scientific and technological importance of micro-channel gas flows, the understanding of these flows, by classical fluid mechanics, remains incomplete including the prediction of flow rates. In this paper, we revisit the problem of micro-channel compressible gas flows and show that the axial diffusion of mass engendered by the density (pressure) gradient becomes increasingly significant with increased Knudsen number compared to the pressure driven convection. The present theoretical treatment is based on a recently proposed modification (Durst et al. in Proceeding of the international symposium on turbulence, heat and mass transfer, Dubrovnik, 3-18 September, pp 25-29, 2006) of the Navier-Stokes equations that include the diffusion of mass caused by the density and temperature gradients. The theoretical predictions using the modified Navier-Stokes equations are found to be in good agreement with the available experimental data spanning the continuum, transition and free-molecular (Knudsen) flow regimes, without invoking the concept of Maxwellian wall-slip boundary condition. The simple theory also results in excellent agreement with the results of linearized Boltzmann equations and Direct Simulation Monte Carlo (DSMC) method. Finally, the theory explains the Knudsen minimum and suggests the design of future micro-channel flow experiments and their employment to complete the present days understanding of micro-channel flows.

Journal ArticleDOI
TL;DR: In this article, the Stokes flow through microchannels employing superhydrophobic surfaces with alternating micro-grooves and ribs was analyzed and the effects of patterning the grooves on one or both channel walls were also analyzed.
Abstract: Superhydrophobic surfaces have been demonstrated to be capable of reducing fluid resistance in micro- and nanofluidic applications. The objective of this paper is to present analytical solutions for the Stokes flow through microchannels employing superhydrophobic surfaces with alternating micro-grooves and ribs. Results are presented for both cases where the micro-grooves are aligned parallel and perpendicular to the flow direction. The effects of patterning the grooves on one or both channel walls are also analyzed. The reduction in fluid resistance has been quantified in terms of a dimensionless effective slip length, which is found to increase monotonically with the shear-free fraction and the periodic extent of each groove–rib combination normalized by the channel half-height. Asymptotic relationships have been derived for the normalized effective slip length corresponding to large and small limiting values of the shear-free fraction and the normalized groove–rib period. A detailed comparison has been made between transverse and longitudinal grooves, patterned on one or both channel walls, to assess their effectiveness in terms of enhancing the effective slip length. These comparisons have been carried out for small and large limiting values, as well as finite values of the shear-free fraction and normalized groove–rib period. Results for the normalized effective slip length corresponding to transverse and longitudinal grooves are further applied to model the Stokes flow through microchannels employing superhydrophobic surfaces containing a periodic array of micro-grooves inclined at an angle to the direction of the applied pressure gradient. Results are presented for the normalized effective slip lengths parallel to the direction of the applied pressure gradient and the normalized cross flow rate perpendicular to the direction of the applied pressure gradient.

Journal ArticleDOI
TL;DR: In this article, a particle focusing technique for DC electrokinetic flow through a planar serpentine microchannel is presented, which stems from the cross-stream dielectrophoretic motion induced within the channel turns.
Abstract: Focusing particles into a tight stream is usually a necessary step prior to separating and sorting them. We present herein a proof-of-concept experiment of a novel particle focusing technique in DC electrokinetic flow through a planar serpentine microchannel. This focusing stems from the cross-stream dielectrophoretic motion induced within the channel turns. The observed particle focusing behavior is consistent with the predicted particle trajectories from a numerical modeling.

Journal ArticleDOI
TL;DR: In this article, the effects of the generalized power law coefficient, the power law exponent and the yield stress on the mechanism of drop breakup, final drop size and frequency of drop formation are studied by using the numerical three-dimensional volume of fluid model.
Abstract: Both numerical and analytical models have been developed to explore the viscosity effect of the continuous phase on drop formation at a T-shaped junction in immiscible liquids. The effects of the generalized power law coefficient, the power law exponent and the yield stress on the mechanism of drop breakup, final drop size and frequency of drop formation are studied by using the numerical three-dimensional volume of fluid model. Droplets coalescence in Bingham fluids is observed in the beginning transient period. The effect of yield stress on drop extension is also discussed. Predictions of drop size by using an analytical force balance show satisfactory agreement with simulation results for Newtonian and power law fluids with different viscosity ratios. The approximation error associated with the analytical model for Bingham fluids is also acceptable. This analytical model can greatly shorten the prediction time as compared with the numerical model, which is helpful for on-line control.

Journal ArticleDOI
TL;DR: In this paper, a new pneumatic micropump integrated with a normally closed valve that is capable of generating a high pumping rate and a high back pressure is presented. But, it is not shown that the back pressure can be as high as 85 cm-H2O (8,610.5 Nt/m2) at the same operation conditions.
Abstract: This study reports on a new pneumatic micropump integrated with a normally closed valve that is capable of generating a high pumping rate and a high back pressure. The micropump consists of a sample flow microchannel, three underlying pneumatic air chambers, resilient polydimethylsiloxane (PDMS) membrane structures and a normally closed valve. The normally closed valve of the micropump is a PDMS-based floating block structure located inside the sample flow microchannel, which is activated by hydraulic pressure created by the peristaltic motion of the PDMS membranes. The valve is used to effectively increase pumping rates and back pressures since it is utilized to prevent backflow. Experimental results indicate that a pumping rate as high as 900 μL/min at a driving frequency of 90 Hz and at an applied pressure of 20 psi (1.378 × 105 Nt/m2) can be obtained. The back pressure on the micropump can be as high as 85 cm-H2O (8,610.5 Nt/m2) at the same operation conditions. The micropump is fabricated by soft lithography processes and can be easily integrated with other microfluidic devices. To demonstrate its capability to prevent cross contamination during chemical analysis applications, two micropumps and a V-shape channel are integrated to perform a titration of two chemical solutions, specifically sodium hydroxide (NaOH) and benzoic acid (C6H5COOH). Experimental data show that mixing with a pH value ranging from 2.8 to 12.3 can be successfully titrated. The development of this micropump can be a promising approach for further biomedical and chemical analysis applications.

Journal ArticleDOI
TL;DR: In this article, analytical and experimental results that compare the laminar flow dynamics through microchannels with superhydrophobic walls featuring ribs and cavities oriented both parallel and transverse to the direction of flow under both wetting and non-wetting conditions are presented.
Abstract: Recent developments in superhydrophobic surfaces have enabled significant reduction in the frictional drag for liquid flow through microchannels. There is an apparent risk when using such surfaces, however, that under some conditions the liquid meniscus may destabilize and, consequently, the liquid will wet the entire patterned surface. This paper presents analytical and experimental results that compare the laminar flow dynamics through microchannels with superhydrophobic walls featuring ribs and cavities oriented both parallel and transverse to the direction of flow under both wetting and non-wetting conditions. The results show the reduction in the total frictional resistance is much greater in channels when the liquid phase does not enter the cavity regions. Further, it is demonstrated that the wetting and non-wetting cavity results represent limiting cases between which the experimental data lie. Generalized expressions enabling prediction of the classical friction factor-Reynolds number product as a function of the relevant governing dimensionless parameters are also presented for both the superhydrophobic and wetting states. Experimental results are presented for a range of parameters in the laminar flow regime.

Journal ArticleDOI
TL;DR: In this paper, the authors combined numerical simulation with molecular-diffusion effect, high-tempo micro-particle image velocimetry (μ-PIV), and probability distribution function (PDF) analysis to investigate the chaotic mixing and hydrodynamics inside a droplet moving through a planar serpentine micromixer.
Abstract: In this work we combined numerical simulation with molecular-diffusion effect, high-tempo micro-particle image velocimetry (μ-PIV), and probability distribution function (PDF) analysis to investigate the chaotic mixing and hydrodynamics inside a droplet moving through a planar serpentine micromixer (PSM). Robust solutions for the distributions of interface and concentration of the droplets were obtained via computational fluid dynamics. The simulated fluid patterns are consistent with those measured with μ-PIV, which serves as a powerful nonintrusive diagnostic approach to observe the droplets. Two mechanisms are proposed to enhance the performance of mixing in a PSM—the deformation of droplets and the asymmetric recirculation within the droplets. On introducing alternating cross sections into a winding channel, this specific design of PSM is found to amplify the fluid disturbance and maximum vorticity difference. Data show that the PDF of the vorticity fields is modified and the fraction with larger vorticity is increased. Accordingly, the PSM is capable of achieving a mixing index 90% within 700 μm (Re = 2), which is eight times better than for a straight microchannel. The results not only demonstrate explicitly the fluid patterns within the droplets but also provide significant insight into the factors dominating the mixing efficiency.

Journal ArticleDOI
TL;DR: In this article, the effect of applied frequency on mixing was investigated over the range 0.5-10 kHz, and it was found that a significant increase in the mixing efficiency was achieved within a window of the frequencies between 1.0 and 5.0 kHz by a piezoelectric disc.
Abstract: Due to small dimensions and low fluid velocity, mixing in microfluidic systems is usually poor. In this study, we report a method of enhancing microfluidic mixing using acoustically induced gas bubbles. The effect of applied frequency on mixing was investigated over the range 0.5-10 kHz. Under either low frequency 0.5 kHz or high frequency 10 kHz, no noticeable improvement in the present mixer was observed. However, a significant increase in the mixing efficiency was achieved within a window of the frequencies between 1.0 and 5.0 kHz. It was found in our present microfluidic structure, single (or multi-) bubble(s) could be acoustically generated under the frequency ranging from 1.0 to 5.0 kHz by a piezoelectric disc. The interaction between bubble and acoustic field causes bubble oscillation which in turn could disturb local flow field to result in mixing enhancement.

Journal ArticleDOI
Majid Nabavi1
TL;DR: The present review reveals that there are many possible areas of research in the field of steady and unsteady flows through microdiffusers and micropumps in order to understand the effects of all important design parameters on the performance of these devices.
Abstract: In recent research, there has been a growing interest in the analysis of flow through microdiffusers and micropumps in order to characterize and optimize the performance of these devices. In this review, the recent advances in the numerical and experimental analysis of the steady and pulsating flows through microdiffusers and valveless micropumps are surveyed. The differences between the performance of microdiffusers and micropumps in steady and unsteady flow regimes are described. Qualitative and quantitative discussions of the effects of different design parameters on the performance of microdiffusers and valveless micropumps in both steady and unsteady flow regimes along with the contradictory results reported in the literature in this regard are provided. In addition, a summary of the latest micropump technologies along with the advantages and disadvantages of each mechanism with the emphasis on the innovative and less-reviewed micropumps are presented. Two important types of fixed microvalves, as part of valveless micropumps are described in details. Experimental flow visualization of steady and pulsating flows through microdiffusers and micropumps as a useful tool for better understanding the underlying micro-fluid dynamics is discussed. The present review reveals that there are many possible areas of research in the field of steady and unsteady flows through microdiffusers and micropumps in order to understand the effects of all important design parameters on the performance of these devices.

Journal ArticleDOI
TL;DR: In this article, a convenient immunoassay using antibody-gold nanoparticle (Ab-AuNP) conjugate as a reporter molecule and a flatbed scanner for the optical scanning and measuring of the immuno-reaction signal is reported.
Abstract: This study reports a convenient immunoassay using antibody-gold nanoparticle (Ab-AuNP) conjugate as a reporter molecule and a flatbed scanner for the optical scanning and measuring of the immuno-reaction signal. The silver enhancement reaction, a signal amplification method in which silver ions are reduced to silver metal, is introduced to magnify the detection signal. The whole framework of the study is based on (1) the direct immunoassay (two-layer format) in which the antigen is directly immobilized on the chip, and (2) the sandwich immunoassay [three-layer format, process similar to enzyme-linked immunosorbent assay (ELISA)], which contains a primary antibody, a secondary antibody (antibody-nanoparticle conjugate) and antigens. The experimental data show that the micro-scale silver precipitation phenomenon is catalyzed by Ab-AuNP conjugates. This reaction can be observed by the naked eye, even at an IgG-AuNPs concentration of 5 pM. The relationships between sample concentration and detection signal are discussed, and the detection limit (sandwich assay) for the sample antigen is 1 ng/mL. Using a flatbed scanner, Ab-AuNP conjugates and a silver enhancement reaction, a new immunoassay is constructed.

Journal ArticleDOI
TL;DR: In this paper, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75.
Abstract: Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with high-speed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the non-uniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 μm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.

Journal ArticleDOI
TL;DR: A review of the latest developments in nanobiosensors with a focus on label-based (fluorescence and nanoparticle) and label-free methods (surface plasmon resonance, micro/nanocantilever, nanowires, and nanopores) is provided in this article.
Abstract: Biosensors based on nanotechnology are rapidly developing and are becoming widespread in the biomedical field and analytical chemistry. For these nanobiosensors to reach their potential, they must be integrated with appropriate packaging techniques, which are usually based on nano/microfluidics. In this review we provide a summary of the latest developments in nanobiosensors with a focus on label-based (fluorescence and nanoparticle) and label-free methods (surface plasmon resonance, micro/nanocantilever, nanowires, and nanopores). An overview on how these sensors interface with nano/microfluidics is then presented and the latest papers in the area summarized.

Journal ArticleDOI
TL;DR: In this article, heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 μm, was analyzed through a parametric investigation, and were also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality.
Abstract: The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 μm, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000 kg/m2 s, and heat fluxes up to 200 kW/m2. Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality.

Journal ArticleDOI
TL;DR: In this paper, the effect of microchannel depths on the droplet formation process was investigated and it was shown that the smaller the depth of the channel the larger the increase of droplet size with temperature.
Abstract: This paper reports experimental investigations on the droplet formation and size manipulation of deionized water (DIW) and nanofluids in a microfluidic T-junction at different temperatures. Investigations of the effect of microchannel depths on the droplet formation process showed that the smaller the depth of the channel the larger the increase of droplet size with temperature. Sample nanofluids were prepared by dispersing 0.1 volume percentage of titanium dioxide (TiO2) nanoparticles of 15 nm and 10 nm × 40 nm in DIW for their droplet formation experiments. The heater temperature also affects the droplet formation process. Present results demonstrate that nanofluids exhibit different characteristics in droplet formation with the temperature. Addition of spherical-shaped TiO2 (15 nm) nanoparticles in DIW results in much smaller droplet size compared to the cylindrical-shaped TiO2 (10 nm × 40 nm) nanoparticles. Besides changing the interfacial properties of based fluid, nanoparticles can influence the droplet formation of nanofluids by introducing interfacial slip at the interface. Other than nanofluid with cylindrical-shaped nanoparticles, the droplet size was found to increase with increasing temperature.

Journal ArticleDOI
TL;DR: In this article, an efficient technique for trapping of magnetic particles in confined spatial locations using customized designs of micro-coils (MCs) was presented, where a large magnetic field gradient was generated and enhanced by two structural parameters: the small width and high aspect ratio of each single conductor and the ferromagnetic pillars positioned at high-flux density locations.
Abstract: This paper presents an efficient technique for trapping of magnetic particles in confined spatial locations using customized designs of micro-coils (MCs). Large magnetic field gradients of up to 20 T/mm and large magnetic forces in the range of 10−8 Newton on magnetic particles with diameter of 1 μm have been achieved using MCs with several planar geometrical configurations. A large magnetic field gradient is generated and enhanced by two structural parameters: the small width and high aspect ratio of each single conductor and the ferromagnetic pillars positioned at high-flux density locations. This arrangement creates very steep magnetic potential wells, in particular at the vicinity of the pillars. The system allowed capturing of suspended magnetic particles as far as 1,000 μm from the center of the device. Magnetic particles/cells have been trapped and confined in single and in arrays of deep magnetic potential wells corresponding to the MCs configuration.

Journal ArticleDOI
TL;DR: In this paper, the authors used an acoustically excited oscillating bubble deposited on a hydrophobic rod tip to capture various millimeter and micron-sized neighboring objects including glass and polystyrene beads.
Abstract: This communication describes novel 3-D manipulations of objects using an acoustically excited oscillating bubble deposited on a hydrophobic rod tip. The oscillating bubble captures various millimeter- and micron-sized neighboring objects including glass and polystyrene beads (~100 μm), fish egg, and live water flea (~1 mm). The captured objects are carried in a 3-D space by traversing the bubble tip, and released at desired positions by simply turning off the oscillation. Carrying performance is characterized along with high-speed imaging of oscillating bubbles by varying the frequency and amplitude of the acoustic excitation and the carrying speed. The higher the oscillation amplitude, the higher the carrying efficiency. The maximum carrying speed is measured at over 3 mm/s. This method is effective with a low-level acoustic excitation (bubble oscillation amplitude relative to the diameter ≤5%), possibly providing a cost-effective, soft-contact manipulating tool for handling biological objects.

Journal ArticleDOI
TL;DR: Concern about evaporative loss in unsealed reactors have been addressed by optimized reactor design, smooth internal reactor surfaces, and incorporation of a localized heating scheme for the reactors, in which isothermal, real-time helicase-dependent amplification was performed.
Abstract: High-throughput microchip devices used for nucleic-acid amplification require sealed reactors. This is to prevent evaporative loss of the amplification mixture and cross-contamination, which may occur among fluidically connected reactors. In most high-throughput nucleic-acid amplification devices, reactor sealing is achieved by microvalves. Additionally, these devices require micropumps to distribute amplification mixture into an array of reactors, thereby increasing the device cost, and adding complexity to the chip fabrication and operation processes. To overcome these limitations, we report microfluidic devices harboring open (unsealed) reactors in conjunction with a single-step capillary based flow scheme for sequential distribution of amplification mixture into an array of reactors. Concern about evaporative loss in unsealed reactors have been addressed by optimized reactor design, smooth internal reactor surfaces, and incorporation of a localized heating scheme for the reactors, in which isothermal, real-time helicase-dependent amplification (HDA) was performed.

Journal ArticleDOI
TL;DR: In this paper, a numerical analysis of magnetophoretic cell separation using magnetic microspheres in a straight and a T-shaped microfluidic channel under the influence of a line dipole is presented.
Abstract: Magnetophoretic isolation of biological cells in a microfluidic environment has strong relevance in biomedicine and biotechnology. A numerical analysis of magnetophoretic cell separation using magnetic microspheres in a straight and a T-shaped microfluidic channel under the influence of a line dipole is presented. The effect of coupled particle–fluid interactions on the fluid flow and particle trajectories are investigated under different particle loading and dipole strengths. Microchannel flow and particle trajectories are simulated for different values of dipole strength and position, particle diameter and magnetic susceptibility, fluid viscosity and flow velocity in both the microchannel configurations. Residence times of the captured particles within the channel are also computed. The capture efficiency is found to be a function of two nondimensional parameters, α and β. The first parameter denotes the ratio of magnetic to viscous forces, while the second one represents the ratio of channel height to the distance of the dipole from the channel wall. Two additional nondimensional parameters γ (representing the inverse of normalized offset distance of the dipole from the line of symmetry) and σ (representing the inverse of normalized width of the outlet limbs) are found to influence the capture efficiency in the T-channel. Results of this investigation can be applied for the selection of a wide range of operating and design parameters for practical microfluidic cell separators.