Design and mixing efficiency of rhombic micromixer with flat angles

In this paper, we used CFD-ACE software to investigate fluid mixing of the rhombic micromixer with flat angles. According to the results of the numerical simulations, this rhombic microchannel with flat angles showed high mixing efficiency compared to cross-shape straight microchannel. Over 95% mixing efficiency had been achieved at Re > 180 due to enhancement of vortices. In processing, CO₂ laser machining is used to fabricate the master mold easily instead of conventional photolithography process. From results of the mixing experiment, high Reynolds number resulted in better fluid mixing because of stronger Dean vortices and recirculation effect.     

Design optimization of micromixer with square-wave microchannel on compact disk microfluidic platform

This paper presents a passive micromixer on a compact disk (CD) microfluidic platform that performs plasma mixing function. The driving force of CD microfluidic platform including, the centrifugal force due to the system rotation, the Coriolis force as a function of the rotation angular frequency and velocity of liquid. Numerical simulations are performed to investigate the flow characteristics and mixing performance of three CD microfluidic mixers with square-wave, curved and zig-zag microchannels, respectively. Of the three microchannels, the square-wave microchannel is found to yield the best mixing performance, and is therefore selected for design optimization. Four CD microfluidic micromixers incorporating square-wave PDMS microchannels with different widths in the x- and y-directions are fabricated using conventional photolithography techniques. The mixing performance of the four microchannels is investigated both numerically and experimentally. The results show that given an appropriate specification of the microchannel geometry and a CD rotation speed of 2,000 rpm, a mixing efficiency of more than 93 % can be obtained within 5 s.     

Design optimization of capillary-driven micromixer with square-wave microchannel for blood plasma mixing

A numerical and experimental investigation is performed into the flow characteristics and mixing performance of three microfluidic polydimethylsiloxane blood plasma mixing devices incorporating square-wave, curved and zigzag microchannels, respectively. For each device, the plasma is introduced into the microfluidic channel under the effects of capillary action alone. Of the three devices, that with the square-wave microchannel is found to yield the best mixing performance, and is therefore selected for design optimization. Four microfluidic micromixers incorporating square-wave microchannels with different widths in the x- and y-directions are fabricated using conventional photolithography techniques. The mixing performance of the four microchannels is investigated both numerically and experimentally. The results show that given an appropriate specification of the microchannel geometry, a mixing efficiency of approximately 76 % can be obtained within 4 s. The practical feasibility of the micromixer is demonstrated by performing prothrombin time (PT) tests using a total liquid volume of 4.0 μL (2.0 μL of plasma and 2.0 μL of PT reagent). It is shown that the mean time required to complete the entire PT test (including loading, mixing and coagulation) is less than 30 s.

     

Analytical, numerical and experimental investigations of mixing fluids in microchannel

This work presents theoretical analysis, numerical simulation, fabrication and test of a micromixer chip for mixing fluids in microchannel. A three-dimensional analytical model is developed using a different mathematical approach to study passive laminar mixing phenomena and predict concentration distribution in a microchannel. The analytical model is validated by comparing with experimental and simulation results. The process of mixing fluids in a microchannel is simulated by solving the continuity, momentum and mass diffusion equations. The simulation results are validated and then parametric studies are performed to investigate the effects of channel aspect ratio, Reynolds number and diffusion coefficient on the mixing performance. The micromixer chip is fabricated with patterned SU-8 photoresist as the microchannel layer on a PMMA substrate using a combination of photolithography and micro-milling. Experiments are performed with different mixing fluids and the results were compared with that obtained from the theoretical model and simulation results.     

Numerical study of the effect on mixing of the position of fluid stream interfaces in a rectangular microchannel

A numerical study has been carried out to investigate the effect on mixing of the position of fluid stream interfaces in a rectangular microchannel. The velocity profile in a microchannel has a parabolic shape which shows maximum local velocity at the center of the channel and minimum velocity near the wall. This velocity profile governs the residence time of the fluid streams and hence effects on the mixing of fluid streams. Single and double interfaces of the fluid streams have been studied in rectangular microchannels for the range of Reynolds numbers (0.1 ≤ Re ≤ 10) where mixing is mainly governed by molecular diffusion. Significant variations in the mixing performance have been shown at various positions of the interfaces at different Reynolds number.     

Numerical study on shape optimization of groove micromixers

The performance of a homogeneous T-mixer can be enhanced significantly by the stimulation of secondary/transverse flows in the microchannel. The groove-based micromixers generate helical flows within the microchannel to augment the mixing performance. These micromixers are extensively studied with respect to planar geometric parameters such as groove width, groove spacing, channel height, etc. The effect of groove shape on mixing performance has not been systematically studied. Previous studies have focused on two or three different predefined groove shapes, typically involving slanted grooves, asymmetric herringbone grooves, and their variations. In this computational study, we analyze the effect of groove shape on micromixing performance and search for the optimal groove shape for a pressure-driven flow across the microchannel. The groove shape is parametrically represented by Bézier curves which could take any shape within a constrained plane. The control points of the Bézier curve are chosen as optimization parameters to identify the optimal groove shape which maximizes the mixing for given operating conditions. The optimization is carried out for pressure-driven flow with and without staggered arrangement of grooves. The resulting single groove optimal design improves the mixing efficiency from 0.18 for T-mixer to 0.85 for the same operating conditions (Re ~0.42, Pe ~4,200). Unlike previous studies, axial mixing index profiles are presented for different micromixers which clearly distinguish the effect of flow field on the mixing performance. Various parametric studies are carried out to compare the optimal groove structure with other common groove type (staggered, herringbone, etc.) micromixers for a range of Pe between 400 and 6,200. The improved mixing performance in optimal designs is due to a continuously growing finger-like structure of the interface which enhances the overall mass transfer.     

Acoustically generated flows in microchannel flexural plate wave sensors: Effects of compressibility

Acoustically generated flowfields in flexural plate wave sensors filled with a Newtonian liquid (water) are considered. A computational model based on compressible flow is developed for the sensor with a moving wall for pumping and mixing applications in microchannels. For the compressible flow formulation, an isothermal equation of state for water is employed. The velocity and pressure profiles for different parameters including flexural wall frequency, channel height, amplitude of the wave and wave length are investigated for four microchannel height/length geometries. It is found that the flowfield becomes pseudo-steady after sufficient number of flexural cycles. Both instantaneous and time averaged results show that an evanescent wave is generated in the microchannel. The predicted flows generated by the FPWs are compared with results available in the literature. The proposed device can be exploited to integrate micropumps with complex microfluidic chips improving the portability of micro-total-analysis systems.     

Centrifuge-based micromixer with three-dimensional square-wave microchannel for blood plasma mixing

A numerical and experimental investigation is performed into the flow characteristics and mixing performance of four centrifuge-based compact disk (CD) microfluidic mixers with square-wave microchannels and different numbers and arrangements of inlet channels. Of the four micromixers, the 3 × 1 3D mixer (three inlet channels, one outlet channel, and a two-layer mixing channel) is found to yield the best mixing performance. The superior mixing performance is attributed to the formation of transverse secondary flows and a 3D stirring effect at the corners of the square-wave channel. The experimental results show that given a CD rotation speed of 1000 rpm, a mixing efficiency of more than 91 % can be obtained within 5 s. The practical feasibility of the micromixer is demonstrated by performing prothrombin time (PT) tests. It is shown that the mean time required to complete the entire PT test (including loading, mixing and coagulation) is less than 45 s.

     

Fabrication of polyimide microfluidic devices by laser ablation based additive manufacturing

Polyimide microfluidic devices (MFDs) have been attached enormous significance because of its excellent organic-solvent inertness, biocompatibility, and thermal stability. In this paper, a novel fabrication method based on the thought of additive manufacturing, which is adding materials layer by layer from bottom to top, was used to construct a multilayer polyimide MFD. The MFD has sophisticated three-dimensional (3D) microchannels with adjustable cross-sectional geometries and high bonding strength, which leads to good reagent mixing performance, large surface-to-volume ratio, and great durability. Starting from a single polyimide film, ultraviolet (UV) laser was utilized to ablate microchannels on the film. Due to the studies over the influence of UV laser on the channel width, the microchannel edge shape is under control, varying from trapezoid to rectangle. From monolayer to multilayer MFDs, thermal bonding with fluorinated ethylene propylene (FEP) nanoparticle dispersion as the adhesive was adopted to stack polyimide films tightly with precise alignment. In this way, microchannels can be connected vertically between layers to form 3D structures. Besides, a homogeneous adhesive interlayer and polyimide-FEP mixing regime were formed, which can provide high bonding strength. Results of computational fluid dynamics simulation of 3D microchannel structures and organic synthesis experiment revealed that our device has great reagent mixing efficiency and promising application prospects in diverse research fields, especially organic chemical and biological studies.

     

Vortex generation in electroosmotic flow passing through sharp corners

A special phenomenon was found when sharp wedges were set in a microchannel where electroosmotic flow occurred, vortices were induced near the wedges when a DC electric field was imposed. The strength of the induced vortices depends on the concentration of electrolytes and the intensity of the electric field. Latex particles are used to aid the flow visualization. Formation of vortices is due to concentration depletion in the microchannel. Furthermore, the vortices are used to enhance mixing in a micromixer. Experimental results showed that the vortex structures created within the mixing section increase the mixing index from a value of 3% in the upstream region of the microchannel to 78% at the outlet of the mixing section.     

Multilamination of flows in planar networks of rotating microchannels

We describe a new multilamination technique to accelerated mixing of centrifugally pumped flows through a simple network of preferentially radial, low-aspect-ratio microchannels. Mixing by multilamination is enforced by planar split-and-recombine structures, consisting of a common inlet for two concurrent centrifugal flows, and a transient region of parallel microchannels which merge again into one common outlet. A repatterning of flow is observed in each parallel channel which is induced by the Coriolis pseudo force. In a distinct regime of the parameter space spanned by the speed of rotation, the channel geometry as the viscosity (and density) of the liquids, a multilamination of flow is achieved at the entrance of the common outlet channel. We also present parallelization and cascading strategies to further enhance the homogeneity and throughput of mixing by multilamination.     

Interfacial configurations and mixing performances of fluids in staggered curved-channel micromixers

We present a parallel laminar micromixer with staggered curved channels for homogeneously mixing two fluids by Dean vortex. The secondary flows are produced in curved rectangular channels by the centrifugal forces; the diffusion distance of two fluids is reduced due to the staggered structures of the flow channels. The mixing strength is increased when one stream is injected into the other. Confocal microscopy and pH indicator have been used to study the mixing. Computational fluid dynamics simulations are utilized to examine the interfacial configurations and the mixing behaviors inside the channels. The interface of the two fluids is heavily distorted and increases the interfacial area because of the unique structures. The mixing index of the staggered curved-channel mixer with tapered channels is higher than those of the other curved-channel mixers. The effects of various Reynolds numbers and channel configurations on mixing performances are investigated in terms of the experimental mixing indices and the computational interfacial patterns. The comparison between the experimental data and numerical results shows a very similar trend.     

Multidirectional vortices mixing in three-stream micromixers with two inlets

In this work, we design, fabricate and compare three types of three-stream curved-straight-curved (CSC) micromixers, including the full three-stream (FTS) CSC micromixer, the CSC microchannel with internal side-wall injection and the CSC microchannel with external side-wall injection. In the three-stream CSC micromixers, there is a core stream sandwiched by two cladding streams into the CSC channel with baffles from two inlets. The sandwiched structure of streams and the multidirectional vortices due to flow separation and channel curvature contribute together to enhance mixing. We examine fluid mixing in the proposed micromixers by numerical simulation and using confocal spectral microscope imaging system. The present results show that the mixing efficiency increases without increasing the pressure applied much by the channel structure forming the sandwiched structure of streams. Besides, it is found that the FTS CSC micromixer is the preferable one among the micromixers considered.     

Cluster formation and growth in microchannel flow of dilute particle suspensions

The lifetime of microfluidic devices depends on their ability to maintain flow without interruption. Certain applications require microdevices for transport of liquids containing particles. However, microchannels are susceptible to blockage by solid particles. Therefore, in this study, the phenomenon of interest is the formation and growth of clusters on a microchannel surface in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flows in microchannels with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel are presented. Although the cluster size seems to be independent of location, more clusters are found at the inlet/outlet regions than in the microchannel center. Similarly as for an individual cluster, as long as particle–cluster interaction is the dominant mode, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size, and concentration are also reported.     

Inertial particle focusing and spacing control in microfluidic devices

Focusing particles into a tight stream is usually a necessary step prior to counting, detecting and sorting them. Meanwhile, particle spacing control in microfluidic devices could also be applied in the field of accurate cell detection, material synthesis and chemical reaction. To achieve simultaneous particle focusing and spacing control, a novel microchannel composed by Dean and sheath flow section was proposed and fabricated according to the elaborated design principle with its manipulating performance in situ visualized. Using microspheres with a few microns as a template, the trajectory of the particles was discovered to follow lateral migration and reach certain equilibrium positions at the end of the designed Dean section. After being focused, the streamline was further concentrated and centralized with a controllable interparticle distance in sheath flow section. For sheath flow section, the angle between symmetrical tributaries and the mainstream channel and abrupt constriction/expansion structure of mainstream channel as important channel geometric features were investigated to minimize the focusing streamline width and optimize spacing control. An modified analytical model for sheath flow with different tributary angles was derived and proved to well describe the microsphere spacing control process.     

Passive microfluidic devices for plasma extraction from whole human blood

Our goal is to analyze and compare different continuous microfluidic principles dedicated to plasma extraction from hardly diluted human blood for lab-on-chip applications. First, the strengths and weaknesses of various emerging passive microfluidic methods (microfiltration- and centrifugation-based methods) were analyzed. Various devices were designed, microfabricated and tested with beads or blood. Filtration may be efficient, but with a high sample dilution, low flow rate and optimized geometry. Due to fast cell clogging, this remains a short-term solution. Separation effects resulting from centrifugal acceleration in curved channel flows are hindered by Dean vortices and anyhow are not pronounced with blood. An innovative device is then proposed and investigated experimentally. This is based on the lateral migration of red cells and the resulting cell-free layer, which is used to supply geometric singularities (an ear-cavity or a corner-edge) and locally enhance the clear plasma region. A maximum extraction of 10.7% is obtained for 1/20 diluted blood, injected at 100 μL/min in the corner-edge design.     

Control of secondary flow in concentrically traveling flow on centrifugal microfluidics

This paper reports a fundamental study of the stripe laminar flow pattern on a centrifugal microfluidic device with the goal of realizing a sedimentation-based, continuous mode particle separation technique. Microfluidic channels were designed with a concentrically integrated microchannel, and the patterning of the flow in the channel was investigated. A significant secondary flow was observed as a preliminary result. We conclude that the origin of this secondary flow was not the Dean force, because it was observed in a straight microchannel, but was not observed in curved channel during the spinning of the system at rest. The transition of the pattern was investigated using a simulation and experiment, and the flow pattern’s dependence on the rotational speed was determined, which suggested that the origin of the secondary flow was the Coriolis force. The significance of the secondary flow was controlled by adjusting the rotational speed of the disk, and the flow rate and laminar flow patterns were controlled by the stripe flow pattern.     

Efficient Micromixing Using Induced-Charge Electroosmosis

Lab-on-a-chip (LOC) devices which utilize electrokinetics for fluid transport are invariably poor mixers due to the nature of low-Reynolds-number flows. For many such devices, efficient mixing is needed for fast analysis, but the predominant mechanism of equalizing concentration differences is often diffusion-a relatively slow form of mass transfer. In this numerical study, we propose a novel micromixer design which utilizes the recent concept of induced-charge electroosmosis for enhancing fluid mixing. As validation, it is shown that numerical simulations of fluid flow in the proposed system are in good agreement with analytical solutions available for electrokinetic flow and electrokinetic mixing in traditional microchannels. The conventional mixing performance index is modified so that it accounts for the length required for desired mixing as well as the concentration gradients across the channel width. With the help of the modified mixing index, the proposed mixer is compared with the traditional mixer design and found to be superior in performance. Furthermore, the effects of design parameters on mixing performance are analyzed for possible device implementation.     

An efficient micromixer actuated by induced-charge electroosmosis using asymmetrical floating electrodes

Efficient microfluid mixing is an important process for various microfluidic-based biological and chemical reactions. Herein we propose an efficient micromixer actuated by induced-charge electroosmosis (ICEO). The microchannel of this device is easy to fabricate for its simple straight channel structure. Importantly, unlike previous design featuring complicated three-dimensional conducting posts, we utilize the simpler asymmetrical planar floating-electrodes to induce asymmetrical microvortices. For evaluating the mixing performance of this micromixer, we conducted a series of simulations and experiments. The mixing performance was quantified using the mixing index, specifically, the mixing efficiency can reach 94.7% at a flow rate of 1500 µm/s under a sinusoidal wave with a peak voltage of 14 V and a frequency of 400 Hz. Finally, we compared this micromixer with different micromixing devices using a comparative mixing index, demonstrating that this micromixer remains competitive among these existing designs. Therefore, the method proposed herein can offer a simple solution for efficient fluids mixing in microfluidic systems.     

Comparative study between computational and experimental results for binary rarefied gas flows through long microchannels

A comparative study between computational and experimental results for pressure-driven binary gas flows through long microchannels is performed. The theoretical formulation is based on the McCormack kinetic model and the computational results are valid in the whole range of the Knudsen number. Diffusion effects are taken into consideration. The experimental work is based on the Constant Volume Method, and the results are in the slip and transition regime. Using both approaches, the molar flow rates of the He–Ar gas mixture flowing through a rectangular microchannel are estimated for a wide range of pressure drops between the upstream and downstream reservoirs and several mixture concentrations varying from pure He to pure Ar. In all cases, a very good agreement is found, within the margins of the introduced modeling and measurement uncertainties. In addition, computational results for the pressure and concentration distributions along the channel are provided. As far as the authors are aware of, this is the first detailed and complete comparative study between theory and experiment for gaseous flows through long microchannels in the case of binary mixtures.