高回流被动式微混合器设计及数值模拟

为了提高混合强度设计了一种高回流被动式微混合器,其反馈通道中的回流延长了混合时间并促进形成涡流,增强了对流和扩散作用。通过优化反馈通道形状和混合腔入口尺寸,该微混合器提升了其回流率和混合强度。利用仿真软件Fluent分析该微混合器的性能,仿真表明对于给定的微混合器其回流率和混合强度仅仅决定于雷诺数,回流率和混合强度均随雷诺数增大而增大。特别地,当雷诺数低至8.3时,仍可在反馈通道中找到回流;当雷诺数为99.6时,回流率达到24%,混合强度则超过60%。通过对反馈通道倾斜角度参数的模拟,该微混合器的回流率和混合强度均有明显提升。     

Effect of baffle height and Reynolds number on fluid mixing

In this paper, both numerical simulation and experiment were performed to investigate the mixing process within mixing chambers of the planar micromixer with three baffles at varied baffle height. The presence of three baffles causes the flow to separate and creates the recirculation and back flow within mixing chambers. The fluid mixing was greatly influenced by the baffle height and Reynolds number (Re) related to the size of recirculation zone. Larger baffle height or Re produces larger recirculation zone and convective mixing. The micromixer with 350 μm high baffles in 400 μm wide channel results in over 95% mixing at Re = 80 from the simulation results. The experiment results confirm the above simulation results qualitatively. This planar micromixer has the merits of simple design and easy fabrication as compared to the three-dimensional passive micromixers.     

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.     

A novel design for 3D passive micromixer based on Cantor fractal structure

In this paper, we design a three-dimensional micromixer based on Cantor structure. According to the principle of enhancing chaotic advection and folding fluid, it can produce better mixing performance. First we design the fractal obstacle based on the Cantor structure. We combine the obstacle with the T type microchannel. And we get multiple combinations of microchannel. We use multi-physics field simulation software COMSOL 5.2a to simulation, which is based on finite element theory. Then we analyze the mixing performance of the Imitate Cantor structure micromixer, x stands for the height of the micromixer (ICSM_x) in the Re of 0.01–100 and explain the mechanisms of mixing enhancement in each structure. We compare the effect of the height of the obstacles, the effect of the spacing between the obstacles, and the effect of fractal obstacles series. By comparison, when Re is more than 50 or less than 0.1, the mixing efficiency of all micromixer can reach above 90%. Finally, we obtain a best micromixer, called Imitate Cantor structure micromixer with height 600 µm (ICSM₆₀₀). The minimum mixing efficiency of ICSM₆₀₀ can reach 85%, so the mixing efficiency of ICSM₆₀₀ is clearly better than others.

     

Millisecond-order rapid micromixing with non-equilibrium electrokinetic phenomena

We report an active micromixer utilizing vortex generation due to non-equilibrium electrokinetics near micro/nanochannel interfaces. Its design is relatively simple, consisting of a U-shaped microchannel and a set of nanochannels. We fabricated the micromixer just using a two-step reactive ion etching process. We observed strong vortex generation in fluorescent microscopy experiments. The mixing performance was evident in a combined pressure-driven and electroosmotic flows, compared with the case with a pure pressure-driven flow. We characterized the micromixer for several conditions: different applied voltages, ion concentrations, flow rates, and nanochannel widths. The experimental results show that the mixing performance is better with a higher applied voltage, a lower ion concentration, and a wider nanochannel width. We quantified the mixing characteristics in terms of mixing time. The lowest mixing time was 2 milliseconds with the voltage of 230 V and potassium chloride solutions of 0.1 mM. We expect that the micromixer is beneficial in several applications requiring rapid mixing.     

Numerical simulation of a novel microfluidic electroosmotic micromixer with Cantor fractal structure

In this paper, we design a novel low voltage of electroosmotic micromixer with fractal structure. Because of the influence of high voltage on electrode and solution, we propose an electroosmotic micromixer of low voltage. In order to optimize the electrode position, we design the Cantor fractal according to Cantor principle, and arrange the electrode pairs on the fractal. Then we study the mixing effect of the electrode pairs length on the mixing performance, the effect of the electrode position and the effect of fractal electrode group spacing on the mixing efficiency. When the electroosmotic micromixer has three electrode groups at alternating voltage of 5 V and alternating frequency of 8 Hz, the best mixing efficiency can reach 95.2% in one second. We call this micromixer Cantor fractal electroosmotic micromixer (CFEM). At the same Re, the mixing efficiency of CFEM is higher than the electrodeless micromixer 50%.

     

Electrokinetically driven flow mixing utilizing chaotic electric fields

This paper presents a novel microfluidic mixing scheme in which the species streams are mixed via the application of chaotic electric fields to four electrodes mounted on the upper and lower surfaces of the mixing chamber. Numerical simulations are performed to analyze the effects of the resulting chaotic electrokinetic driving forces on the fluid flow characteristics within the micromixer and the corresponding mixing performance. During simulation, chaotic oscillating electric potentials are derived using a Duffing–Holmes chaos system. Simulation results indicate that the chaotic electrokinetic driving forces induce a complex flow behavior within the micromixer which results in efficient mixing of the two species streams. It is shown that mixing efficiencies up to 95% can be obtained in the novel micromixer.     

A high-efficiency three-dimensional helical micromixer in fused silica

True three-dimensional (3D) micromixers in fused silica are highly desirable for efficient and compact mixing in microfluidic applications. However, realization of such devices remains technically challenging. Here, we report high-quality fabrication of 3D helical microchannels in fused silica by taking the full advantage of an improved femtosecond laser irradiation followed by chemical etching process, and a glass-PDMS interface structure is introduced for assembling 3D helical micromixer. Highly efficient mixing is achieved in the helical micromixer at low Reynolds numbers, whose excellent mixing performance is approved by the experimental evaluation and computational fluid dynamics simulation.     

Early induction of secondary vortices for micromixing enhancement

In the present work a new type of micromixer has been proposed and its mixing characteristic has been analyzed. The micromixer can be viewed as a U-tube with a side inlet. Here micromixing is enhanced by the secondary vortex generation induced by the curvature of the tube. The flow in the mixer geometry is investigated theoretically to understand micro-mixing using computational fluid dynamics (CFD). For this we use the Navier–Stokes equations coupled with species transport. Mixing is quantified using mixing quality which is a measure of the uniformity of the concentration in a given geometry. Special attention is paid to the occurrence of the secondary vortices close to the mid point of the outer wall and its role in mixing. Simulations are also done to study the flow in U shaped channels. The simulation results show that the new design leads to an early introduction of secondary vortices than a simple U tube. Thus in the new design the secondary vortices are induced at a Re = 120 as opposed to the classical value of Re = 400 (when there is no side inlet) reported in the literature. Mixing is studied for different diffusivities and combination of inlet velocities. We also compare the performance of our design with the classical T and Y mixers. The early induction implies that we can have good mixing at low Re. Consequently, when used as a micro-reactor we can combine good mixing with high residence times to obtain good conversions in our system.     

An efficient passive planar micromixer with ellipse-like micropillars for continuous mixing of human blood

In this paper, a passive planar micromixer with ellipse-like micropillars is proposed to operate in the laminar flow regime for high mixing efficiency. With a splitting and recombination (SAR) concept, the diffusion distance of the fluids in a micromixer with ellipse-like micropillars was decreased. Thus, space usage for micromixer of an automatic sample collection system is also minimized. Numerical simulation was conducted to evaluate the performance of proposed micromixer by solving the governing Navier–Stokes equation and convection–diffusion equation. With software (COMSOL 4.3) for computational fluid dynamics (CFD) we simulated the mixing of fluids in a micromixer with ellipse-like micropillars and basic T-type mixer in a laminar flow regime. The efficiency of the proposed micromixer is shown in numerical results and is verified by measurement results.     

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.     

Investigations into mixing of fluids in microchannels with lateral obstructions

This work presents a study of a passive micromixer with lateral obstructions along a microchannel. The mixing process is simulated by solving the continuity, momentum and diffusion equations. The mixing performance is quantified in terms of a parameter called ‘mixing efficiency’. A comparison of mixing efficiencies with and without obstructions clearly indicates the benefit of having obstructions along the microchannel. The numerical model was validated by comparing simulation results with experimental results for a micromixer. An extensive parametric study was carried out to investigate the influence of the geometrical and operational parameters in terms of the mixing efficiency and pressure drop, which are two important criteria for the design of micromixers. A very interesting observation reveals that there exists a critical Reynolds number (Re _cr  ~ 100) below which the mixing process is diffusion dominated and thus the mixing efficiency is reduced with increase in Re and above which the mixing process is advection dominated and mixing efficiency increases with increase in Re. Microchannels with symmetric and staggered protrusion arrangements were studied and compared. The mixing performance of the staggered arrangement was comparable with that of symmetric arrangement but the pressure drop was lower in the case of staggered arrangements making it more suitable.     

Flow feature analysis of an eye shaped split and collision (ES-SAC) element based micromixer for lab-on-a-chip application

An eye shaped split and collision micromixer having low-pressure drop is proposed, which works on the concept of unbalanced splits and cross-collisions of fluid streams. The 3-D Navier–Stokes equations in combination with an advection–diffusion model were solved for the analysis with water and ethanol as working fluids. The in-depth analysis of the flow features and the mixing performance parameters viz. mixing index and pressure drop of the micromixer has been carried out. The micromixer model is composed of two sub-channels of equal/unequal widths which repeatedly undergo splitting and collision of fluid streams along the flow direction. The numerical study has been carried out on the micromixer at Reynolds numbers ranging from 0.1 to 45. The difference between the mass flow rates in the two sub-channels creates an unbalanced collision of the two fluid streams. Mixing enhancement is mainly due to the effect of unbalanced collisions of the fluid streams. The micromixers show exciting flow features for different ratios of the widths of the sub-channels. The ratios of a width of subchannels viz. 1, 1.4 and 2 are considered. The highest mixing performance has been observed for the width ratio of 2, whereas poor mixing performance has been observed for the width ratio of 1.

     

Transverse momentum micromixer optimization with evolution strategies

We conduct a numerical study of mixing in a transverse momentum micromixer. Good values for actuation frequencies can be determined using simple kinematic arguments, and evolution strategies are introduced for the optimization of mixing by adjusting the control parameters in micromixer devices. It is shown that the chosen optimization algorithm can identify, in an automated fashion, effective actuation parameters. We find that optimal frequencies for increasing number of transverse channels are superposable despite the non-linear nature of the mixing process.     

Quantitative analysis of microfluidic mixing using microscale schlieren technique

In this study, we discuss the employment of microscale schlieren technique to facilitate measurement of inhomogeneities in a micromixer. By mixing dilute aqueous ethanol and water in a T-microchannel, calibration procedures are carried out to obtain the relation between the concentration gradients and grayscale readouts under various incident illuminations, concentrations of aqueous ethanol solution, and knife-edge cutoffs. We find that to broaden measuring range with minimal error, the luminous exitance should be tuned to have a reference background with an average grayscale readout of 121, and dilute aqueous ethanol solution with a mass fraction of 0.05 should be used along a 50 % cutoff. For concentration gradients greater than 6.8 × 10^?3 or below ?2.5 × 10^?2 μm^?1, the calibration curves show great linearity. Correspondingly, the discernable limit of our microscale schlieren system is 2.3 × 10^?5 μm^?1 for a positive refractive index gradient and ?8.6 × 10^?5 μm^?1 for a negative refractive index gradient. Once the relation between concentration gradients and grayscale readouts is known, the concentration distribution in a microfluidic can be reconstructed by integrating its microscale schlieren image with appropriate boundary conditions. The results prove that the microscale schlieren technique is able to provide spatially resolved, noninvasive, full-field measurements. Since the microscale schlieren technique is directly linked to the measurement of a refractive index gradient, the present method can be easily extended to other scalar quantifications that are related to the variation of refractive index.     

Numerical simulation on fluid mixing by effects of geometry in staggered oriented ridges micromixers

Owing to the enhancement of surface effects at the micro-scale, patterned grooves on a microchannel remain a powerful method to induce chaotic advection within a pressure driven system. Since the staggered oriented ridges static micromixers are presented, there are few results in the literature about the geometric effects of such micromixer on the fluid mixing. This paper presents simulations within the micromixer and identifies geometric factors that affect the generation of advection flow over staggered oriented ridges. By varying the inflow directions, the ridge height ratio and the ridge asymmetry index, the modes of fluid motion and the pressure drops are studied respectively. Furthermore, through a set of numerical simulations, the relation expression between a mixing index to evaluate the mixing performance and the mentioned geometric parameters is obtained and the value of this mixing index could be calculated continuously. It indicates that the mixing performance of every staggered oriented ridges static micromixer could be estimated.     

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.     

The mapping method as a toolbox to analyze,design, and optimize micromixers

The mapping method is employed as an efficient toolbox to analyze, design, and optimize micromixers. A new and simplified formulation of this technique is introduced here and applied to three micromixers: the staggered herringbone micromixer (SHM), the barrier-embedded micromixer (BEM), and the three-dimensional serpentine channel (3D-SC). The mapping method computes a distribution matrix that maps the color concentration distribution from inlet to outlet of a micromixer to characterize mixing in a quantitative way. Once the necessary distribution matrices are obtained, computations are fast and numerous layouts of the mixer are easily evaluated, resulting in an optimal design. This approach is demonstrated using the SHM and the BEM as typical examples. Mixing analysis in the 3D-SC illustrates that also complex flows, for example in the presence of back-flows, can be efficiently dealt with by using the new formulation of the mapping method.     

Novel prototyping method for microfluidic devices based on thermoplastic polyurethane microcapillary film

Thermoplastic polyurethane microcapillary film (TPU-MCF), as a novel extruded product, inherently contains an array of circular micron-sized capillaries embedded inside the polymer matrix. With the aid of simple laser cutting and conventional sealing technologies, a rapid prototyping method for microfluidic devices is proposed based on the ready-made microstructure of MCFs. Two functionalized microfluidic devices: serpentine micromixer and multi-droplet generator, are rapidly fabricated to demonstrate the advantages and potential of employing this new method. The whole proof-of-concept fabrication process can be completed in 8–10 min in a simple way; each procedure is repeatable with stable performance control of microfluidic devices; and the material cost can be as low as $0.01 for each device. The TPU-MCF and this novel method are expected to provide a new perspective and alternative in microfluidic community with particular requirements.     

Improved serpentine laminating micromixer with enhanced local advection

It is a complicated task to achieve high level of mixing inside a microchannel because the flow is characterized by low Reynolds number (Re). Recently, the serpentine laminating micromixer (SLM) was reported to achieve efficient chaotic mixing by introducing “F”-shape mixing units successively in two layers such that two mixing mechanisms, namely splitting/recombination and chaotic advection, enhance the mixing performance in combination. The present paper proposes an improved serpentine laminating micromixer (ISLM) with a novel redesign of the “F”-shape mixing unit: reduced cross-sectional area at the recombination region locally enhances advection effect which helps better vertical lamination, resulting in improved mixing performance. Flow characteristics and mixing performances of SLM and ISLM are investigated numerically and verified experimentally. Numerical analysis system is developed based on a finite element method and a colored particle tracking method, while mixing entropy is adopted as a mixing measure. Numerical analysis result confirms enhanced vertical lamination performance and consequently improved mixing performance of ISLM. SLM and ISLM were fabricated by polydimethylsiloxane (PDMS) casting against SU-8 patterned masters. Mixing performance is observed by normalized purple color intensity change of phenolphthalein along the downchannel. Flow characteristics of SLM and ISLM are investigated by tracing the purple interface of two streams via optical micrograph. The normalized mixing intensity behavior confirms improved mixing performance of ISLM, which is consistent with numerical analysis result.