Serpentine Microchannel Research Articles

Effects of channel diameter on flow pattern and pressure drop for air–water flow in serpentine gas channels of PEM fuel cell-An Ex situ experiment

Ex-situ experimental investigation of air–water two phase flow characteristics are carried out in rectangular serpentine mini and micro-channels. The test sections to be studied are made up of acrylic and are of three different hydraulic diameters of 1.65 mm, 1.00 mm and 0.65 mm with the corresponding length of 50 mm. Experiments are performed for different combinations of superficial velocities of air and water ranging between 0.0011 m/s and 0.33 m/s and 0.33 m/s to 16.66 m/s, respectively. The effect of channel orientation on various parameters is also studied. The visualisation of two phase is accomplished by high-speed camera. The flow pattern maps developed by closely scrutinizing each flow rate combinations are presented. Mal-distribution occurred in horizontal orientation of micro channel. However, vertical orientation of micro channel favoured draining of water, thereby encountered lesser pressure drop than mini channel. New correlation based on various dimensionless numbers has been proposed for modelling the pressure drop.

Lipoplex‐Mediated Single‐Cell Transfection via Droplet Microfluidics

While lipoplex (cationic lipid-nucleic acid complex)-mediated intracellular delivery is widely adopted in mammalian cell transfection, its transfection efficiency for suspension cells, e.g., lymphatic and hematopoietic cells, is reported at only ≈5% or even lower. Here, efficient and consistent lipoplex-mediated transfection is demonstrated for hard-to-transfect suspension cells via a single-cell, droplet-microfluidics approach. In these microdroplets, monodisperse lipoplexes for effective gene delivery are generated via chaotic mixing induced by the serpentine microchannel and co-confined with single cells. Moreover, the cell membrane permeability increases due to the shear stress exerted on the single cells when they pass through the droplet pinch-off junction. The transfection efficiency, examined by the delivery of the pcDNA3-EGFP plasmid, improves from ≈5% to ≈50% for all three tested suspension cell lines, i.e., K562, THP-1, Jurkat, and with significantly reduced cell-to-cell variation, compared to the bulk method. Efficient targeted knockout of the TP53BP1 gene for K562 cells via the CRISPR (clustered regularly interspaced short palindromic repeats)-CAS9 (CRISPR-associated nuclease 9) mechanism is also achieved using this platform. Lipoplex-mediated single-cell transfection via droplet microfluidics is expected to have broad applications in gene therapy and regenerative medicine by providing high transfection efficiency and low cell-to-cell variation for hard-to-transfect suspension cells.

Continuous flow microreactor for protein PEGylation.

PEGylation is increasingly being utilized to enhance the therapeutic efficacy of biopharmaceuticals. Various chemistries and reaction conditions have been established to synthesize PEGylated proteins and more are being developed. Both the extent of conversion and selectivity of protein PEGylation are highly sensitive to process variables and parameters. Therefore, microfluidic-based high-throughput screening platforms would be highly suitable for optimization of protein PEGylation. As part of this study, a poly-dimethylsiloxane-based continuous flow microreactor system was designed and its performance was compared head-to-head with a batch reactor. The reactants within the microreactor were contacted by passive micromixing based on chaotic advection generated by staggered herringbone grooves embedded in serpentine microchannels. The microreactor system was provided with means for on-chip reaction quenching. Lysozyme was used as the model protein while methoxy-polyethylene glycol-(CH2)5COO-NHS was used as the PEGylation reagent. Full mixing was achieved close to the microreactor inlet, making the device suitable for protein PEGylation. The effect of mixing type, i.e., simple stirring versus chaotic laminar mixing on PEGylation, was investigated. Higher selectivity (as high as 100% selectivity) was obtained with the microreactor while the conversion was marginally lower.

Numerical and Experimental Study on Mixing Performances of Simple and Vortex Micro T-Mixers.

Vortex flow increases the interface area of fluid streams by stretching along with providing continuous stirring action to the fluids in micromixers. In this study, experimental and numerical analyses on a design of micromixer that creates vortex flow were carried out, and the mixing performance was compared with a simple micro T-mixer. In the vortex micro T-mixer, the height of the inlet channels is half of the height of the main mixing channel. The inlet channel connects to the main mixing channel (micromixer) at the one end at an offset position in a fashion that creates vortex flow. In the simple micro T-mixer, the height of the inlet channels is equal to the height of the channel after connection (main mixing channel). Mixing of fluids and flow field have been analyzed for Reynolds numbers in a range from 1–80. The study has been further extended to planar serpentine microchannels, which were combined with a simple and a vortex T-junction, to evaluate and verify their mixing performances. The mixing performance of the vortex T-mixer is higher than the simple T-mixer and significantly increases with the Reynolds number. The design is promising for efficiently increasing mixing simply at the T-junction and can be applied to all micromixers.

Dynamic changes in gas-liquid mass transfer during Taylor flow in long serpentine square microchannels

The present work focuses on the hydrodynamics variation and mass transfer characteristics of Taylor flow along long serpentine microchannels with a square cross-section. The volumetric mass transfer coefficient (kLa) is regarded as the transient change value to characterize the gas-liquid mass transfer process of CO2 in water. All experimental data of Taylor bubble are obtained from 1000 continuously captured images. An online high-speed imaging method and the unit cell model are adopted in this study. The effects of gas and liquid flow rates, together with microchannel geometry are investigated on Taylor bubble characteristics in terms of length, velocity and the mass transfer performance.Taylor bubble length shrinks and subsequently plateaus out along the flow direction from the T-junction, resulting in the decrease in Taylor bubble velocity. kLa in a unit cell gradually decreases along the serpentine microchannel, and increases as the channel cross-sectional area decreases. As the gas flow rate increases under a given liquid flow rate, a critical point is found for the evolution of kLa and kL (that is the liquid phase mass transfer coefficient). The results indicate that the contribution of the circulation in the liquid slug to kL is dominant before the critical point compared to the leakage flow in the liquid film. All these findings in this work give important information to understand the dynamic change in gas-liquid Taylor flow mass transfer within microchannels. They will serve as basis for designing and optimizing gas-liquid multiphase microreactors in the future.

Numerical and experimental investigations on the performance of a serpentine microchannel with semicircular obstacles

This paper focuses on computational and experimental analysis of a simple serpentine microchannel without obstacles and serpentine microchannel with semicircular obstacles of different sizes. The work has been performed in three phases- computational analysis, experimentation and flow physics study. The 3D models of a simple serpentine microchannel (without obstacles) and serpentine microchannels with semicircular obstacles (with radius as 50, 100, 150 and 200 µm) have been developed using COMSOL Multiphysics software. The effect of obstacle size on pressure drop and mixing length for achieving index 1 has been analyzed. A simple serpentine qmicrochannel and a serpentine microchannel with 150 µm radius semicircular obstacles have been fabricated with polydimethylsiloxane using soft lithography process. The experimental analysis for pressure drop as well as mixing index has been performed. A good agreement has been observed between experimental and computational results. The validated computational model is then used to study the mixing index for the same microchannels for different flow conditions, i.e. for different Reynolds numbers. The mixing lengths are observed to be lesser for Re 0.28 and 30. Further, the effect of diffusion and generation of secondary flow due to advection on mixing length for the considered Reynolds numbers are analyzed through the flow physics study.

Investigation of rarefied gas flow through bended microchannels

Due to complexities of microfluidic devices, not all of the micro-ducts are straight. As a result, there is a need for utilizing geometric turns and serpentine microchannels. The main purpose of this study is to investigate the effect of applying sharp bends and curved corners to the microchannel geometry on the thermal and hydrodynamic behaviors of rarefied gas flow. In this regard Direct Simulation Monte Carlo method has been employed with pressure driven implicit boundary condition. Comparisons of mass flow rate, slip velocity, shear stress and heat transfer have been carried out between these cases. The obtained results depicted that the mass flow rate is increased by applying curvatures of various radii and the maximum enhancement in mass flow rate is for a radius-to-width ratio equal to 0.6.

Three-Dimensional Computational Fluid Dynamics modelling of a Proton Exchange Membrane Fuel Cell with a serpentine micro-channel design

Three-Dimensional Computational Fluid Dynamics modelling of a Proton Exchange Membrane Fuel Cell with a serpentine micro-channel design

Analysis of Passive Mixing in a Serpentine Microchannel with Sinusoidal Side Walls.

Sample mixing is difficult in microfluidic devices because of laminar flow. Micromixers are designed to ensure the optimal use of miniaturized devices. The present study aims to design a chaotic-advection-based passive micromixer with enhanced mixing efficiency. A serpentine-shaped microchannel with sinusoidal side walls was designed, and three cases, with amplitude to wavelength (A/λ) ratios of 0.1, 0.15, and 0.2 were investigated. Numerical simulations were conducted using the Navier–Stokes equations, to determine the flow field. The flow was then coupled with the convection–diffusion equation to obtain the species concentration distribution. The mixing performance of sinusoidal walled channels was compared with that of a simple serpentine channel for Reynolds numbers ranging from 0.1 to 50. Secondary flows were observed at high Reynolds numbers that mixed the fluid streams. These flows were dominant in the proposed sinusoidal walled channels, thereby showing better mixing performance than the simple serpentine channel at similar or less mixing cost. Higher mixing efficiency was obtained by increasing the A/λ ratio.

Fabrication of serpentine micro-channels on glass by ultrasonic machining using developed micro-tool by wire-cut electric discharge machining

A rapid method for fabrication of micro-channel on glass substrates is introduced and investigated in this paper. Wire-cut electro discharge machining (WEDM) and ultrasonic machining (USM) were used as manufacturing processes for machining the micro-tools and micro-channels, respectively. The copper micro-tool was machined using WEDM and the optimal design of the tool was achieved using Taguchi design of experiment. After overall evaluation criteria (OEC) analysis, the optimum conditions were achieved as Ton as 25 μs, Toff as 6 μs, Ip as 1 A, and Wf as 75 mm/s. Further, it was used for fabrication of micro-tools with different profiles such as square, circular, and zigzag. The fabricated micro-tools were used to machine serpentine micro-channels on glass using USM at various power ratings such as 40, 50, and 60%. After machining, net overcut of the micro-channels were found as 74.65, 75.86, and 81.75 μm for square, circular, and zigzag profiles, respectively.

Newtonian and generalized Newtonian reacting flows in serpentine microchannels: Pressure driven and centrifugal microfluidics

This paper presents a comprehensive 3D numerical simulation of reacting flows in micro scale dimension through centrifugal, or Lab-On-a-CD (LOCD), and pressure-driven, or Lab-On-a-Chip (LOC) devices. Three different serpentine channel configurations (rectangular, triangular and sinusoidal) are investigated. In these configurations, two chemical species enter from two inlets and according to an irreversible chemical reaction, start yielding other species. Both Newtonian and generalized Newtonian fluids are considered in the simulations and the results are compared for both LOC and LOCD devices. Besides, the effects of different parameters such as the aspect ratio of channels’ cross section, inlet flow rate, angle of fabrication in LOCD devices and the radial distance are also investigated. It is found that the products are generated in the vicinity of the LOCD microchannel corners, but in LOC devices, reactants yield their product on the top and bottom walls of the microchannel. The best fabrication angle depends on the configuration of the microchannels and the rectangular configuration has the maximum production factor for AR = 1. Besides, secondary vortices for Newtonian and generalized Newtonian fluids in the LOC and LOCD devices are compared in detail.

Thermal cycling characteristics of a 3D-printed serpentine microchannel for DNA amplification by polymerase chain reaction

A polymerase chain reaction (PCR) device with integrated heaters for DNA amplification is proposed by using 3D-printing technology, which has the advantages of fast prototyping, design flexibility, and low cost. The thermal characteristics of the 3D-printed device for PCR are reported for the first time. The overall dimensions of the PCR device are 30mm×40mm where a serpentine microchannel is created to implement 27 thermal cycles. The serpentine microchannel of 260μm in depth, 450μm in width and 1470mm in length has been designed to inspect shape conformity and temperature variations. Thermal cycling experiments has showed that three temperature zones for denaturation (90–95°C), annealing (55–65°C) and extension (70–77°C) were successfully produced for DNA amplification. The thermal cycling efficiency ranges 67.4% to 47.8% when the flow rate is changed from 5μL/min to 10μL/min. The study demonstrates the feasibility of a low-cost 3D-printed PCR device that enables DNA amplification by thermal cycling. This paper concludes that 3D-printing technology can be applied for bio-microfluidic devices that require precise temperature control.

High-Throughput Separation of White Blood Cells From Whole Blood Using Inertial Microfluidics.

White blood cells (WBCs) constitute only about 0.1% of human blood cells, yet contain rich information about the immune status of the body; thus, separation of WBCs from the whole blood is an indispensable and critical sample preparation step in many scientific, clinical, and diagnostic applications. In this paper, we developed a continuous and high-throughput microfluidic WBC separation platform utilizing the differential inertial focusing of particles in serpentine microchannels. First, separation performance of the proposed method is characterized and evaluated using polystyrene beads in the serpentine channel. The purity of 10-μm polystyrene beads is increased from 0.1% to 80.3% after two cascaded processes, with an average enrichment ratio of 28 times. Next, we investigated focusing and separation properties of Jurkat cells spiked in the blood to mimic the presence of WBCs in whole blood. Finally, separation of WBCs from human whole blood was conducted and separation purity of WBCs was measured by the flow cytometry. The results show that the purity of WBCs can be increased to 48% after two consecutive processes, with an average enrichment ratio of ten times. Meanwhile, a parallelized inertial microfluidic device was designed to provide a high processing flow rate of 288 ml/h for the diluted (×1/20) whole blood. The proposed microfluidic device can potentially work as an upstream component for blood sample preparation and analysis in the integrated microfluidic systems.

Droplet-based flows in serpentine microchannels: Chemical reactions and secondary flows

Mixing is an essential operation in many microfluidic devices. Droplet-based micromixers utilize droplets for mixing enhancement. In the present study, a novel three-dimensional simulation is conducted which has the ability to capture not only the mixing process, but also the chemical reactions inside liquid droplets. This two-phase model is used for simulating the reacting flow inside a serpentine microchannel and explores the effects of droplet size and reaction rate on the production and consumption of species in droplets. It is observed that the chemical reaction in each droplet, begins from its front area. Furthermore, it is shown that the production of species does not depend on water fraction (ratio of water flow rate to total flow rate) and for all droplet sizes, only depends on the reaction rate. Moreover, different transient generated vortices and secondary flows are studied in the presence and absence of droplets and explained in details based on the position of droplets. It is found that during the passage of droplets through a microchannel bend, the number of cross-sectional vortices changes and the velocity magnitude in these secondary flows increases dramatically.

Characterisation of elastic turbulence in a serpentine micro-channel

A systematic experimental investigation of the onset, development, and statistical and scaling properties of elastic turbulence in a curvilinear micro-channel of a dilute solution of a high molecular weight polymer is presented. By measurements of time series of high spatial resolution flow fields performed over a time 320 times longer than the average relaxation, we show that the transition to elastic turbulence occurs via an imperfect bifurcation. Slightly above the onset of the primary elastic instability, rare events manifested through a local deceleration of the flow are observed. By measurements of the spatial distributions and statistics of the second invariant of the rate of strain tensor, we show that the main prediction of the theory regarding the saturation of root mean square of fluctuations of the velocity gradients is qualitatively verified though a quantitative agreement could not be found. A systematic analysis of the statistics of the fluctuations of flow fields in terms of spatial and temporal correlations, power spectra, and probability distributions is presented. The scaling properties of structure functions of the increments of the velocity gradients are discussed. Our experimental findings call for further developments of the theory of elastic turbulence in bounded flow channels.

Numerical and experimental study of passive fluids mixing in micro-channels of different configurations

This paper represents a numerical and experimental investigation of fluids mixing in straight and serpentine micro-channels. These microchannels are fabricated with the help of soft lithography technique. The width of straight and serpentine micro-channels after fabrication is obtained around 335 and 282 µm respectively. Numerical simulations of fluids mixing in micro-channels are performed by solving momentum, continuity and mass diffusion equations. Mixing performance has been evaluated in three average inlet velocities such as 0.001, 0.007 and 0.0167 m/s. The numerical results show that in serpentine micro-channel, an effective mixing has been achieved in almost all the flow conditions whereas in straight channel that is achieved for 0.001 m/s velocity only. Water and water-KMnO4 solution are used as working fluids in this study. The experimental results show that the mixing efficiency of around 95.5, 70.20, and 48.54% has been achieved for straight channel at inlet velocities 0.001, 0.007 and 0.0167 m/s respectively. However, in serpentine micro-channel, the mixing efficiency has been achieved around 97.12, 95.10, and 65.21% at inlet velocities 0.007, 0.0167 and 0.03 m/s respectively. Moreover, the experimental results are compared with the numerical results where it has been observed that the results are quite identical to each other.

COMSOL-Based Design and Validation of Dilution Algorithm with Continuous-Flow Lab-on-Chip

Sample preparation is an essential component in any biochemical analysis, which involves, among others, the task of producing dilutions of raw sample, and fluid-mixtures of different reagents with prescribed proportions. In this work, a dilution method using continuous-flow microfluidic (CFM) lab-on-chip (LoC) is proposed, and a universal fluidic network for its implementation is designed. The mixing of fluids is effected by the convective/diffusive transport through specially designed Y-shaped serpentine microchannels. Our method does not need any valve-control, and instead of emitting waste droplets at each mix-split step as practiced earlier in the case of digital microfluidics, we adjust fluid-injection velocities or the dimension of inlet tubes as needed. The target concentration factor of the fluid computed via COMSOL simulation of the network matches favorably with the expected value. Furthermore, this investigation provides an effective procedure towards the design of CFM biochips needed for automated sample preparation with any given input ratio.

Rapid fabrication of rounded microchannels via extrusion printing of molds using a thixotropic ink

This paper reports a direct-write strategy that rapidly produces rounded cross-sectional molds for casting of microfluidic channels in polydimethylsiloxane (PDMS). Robotically controlled microextrusion of a thixotropic ink through a micronozzle onto a substrate surface generates user-defined positive relief structures that serve as molds. Printed lateral resolutions of less than 10μm can be achieved by using a micronozzle with dimensions of a few micrometers and microscopic visual feedback. The cross-sectional geometries of the microchannels can be easily adjusted by regulating parameters such as the printing speed, applied pressure, micronozzle-substrate distance, and number of stacks. As a result of the “liquid rope coiling” effect, this technique provides a straightforward way of producing a serpentine microchannel. To demonstrate the feasibility of extrusion printing, several complex microfluidic structures were cast in PDMS from their printed molds. This strategy provides a simple but flexible approach to on-demand prototyping of microfluidic molds with controllable, rounded cross-sections for microfluidic structures.

A microfluidic chip for rapid single nucleotide polymorphism (SNP) genotyping using primer extension on microbeads

A microfluidic device, which performs a primer extension technique onto microbeads, is demonstrated to genotype Single Nucleotide Polymorphism (SNP) on genomic DNA. The device has on-chip heaters and sensors to create a uniform and yet stable temperature gradient between 56°C and 94°C across an array of serpentine microchannels, where the microbeads carry allele-specific primers passing through for thermal cycling. As the primers on the beads is designed to be only extended when perfectly matched with the SNP site of the template, the mismatched samples will not be amplified. The SNP discrimination is achieved. In addition that the microfluidic environment promote enhanced mass transport and better hybridization kinetics, the microbeads offer an advantage for faster thermal response and higher signal-to-noise ratio. Three most-frequent variants in one SNP location from genomic DNA of Taiwan country chicken are successfully distinguished in only five thermal cycles, compared to thirty (30) thermal cycles in traditional approaches. Moreover, the SNP genotyping procedures, which typically involves multiple steps in amplification and detection, were integrated onto a single chip.

An experimental and numerical investigation of the use of liquid flow in serpentine microchannels for microelectronics cooling

This paper presents a combined experimental and numerical investigation of single-phase water flow and heat transfer in serpentine rectangular microchannels embedded in a heated copper block. The performance of four different microchannel heat sink (MCHS) configurations are investigated experimentally, the first having an array of straight rectangular microchannels (SRMs), while the other have single (SPSMs), double (DPSMs) and triple path multi-serpentine rectangular microchannels (TPSMs). Three-dimensional conjugate heat transfer models are developed for both laminar and turbulent single-phase water flows in each of these MCHSs and the governing flow and energy equations solved numerically using finite elements. The numerical predictions of pressure drop (ΔP) and average Nusselt number (Nuavg) are in good agreement with experimental data, and indicated that the single path serpentine microchannel (SPSM) leads to a 35% enhancement of the Nuavg at a volumetric flow rate of 0.5l/min and a 19% reduction in total thermal resistance (Rth) compared to the conventional SRM heat sink. However, this enhancement is at the expense of a large (up to ten-fold) increase in ΔP compared to the SRM heat sink, so that a suitable compromise must be struck between heat transfer and pressure drop in practical MCHS designs.