A novel design for passive misscromixers based on topology optimization method

An overview on state-of-art of micromixer designs, characteristics and applications

Micromixers are characterized based on characteristics such as excellent mixing efficiency, low reagent cost and flexible controllability compared with conventional reactors in terms of macro size. A variety of designs and applications of micromixers have been proposed. The focus of current reviews is restricted to micromixer structures. Each type of micromixer has characteristics corresponding to its structure, which determines the suitable application areas. This paper provides an overview connecting micromixer designs and their applications. First, the typical designs and mixing mechanisms of both passive and active micromixers are summarized. Then, application cases of micromixers, including chemical, biological and medical applications, are presented. The characteristics, including the advantages and restrictions of different micromixers, are discussed. Finally, the future perspective of micromixer design is proposed. It is predictable that micromixers will have widespread applications by integrating two or more different mixing methods together. This review would be beneficial to guide the design of micromixers applied for specific purposes.

A Mini Review on Fluid Topology Optimization

Topology optimization holds great potential to achieve the best performance for various fluid-related applications like aircraft components and microfluidic mixers. This paper reviews comprehensively the technical progress of this field over the last decade from the viewpoint of structural expression. The density-based approach has been widely adopted to design structures due to its simple concept, ease of implementation, and robustness. Different designs using such a pointwise method for systems under Stokes, laminar Navier-Stokes, turbulent, non-Newtonian, and steady-state/unsteady-state fluid flows are reviewed and discussed in depth. Both isogeometric analysis and the moving morphable components/voids methods will demonstrate their advantages regarding integration with computer-aided design. The moving morphable components/voids method also significantly reduces computing costs. From the viewpoint of boundary smoothness, we are most concerned about whether smoother boundaries can reduce objective functions such as energy dissipation even lower. Therefore, this work also concentrates on level set and spline expression methods. Furthermore, we identify isogeometric analysis and machine learning in shaping the field's future. In addition, the review highlights the following two challenges: achieving accurate fluid model construction and the relatively limited experimental validation of fluid topology optimization outcomes.

A review on acoustic field-driven micromixers

A review on acoustic field-driven micromixers is given. This is supplemented by the governing equations, governing non-dimensional parameters, numerical simulation approaches, and fabrication techniques. Acoustically induced vibration is a kind of external energy input employed in active micromixers to improve the mixing performance. An air bubble energized by an acoustic field acts as an external energy source and induces friction forces at the interface between an air bubble and liquid, leading to the formation of circulatory flows. The current review (with 200 references) evaluates different characteristics of microfluidic devices working based on acoustic field shaking.

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

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.

A cost-effective serpentine micromixer utilizing ellipse curve

Due to high mixing performance and simple geometry structure, serpentine micromixer is one typical passive micromixer that has been widely investigated. Traditional zigzag and square-wave serpentine micromixers can achieve sufficient mixing, but tend to induce significant pressure drop. The excessive pressure drop means more energy consumption, which leads to low cost-performance of mixing. To mitigate excessive pressure drop, a novel serpentine micromixer utilizing ellipse curve is proposed. While fluids flowing through ellipse curve microchannels, the flow directions keep continuous changing. Therefore, the Dean vortices are induced throughout the whole flow path. Numerical simulation and visualization experiments are conducted at Reynolds number (Re) ranging from 0.1 to 100. Dean vortices varies with the changing curvature in different ellipse curves, and local Dean numbers are calculated for quantitative evaluation. The results suggest that the ellipse with a larger eccentricity induces stronger Dean vortices, thus better mixing performance can be obtained. A parameter, named mixing performance cost (Mec), is proposed to evaluate the cost-performance of micromixers. Compared with the zigzag, square-wave and other improved serpentine micromixers, the ellipse curve micromixer produces lower pressure drop while have the capability to maintain excellent mixing performance. The ellipse curve micromixer is proved to be more cost-effective for rapid mixing in complex microfluidic systems.

Mixing performance of an electroosmotic micromixer with Koch fractal structure

In this paper, we have designed a Koch fractal electroosmotic micromixer (KFEM). A low-voltage electroosmotic micromixer. In order to optimize the electrode position, Koch microchannel is designed according to the Koch fractal principle and the electrode pairs based on the fractal are arranged. Then the effect of electrode voltage, electrode distribution positions, the number of electrode pairs, two kinds of Koch fractal structures, Reynolds (Re) number and the frequency of alternating current (AC) on the mixing performance are studied. The results show that the mixing efficiency can reach 99% in a short time when the AC voltage is 1 V, the AC frequency is 12 Hz and the electroosmotic micromixer has two sets of electrode pairs.

Fundamental Studies of Rapidly Fabricated On-Chip Passive Micromixer for Modular Microfluidics

Micromixers play an important role in many modular microfluidics. Complex on-chip mixing units and smooth channel surfaces ablated by lasers on polymers are well-known problems for microfluidic chip fabricating techniques. However, little is known about the ablation of rugged surfaces on polymer chips for mixing uses. This paper provides the first report of an on-chip compact micromixer simply, easily and quickly fabricated using laser-ablated irregular microspheric surfaces on a polymethyl methacrylate (PMMA) microfluidic chip for continuous mixing uses in modular microfluidics. The straight line channel geometry is designed for sequential mixing of nanoliter fluids in about 1 s. The results verify that up to about 90% of fluids can be mixed in a channel only 500 µm long, 200 µm wide and 150 µm deep using the developed micromixer fabricating method under optimized conditions. The computational flow dynamics simulation and experimental result agree well with each other.

MONOPHASIC REACTION WITH A PACKED BED MICROREACTOR: CHARACTERIZATION OF MASS TRANSFER AND REACTION

In this paper, the monophasic catalytic reaction in the microreactor is studied. Several factors that may affect the catalytic reaction are discussed, including the pressure drop, the size of catalyst particles, and the channel structure. Finally, some important conclusions can be reached. The change of pressure drop has an effect on the reaction. For example, the C3H6 conversion rate is 62.88% when the pressure drop is 8[Formula: see text]atm, and the C3H6 conversion rate is 61.78% when the pressure drop is 11[Formula: see text]atm. The effect of the change particle radius is not obvious on the reaction. Enhancing the mixing of substances before entering the reaction domain is helpful to the catalytic reaction. There are different substances concentration in catalyst particles at different positions in microreactors. But from the surface to the inside of catalyst particles, the substances concentration has a clear change rule.

A Review of Topology Optimisation for Fluid-Based Problems

This review paper provides an overview of the literature for topology optimisation of fluid-based problems, starting with the seminal works on the subject and ending with a snapshot of the state of the art of this rapidly developing field. “Fluid-based problems” are defined as problems where at least one governing equation for fluid flow is solved and the fluid–solid interface is optimised. In addition to fluid flow, any number of additional physics can be solved, such as species transport, heat transfer and mechanics. The review covers 186 papers from 2003 up to and including January 2020, which are sorted into five main groups: pure fluid flow; species transport; conjugate heat transfer; fluid–structure interaction; microstructure and porous media. Each paper is very briefly introduced in chronological order of publication. A quantititive analysis is presented with statistics covering the development of the field and presenting the distribution over subgroups. Recommendations for focus areas of future research are made based on the extensive literature review, the quantitative analysis, as well as the authors’ personal experience and opinions. Since the vast majority of papers treat steady-state laminar pure fluid flow, with no recent major advancements, it is recommended that future research focuses on more complex problems, e.g., transient and turbulent flow.

Mixing Optimization in Grooved Serpentine Microchannels

Computational fluid dynamics modeling at Reynolds numbers ranging from 10 to 100 was used to characterize the performance of a new type of micromixer employing a serpentine channel with a grooved surface. The new topology exploits the overlap between the typical Dean flows present in curved channels due to the centrifugal forces experienced by the fluids, and the helical flows induced by slanted groove-ridge patterns with respect to the direction of the flow. The resulting flows are complex, with multiple vortices and saddle points, leading to enhanced mixing across the section of the channel. The optimization of the mixers with respect to the inner radius of curvature (R_in) of the serpentine channel identifies the designs in which the mixing index quality is both high (M > 0.95) and independent of the Reynolds number across all the values investigated.

Flow Feature Analysis of T-Junction Wavy Micromixer for Mixing Application

The mixing of fluids in wavy micromixer and wavy micromixer with obstacles on its side walls has been numerically investigated. The effect of frequency of wavy channel on mixing performance is studied over a range of Reynolds number from 0.1 to 45. Various performance characteristics viz. the mixing index, pressure drop, performance index, and pumping power are used to analyze the overall mixing performance. The results show that the wavy micromixer with obstacles produces better mixing performance than the wavy micromixer. Also, the mixing index is sensitive to the wavy frequency of the channel. The wavy micromixer exhibits the smallest pressure drop as compared to micromixer with obstacles in all cases.

Fractal Reactor in Micro-Scale for Process Intensification

Fractal theory, with its novel architectures inspired by nature, provides some novel concepts for smart reactor design. Here, researches on the applications of fractal theory to micro-reactor design are reviewed, in term of its high surface area-to-volume ratio, rapid and direct numbering-up, safety, and precise control. In addition, two designs of fractal micro-reactor are introduced as typical examples. First, the H-type fractal structure is considered in the context of the design of a double-plate micro-reactor, which is used for photocatalytic reactions of CO2. Second, applications of fractal Hilbert curves are considered in the design of channel structures for gas-liquid reactions. These two fractal micro-reactors can be fabricated via 3D printing technology and used for CO2conversion under mild conditions.

Fabrication of Polyethylene Terephthalate Microfluidic Chip Using CO2 Laser System

CO2 laser machining technology is a wide and low-cost method for fabrication of microfluidic chips on polyethylene terephthalate (PET). In this paper, the influence of CO2 laser parameters including laser power and laser moving velocity on the depth and width of PET microchannel are studied. Laser power is set from 4 W to 20 W and laser moving velocity is set from 5 mm/s to 25 mm/s in the experiment. Compared with experimental results, some rules for the depth and width on laser parameters are obtained. The depth and width of the microchannel increase with the increase of laser power at the same laser moving velocity. However, the depth and width of the microchannel first increase and then decrease with the increase of laser moving velocity at the same laser power. The PET microfluidic chip is fabricated by a hot bonding machine.

Mixing Enhancement in Serpentine Micromixers with a Non-Rectangular Cross-Section

In this numerical study, a new type of serpentine micromixer involving mixing units with a non-rectangular cross-section is investigated. Similar to other serpentine/spiral shaped micromixers, the design exploits the formation of transversal vortices (Dean flows) in pressure-driven systems, associated with the centrifugal forces experienced by the fluid as it is confined to move along curved geometries. In contrast with other previous designs, though, the use of non-rectangular cross-sections that change orientation between mixing units is exploited to control the center of rotation of the transversal flows formed. The associated extensional flows that thus develop between the mixing segments complement the existent rotational flows, leading to a more complex fluid motion. The fluid flow characteristics and associated mixing are determined numerically from computational solutions to Navier⁻Stokes equations and the concentration-diffusion equation. It is found that the performance of the investigated mixers exceeds that of simple serpentine channels with a more consistent behavior at low and high Reynolds numbers. An analysis of the mixing quality using an entropic mixing index indicates that maximum mixing can be achieved at Reynolds numbers as small as 20 in less than four serpentine mixing units.

A Review on Micromixers

Microfluidic devices have attracted increasing attention in the fields of biomedical diagnostics, food safety control, environmental protection, and animal epidemic prevention. Micromixing has a considerable impact on the efficiency and sensitivity of microfluidic devices. This work reviews recent advances on the passive and active micromixers for the development of various microfluidic chips. Recently reported active micromixers driven by pressure fields, electrical fields, sound fields, magnetic fields, and thermal fields, etc. and passive micromixers, which owned two-dimensional obstacles, unbalanced collisions, spiral and convergence-divergence structures or three-dimensional lamination and spiral structures, were summarized and discussed. The future trends for micromixers to combine with 3D printing and paper channel were brought forth as well.

Numerical and Experimental Study on a Microfluidic Concentration Gradient Generator for Arbitrary Approximate Linear and Quadratic Concentration Curve Output

This work introduces a simple and versatile method for researching the concentration gradient generator (CGG) which can present the arbitrary approximate linear and quadratic concentration gradient curves output. The concentration gradients of arbitrary approximate linear curves with two inlets and arbitrary quadratic curves in the CGG with three inlets are obtained with the corresponding flow velocities. The CGG was simulated basing on the finite element method (FEM). The fluid-dynamic and mass-transport about the CGG was studied. Moreover, the feasibility of simulation was clearly verified by an experiment which two microfluidic chips of CGG on the PMMA substrate were processed using CNC engraving and milling machine. The paper successfully demonstrates the controllability of concentration gradient profiles in CGG with two inlets and three inlets. The study on the CGG can help the trends study of cell and molecule in different samples in the biochemical engineering.

A Novel Electroosmotic Micromixer with Asymmetric Lateral Structures and DC Electrode Arrays

We present a novel electroosmotic micromixer that consists of arrays of direct current (DC) asymmetric electrode and asymmetric lateral structures. By embedding asymmetric electrode arrays on the top and bottom walls of a rectangular microchannel appropriately, the flow perturbations and vortexes can be induced when a DC electric field is imposed. An efficient lateral structure is then sequentially combined with the rectangular microchannel, which enhances the mixing effect significantly. The effects of operational parameters such as the Reynolds number, the applied potential, and the Peclet number on the mixing performance are analyzed in detail by numerical simulations. The results indicate that an enhanced mixing performance can be achieved with low applied potential. The novel method proposed in this paper provides a simple solution for mixing in the field of micro-total-analysis systems.

An Enhanced Electroosmotic Micromixer with an Efficient Asymmetric Lateral Structure

Homogeneous and rapid mixing in microfluidic devices is difficult to accomplish, owing to the low Reynolds number associated with most flows in microfluidic channels. Here, an efficient electroosmotic micromixer based on a carefully designed lateral structure is demonstrated. The electroosmotic flow in this mixer with an asymmetrical structure induces enhanced disturbance in the micro channel, helping the fluid streams' folding and stretching, thereby enabling appreciable mixing. Quantitative analysis of the mixing efficiency with respect to the potential applied and the flow rate suggests that the electroosmotic microfluidic mixer developed in the present work can achieve efficient mixing with low applied potential.