Micromixing Efficiency Research Articles

Numerical simulation of a three dimensional electroosmotic micromixer with a flexible and controllable Rubik's cube module

Based on the incompressible Navier Stokes equation (N-S) and the coupling principle of laminar flow and electric field, a new 3D electroosmotic micromixer with a variable module is designed in this paper. A microchannel is designed throughout the variable module to improve the mixing efficiency. The mixing efficiency of the electroosmotic micromixer in different directions, different positions, different alternating voltages and different times was studied. It can be seen from the simulation results that under the influence of electric field, the number, size and position of vortexes formed by the fluid in the microchannel are different. When the Reynolds number (Re) < 10, the electroosmotic micromixer shows a good mixing effect. When Re > 10, most of the micromixers with several torsion modes show a rapid downward trend. Different from the other variable methods, when the first variable module is rotated in the YZ plane, the mixing efficiency decreases slowly and changes steadily, and the maximum mixing efficiency can reach 98.89%. The new electroosmosis micromixer has a smaller pressure drop and provides high flexibility for rapid mixing and a very stable high mixing efficiency.

Sucrose solution as a new viscous test fluid with tunable viscosities up to 2 Pas for micromixing characterization by the Villermaux\u2013Dushman reaction

For the first time, micromixing characterization for the Villermaux–Dushman reaction could be performed with a non-reactive viscous medium at viscosities up to 2 Pas. As viscous medium, sucrose solution was used with the benefit of being a Newtonian fluid with tuneable viscosity. Due to the higher viscosities in comparison to established media for micromixing investigations, a new protocol for the experimental implementation was developed. Micromixing experiments were conducted and the applicability of viscous sucrose solutions was proven in a stirred tank reactor. Major challenges in characterizing micromixing efficiency in high viscous solution were consolidated.

Intensification of micromixing efficiency in a spinning disk reactor: Experimental investigation

Spinning disk reactor (SDR) exhibits excellent performance for liquid-liquid micromixing. For traditional SDR, there is still much room to intensify micromixing efficiency through structure design of disk surface and pre-mixer type. In this work, SDR consisted of a spinning disk and a pre-mixer. Three kinds of baffles on the disk were designed for enhancing disturbance of liquid film. Besides, two types of pre-mixers were adopted in the SDR for improving micromixing environment. The iodide-iodate reaction was employed as working system. Compared with other kinds of baffles, the radial baffle exhibited better micromixing efficiency. The incorporation model was adopted to determine the corresponding micromixing time of SDR with radial baffle which reduced about 60% than that of SDR with smooth disk. Pre-mixer could also enhance the micromixing efficiency of SDR, especially at relatively low rotational speed. Considering different rotational speed requirements, suitable intensification mode was proposed for improving micromixing performance of SDR.

Micromixing Efficiency in the Presence of an Inert Gas in a Rotor–Stator Spinning Disk Reactor

The effects on micromixing of co-feeding an inert gas with the bulk liquid are studied for a rotor–stator spinning disk reactor by means of the Villermaux–Dushman test reaction. The results of varying rotational speeds and gas–liquid ratios are reported for three configurations: injecting the acid into the dispersed region, into the bottom thin-film region, and into the upper thin-film region. The results show that injecting in the dispersed region is the optimal configuration, as the liquid experiences the shear stress between the rotor and the stator, and with increasing rotational speed, micromixing efficiency subsequently increases. Injecting in the thin-film regions leads to poor micromixing efficiency, since the gas layer reduces the turbulence levels experienced by the liquid. For the bottom thin film, increasing rotational speed improves micromixing efficiency due to inertial rotation of the liquid film. However, in the upper thin film, micromixing is significantly worse, being affected by the dripping of liquid on the stator at the injection point. Gas holdup measurements are compared to previous studies, showing that most of the gas volume is present in the thin-film regions. These results are relevant for the design of modular chemical processes using the spinning disk technology, when fast competitive chemical reactions occur in the presence of gas and liquid.

Intensified micro-mixing effects on evolution of oxygen vacancies of CeO2-based catalysts for improved CO oxidation

The oxygen vacancies in CeO2-based catalysts play a vital role in the oxidation reactions due to the repeatable Ce4+/Ce3+ redox cycles. However, the influence of micro-mixing efficiency during co-precipitation reaction on the oxygen vacancy concentration in CeO2-based catalysts was scarce. In this work, a rotating packed bed (RPB) reactor was employed to intensify the micro-mixing of reactants before crystal nucleation, aiming to increase the oxygen vacancy concentration of CeO2-based catalysts. The MOx-CeO2 (M = Cu, Co, Fe) catalysts prepared in the RPB reactor featured more M-O-Ce species and 1.3 – 1.4 times concentration of oxygen vacancy compared to those prepared in a stirred tank reactor (STR). The RPB-CuO-CeO2 catalyst showed an elevated reaction rate of CO (6.70 × 10-4 mol gcat-1 s-1) at 90 °C and remained unchanged after 8 cycles in CO oxidation. This work demonstrates a feasible “nonchemical” strategy to prepare the high-activity solid solution catalysts with increased oxygen vacancies.

Effect of Thermal Energy and Ultrasonication on Mixing Efficiency in Passive Micromixers

Micromixing is a key process in microfluidics technology. However, rapid and efficient fluid mixing is difficult to achieve inside the microchannels due to unfavourable laminar flow. Active micromixers employing ultrasound and thermal energy are effective in enhancing the micromixing process; however, integration of these energy sources within the devices is a non-trivial task. In this study, ultrasound and thermal energy have been extraneously applied at the upstream of the micromixer to significantly reduce fabrication complexity. A novel Dean micromixer was laser-fabricated to passively increase mixing performance and compared with T- and Y-micromixers at Reynolds numbers between 5 to 100. The micromixers had a relatively higher mixing index at lower Reynolds number, attributed to higher residence time. Dean micromixer exhibits higher mixing performance (about 27% better) than T- and Y-micromixers for 40 ≤ Re ≤ 100. Influence of ultrasound and heat on mixing is more significant at 5 ≤ Re ≤ 20 due to the prolonged mechanical effects. It can be observed that mixing index increases by about 6% to 10% once the temperature of the sonicated fluids increases from 30 °C to 60 °C. The proposed method is potentially useful as direct contact of the inductive energy sources may cause unwanted substrate damage and structural deformation especially for applications in biological analysis and chemical synthesis.

Superhydrophobic coating modified nozzles for energy-saving rapid micro-mixing

Applying superwetting materials to tailor liquid dynamics has demonstrated advanced applications in directed liquid transportation, droplet manipulation etc., and holds promise to be extended to industrialized processes (e. g. micro-mixing). Until now, most research focus of extreme wettability has been on liquid droplets; tailoring the flow patterns via superwetting materials still lacks understanding and demonstrations. As a widely concerned issue in chemical engineering, the conversion of flow patterns is ubiquitous in most processes and reactors. Herein, we have induced a superhydrophobic coating to the inlet nozzles of an industrialized chemical reactor, Higee (high gravity machine), and demonstrated an energy-saving strategy to realize rapid micro-mixing. Compared with common hydrophilic metal nozzles, the superhydrophobic nozzles show advantages in (1) lowering the minimum flow flux requisite to form stream flows by 20% owing to low liquid retaining on the nozzles; (2) improving the micro-mixing efficiency by one magnitude under the same flow flux; (3) having reduced the energy cost by 36% and 46% in the mixing and mass transfer units, respectively. Moreover, applying superhydrophobic nozzles in the production of calcium carbonate particles contributes to a narrow size distribution of particles. We envision that the strategy of inducing superwetting materials could provide new energy-saving solutions to diverse chemical engineering processes with demands in improving the micro-mixing efficiency, and facile methods to tailor flow patterns in research fields such as bio-analysis, micro-fluidic channels, micro-reactors etc. Meanwhile, the application of superwetting materials could be extended, especially in industrialized equipment.

The effect of varying radius of curvature on mixing in elliptical spiral microchannels

Effective mixing is essential for a wide range of microfluidics applications. Passive micromixers have attracted much attention during recent years due to their low-cost and simple fabrication procedures with less power input in their implementation to high-throughput microfluidics platforms. Increasing the efficiency of micromixers is possible with an optimum geometry of inertial microfluidic channels, which utilize Dean vortices and Dean flows for the enhancement of mixing. Recently, micromixers with curved microchannels have been introduced to the literature. Yet, the enormous potential of elliptical spiral microchannels has not been adequately revealed. This study aims to assess the mixing performance of five-loop spiral polydimethylsiloxane micromixers with spiral structures and elliptic structures, which have different initial aspect ratios with varying radii of curvature along the channel. The performances of these micromixers were evaluated by comparing the mixing indices and intensity profiles obtained from inverted fluorescence microscopy images over a broad range of Reynolds numbers between 10 and 100. Elliptical spiral microchannels were able to provide mixing indices up to 98% at a Reynolds number of 20. Overall, the utilization of ellipse-shaped microchannels appear to provide a passive control over the mixing performance at several locations through the microchannel.

Micromixing in a gas–liquid vortex reactor

AbstractThe gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas‐only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10−4 ~ 10−3 s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.

Numerical simulation of three-dimensional passive micromixer based on the principle of Koch fractal

Abstract In this paper, We arrange the obstacles based on the Koch fractal principle (OKF) in the micromixer. By changing the fluid flow and folding the fluid, a better mixing performance is achieved. We improve the mixing efficiency by placing OKF and changing the position of OKF, then we studied the influence of the number of OKF and the height of the micromixer on the mixing performance. The results show that when eight OKF are staggered in the microchannel and the height is 0.2 mm, the mixing efficiency of the OKF micromixer can reach 97.1%. Finally, we compared the velocity cross section and velocity streamline of the fluid, and analyzed the influence of OKF on the concentration trend. Through analysis, it is concluded that OKF can generate chaotic convection in the fluid, and enhance the mixing of fluids by generating vortices and folding the fluid. It can effectively improve the mixing efficiency of the micromixer.

Micromixer structure design based on Koch fractal principle and baffle distribution on the lower surface

In this paper, the effect of the baffle on the mixing efficiency is studied when the baffle is distributed on the lower surface of the microchannel. The mixing efficiency of the micromixer was analyzed by numerical simulation. To study the change of mixing efficiency of micromixer, we changed the fractal number of baffles, height of baffles, spacing of baffles, angle of baffles, the number of baffles. Through analysis and comparison, we finally chose a micromixer with fractal number of 2, baffle height of 0.2 mm, baffle spacing of 0.25 mm, baffle angle of 45° and baffle number of 15.

Contributions to the kinetics of the iodide–iodate test reaction for micromixing time calculation with extended incorporation models

In order to characterize the efficiency of micromixing in chemical reactors, Villermaux used a borate buffer in the reaction system named after him. For micromixing time calculations, kinetic data of the Dushman reaction are essential. The choice of the acid and the influence of the salt on the reaction rate constant has led to a debate on the kinetics of the Dushman reaction. In the present work, a comprehensive kinetic study was performed to obtain the reaction rate equation of the Dushman reaction. A fifth-order rate equation with the partial reaction rate orders: H+2;IO3-1;I-2 could be confirmed.Furthermore, the influence of ionic strength μ was investigated using sodium perchlorate as the ion source. The reaction rate constant k for the Dushman reaction was determined as a function of the ionic strength k=k0·fDushmanlogfDushman=-1.93(±0.06)·μ1+μ+0.40±0.02·μ.The impact of the newly obtained reaction rate constant on micromixing time calculations was investigated using the incorporation model. It could be shown, that micromixing times can differ by a power of ten depending on the chosen reaction rate constant. Moreover, an extension of the incorporation model for sulphuric acid is provided, which includes the acid dissociation constants. This model unites the obtained reaction rate constant from this work and the equilibrium reactions of sulphuric acid. Micromixing experiments can now be performed independent of the acid choice, since similar micromixing times are calculated.According to the CLP Regulation and REACH, boric acid is classified as toxic for reproduction. Therefore, non-toxic chemicals have recently been preferred which has led to the implementation of a phosphate buffer by Pinot et al. and Baqueiro et al. This has risen questions about the definition of the segregation index and micromixing time calculations. Comprehensive micromixing time calculations have been performed with an extended incorporation model for a phosphate buffer. The results suggest that both HPO42- and H2PO4- species only take partially part in the acid–base reaction.

Effect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow reactor with variable configurations of inner cylinder

AbstractEffect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow (TC) reactor with variable configurations of inner cylinder has been investigated by adoption of a parallel competing iodide‐iodate reaction system. Two types of inner cylinder, circular inner cylinder and lobed inner cylinder (CTC and LTC), were used to generate hydrodynamic heterogeneity, focusing on the effects of the Reynolds number, the acid concentration, and the feeding time on the micromixing performance. Segregation index (Xs) was employed to evaluate the micromixing efficiency. It is revealed that Xs decreases with the increase of Reynolds number and feeding time but increases with the increase of acid concentration for both the CTC and LTC. However, the LTC does present a better micromixing performance at various operating conditions than that of the CTC as affirmed by both the experimental and computational fluid dynamics simulation results.

Mixing performance and continuous production of nanomaterials in an advanced-flow reactor

The Corning Advanced-FlowTM Reactor (AFR) whose productivity is ready for scale-up with limited loss of transport properties was implemented in the continuous synthesis of Ag nanoparticles (NPs). To demonstrate the effect of mixing on Ag NPs, the flow pattern and mixing characteristic (mainly on the meso- and macroscale) in AFR were studied by flow visualisation and computational fluid dynamics simulation first. It was found that increasing the total flow rate from 1 to 9 mL·min−1 could enhance mixing by the synergetic combination of secondary flows created by different mechanisms. The flow rate ratio in the range of 0.25–8 affected the degree of mixing by changing the initial concentration distribution profile across the combined stream. The highest degree of mixing was obtained at the flow rate ratios of 0.25 and 8. Subsequently, the micromixing performance was quantified by Villermaux-Dushman method. The micromixing time decreased from 17 to 4 ms as the total flow rate increased from 1 to 9 mL·min−1, while the micromixing time was nearly independent of the flow rate ratio. At last, the link between the mixing characteristics and average size and particle size distribution (PSD) of Ag NPs was established. As the total flow rate increased, the average particle size decreased, and PSD became narrower due to better micromixing efficiency. When the flow rate ratio varied, the PSD of Ag NPs was dependent on the width of the Ag precursor stream rather than the degree of mixing, because an excessive amount of NaBH4 was used.

Exploring the Design Efficiency of Random Microfluidic Mixers

With the development of microfluidic technologies, the increasing application of microfluidic devices in various research fields has brought up the urgent need for device performance simulation before fabrication to increase the design efficiency and reduce unnecessary trials and errors. As a fundamental function unit, the mixing performance is critical for a successful microfluidic device. Here in this study, we present a method of increasing the design efficiency of random micromixers. We have first generated the random libraries of micromixers constrained to different rectilinear grid configurations from $3\times3$ to $8\times8$ using MATLAB, and finite element analysis was used to simulate the performance of the micromixers. Then the most appropriate grid configuration was investigated on the aspects of fluid velocity and concentration distribution, simulation cost, and micromixer chip size. Our study results showed that the $5\times5$ grid configuration of random micromixer is an optimal balance to achieve the most abundant fluid velocity and solute concentration, while has acceptable simulation cost and reduced chip size. In summary, the $5\times5$ grid configuration is appropriate to provide enough random micromixer designs to meet user desire, not only time-effective but also cost-effective.

Residence Time Distribution and Micromixing Efficiency of a Dynamic Inline Rotor–Stator Mixer

Residence Time Distribution and Micromixing Efficiency of a Dynamic Inline Rotor–Stator Mixer

Twisted architecture for enhancement of passive micromixing in a wide range of Reynolds numbers

Micromixers present essential roles in providing homogeneous mixtures in microfluidic systems. It is of critical importance to introduce strategies to increase the mixing efficiency of passive micromixers, capable of operating at high efficiency levels over a wide range of Reynolds (Re) numbers. To this end, a novel design of twisted microstructure for enhancing mixing performance in a wide range of Reynolds numbers was introduced. Incorporating this microstructure with straight and serpentine micromixers was numerically and experimentally investigated. Micromixers with twisted microstructures were fabricated in Poly(methyl methacrylate) (PMMA) using high-precision micromilling. The effects of Reynolds number, pitch number, and channel hydraulic diameter on mixing efficiency and pressure drop were analyzed. Results revealed that the twist architecture could increase mixing efficiency significantly with very low pressure drop of up to 0.89 kPa. The twisted serpentine micromixer could narrow the disparity of mixing efficiency from 87% (Re = 10) to 98% (Re = 400). High mixing efficiency could be achieved within a length of 4.8 mm in the twisted serpentine micromixer with a hydraulic diameter of 300 μm. Taken together, the twisted structure could be incorporated with various geometries to create compact and high efficiency micromixers for operation in a wide range of Re numbers.

Volume-preserving strategies to improve the mixing efficiency of serpentine micromixers

In this study, we have proposed volume-preserving strategies to boost chaoticadvection and improve the mixing efficiency of serpentine micromixers. The proposed strategies revolve around the point that the volume of the micromixer is kept constant during the manipulation. The first strategy involves the utilization of a nozzle-diffuser (ND) shaped microchannel. Using this, the velocity of the fluids fluctuates in an alternating pattern, leading to additional chaotic advection, a decrease in the mixing path, and an increase in the mixing index. The second strategy uses non-aligned inlets to generate swirl inducing effects at the microchannel entrance, where the collision of two fluids generates angular momentum in the flow, providing more chaotic advection. These strategies proved to be effective in boosting the mixing efficiency over wide ranges of Re in which 60% enhancement (from 20.53% to 80.31%) was achieved for Re of 30 by applying an ND shaped microchannel, and 20% enhancement (from 12.71% to 32.21%) was achieved for a critical Re of 15 by applying both of the strategies simultaneously.

Numerical investigations of a passive micromixer with tree shape obstacles based on fractal principle and Murray’s law

This work mainly studies the mixing efficiency and mixing energy cost (MEC) of the tree-shaped obstacle micromixer based on fractal principle and Murray’s law. The obstacles are regularly arranged on both sides of the T-type microchannel. The mixing efficiency and MEC of primary tree-shaped obstacle (PTSO) micromixer, secondary tree-shaped obstacle (STSO) micromixer, and tertiary tree-shaped obstacle (TTSO) micromixer are compared at five kinds of Reynolds (Res). The MEC of STSO micromixer and TTSO micromixer is higher than that of PTSO micromixer under different δ conditions. However, PTSO micromixer has higher mixing efficiency. Especially when δ = 45° and D = 0.5 mm, the mixing efficiency of PTSO micromixer reaches 95%. On the other hand, the convection phenomenon caused by PTSO causes the contact area of the two fluids to increase.

Study on Flow Characteristics of Microfluid in Electroosmotic Flow Mixer

As one of the important components of modern microfluidic devices, micromixers have attracted more and more attention. Among many microfluidic mixers, electro-osmotic flow mixers have been widely studied and applied due to their advantages such as simple structure, convenient control and remarkable mixing effect. In this paper, a two-dimensional numerical model of an electro-osmotic flow micromixer with an annular mixing chamber is established by means of the simulation software COMSOL Multiphysics. The mixing process of fluid in the electro-osmotic flow micromixer is simulated, and the flow characteristics of fluid in the micromixer are analyzed. The analysis shows that the electroosmotic flow micromixer generates electroosmotic flow under the action of alternating electric field, and disturbs the fluid to form vortex, thus greatly improving the mixing efficiency. The results show that the micromixing efficiency increases with the increase of zeta potential and applied alternating electric field potential, and in a certain range, the micromixing efficiency increases first and then decreases with the increase of alternating electric field frequency.