scholarly journals Numerical Analysis of the Heterogeneity Effect on Electroosmotic Micromixers Based on the Standard Deviation of Concentration and Mixing Entropy Index

Micromachines ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1055
Author(s):  
Alireza Farahinia ◽  
Jafar Jamaati ◽  
Hamid Niazmand ◽  
Wenjun Zhang
Keyword(s):  
Reynolds Number ◽  
Zeta Potential ◽  
Electroosmotic Flow ◽  
Molecular Diffusion ◽  
Slip Surface ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Slip Coefficient ◽  
Mixing Rate ◽  
Heterogeneous Distribution

One approach to achieve a homogeneous mixture in microfluidic systems in the quickest time and shortest possible length is to employ electroosmotic flow characteristics with heterogeneous surface properties. Mixing using electroosmotic flow inside microchannels with homogeneous walls is done primarily under the influence of molecular diffusion, which is not strong enough to mix the fluids thoroughly. However, surface chemistry technology can help create desired patterns on microchannel walls to generate significant rotational currents and improve mixing efficiency remarkably. This study analyzes the function of a heterogeneous zeta-potential patch located on a microchannel wall in creating mixing inside a microchannel affected by electroosmotic flow and determines the optimal length to achieve the desired mixing rate. The approximate Helmholtz–Smoluchowski model is suggested to reduce computational costs and simplify the solving process. The results show that the heterogeneity length and location of the zeta-potential patch affect the final mixing proficiency. It was also observed that the slip coefficient on the wall has a more significant effect than the Reynolds number change on improving the mixing efficiency of electroosmotic micromixers, benefiting the heterogeneous distribution of zeta-potential. In addition, using a channel with a heterogeneous zeta-potential patch covered by a slip surface did not lead to an adequate mixing in low Reynolds numbers. Therefore, a homogeneous channel without any heterogeneity would be a priority in such a range of Reynolds numbers. However, increasing the Reynolds number and the presence of a slip coefficient on the heterogeneous channel wall enhances the mixing efficiency relative to the homogeneous one. It should be noted, though, that increasing the slip coefficient will make the mixing efficiency decrease sharply in any situation, especially in high Reynolds numbers.

scholarly journals A stretchable micromixer with enhanced performance for intermediate Reynolds numbers

2021 ◽  
Author(s):  
Hedieh Fallahi ◽  
Jun Zhang ◽  
Jordan Nicholls ◽  
Pradip Singha ◽  
Nhat-Khuong Nguyen ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Molecular Diffusion ◽  
Inertial Force ◽  
Reynolds Numbers ◽  
Chaotic Advection ◽  
Low Flow ◽  
Viscous Force ◽  
Mixing Efficiency ◽  
Potential Candidate ◽  
Low Reynolds

Abstract Chemical reactions in microscale require good mixing at a relatively low flowrate. However, mixing in microscale faces the major challenge of stable laminar flow associated with the low Reynolds number, the relative ratio between inertial force and viscous force. For low Reynolds numbers of less than unity, mixing occurs due to molecular diffusion. For high Reynolds number of more than several tens, chaotic advection enhances mixing. However, in the intermediate regime, mixing is not efficient. This paper reports a stretchable micromixer with dynamically tuneable channel dimensions. Periodically stretching the device changes the channel geometry and the curvature induced secondary Dean flows. The dynamically evolving secondary and main flows in the mixing channel result in chaotic advection and enhance mixing. The concept was demonstrated in a stretchable micromixer with a serpentine channel. We evaluated the performance of this stretchable micromixer both experimentally and numerically. At the intermediate range of Reynolds numbers from 4 to 17, the periodically stretched micromixer showed a better mixing efficiency than the non-stretched counterpart. Therefore, our stretchable micromixer is a potential candidate for applications where precious reagents need to be mixed at relatively low flow rate conditions.

Parametric Study of Obstacle Geometry Effect on Mixing Performance in a Convergent-Divergent Micromixer with Sinusoidal Walls

2017 ◽  
Vol 12 (1) ◽  
Author(s):  
Fazlollah Heshmatnezhad ◽  
Halimeh Aghaei ◽  
Ali Reza Solaimany Nazar
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Mixing Efficiency ◽  
Main Concept ◽  
Mixing Index ◽  
Operational Parameter

Abstract This study presents a numerical simulation through computational fluid dynamics on mixing and flow structures in convergent-divergent micromixer with a triangular obstacle. The main concept in this design is to enhance the interfacial area between the two fluids by creating a transverse flow and split, and recombination of fluids flow due to the presence of obstacles. The effect of triangular obstacle size, the number of units, changing the position of an obstacle in the mixing channel and operational parameter like the Reynolds number on the mixing efficiency and pressure drop are assessed. The results indicate that at inlet Reynolds numbers below 5, the molecular diffusion is the most important mechanism of mixing, and the mixing index is almost the same for all cases. By increasing the inlet Reynolds number above 5, mixing index increases dramatically, due to the secondary flows. Based on the simulation results, due to increasing the effect of dean and separation vortices as well as more recirculation of flow in the side branches and behind the triangular obstacle, the highest mixing index is obtained at the aspect ratio of 2 for the triangular obstacle and its position at the front of the mixing unit. Also the highest value of mixing index is obtained by six unit of mixing chamber. The effect of changing the position of the obstacle in the channel and changing the aspect ratio of the obstacle is evident in high Reynolds numbers. An increase in the Reynolds number in both cases (changing the aspect ratio and position of the obstacle) leads to pressure drop increases.

scholarly journals A stretchable micrometer with enhanced performance for intermediate Reynolds numbers

2021 ◽  
Author(s):  
Hedieh Fallahi ◽  
Jun Zhang ◽  
Jordan Nicholls ◽  
Pradip Singha ◽  
Nhat-Khuong Nguyen ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Molecular Diffusion ◽  
Inertial Force ◽  
Reynolds Numbers ◽  
Chaotic Advection ◽  
Low Flow ◽  
Viscous Force ◽  
Mixing Efficiency ◽  
Potential Candidate ◽  
Low Reynolds

Abstract Chemical reactions in microscale require good mixing at a relatively low flowrate. However, mixing in microscale faces the major challenge of stable laminar flow associated with the low Reynolds number, the relative ratio between inertial force and viscous force. For low Reynolds numbers of less than unity, mixing occurs due to molecular diffusion. For high Reynolds number of more than several tens, chaotic advection enhances mixing. However, in the intermediate regime, mixing is not efficient. This paper reports a stretchable micromixer with dynamically tuneable channel dimensions. Periodically stretching the device changes the channel geometry and the curvature induced secondary Dean flows. The dynamically evolving secondary and main flows in the mixing channel result in chaotic advection and enhance mixing. The concept was demonstrated in a stretchable micromixer with a serpentine channel. We evaluated the performance of this stretchable micromixer both experimentally and numerically. At the intermediate range of Reynolds numbers from 4 to 17, the periodically stretched micromixer showed a better mixing efficiency than the non-stretched counterpart. Therefore, our stretchable micromixer is a potential candidate for applications where precious reagents need to be mixed at relatively low flow rate conditions.

A Microfluidic Mixer Fabricated From Compliant Thermoplastic Films

2008 ◽  
Vol 2 (1) ◽  
Author(s):  
N. Paya ◽  
T. Dankovic ◽  
A. Feinerman
Keyword(s):  
Reynolds Number ◽  
Lab On A Chip ◽  
Microfluidic Devices ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Minimal Amount ◽  
Microfluidic Mixer ◽  
Mixing Performance ◽  
Rapid Mixing ◽  
Microfluidic Flows

Mixing is often crucial to the operation of various microfluidic devices. And the most common objective is rapid mixing between two initially segregated fluid streams in a minimal amount of space. In microfluidic flows characterized by incompressibility and low Reynolds number, however, turbulence is almost entirely absent and mixing generally relies on diffusion. Therefore, based on the properties of the fluids involved, it can take impractically long to achieve high mixing efficiency in some cases. To resolve this problem, this paper demonstrates a novel compliant micromixer made of thermoplastic films for lab-on-a-chip applications. The microfluidic mixer utilizes self-rotation effects to achieve high mixing efficiency at Reynolds numbers below 100. In addition, a possible design is suggested for a thermoplastic voltage-actuated micromixer which can lead to even better mixing performance at Reynolds numbers below 1.

scholarly journals The influence of far field stratification on shear-induced turbulent mixing

2021 ◽  
Vol 928 ◽  
Author(s):  
S.F. Lewin ◽  
C.P. Caulfield
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Richardson Number ◽  
Scaling Laws ◽  
Mixing Efficiency ◽  
Far Field ◽  
Mixing Rate ◽  
Gradient Richardson Number ◽  
Background Density ◽  
Secondary Instabilities

We compare the properties of the turbulence induced by the breakdown of Kelvin–Helmholtz instability (KHI) at high Reynolds number in two classes of stratified shear flows where the background density profile is given by either a linear function or a hyperbolic tangent function, at different values of the minimum initial gradient Richardson number ${{Ri}}_0$ . Considering global and local measures of mixing defined in terms of either the irreversible mixing rate $\mathscr {M}$ associated with the time evolution of the background potential energy, or an appropriately defined density variance dissipation rate $\chi$ , we find that the proliferation of secondary instabilities strongly affects the efficiency of mixing early in the flow evolution, and also that these secondary instabilities are highly sensitive to flow perturbations that are added at the point of maximal (two-dimensional) billow amplitude. Nevertheless, mixing efficiency does not appear to depend strongly on the far field density structure, a feature supported by the evolution of local horizontally averaged values of the buoyancy Reynolds number ${Re}_b$ and gradient Richardson number ${Ri}_g$ . We investigate the applicability of various proposed scaling laws for flux coefficients $\varGamma$ in terms of characteristic length scales, in particular discussing the relevance of the overturning ‘Thorpe scale’ in stratified turbulent flows. Finally, we compare a variety of empirical model parameterizations used to compute diapycnal diffusivity in an oceanographic context, arguing that for transient flows such as KHI-induced turbulence, simple models that relate the ‘age’ of a turbulent event to its mixing efficiency can produce reasonably robust mixing estimates.

scholarly journals Numerical Simulation of Mixing Process in a Splitting-and-Recombination Microreactor

2022 ◽  
Vol 3 ◽  
Author(s):  
Lifang Yan ◽  
Shiteng Wang ◽  
Yi Cheng
Keyword(s):  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Horizontal Distance ◽  
Mixing Process ◽  
Mixing Rate ◽  
Navier Stokes Equation ◽  
Vertical Distance ◽  
Wide Range ◽  
Spacing Distance ◽  
Comb Tooth

The mixing process between miscible fluids in a splitting-and-recombination microreactor is analyzed numerically by solving the Navier–Stokes equation and species transfer equation. The commercial microreactor combines rectangular channels with comb-shaped inserts to achieve the splitting-and-recombination effect. The results show that the microreactor with three-layer standard inserts have the highest mixing rate as well as good mixing efficiency within a wide range of Reynolds numbers from 0.1 to 160. The size parameters of the inserts, both the ratio of the width of comb tooth (marked as l) and the spacing distance (marked as s) between two comb teeth, and the ratio of the vertical distance (marked as V) of comb teeth and the horizontal distance (marked as H) are essential for influencing the liquid–liquid mixing process at low Reynolds numbers (e.g., Re ≤ 2). With the increase of s/l from 1 to 4, the mixing efficiency drops from 0.99 to 0.45 at Re = 0.2. Similarly, the increase in V/H is not beneficial to promote the mixing between fluids. When the ratio of V/H changes from 10:10 to 10:4, the splitting and recombination cycles reduce so that the uniform mixing between different fluids can be hardly achieved. The width of comb tooth (marked as l) is 1 mm and the spacing distance (marked as s) between two comb teeth is 2 mm. The vertical distance (marked as V) of comb teeth and the horizontal distance (marked as H) are both 10 mm.

scholarly journals High mixing performances of shear-thinning fluids in two-layer crossing channels micromixer at very low Reynolds numbers

2019 ◽  
Vol 13 (4) ◽  
pp. 5938-5960
Author(s):  
A. Kouadri ◽  
Y. Lasbet ◽  
M. Makhlouf
Keyword(s):  
Reynolds Number ◽  
Power Law ◽  
Transport Model ◽  
Tangential Velocity ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Species Transport ◽  
Industrial Sectors ◽  
Power Law Fluids ◽  
Power Law Index

In a recent study, the Two-Layer Crossing Channels Micromixer (TLCCM) exhibited good mixing capacities in the case of the Newtonian fluids (close to 100%) for all considered Reynolds number values. However, since the majority of the used fluids in the industrial sectors are non-Newtonians, this work details the mixing evolution of power-law fluids in the considered geometry. In this paper, the power-law index ranges from 0.73 to 1 and the generalized Reynolds number is bounded between 0.1 and 50. The conservation equations of momentum, mass and species transport are numerically solved using a CFD code, considering the species transport model. The flow structure at the cross-sectional planes of our micromixer was studied using the dynamic systems theory. The evolutions of the intensity, also the axial, radial and tangential velocity profiles were examined for different values of the Reynolds number and the power-law index. Besides, the pressure drop of the power-law fluids under different Reynolds number was calculated and represented. Furthermore, the mixing efficiency is evaluated by the computation of the mixing index (MI), based on the standard deviation of the mass fraction in different cross-sections. In such geometry, a perfect mixing is achieved with MI closed to 99.47 %, at very small Reynolds number (from the value 0.1) whatever the power-law index and generalized Reynolds numbers taken in this investigation. Consequently, the targeted channel presents a useful tool for pertinent mass transfer improvements, it is highly recommended to include it in various microfluidic systems.

Passive Microextractor with Internal Fluid Recirculation for Two Immiscible Liquids

2014 ◽  
Vol 12 (1) ◽  
pp. 285-293 ◽  
Author(s):  
Cong Xu ◽  
Jiao Wang
Keyword(s):  
Mass Transfer ◽  
Reynolds Number ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Immiscible Liquid ◽  
Chaotic Advection ◽  
Inlet Channel ◽  
External Energy ◽  
Outlet Channel ◽  
Internal Fluid

Abstract A microextractor comprising an inlet channel, a mixing chamber, two feedback channels, and an outlet channel and having no moving parts was designed for immiscible liquid–liquid extraction. Two liquids were mixed passively without any external energy input, and the extraction was completed in the microextractor. The extractor performance with or without a splitter was investigated by visualization and mass transfer experiments. Two mixing mechanisms were observed: (i) molecular diffusion at lower Reynolds number and (ii) chaotic advection at higher Reynolds number. The transition point between the two mechanisms was at Reynolds numbers 375.2 and 179.9 for the aqueous phase (3 mol/L HNO3 solution) and the organic phase (30% tributyl phosphate (TBP)–kerosene solution), respectively. In the chaotic advection mode, two vortexes rotating in opposite directions were formed on both sides of the main flow, which enhanced the mass transfer between the two liquids. Mass transfer between the 3 mol/L HNO3 and 30% TBP–kerosene solutions was achieved with an efficiency of 92.8% at the extractor exit when the extractor operated in the chaotic advection mode.

Correlation of Mixing Efficiency and Entropy Generation Rate in a Square Cross Section Tee Junction Micromixer

2017 ◽  
Author(s):  
Aric M. Gillispie ◽  
Evan C. Lemley
Keyword(s):  
Reynolds Number ◽  
Entropy Generation ◽  
Molecular Diffusion ◽  
Chaotic Advection ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Analytical Relationship ◽  
Mixing Efficiency ◽  
Mixed Solution ◽  
Tee Junction

The potential applications of micromixers continues to expand in the bio-sciences area. In particular, passive micromixers that may be used as part of point-of-care (POC) diagnostic testing devices are becoming commonplace and have application in developed, developing, and relatively undeveloped locales. Characterizing and improving mixing efficiency in these devices is an ongoing research effort. Micromixers are used in some lab-on-chip (LOC) devices where it is often necessary to combine two or more fluids into a mixed solution for testing or delivery. The simplest micromixer incorporates a tee junction to combine two fluid species in anti-parallel branches, with the mixed fluid leaving in a branch perpendicular to the incoming branches. Micromixers rely on two modes of mixing: chaotic advection and molecular diffusion. In micro-mixers flow is typically laminar, making chaotic advection occur only via induced secondary flows. Hence, micromixers, unless carefully designed, rely almost exclusively on molecular diffusion of fluid species. A well designed micromixer should exhibit significant chaotic advection; which is also a sign of large strain rates and large entropy generation rates. This paper describes the development of an analytical relationship for the entropy generation rate and the mixing efficiency as function of the outgoing branch Reynolds number. Though there has been extensive research on tee junctions, entropy generation, and the mixing efficiencies of a wide variety of micromixers, a functional relationship for the mixing efficiency and the entropy generation rate has not been established. We hypothesize a positive correlation between the mixing index and the entropy generation rate. The worked described here establishes a method and provides the results for such a relationship. A basic tee junction with square cross sections has been analyzed using computational fluid dynamics to determine the entropy generation rate and outgoing mixing efficiencies for Reynolds numbers ranging from 25–75. The mixing efficiency is determined at a location in the outgoing branch where the effects of molecular diffusive mixing is minimized and chaotic advective mixing is the focus. The entropy generation rate has been determined for the indicated range of Reynolds number and decomposed into its viscous and diffusive entropy terms. The functional relationships that have been developed are applicable for micromixer design and serve as a reference for more complex passive micromixer designs.

The Numerical Investigation of an Interdigital Micromixer With the Circular-Sector Obstacles

2009 ◽  
Author(s):  
Yan Feng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Surface Area ◽  
Molecular Diffusion ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Mixing Length ◽  
Circular Sector ◽  
Lateral Direction ◽  
Low Reynolds Numbers ◽  
Low Reynolds

In this paper, a passive interdigital micromixer with the circular-sector obstacles is proposed and the mixing performance is estimated by numerical simulation. The tested Reynolds numbers range from 0.01 to 10. Flow recirculation or vortices seems impossible to generate to enhance the mixing at such low Reynolds numbers. Hence, molecular diffusion is the dominant mixing mechanism. Based on the diffusion principle, enlarging the mixing length, reducing the diffusion length and increasing the surface area between species are major methods to obtain mixing enhancement. In order to achieve rapid mixing, shortening the mixing length is necessary. However, the reduced mixing length induces the decreased mixing time which the species take to mix. The circular-section obstacles are placed in the straight microchannels to enlarge the contact surface area between species. The flow path is distorted after passing the obstacles so that the real mixing length increases compared with traditional T-shape micromixers. Furthermore, flow advection takes a part role in mixing since the velocity direction is no longer perpendicular to diffusion direction. Different geometries and layouts of obstacles are analyzed for optimization. The results of optimal design show the worst mixing efficiency, around 50%, occurs at Re = 1. In order to improve the lower limitation of mixing efficiency, the duplicated layouts of obstacles in lateral direction with interdigital inlet are applied to reduce the diffusion path and increase the interface area so that the mixing efficiency could be enhanced. The results show that the mixing efficiency could achieve 85% at Re ≤ 1 with a low pressure drop of 100 Pa. It has the potential to be used in applications with low Reynolds numbers.

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