A novel in-plane passive microfluidic mixer with modified Tesla structures

An innovative in-plane passive micromixer using modified Tesla structures, which are used as passive valves, has been designed, simulated, fabricated and successfully characterized in this paper. Simulation and experimental results of the developed novel micromixer have shown excellent mixing performance over a wide range of flow conditions in the micro scale. The micromixer realized in this work has achieved even better mixing performance at a higher flow rate, and its pressure drop is less than 10 KPa at the flow rate of 100 microl min(-1). This micromixer shows characteristics similar to Taylor dispersion, with contributions from both diffusion and convection. The mixer has a diffusion domain region at low flow rate, but it moves to a convection domain region at high flow rate. Due to the simple in-plane structure of the novel micromixer explored in this work, the mixer can be easily realized and integrated with on-chip microfluidic devices and micro total analysis systems (micro-TAS).     

Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis

A multifunctional and high-efficiency microfluidic device for droplet generation and fusion is presented. Through unique design of the micro-channels, the device is able to alternately generate droplets, generating droplet ratios ranging from 1 ratio 5 to 5 ratio 1, and fuse droplets, enabling precise chemical reactions in several picoliters on a single chip. The controlled fusion is managed by passive control based on the channel geometry and liquid phase flow. The synthesis of CdS nanoparticles utilizing each fused droplet as a microreactor for rapid and efficient mixing of reagents is demonstrated in this paper. Following alternating droplet generation, the channel geometry allows the exclusive fusion of alternate droplets with concomitant rapid mixing and produces supersaturated solution of Cd2+ and S2- ions to form CdS nanoparticles in each fused droplet. The spectroscopic properties of the CdS nanoparticles produced by this method are compared with CdS prepared by bulk mixing.     

Fabrication of antigen-containing nanoparticles using microfluidics with Tesla structure

The polyethyleneimine (PEI)-based antigen delivery system has been proved valuable for therapeutic vaccines. However, the previous bulk mixing method is not ideal to prepare PEI-base antigen- containing nanoparticles. The wide-size distribution and poor reproducibility limit its further application. In this research, we developed a microfluidic method to prepare nanopolyplexes on chips with Tesla structure to improve the fabrication. The structure and bioactivity of protein were not hampered by the shearing force in microfluidics. Comparison between physiochemical parameters suggested that the polyplexes prepared by Tesla chips were more uniform than those prepared by bulk mixing and non-Tesla chips. The reproducibility was improved obviously. The preparation was not influence much by the operating parameters such as flow rates, reagents concentration, and switching of inlets. The results indicated that it was a robust and reliable method. The data that were obtained from BMDC model demonstrated that the nanopolyplexes with optimal weight ratio had higher antigen cross-presentation efficiency than those in free antigen group. In conclusion, Tesla structured microfluidics offers a better method over bulk mixing in the preparation of PEI-based antigen-containing nanopolyplexes. It greatly expands the scope of our study and increases the potential of PEI-based antigen delivery system.     

A pneumatic micromixer facilitating fluid mixing at a wide range flow rate for the preparation of quantum dots

A novel fluid micromixer based on pneumatic perturbation and passive structures was developed. This micromixer facilitates integration and is applicable to fluid mixing over a wide range of flow rates. The microfluidic mixing device consists of an S-shaped structure with two mixing chambers and two barriers, and two pneumatic chambers designed over the S-shaped channel. The performance of the micromixer for fluids with wide variation of flow rates was significantly improved owing to the integration of the pneumatic mixing components with the passive mixing structures. The mixing mechanism of the passive mixing structures was explored by numerical simulation, and the influencing factors on the mixing efficiency were investigated. The results showed that when using a gas pressure of 0.26 MPa and a 100 m-thick polydimethylsiloxane (PDMS) pneumatic diaphragm, the mixing of fluids with flow rates ranging from 1 to 650 L/min was achieved with a pumping frequency of 50 Hz. Fast synthesis of CdS quantum dots was realized using this device. Smaller particles were obtained, and the size distribution was greatly improved compared with those obtained using conventional methods.     

Numerical study of the vortex-induced electroosmotic mixing of non-Newtonian biofluids in a nonuniformly charged wavy microchannel: Effect of finite ion size

We propose a micromixer for obtaining better efficiency of vortex induced electroosmotic mixing of non-Newtonian bio-fluids at a relatively higher flow rate, which finds relevance in many biomedical and biological applications. To represent the rheology of non-Newtonian fluid, we consider the Carreau model in this study, while the applied electric field drives the constituent components in the micromixer. We show that the spatial variation of the applied field, triggered by the topological change of the bounding surfaces, upon interacting with the non-uniform surface potential gives rise to efficient mixing as realized by the formation of vortices in the proposed micromixer. Also, we show that the phase-lag between surface potential leads to the formation of asymmetric vortices. This behavior offers better mixing performance following the appearance of undulation on the flow pattern. Finally, we establish that the assumption of a point charge in the paradigm of electroosmotic mixing, which is not realistic as well, under-predicts the mixing efficiency at higher amplitude of the non-uniform zeta potential. The inferences of the present analysis may guide as a design tool for micromixer where rheological properties of the fluid and flow actuation parameters can be simultaneously tuned to obtain phenomenal enhancement in mixing efficiency.     

Ultrafast active mixer using polyelectrolytic ion extractor

We report on a low voltage, straight/smooth surface, and efficient active micromixer. The mixing principle is based on alternative ion depletion-enrichment using a pair of positively charged polyelectrolytic gel electrodes (pPGEs), which face each other joined by a microchannel. This system has an external AC signal source electrically connected to the pPGEs via the respective 1 M KCl solutions and Ag/AgCl electrodes. When an electric bias is applied between the two pPGEs, anions are extracted through one of the pPGEs to create a local ion-deficient region. Simultaneously, an ion-rich area appears near the other pPGE due to an inward anionic flux. As the direction of the charge flow is periodically reversed by the AC signal source, the ion depletion-enrichment regions are alternately swapped with each other on the 'push-pull' basis. The turmoil between the pPGEs quickly mixes the solutions in the microchannel without any mechanical moving part or specially machined structures. In the proposed system, both AC frequency and current density can be easily and finely controlled so that one can quickly find the optimal conditions for a given sample. The micromixer as made showed a mixing efficiency higher than 90% for sample solutions of 1 mM Rhodamine 6G and PBS at pH 7.4 when the flow rate was under 6 mm s(-1). In addition to the solution-solution mixing, the micromixer can effectively mix suspended microparticles with solution. As a representative example, rapid and efficient lysis of human red blood cells was demonstrated allowing minimal damage of the white blood cells.     

Novel electroosmotic micromixer configuration based on ion-selective microsphere

In this paper, a micromixer of a new configuration is presented, consisting of a spherical chamber in the center of which an ion-selective microsphere is placed. Stratified liquid is introduced through the chamber via inlet and outlet holes under an external pressure gradient and an external electric field is directed in such a way that the resulting electroosmotic flow is directed against the pressure-driven flow, resulting in mixing. The investigation is carried out by direct numerical simulation on a super-computer. Optimal values of the applied electric field are determined to yield strong mixing. Above this optimal mixing regime, a number of instabilities and bifurcations are realized, which qualitatively coincide with those occurring during electrophoresis of an ion-selective microgranule. As shown by our calculation, these instabilities do not lead to an enhanced mixing. The resulting electroconvective vortices remain confined near the surface of the microgranule, and do not sufficiently perturb the stratified fluid flow further from the granule. On the other hand, another type of instability caused by the salt concentration gradient can generate sufficiently strong oscillations to enhance mixing. However, this only occurs when the external electric field is sufficiently high that the electroosmotic flow is comparable to the pressure-driven flow. This ultimately leads to creation of reverse flows of the liquid and cessation of the device operation. Thus, it was shown that the best mixing occurs in the absence of electrokinetic instability. Based on the data obtained, it is possible to select the necessary geometric characteristics of the micromixer to achieve the optimal mixing mode for a given set of liquids, which may be ten times more effective than passive mixers at the same flow rates. A comparison with the experimental data of the other authors confirms the effectiveness of this device and its other capabilities. Furthermore, the basic device design can be operated in other modes, for example, an electrohydrodynamic pump, a streaming current generator, or even a micro-reactor, depending on the system parameters and choice of an ion-selective granule.     

A serpentine laminating micromixer combining splitting/recombination and advection

Mixing enhancement has drawn great attention from designers of micromixers, since the flow in a microchannel is usually characterized by a low Reynolds number (Re) which makes the mixing quite a difficult task to accomplish. In this paper, a novel integrated efficient micromixer named serpentine laminating micromixer (SLM) has been designed, simulated, fabricated and fully characterized. In the SLM, a high level of efficient mixing can be achieved by combining two general chaotic mixing mechanisms: splitting/recombination and chaotic advection. The splitting and recombination (in other terms, lamination) mechanism is obtained by the successive arrangement of "F"-shape mixing units in two layers. The advection is induced by the overall three-dimensional serpentine path of the microchannel. The SLM was realized by SU-8 photolithography, nickel electroplating, injection molding and thermal bonding. Mixing performance of the SLM was fully characterized numerically and experimentally. The numerical mixing simulations show that the advection acts favorably to realize the ideal vertical lamination of fluid flow. The mixing experiments based on an average mixing color intensity change of phenolphthalein show a high level of mixing performance was obtained with the SLM. Numerical and experimental results confirm that efficient mixing is successfully achieved from the SLM over the wide range of Re. Due to the simple and mass producible geometry of the efficient micromixer, SLM proposed in this study, the SLM can be easily applied to integrated microfluidic systems, such as micro-total-analysis-systems or lab-on-a-chip systems.     

A three-dimensional vortex microsystem designed and fabricated for controllable mixing

A three-dimensional micromixer is designed and fabricated by using glass-poly(dimethylsiloxane) (PDMS) hybridized materials. The improvement of the fabrication process makes the micromixer endure much higher flow rate. Based on the self-rotation effect of the fluid, the fast mixing can be achieved. The mixing process is evaluated by connecting the micromixer to a UV-Vis detector. The results show that by adjusting the infuse flow rate, the mixing process can be accurately controlled. Supported by the National Natural Science Foundation of China (Grant Nos. 20735002 & 29877019) and the Key Natural Science Foundation of Fujian Province, China (Grant No. D0520001)     

Microfluidic cell disruption system employing a magnetically actuated diaphragm

A microfluidic cell lysis chip equipped with a micromixer and SPE unit was developed and used for quantitative analysis of intracellular proteins. This miniaturized sample preparation system can be employed for any purpose where cell disruption is needed to obtain intracellular constituents for the subsequent analysis. This system comprises a magnetically actuated micromixer to disrupt cells, a hydrophobic valve to manipulate the cell lysate, and a packed porous polymerized monolith chamber for SPE and filtering debris from the cell lysate. Using recombinant Escherichia coli expressing intracellular enhanced green fluorescent protein (EGFP) and lipase as model bacteria, we optimized the cell disruption condition with respect to the lysis buffer composition, mixing time, and the frequency of the diaphragm in the micromixer, which was magnetically actuated by an external magnetic stirrer in the micromixer chamber. The lysed sample prepared under the optimal condition was purified by the packed SPE in the microfluidic chip. At a frequency of 1.96 Hz, the final cell lysis efficiency and relative fluorescence intensity of EGFP after the cell disruption process were greater than 90 and 94%, respectively. Thus, this microfluidic cell disruption chip can be used for the efficient lysis of cells for further analysis of intracellular contents in many applications.     

Microfluidic synthesis and catalytic application of PVP-stabilized, approximately 1 nm gold clusters

Small PVP-stabilized gold clusters were successfully prepared by the homogeneous mixing of continuous flows of aqueous AuCl 4 (-) and BH 4 (-) in a micromixer. Spectroscopic characterization revealed that microfluidic synthesis could yield monodisperse Au:PVP clusters with an average diameter of approximately 1 nm, which is smaller than clusters produced by conventional batch methods. These approximately 1 nm Au:PVP clusters exhibited higher catalytic activity for the aerobic oxidation of p-hydroxybenzyl alcohol than did Au:PVP clusters prepared by batch methods.     

Design and Fabrication of a Three Dimensional Spiral Micromixer

In this work, we present a three dimensional micromixer which consists of two layers of spiral channels overlapped together in the vertical direction. This micromixer is designed by using a smooth channel twisted into double‐layer spiral geometry with simple topological structure. Based on the principle of Dean effects, this kind of structure is beneficial to produce, enhance and sustain the Dean vortices, which can perturb the laminar fluid effectively. In order to improve the mixing performance, the detailed parameters have been optimized by using the computational fluid dynamics software. The results indicate that the erect channel which is connected with the two layers of spiral channels plays a critical role for well mixing. Meanwhile, the effect of mixing has been identified in a fabricated glass‐micromixer. The mixing ef?ciency of 90% has been achieved by optimizing the flow rate and the structure of the erect channel. Thus, this micromixer has manifested high mixing efficiency and presents good practicability in the versatile microfluidic systems.     

Design and simulation of the micromixer with chaotic advection in twisted microchannels

Chaotic mixers with twisted microchannels were designed and simulated numerically in the present study. The phenomenon whereby a simple Eulerian velocity field may generate a chaotic response in the distribution of a Lagrangian marker is termed chaotic advection. Dynamic system theory indicates that chaotic particle motion can occur when a velocity field is either two-dimensional and time-dependent, or three-dimensional. In the present study, micromixers with three-dimensional structures of the twisted microchannel were designed in order to induce chaotic mixing. In addition to the basic T-mixer, three types of micromixers with inclined, oblique and wavelike microchannels were investigated. In the design of each twisted microchannel, the angle of the channels' bottoms alternates in each subsection. When the fluids enter the twisted microchannels, the flow sways around the varying structures within the microchannels. The designs of the twisted microchannels provide a third degree of freedom to the flow field in the microchannel. Therefore, chaotic regimes that lead to chaotic mixing may arise. The numerical results indicate that mixing occurs in the main channel and progressively larger mixing lengths are required as the Peclet number increased. The swaying of the flow in the twisted microchannel causes chaotic advection. Among the four micromixer designs, the micromixer with the inclined channel most improved mixing. Furthermore, using the inclined mixer with six subsections yielded optimum performance, decreasing the mixing length by up to 31% from that of the basic T-mixer.     

AC electroosmotic micromixer for chemical processing in a microchannel

A rapid micromixer of fluids in a microchannel is presented. The mixer uses AC electroosmotic flow, which is induced by applying an AC voltage to a pair of coplanar meandering electrodes configured in parallel to the channel. To demonstrate performance of the mixer, dilution experiments were conducted using a dye solution in a channel of 120 microm width. Rapid mixing was observed for flow velocity up to 12 mm s(-1). The mixing time was 0.18 s, which was 20-fold faster than that of diffusional mixing without an additional mixing mechanism. Compared with the performance of reported micromixers, the present mixer worked with a shorter mixing length, particularly at low Peclet numbers (Pe < 2 x 10(3)).     

Active mixing inside microchannels utilizing dynamic variation of gradient zeta potentials

This study presents a new active micromixer with high mixing efficiency achieved by means of a gradient distribution of the surface zeta potential controlled by changing the frequency of voltage applied on shielding electrodes. Gradient surface zeta potential is generated by applying a high voltage to inclined buried shielding electrodes. While alternating the frequency of driving voltage, the zeta potential could be changed accordingly, thus providing a significant mixing effect inside microchannels. A theoretical model is proposed to predict the distribution of zeta potential. The results from this model are critically compared with the well-developed three-capacitor model. Additionally, two time-factor scales, the charge time of capacitor and mixing length flow time, are used to predict the optimum frequency. The prediction of optimum frequency, 0.5 Hz, is consistent with experimental results. Moreover, a five-pair inclined shielding electrode with a frequency of 0.5 Hz leads to a significant improvement in the mixing performance of the active micromixer. Numerical results indicate that a localized flow circulation is generated when the control voltage is applied to the inclined shielding electrodes. Furthermore, the streamlines are experimentally observed by using fluorescent beads. The shape of this circulation is dependent on the distribution of gradient zeta potential, which is determined by the arrangement of electrodes. The effects of the number of electrode pairs and the layout of shielding electrodes on the mixing performance of micromixer are also explored both numerically and experimentally. It is revealed that five-pair inclined electrodes at 0.5 Hz provide the highest mixing efficiency. Finally, a reaction between N-benzoyl-L-arginine-p-nitroanilide and trypsin enzyme is performed to verify the capability of micromixers. The experimental results reveal that the reaction can achieve a higher performance indicating a higher mixing efficiency. The active micromixers could be used in microfluidic systems for improving the mixing efficiency and thus enhancing the bioreaction.     

Rapid method for design and fabrication of passive micromixers in microfluidic devices using a direct-printing process

We developed a facile and rapid one-step technique for design and fabrication of passive micromixers in microfluidic devices using a direct-printing process. A laser printing mechanism was dexterously adopted to pattern the microchannels with different gray levels using vector graphic software. With the present method, periodically ordered specific bas-relief microstructures can be easily fabricated on transparencies by a simple printing process. The size and shape of the resultant microstructures are determined by the gray level of the graphic software and the resolution of the laser printer. Patterns of specific bas-relief microstructures on the floor of a channel act as obstacles in the flow path for advection mixing, which can be used as efficient mixing elements. The mixing effect of the resultant micromixer in microfluidic devices was evaluated using CCD fluorescence spectroscopy. We found that the mixing performance depends strongly on the gray level values. Under optimal conditions, fast passive mixing with our periodic ordered patterns in microfluidic devices has been achieved at the very early stages of the laminar flow. In addition, fabrication of micromixers using the present versatile technique requires less than an hour. The present method is promising for fabrication of micromixers in microfluidic devices at low cost and without complicated devices and environment, providing a simple solution to mixing problems in the micro-total-analysis-systems field.     

T型微混合器混合特性的浓度分布评价法

提出一种微混合器混合性能的评价方法.在样品盒中注入不同浓度罗丹明B溶液并用体视显微镜观察捕获图像,通过Image J软件读取图像灰度值,建立不同深度下溶液的浓度-灰度值函数关系,运用此关系式将T型微混合器3种不同深度(0.1、 0.2和0.4 mm,质量浓度0.05 %的罗丹明B溶液和去离子水作为配对流体)混合实验中捕获的图像中各像素点上的灰度值转换为浓度值,绘制浓度等高线图及浓度频数分布图,分析各自混合情况,最后引入浓度混合指数概念及计算公式,分析3种深度混合器内不同截面上的混合程度.此方法从定性和定量两方面分析了微尺度下混合腔深度对微混合的影响程度,具有一定的应用价值.     

A model for laminar diffusion-based complex electrokinetic passive micromixers

This paper presents a model for the efficient and accurate simulations of laminar diffusion-based complex electrokinetic passive micromixers by representing them as a system of mixing elements of relatively simple geometry. Parameterized and analytical models for such elements are obtained, which hold for general sample concentration profiles and arbitrary flow ratios at the element inlet. A lumped-parameter and system-level model is constructed for a complex micromixer, in which the constituent mixing elements are represented by element models, in such a way that an appropriate set of parameters are continuous at the interface between each pair of adjacent elements. The system-level model, which simultaneously computes electric circuitry and sample concentration distributions in the entire micromixer, agrees with numerical and experimental results, and offers orders-of-magnitude improvements in computational efficiency over full numerical simulations. The efficiency and usefulness of the model is demonstrated by exploring a number of laminar diffusion based mixers and mixing networks that occur in practice.     

Rapid mixing with high‐throughput in a semi‐active semi‐passive micromixer

In this paper, we investigate a novel alternating current electrothermal (ACET) micromixer driven by a high efficiency ACET micropump. The micromixer consists of thin film asymmetric pairs of electrodes on the microgrooved channel floor and array of electrode pairs fabricated on the top wall. By connecting electrodes with AC voltage, ACET forces are induced. Asymmetric microgrooved electrodes force the fluids along the channel, while lateral vortex pairs are generated by symmetric electrode pairs located on the top wall. Waviness of the floor increases contact area between two confluent streams within a narrow confinement. An active mixer operates as a semi active semi passive mixer. Effects of various parameters are investigated in details in order to arrive at an optimal configuration that provides for efficient mixing as well as appreciable transport. It is found that using a specific design, uniform and homogeneous mixing quality with mixing efficiency of 97.25% and flow rate of per unit width of the channel can be achieved.     

Passive and Active Mixing in Microfluidic Devices

In microfluidics the Reynolds number is small, preventing turbulence as a tool for mixing, while diffusion is that slow that time does not yield an alternative. Mixing in microfluidics therefore must rely on chaotic advection, as well-known from polymer technology practice where on macroscale the high viscosity makes the Reynolds numbers low and diffusion slow. The mapping method is used to analyze and optimize mixing also in microfluidic devices. We investigate passive mixers like the staggered herringbone micromixer (SHM), the barrier embedded micromixer (BEM) and a three-dimensional serpentine channel (3D-SC). Active mixing is obtained via incorporating particles that introduce a hyperbolic flow in e.g. two dimensional serpentine channels. Magnetic beads chains-up in a flow after switching on a magnetic field. Rotating the field creates a physical rotor moving the flow field. The Mason number represents the ratio of viscous forces to the magnetic field strength and its value determines the fate of the rotor: a single, an alternating single and double, or a multiple part chain-rotor results. The type of rotor determines the mixing quality with best results in the alternating case where crossing streamlines introduce chaotic advection. Finally, an active mixing device is proposed that mimics the cilia in nature. The transverse flow induced by their motion indeed enhances mixing at the microscale.