Passive Micromixer Research Articles

New insights into fluid mixing in micromixers with fractal wall structure

Abstract Microfluidics is thought to have a high development potential and a wide range of applications in biomedical research. The design of micromixers has gotten a lot of attention because they are such a crucial aspect of microfluidic devices. The passive micromixer has the advantages of simple construction and steady performance over the active micromixer. In this paper, a fractal wall micromixer is proposed, and the flow characteristics and mixing process of the secondary fractal double wall micromixer are studied using intuitive flow patterns and quantitative calculation methods. The results show that the mixing efficiency of secondary fractal wall is higher than that of primary fractal wall, and with the increase of h, the mixing efficiency and pressure drop begin to decrease gradually. When there is a secondary fractal wall structure on both sides, when Reynolds number (Re) = 0.1, the mixing efficiency of the outlet can reach 95%, and when Re = 100, the mixing efficiency of the outlet can reach 99%, almost complete mixing. The fractal wall micromixer has good mixing effect and shows great application potential in chemical engineering and biological engineering.

INSIGHT INTO AN EFFICIENT MICROMIXER WITH STAGGER ABREAST BAFFLES

Passive micromixers utilize no external energy input. It mainly depends on the structure change of microchannel to produce chaotic convection effect, which increases the contact surface and contact time between the fluid. The mixing efficiency of stagger non-abreast baffles (SNB) micromixer and stagger abreast baffles (SAB) micromixer is compared at five kinds of Reynolds (Res). It may affect the mixing efficiency including the height ([Formula: see text]) and number ([Formula: see text]) of baffles, the distance between two adjacent baffles, non-abreast baffles, and abreast baffles. As the [Formula: see text] of the baffles changes, it contributes to enhance the chaotic convection of the micromixer. When Re [Formula: see text], the worst mixing efficiency of the most efficient seven SABs ([Formula: see text], [Formula: see text] mm, [Formula: see text] mm, [Formula: see text]) micromixer reaches an astonishing 98%. The concentration change along the microchannel is studied in this paper. At low Re, SAB makes fluid contact zigzag in microchannel and enhances molecular diffusion. At high Re, SAB causes chaotic convection in the microchannel and completes the mixing quickly.

Mixing Enhancement of a Passive Micromixer with Submerged Structures.

A passive micromixer combined with two different mixing units was designed by submerging planar structures, and its mixing performance was simulated over a wider range of the Reynolds numbers from 0.1 to 80. The two submerged structures are a Norman window and rectangular baffles. The mixing performance was evaluated in terms of the degree of mixing (DOM) at the outlet and the required pressure load between inlet and outlet. The amount of submergence was varied from 30 μm to 70 μm, corresponding to 25% to 58% of the micromixer depth. The enhancement of mixing performance is noticeable over a wide range of the Reynolds numbers. When the Reynolds number is 10, the DOM is improved by 182% from that of no submergence case, and the required pressure load is reduced by 44%. The amount of submergence is shown to be optimized in terms of the DOM, and the optimum value is about 40 μm. This corresponds to a third of the micromixer depth. The effects of the submerged structure are most significant in the mixing regime of convection dominance from Re = 5 to 80. In a circular passage along the Norman window, one of the two Dean vortices burst into the submerged space, promoting mixing in the cross-flow direction. The submerged baffles in the semi-circular mixing units generate a vortex behind the baffles that contributes to the mixing enhancement as well as reducing the required pressure load.

An open-source topology optimization modeling framework for the design of passive micromixer structure

In the microfluidic systems, the structure of micromixer is one of the important influence factors for the mixing performance. The traditional trial-and-error method for the optimization design of micromixer structure is difficult to consider both the mixing performance and power dissipation. Here we show the topology optimization method of designing the passive micromixer with the minimum power dissipation. The micromixer structure is updated with the adjoint method and adjoint variables are solved by the continuous adjoint equation derived manually. The forward fluid problem and adjoint equation are constructed using OpenFOAM. The optimal structure of micromixer is generated at different mixing index constraint and Re in two-dimensional design problem. Furthermore, in the three-dimensional optimal design, the multiple spiral slice structures are produced along the micromixer, which generate chaotic advection. This open-source topology optimization modeling framework is effective to design optimal structure of micromixer.

Evaluation of Hydrodynamic and Thermal Behaviour of Non-Newtonian-Nanofluid Mixing in a Chaotic Micromixer.

Three-dimensional numerical investigations of a novel passive micromixer were carried out to analyze the hydrodynamic and thermal behaviors of Nano-Non-Newtonian fluids. Mass and heat transfer characteristics of two heated fluids have been investigated to understand the quantitative and qualitative fluid faction distributions with temperature homogenization. The effect of fluid behavior and different Al2O3 nanoparticles concentrations on the pressure drop and thermal mixing performances were studied for different Reynolds number (from 0.1 to 25). The performance improvement simulation was conducted in intervals of various Nanoparticles concentrations (φ = 0 to 5%) with Power-law index (n) using CFD. The proposed micromixer displayed a mixing energy cost of 50–60 comparable to that achieved for a recent micromixer (2021y) in terms of fluid homogenization. The analysis exhibited that for high nanofluid concentrations, having a strong chaotic flow enhances significantly the hydrodynamic and thermal performances for all Reynolds numbers. The visualization of vortex core region of mass fraction and path lines presents that the proposed design exhibits a rapid thermal mixing rate that tends to 0.99%, and a mass fraction mixing rate of more than 0.93% with very low pressure losses, thus the proposed micromixer can be utilized to enhance homogenization in different Nano-Non-Newtonian mechanism with minimum energy.

Enhanced Fluid Mixing Using a Reversed Multistage Tesla Micromixer

AbstractA two‐dimensonal Tesla micromixer is experimentally characterized at varying Reynolds numbers (Re) and valve stages with the aim to acquire sufficiently high mixing performance. To ease fabrication, a simplified Tesla valve design is adopted. Results show two distinctive regimes of low and highRe. In the low‐Reregime, a steady incremental mixing was observed as the fluid passes by each valve, whereas an enhanced mixing was identified right in the first valve in the high‐Reregime. This is predominantly due to the amplified opposing flow from the helix branch which promotes stronger chaotic advection in the main microchannel. Interestingly, the measured mixing performance was found comparable to that of three‐dimensional passive micromixers reported in the literature.

Hydrodynamic and Kinematic Study to Analyze the Mixing Efficiency of Short Passive Micromixers

Hydrodynamic and Kinematic Study to Analyze the Mixing Efficiency of Short Passive Micromixers

Enhanced mixing in dual-mode cylindrical magneto-hydrodynamic (MHD) micromixer

The proposed cylindrical magneto-hydrodynamic (MHD) micromixer developed in this study was a geometrically modified conventional T-micromixer combining the characteristics of passive and active micromixers. Characterization was achieved by observing the mixing efficiencies of NaCl solution (1% concentration) based on supplied electrical potential and inlet flow rates. The design mainly aimed to utilize the pumping capability of the magneto-hydrodynamic principle as a secondary element to give counter pumping energy towards inlet flows, such that perturbation of fluid increased mixing performance. NaCl solution was mixed by advection through the stretching and folding by micro-vortices generated in the mixing reservoir. The cylindrical MHD micromixer achieved its highest mixing index of 99.42% at Re = 40 with 3 V of direct-current potential (VDC) supplied to the electrodes. Mixing efficiency increased in a considerably similar and linear pattern for Re = 5, 10, 20, and 40 within the electrical potential range of 0.5 ≤ V ≤ 3.0. Control was gained by manipulating the external electrical potential source which only required a smaller capacity of direct-current voltage sources. Overall, the proposed cylindrical MHD micromixer, which emphasizes the use of inexpensive material and simple design, has been experimentally proven to be practical as compared to the passive and active micromixers found in the literature.

Numerical study of the passive micromixer with the novel blocks

The micromixer is a key component of the microfluidic chip analysis system. Micromixers are widely used in applications, such as DNA hybridization and protein synthesis. A high-efficiency mixer can speed up the biochemical analysis process. In order to study how to improve the mixing efficiency of the mixer, this paper designs passive micromixers with three different blocks: cylindrical, equilateral triangle, and square. The effects of them on the mixing performance and pressure drop of the mixer were studied, respectively. Through numerical simulation, the study shows that the mixing efficiency of the mixer with equilateral triangle blocks is 96% at Re = 100, and the maximum pressure drop is 18 135.8 Pa. In addition, through the analysis of three-dimensional numerical simulation, the block causes the fluid to generate a horizontal and vertical vortex flow state in the mixing unit, thereby breaking the laminar flow and greatly improving the mixing efficiency. Through structural optimization, ETOM4, which has four mixing units and a side length of 150 μm equilateral triangle blocks, is the best passive micromixer with its mixing efficiency of 99.1%.

Numerical and experimental investigation of mixing enhancement in the passive planar mixer with bent baffles

As an important pre-treatment part of the micro-total analysis system, the micromixer that ensures the uniform mixing of samples and reagents in a short distance is essential for the improvement of the detection accuracy. Here, a type of passive planar micromixer with narrow gaps and obstacles arranged in the mixing units is proposed. The mixer utilizes the micro-jets and bent baffles to split and recombine streams and promote vortex formation, which breaks the laminar flow when the Reynolds number (Re) varies from 0.1 to 50. The performance of the unoptimized mixer decreases first and then increases with the increase of Re. However, a poor mixing quality is found in the range of 0.5 ≤ Re ≤ 15. To surmount the above shortcomings, a comprehensive geometrical study is performed on this micromixer and the effects of various parameters such as the width and length of the gap, the shape of the baffle, and mixing unit on the mixing quality and pressure drop are analyzed. The results show that the reduction of gap width and the increase of the gap length result in the improvement of mixing efficiency. The staggered Z-shaped baffles accelerate the mixing process compared to traditional V-shaped baffles owing to asymmetric flow and vortex, which leads to excellent mixing. The results also show that the octagonal mixing unit increases mixing efficiency without basically changing pressure drop because the progressive and tapered structure enhances the convection. Lastly, a novel micromixer whose mixing performance is close to 0.8 at Re = 0.5 at the short distance of 2800 µm is proposed, fabricated and tested. This work provides a reference for the development of high-efficiency mixers in the pre-processing part of the micro-total analysis system.

A novel design of split and recombination multilayer micromixers with excellent hydraulic and mixing performance based on the baker's transformation

The objective of this study is to introduce passive micromixers that provide great mixing efficiency in a wide range of Reynolds numbers. In this regard, six novel micromixers are designed based on the splitting and recombination mechanism and investigated in the Reynolds number range of 0.5 to 100. Differences in proposed configurations are in the number and arrangement of the mixing units. These units are classified into two types of clockwise and non-clockwise based on the direction of flow deviation in upper and lower unit branches. It is found that the mixing index (MI) higher than 0.92 is achieved for the micromixer with seven alternate mixing units at all studied Reynolds numbers. MI is more than 0.98 at Reynolds numbers 0.5, 1, and 100. Moreover, pressure drop (ΔP) in all the proposed micromixers is very acceptable. The maximum increase of 31% in ΔP is reported for the micromixer with seven non-clockwise mixing units with respect to a simple T-mixer at Re=100. Also, the figure of merit (FoM) values over 1.63 illustrate the excellence of the proposed configurations. FoM increases with the Reynolds number and touches the amazing value of 32.5 in the micromixer with five alternate mixing units at Re=100.

Mathematical analysis of mixing index for 3D-helical micromixture with rectangular cross-section by providing convergent divergent profile

The sole purpose of this research is to simulate and examine the characteristics of mixing and pressure drop inside a 3D helical micromixer with a convergent-divergent profile and having rectangular cross-section. A passive micromixer only requires flow-driven energy, unlike an active micromixer which requires flows driven as well as mixing energy. Mixing inside a simple passive micromixer mainly depends on the mass diffusion process, which is time-consuming and requires an elongated microchannel length. So, it becomes requisite to design a micromixer that enhances mixing as well as reduces the prolonged length. For the proposed micromixer, before running the actual simulations, the computational method used in this study was tested against earlier work, and the results demonstrate good agreement with those found in the literature. Using the Continuity and Navier–Stokes equations with water as the working fluid, a thorough numerical examination of mixing performance and fluid flow patterns was carried out at the outlet and five different cross-sections inside the microchannel with a wide range of Reynolds numbers from 2 to 260. The mixing index of the C-D Tdhm is 10% higher at Re = 260, and the pressure drop is almost 40% lower when compared to simple Tdhm mainly because of secondary flow and formation of separation and dean vortices. According to the numerical analysis presented in this paper, the C-D Tdhm delivers superior results at the same mixing length compared to simple Tdhm for all Reynolds number values evaluated in the study. Therefore, C-D 3D helical micromixer can be used in a variety of biological, electrical, biomedical, and Microtech applications.

Numerical study of microscale passive mixing in a 3-Dimensional spiral mixer design

Mixing of fluids becomes difficult if the length of the mixing channel is very small with size in microns. This is due to the lack of inertia and pre-eminence of viscous forces at low Reynolds numbers. The present study proposes a passive 3-dimensional spiral micromixer (SMM) and its Computational Fluid dynamics (CFD) study to scrutinize the mixing performance of water over a broad range of Reynolds numbers (2 ≤ Re ≤ 320). The obtained results were compared with a simple straight T-micromixer (STM) keeping the axial length same for both micromixers, which is 3000 µm. SMM appears to be much better and shows promising results with complete mixing (97.5%) of fluids species achieved at Re = 320 and minimum efficiency of 74.3 % achieved at Re = 66. Contrarily, STM shows comparatively poor results with a maximum mixing efficiency of 32 % for the highest Reynolds number (Re = 320). Thus, it is concluded that the proposed SMM design is an effective and significant choice for various micromixing applications.

Numerical study of 3-D helical passive micromixer having both inlets at offset with blood as fluid

In the present study, two different micromixers have been evaluated for their mixing efficiencies with non-Newtonian working fluid blood. This helical micromixer has been chosen since it solves the problem of high mixing time during the mixing of fluids at the micro-level. This helical micromixer has both of its inlets at an offset to each other and is named TDHM-TO, i.e., 3D helical micromixer with inlet at offset. When this helical micromixer was compared with standard simple T-micromixer (STM), it is found that TDHM-TO requires much less mixing length than that of STM and still provides better mixing efficiency. Continuity equation, Navier-Stokes and species transport equations have been solved to investigate the mixing efficiency of the micromixers for non-Newtonian fluid blood. The study considers a wide range of mass flow rates (0.0004 kg/hr.–0.10 kg/hr.). It is being found that, for all values of mass flow rate, TDHM-TO performed much better than STM. E.g., for flow rate = 0.07 kg/hr., TDHM-TO provide a mixing efficiency of 74.41% compared to 4.93 of STM which is about 65.2% more than that of STM. STM showed a very poor result compared to TDHM-TO with only 7.64% of maximum mixing efficiency obtained with the flow rate = 0.1 kg/hr. compared to 72.84% of TDHM-TO. This TDHM-TO micromixer can be, thus, utilized in many chemicals, biochemical, and biomedical industries because of its higher efficiency and lower mixing length.

Efficient Mixing of Microfluidic Chip with a Three-Dimensional Spiral Structure.

In this paper, a helical three-dimensional (3D) passive micromixer is presented. A three-dimensional spiral passive micromixer is fabricated through the 3D printing technology and the polymer dissolution technology. The main process is as follows: First of all, a high-impact polystyrene (HIPS) material was used to make a 3D spiral channel mold. Second, the channel mold was dissolved in limonene solvent. The mixing experiment shows that the single helix structure can improve the mixing efficiency to 0.85, compared with the mixing efficiency of 0.78 in the traditional T-shaped two-dimensional (2D)-plane channel. Different screw diameters, screw number structures, and flow rates are used to test the mixing effect. The optimal helical structure is 5 mm, and the flow rate is 2.0 mL/min. Finally, the mixing efficiency of the 3D helical micromixer can reach 0.948. The results show that the three-dimensional helical structure can effectively improve the mixing efficiency.

Pressure-Free Assembling of Poly(methyl methacrylate) Microdevices via Microwave-Assisted Solvent Bonding and Its Biomedical Applications.

Poly(methyl methacrylate) (PMMA) has become an appealing material for manufacturing microfluidic chips, particularly for biomedical applications, because of its transparency and biocompatibility, making the development of an appropriate bonding strategy critical. In our research, we used acetic acid as a solvent to create a pressure-free assembly of PMMA microdevices. The acetic acid applied between the PMMA slabs was activated by microwave using a household microwave oven to tightly merge the substrates without external pressure such as clamps. The bonding performance was tested and a superior bond strength of 14.95 ± 0.77 MPa was achieved when 70% acetic acid was used. Over a long period, the assembled PMMA device with microchannels did not show any leakage. PMMA microdevices were also built as a serpentine 2D passive micromixer and cell culture platform to demonstrate their applicability. The results demonstrated that the bonding scheme allows for the easy assembly of PMMAs with a low risk of clogging and is highly biocompatible. This method provides for a simple but robust assembly of PMMA microdevices in a short time without requiring expensive instruments.

Enabling Technologies for Obtaining Desired Stiffness Gradients in GelMA Hydrogels Constructs

AbstractThis work presents enabling technologies for the optimization of the manufacturing of GelMA‐based hydrogels constructs with desired stiffness gradients. The manufacturing technique combines dynamic mixing for gradient generation and a passive micromixer for efficient hydrogel blending. A digital replica of the fabrication process is developed, integrating theoretical and computational models, as well as experimental data, in order to predict and control the stiffness profile obtained within the constructs. The workflow for the development of the in silico framework, based on rigorous verification, validation, and uncertainty quantification steps, is presented. The validation of the digital replica is based on reference settings of process variables, which result in constructs with an exponential stiffness profile. The developed in silico model has been employed for optimizing process variables in order to obtain a linear stiffness profile in the extruded construct without the need of expensive and time‐consuming trial‐and‐error procedures. The developed digital replica is now a powerful tool for the creation of hydrogel gradient constructs for tissue engineering applications or for the screening of optimal 3D cell culture conditions.

Mixing Performance of a Passive Micro-Mixer with Mixing Units Stacked in Cross Flow Direction.

A new passive micro-mixer with mixing units stacked in the cross flow direction was proposed, and its performance was evaluated numerically. The present micro-mixer consisted of eight mixing units. Each mixing unit had four baffles, and they were arranged alternatively in the cross flow and transverse direction. The mixing units were stacked in four different ways: one step, two step, four step, and eight step stacking. A numerical study was carried out for the Reynolds numbers from 0.5 to 50. The corresponding volume flow rate ranged from 6.33 μL/min to 633 μL/min. The mixing performance was analyzed in terms of the degree of mixing (DOM) and relative mixing energy cost (MEC). The numerical results showed a noticeable enhancement of the mixing performance compared with other micromixers. The mixing enhancement was achieved by two flow characteristics: baffle wall impingement by a stream of high concentration and swirl motion within the mixing unit. The baffle wall impingement by a stream of high concentration was observed throughout all Reynolds numbers. The swirl motion inside the mixing unit was observed in the cross flow direction, and became significant as the Reynolds number increased to larger than about five. The eight step stacking showed the best performance for Reynolds numbers larger than about two, while the two step stacking was better for Reynolds numbers less than about two.

Analysis of different shapes of cross-sections and obstacles in variable-radius spiral micromixers on mixing efficiency

Spiral micromixers are one of the most widely used passive micromixers, but their mixing efficiency in the medium Reynolds number range (0.5∼50) is not ideal. To enhance the mixing efficiency of variable-radius spiral micromixers, VSR-C,VSR-T,VSR-H micromixers were designed by inserting cylindrical,triangular and hexagonal obstacles into the microchannel, and VSE, VST micromixers were designed by turning the cross section of the microchannel into triangle and ellipse. All micromixers were fabricated by LCD 3D printing. Simulation analysis and mixing experiments indicate that variable-radius spiral micromixers with obstacles can only increase mixing efficiency at high Reynolds number, while micromixers with triangular or elliptical cross sections can improve the mixing efficiency not only at high Reynolds number (Re > 5), but also at low and medium Reynolds number (Re < 3). The results suggest more Dean vortexes could be generated in spiral micromixer with an elliptical cross section (VSE) at the same Dean number,and the effective action area of Dean flow is enlarged simultaneously. As compared to VSR micromixer, the mixing efficiency of VSE exhibited 28% improvement on average, which implys the optimization of cross-section shape provides a potential way to improve the performance of spiral micromixers.

Numerical analysis of vortex T micromixer with diffuser plates and obstacles

The design of passive micromixers is a trade-off among the mixing efficiency, pressure drop, length/time scale and the complexity (or cost) of the mixing channel. The advent of chaotic advection in micromixers has encouraged researchers to explore the untapped potential of disturbance in the flow. Over the past years, the performance of T micromixers has increased significantly, however, the mixing efficiency is still far behind the conventional threshold. In order to enhance the mixing further, non-aligned inlets with rigid diffuser plates (or vortex generators) and cylindrical obstacles are proposed with the present study. The vortex formed at the T-junction is intensified at the leading edge of the diffuser plate and each diffuser plate creates two vortices in opposite directions. These vortices are energized across each pair of the diffuser plate and gets propelled along the mixing chamber. The common flow-up approach with the staggered arrangement is suggested for enhancing the mixing efficiency. For the Reynolds number around 30; mixing efficiency as high as 85% is achieved within the pressure loss of 2300 Pa. The proposed design is simple and similar approach can be used for desired mixing efficiency at specific Reynolds number. Configurations such as in-line & staggered arrangement, flow-up & flow-down arrangement, presence of obstacles etc. are compared and their implementation is proposed accordingly.