Serpentine Microchannel Research Articles

Multifunctional Droplet Microfluidic Platform for Rapid Immobilization of Oligonucleotides on Semiconductor Quantum Dots.

Quantum dot-DNA oligonucleotide (QD-DNA) conjugates have been used in many fields such as nucleic acid bioassays, intracellular probes, and drug delivery systems. A typical solid-phase method that achieves rapid loading of oligonucleotides on surfaces of QDs involves a two-step reaction and is performed in a batch-based approach. In contrast, droplet microfluidics offers many advantages that are unavailable when using batch processing, providing rapid and dense immobilized DNA oligonucleotides on QDs. The presented droplet microfluidic approach allows high-quality QD-DNA conjugates to be produced using one single device, which is designed to have two droplet generators, one droplet merger, and one mixer. One of the droplet generators coencapsulates QDs and magnetic beads (MBs) into nanoliter-sized droplets for the production of QD-MB conjugates and the other encapsulates oligonucleotides in nanoliter-sized droplets. These two streams of droplets then merge at a one-to-one ratio in a chamber. The merged droplets travel along the mixer, which is a serpentine microchannel with 30 turns, resulting in QD-DNA conjugation structures of high quality. This multifunctional microfluidic device provides advantages such as higher degree of control over the reaction conditions, minimized cross-contamination and impurities, and reduction of reagent consumption while eliminating any need for external vortexing and pipetting. To evaluate the quality of the QD-DNA conjugates, they were used as Forster resonance energy transfer (FRET) probes to quantify oligonucleic targets.

On the 3D distribution and size fractionation of microparticles in a serpentine microchannel

Suitable methods to realize a multi-dimensional fractionation of microparticles smaller than {10},upmu mathrm{{m}} diameter are still rare. In the present study, size and density fractionation is investigated for 3.55,upmu mathrm{{m}} and {9.87},upmu mathrm{{m}} particles in a sharp-corner serpentine microchannel of cross-sectional aspect ratio h/w = 0.25. Experimental results are obtained through Astigmatism Particle Tracking Velocimetry (APTV) measurements, from which three-dimensional particle distributions are reconstructed for Reynolds numbers between 100 and 150. The 3D reconstruction shows for the first time that equilibrium trajectories do not only develop over the channel width, i.e. in-plane equilibrium positions but also over the channel height at different out-of-plane positions. With increasing Reynolds number, 9.87,upmu mathrm{{m}} polystyrene left( rho _{mathrm{{PS}}} = 1.05,mathrm{{g}};mathrm{{cm}}^{-3} right) and melamine left( rho _mathrm{{MF}} = 1.51,mathrm{{g}};mathrm{{cm}}^{-3} right) particles focus on two trajectories near the channel bisector. In contrast to this, it is shown that 3.55,upmu mathrm{{m}} polystyrene particles develop four equilibrium trajectories at different in-plane and out-of-plane positions up to a critical Reynolds number. Beyond this critical Reynolds number, also these particles merge to two trajectories at different channel heights. While the rearrangement of 3.55,upmu mathrm{{m}} polystyrene particles just starts beyond mathrm{{Re}}>140, 9.87,upmu mathrm{{m}} polystyrene particles undergo this rearrangement already at Re=100. As the equilibrium trajectories of these two particle groups are located at similar out-of-plane positions, outlet geometries that aim to separate particles along the channel width turn out to be a good choice for size fractionation. Indeed, polystyrene particles of different size assume laterally well-separated equilibrium trajectories such that a size fractionation of nearly 100% at Re=110 can be achieved.

Transient flow behavior in serpentine curved microchannel of inertial microfluidic devices

The serpentine microchannel or bent microchannel has been identified as one of the essential elements for inertial microfluidic devices. Understanding the transient flow of liquid is important in predicting flow time and evaluating device performance. In this paper, based on the unsteady Bernoulli equation, the inertial transient flow model was derived by analyzing the microchannel resistance factor using back propagation (BP) artificial neural network (ANN). In order to train the ANN, an improved BP algorithm was used to make the resistance model more accurate. A centrifugal test platform was employed to measure flow time in the microchannel with different structure sizes. The results show that the transient flow model has a good agreement with the experiment values, the results also show that the method using BP neural network can also be used to solve the problem of nonlinear inertial flow.

Mixing Optimization in Grooved Serpentine Microchannels.

Computational fluid dynamics modeling at Reynolds numbers ranging from 10 to 100 was used to characterize the performance of a new type of micromixer employing a serpentine channel with a grooved surface. The new topology exploits the overlap between the typical Dean flows present in curved channels due to the centrifugal forces experienced by the fluids, and the helical flows induced by slanted groove-ridge patterns with respect to the direction of the flow. The resulting flows are complex, with multiple vortices and saddle points, leading to enhanced mixing across the section of the channel. The optimization of the mixers with respect to the inner radius of curvature (Rin) of the serpentine channel identifies the designs in which the mixing index quality is both high (M > 0.95) and independent of the Reynolds number across all the values investigated.

Correlations of the capture efficiency with the Dean number and its constituents in heterogeneous microfluidic immunosensors

In heterogeneous microfluidic immunosensors, enhanced capture efficiencies of the antigens (Ag) in the carrier fluids by the surface-immobilized antibodies (Ab) facilitate lower detection limits and thus early detection of disease. Capture efficiency depends on the interplay of transport, reaction parameters and the geometry of the system. A detailed analysis on the enhanced capture efficiencies due to secondary flows in heterogeneous immunosensors has not received significant attention and is the theme of the present work. We conducted a systematic study to observe the significance of secondary forces on the capture efficiency, manifested as the average surface concentration (Cs,avg), in serpentine channels of different lengths (l) and radius of curvature (Rc) as a function of the Reynolds number (Re). Experimental observations were validated with numerical simulations. Micro-PIV studies at different planes and sections of the serpentine microchannels were conducted and matched with the simulated velocity profiles. Further investigation of the process and the geometrical parameters was conducted using numerical simulation and the behaviour of Cs,avg as a function of Re and Rc was plotted for different cases. A highlight of the present work are correlations of Cs,avg as a function of the Dean number (De), as well as its constituents (Re and α). The scientific studies of the geometrical and process parameters which affect the analyte capture advance the understanding of the phenomena and the proposed engineering correlations would be useful in the design of more efficient flow-based heterogeneous immunosensors.

Mixing enhancement via a serpentine micromixer for real-time activation of carboxyl

Abstract The amide condensation reaction is crucial for preparing biological probes with a polypeptide backbone. The traditional condensation reaction requires the monomer and condensing agent to be pre-mixed and activated by physical agitation mixing, which entail long time, require many reagents, and yield several by-products. In microfluidic mixing devices, the high specific surface area of microchannel enables traditional chemical reactions to be accomplished efficiently and quickly using only trace amounts of reagents in real time. This paper presents a serpentine mixing channel for the activation of amino acid monomers. A number of outward convex elliptical structures are introduced to both sides of the improved serpentine microchannel, creating a Dean vortex that increases the mixing efficiency at low Reynolds condition (90% at Re = 40). Effective monomer activation can be realized directly using a micromixing reactor and leads to less by-products as verified by ultraviolet spectroscopy and high-performance liquid chromatography analysis. The practicability of the micromixer is successfully tested by utilizing it as a microreactor for the real-time activation of amino acids for the in situ synthesis of polypeptide array at a flow rate of 300 μL/min (Re = 40). The proposed micromixer has the potential to be an excellent alternative for conventional flask reactions and integrated in automated miniaturized biochip preparation systems.

Catalytic CO2 absorption in an amine solvent using nickel nanoparticles for post-combustion carbon capture

In industrial post-carbon capture processes, monoethanolamine (MEA) has been mainly used as an absorption solvent. However, this approach generates significant amounts of toxic wastewater containing a heavy chemical difficult to treat and also raises concerns about acute corrosion of metal structures in the facility. To reduce the use of MEA in carbon capture, this work evaluates the catalytic performance of nickel nanoparticles (NiNPs) for CO2 capture as a possible additive in an MEA solvent. We test the CO2 absorption rate in MEA catalyzed by NiNPs in both limited and high mixing conditions to model real capturing processes in the packed column of industrial absorption reactors. For this purpose, a microreactor and a long serpentine microchannel are employed. The catalytic absorption performance of NiNPs for CO2 in aqueous MEA is evaluated using CO2 microbubbles by monitoring changes in size upon their time-dependent absorption. We find that the average CO2 absorption rate with NiNPs is accelerated by 34% in the limited mixing condition in the microreactor. This increase is mainly due to NPs’ catalytic CO2 absorption driven by a Brownian motion. On the other hand, in the high mixing condition in the long serpentine microchannel, the catalytic activity of NiNPs improves the average CO2 absorption rate further to 54%. This improvement makes it possible to shorten the timescale for reaching CO2 absorption equilibrium and therefore to reduce the size of the reactors significantly. The test results demonstrate that NiNPs serve as suitable additives in the MEA-based CO2 absorption system.

On thermal performance of serpentine silicon microchannels

In the present study, three different serpentine microchannels (Rectangular, U and V) of rectangular cross-section with a channel span 24 mm were fabricated on a 0.525 mm thick (100) silicon substrate using micro-ultrasonic machining (micro-USM) technique. Thermal performance of the microchannels was experimentally investigated by making the working fluid (water) to flow through them at different Reynolds numbers (100–400) and at different heat fluxes (10 kW/m2, 20 kW/m2 and 30 kW/m2). Similar microchannels were modeled and analysed numerically simulating the experimental conditions. The performances of the microchannels obtained with the experimental as well as numerical results were compared in terms of pressure drop, substrate and fluid temperatures and heat transfer coefficient. The results indicate that the overall thermal performance of the microchannel heat sink mostly depends on the type of pattern as the fluid-substrate interaction area, defined by the Sink Area Factor (SAF), changes appreciably with the pattern. It was found that the U-serpentine microchannel exhibited the best thermal performance while compared to the other two serpentine microchannels. The sharp bends of the microchannel patterns and the surface roughness of the microchannel walls adversely affect the overall thermal performance of the microchannels. The study also outlines the need for a compromise in the Reynolds number, as higher Reynolds number tends to lower the overall thermal performance of the microchannels.

Mixing Performance and Application of a Three-Dimensional Serpentine Microchannel Reactor with a Periodic Vortex-Inducing Structure

A new type of three-dimensional (3D) serpentine microchannel reactor was fabricated by thermally bonding stacked polyimide films to greatly improve the mixing efficiency by transversal vortices at ...

Numerical simulation of hydrodynamic and heat transfer characteristics of slug flow in serpentine microchannel with various curvature ratio

A numerical investigation is carried out to study hydrodynamic behavior and thermal performance of slug flow in serpentine microchannel with different curvature ratio and wall properties. By using coupled Level Set and Volume of Fluid (CLSVOF) method implemented in Fluent, gas-liquid interface shows good consistency with experimental results especially for slug flow. Distribution of sectional velocity profile in curved microchannel with various curvature ratios was investigated. On the basis of fluid flow in microchannel with non-slip boundary condition, by means of user defined function (UDF), influence of slip velocity on hydrodynamic characteristics was performed numerically. For a better prediction on thermal performance, further analysis of combined effect of wall properties and curvature ratio on fluid flow characteristics was also conducted. As a consequence, the increasing slip velocity on channel wall can reduce hydraulic resistance. Both the contact angle and the curvature ratio have an appreciable influence on thermal performance of gas-liquid two-phase flow in serpentine microchannel.

Mixing efficiency and pressure drop analysis of liquid-liquid two phases flow in serpentine microchannels

When fluids flow in microchannels, due to the relative low hydraulic diameter and low flow velocity, the flow is usually laminar flow, which hinders the effective mixing between two liquid-liquid phases. In this paper, the mixing efficiency at both droplet forming stage and droplet moving stage are investigated with the volume of fluid (VOF) method. The user defined scalar (UDS) is defined in the dispersed phase to analyze the mixing efficiency quantitatively. The droplet moves in the microchannel with the constant velocity, while serpentine microchannels are designed with different bend radius ranging. \( \overline{R} \) and ed are defined as the ratio of the bend radius to the width of the microchannel, and the ratio of the dispersed phase velocity to the droplet moving velocity, respectively. They are used for illustrating the effect of the bend radius and the dispersed phase fraction on the mixing efficiency in the droplet. Results indicate that the smaller dispersed phase fraction and smaller bend radius show the better mixing efficiency. However, the droplet production frequency changes in a parabolic trend with the variation of the dispersed phase fraction. The suitable dispersed phase fraction should be less than 0.5 in order to achieve a higher mixing efficiency and higher droplet formation frequency. In addition, the pressure drop in microchannel is increased with the decrease of the bend radius. The pressure drop observed in the micorchannel with square turn (\( \overline{R} \) = 0) and the microchannel with a bend radius of \( \overline{R} \) = 1 is nearly the same. This paper can be referred by someone dealing with the micromixer design and mass transfer in micro scale.

Performance and implementation of centrifugal serpentine micromixers with non-rectangular cross-section

The fluid flow and mixing characteristics of planar serpentine microchannels consisting of repeating S-shaped units was characterized both computationally and experimentally for Reynolds numbers Re < 100. In particular, it was found that using non-rectangular cross-sections with changing orientation for the geometrical units greatly enhances their mixing performance relative to standard rectangular profile serpentine channels. The optimization of the cross-section enables efficient mixing of two-fluid components at Reynolds numbers as low as 20, making these designs practical for chemical reaction engineering and biological assays platforms. A computational analysis of the cross-sectional flow structures present in this type of channels indicates that their enhanced performance is correlated with the formation of secondary transversal vortexes. In the optimal designs, their presence increases the mass transfer in the microchannel by both stretching the interface between the fluid components to be mixed, as well as by promoting chaotic advection.

Numerical simulation of geometry effect on mixing performance in L-shaped micromixers

This study accomplishes a numerical analysis of mixing in a microchannel with repeating L-shaped units in order to research the effect of the extension of L-shaped units in a three-dimensional (3D) space and the angle of repeating units on the process of mixing. In the first part, Geometry 2 and Geometry 3 are designed by extending the units of Geometry 1 in a 3D space. In the second part, an L-shaped micromixer, a 90° V-shaped micromixer, and a 60° V-shaped micromixer are analyzed. It is observed that Geometry 1 and Geometry 2 perform better than Geometry 3 in terms of mixing due to the spiral path with 360° rotation of the flow. The L-shaped micromixer is more efficient than the 90° and 60° V-shaped micromixers. A maximum mixing index of about 88% is achieved in all serpentine microchannels at the following Reynolds numbers: Re = 150 and Re = 200.

Design and fabrication of a two-phase diamond nanoparticle aided fast PCR device

Polymerase Chain Reaction (PCR) is an important and prevalent technique in biotechnology because of its crucial role in cloning DNA fragments and diagnostic applications. In the present study, a high-throughput two-phase PCR device is designed and fabricated which utilizes a serpentine microchannel together with a spiral structure. The former is for the droplet-generation and mixing and the latter is for the thermal cycling process. Moreover, the effect of diamond nanoparticles (diamondNP) on the performance of PCR is also investigated while using commercial PCR devices and the fabricated PCR device designed in this study. Using numerical simulation, it is shown that within the simple and easy-to-make microchannel structures designed in this study, there are different transient secondary flows which can help mixing the contents of the droplets including PCR solution and diamondNP. Moreover, a new fabrication process is used for the design of the droplet-generation chip which is applicable for the fabrication of all high-pressure microfluidic devices. It is found that the effect of diamondNPs on the PCR performance depends on the nanoparticle concentration; diamaondNPs can improve the PCR performance more than 5 times. On the other hand, if a high concentration of diamondNPs is used, PCR performance decreases significantly. Moreover, it is shown that the fabricated device is able to do the same PCR procedure faster than the commercial PCR devices.

Dean-flow-coupled elasto-inertial particle and cell focusing in symmetric serpentine microchannels

This work investigates particle focusing under Dean-flow-coupled elasto-inertial effects in symmetric serpentine microchannels. A small amount of polymers were added to the sample solution to tune the fluid elasticity, and allow particles to migrate laterally and reach their equilibriums at the centerline of a symmetric serpentine channel under the synthesis effect of elastic, inertial and Dean-flow forces. First, the effects of the flow rates on particle focusing in viscoelastic fluid in serpentine channels were investigated. Then, comparisons with particle focusing in the Newtonian fluid in the serpentine channel and in the viscoelastic fluid in the straight channel were conducted. The elastic effect and the serpentine channel structure could accelerate the particle focusing as well as reduce the channel length. This focusing technique has the potential as a pre-ordering unit in flow cytometry for cell counting, sorting, and analysis. Moreover, focusing behaviour of Jurkat cells in the viscoelastic fluid in this serpentine channel was studied. Finally, the cell viability in the culture medium containing a dissolved polymer and after processing through the serpentine channel was tested. The polymer within this viscoelastic fluid has a negligible effect on cell viability.

Effects of structural parameters on fluid flow and heat transfer in a serpentine microchannel with fan-shaped reentrant cavities

A three-dimensional numerical model has been conducted to study the laminar flow and heat transfer characteristics in serpentine microchannel with different geometric fan-shaped reentrant cavities for Reynolds number ranging from 150 to 980. The microchannel has the width of 0.1 mm and depth of 0.2 mm in the rectangular cross-section. The geometric parameters of fan-shaped reentrant cavities examined in this paper are 0.04058 mm for the width (Wc), 0.0084–0.02029 mm for the height (Hc) and 0.04058–0.12174 mm for the spacing (Sc).This study firstly presents the comparison of the thermal and hydraulic characteristics of serpentine microchannel with fan-shaped reentrant cavities with the conventional microchannels heat sink. Then the effects of fan-shaped cavities on velocity contour, temperature distribution and secondary flow features in such microchannel heat sinks are given. It is apparent that the fan-shaped cavities can effectively reduce the flow resistance and improve the temperature distribution uniformity. The effects of developed non-dimensional variables on the frictional pressure drop, average Nusselt number and the relationship between thermal resistance and pumping power in the novel channel are investigated. Compared with rectangular serpentine channel, heat transfer is enhanced for all cases and flow resistance is decreased besides Re = 980 and Hc/Wc = 0.2887. The best performance of serpentine microchannel with fan-shaped reentrant cavities is found at Sc/Wc = 3 and Hc/Wc = 0.2887.

Secondary flows of viscoelastic fluids in serpentine microchannels

Secondary flows are ubiquitous in channel flows, where small velocity components perpendicular to the main velocity appear due to the complexity of the channel geometry and/or that of the flow itself such as from inertial or non-Newtonian effects, etc. We investigate here the inertialess secondary flow of viscoelastic fluids in curved microchannels of rectangular cross-section and constant but alternating curvature: the so-called "serpentine channel" geometry. Numerical calculations (Poole et al, 2013) have shown that in this geometry, in the absence of elastic instabilities, a steady secondary flow develops that takes the shape of two counter-rotating vortices in the plane of the channel cross-section. We present the first experimental visualization evidence and characterization of these steady secondary flows, using a complementarity of microPIV in the plane of the channel, and confocal visualisation of dye-stream transport in the cross-sectional plane. We show that the measured streamlines and the relative velocity magnitude of the secondary flows are in qualitative agreement with the numerical results. In addition to our techniques being broadly applicable to the characterisation of three-dimensional flow structures in microchannels, our results are important for understanding the onset of instability in serpentine viscoelastic flows.

A Microfluidic Spheroid Culture Device with a Concentration Gradient Generator for High-Throughput Screening of Drug Efficacy.

Three-dimensional (3D) cell culture is considered more clinically relevant in mimicking the structural and physiological conditions of tumors in vivo compared to two-dimensional cell cultures. In recent years, high-throughput screening (HTS) in 3D cell arrays has been extensively used for drug discovery because of its usability and applicability. Herein, we developed a microfluidic spheroid culture device (μFSCD) with a concentration gradient generator (CGG) that enabled cells to form spheroids and grow in the presence of cancer drug gradients. The device is composed of concave microwells with several serpentine micro-channels which generate a concentration gradient. Once the colon cancer cells (HCT116) formed a single spheroid (approximately 120 μm in diameter) in each microwell, spheroids were perfused in the presence of the cancer drug gradient irinotecan for three days. The number of spheroids, roundness, and cell viability, were inversely proportional to the drug concentration. These results suggest that the μFSCD with a CGG has the potential to become an HTS platform for screening the efficacy of cancer drugs.

Fast Start-Up Microfluidic Microbial Fuel Cells With Serpentine Microchannel.

Microfluidic microbial fuel cells (MMFCs) are promising green power sources for future ultra-small electronic devices. The MMFCs with co-laminar microfluidic structure are superior to other MMFCs according to their low internal resistance and relative high power density. However, the area for interfacial electron transfer between the bacteria and the anode is quite limited in the typical Y-shaped device, which apparently restricts the current generation performance. In this study, we developed a membraneless MMFC with serpentine microchannel to enhance the interfacial electron transfer and promote the power generation of the device. Owing to the merit of laminar flow, the proposed MMFC was working well without any proton exchange membrane (PEM). At the same time, the serpentine microchannel greatly increased the power density. The S-MMFC catalyzed by Shewanella putrefaciens CN32 achieves a peak power density of 360 mW/m2 with the optimal channel configuration and the flow rate of 5 ml/h. Meanwhile, this device possesses much shorter start-up time and much longer duration time at high current plateau than the previous reported MMFCs. The presented MMFC appears promising for biochip technology and extends the scope of microfluidic energy.

Liquid-liquid two-phase flow patterns in ultra-shallow straight and serpentine microchannels

Water-butanol and water-hexane flows were visualized in ultra-shallow straight and serpentine microchannels with a cross-junction. At the inlet cross-junction, three major flow patterns including tubing/threading, dripping and jetting were mapped using the aqueous Capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular flow, slug flow and droplet flow were mapped using combined dimensionless numbers (Weber number times Ohnesorge number) of both phases. The flow pattern transitions were explained based on a force analysis, considering the phase flow rates, junction angle between the side feeding channels and the central feeding channel as well as aspect ratios. Compared to the straight microchannel, the dripping regime at the inlet junction and the slug flow occupy larger zones in serpentine microchannels because the centrifugal force tends to break up the organic annular core into slugs and droplets over the bends.