Mixing In Microchannels Research Articles

Microfluidic Mixing: A Physics-Oriented Review.

This comprehensive review paper focuses on the intricate physics of microfluidics and their application in micromixing techniques. Various methods for enhancing mixing in microchannels are explored, with a keen emphasis on the underlying fluid dynamics principles. Geometrical micromixers employ complex channel designs to induce fluid-fluid interface distortions, yielding efficient mixing while retaining manufacturing simplicity. These methods synergize effectively with external techniques, showcasing promising potential. Electrohydrodynamics harnesses electrokinetic phenomena like electroosmosis, electrophoresis, and electrothermal effects. These methods offer dynamic control over mixing parameters via applied voltage, frequency, and electrode positioning, although power consumption and heating can be drawbacks. Acoustofluidics leverages acoustic waves to drive microstreaming, offering localized yet far-reaching effects. Magnetohydrodynamics, though limited in applicability to certain fluids, showcases potential by utilizing magnetic fields to propel mixing. Selecting an approach hinges on trade-offs among complexity, efficiency, and compatibility with fluid properties. Understanding the physics of fluid behavior and rationalizing these techniques aids in tailoring the most suitable micromixing solution. In a rapidly advancing field, this paper provides a consolidated understanding of these techniques, facilitating the informed choice of approach for specific microfluidic mixing needs.

Analysis and optimization of heterogeneously charged surface features in electrokinetic micromixing of non-Newtonian fluids

This study investigates the mixing of non-Newtonian fluids in microfluidic devices using electrokinetic phenomena Micromixing is a crucial operation that finds multiple applications in chemical research and biomedical diagnostics. In order to improve mixing, multiple combinations of heterogeneously charged triangular elements are considered for investigation. The power-law model was used to characterize the non-Newtonian behavior of fluids. The effect of different geometrical parameters (hurdle length, height, number, and shape) and electrical parameters (applied external electric field and zeta potential on hurdles) on mixing are evaluated. The results indicate that the element shape, number, and fluid viscosity significantly affect mixing in microchannels. Moreover, the homogeneity of the mixed fluids increases with the flow behavior index (n). We found that dilatant fluids mix relatively better than pseudoplastic fluids. Finally, the Taguchi technique used to evaluate the combined effect of all factors and obtained optimal mixing conditions for various flow behavior indices (n) of non-Newtonian fluids.

A Review on Fluid Flow and Mixing in Microchannel and their Design and Manufacture for Microfluidic Applications

Abstract: The present time has witnessed a never-before-heard interest in and applications of microfluidic devices and systems. In microfluidic systems, fluid flows and is manipulated in microchannels. Mixing is one of the most important criteria for a majority of microfluidic systems, whose laminar nature hinders the efficiency of micromixing. The interface between the flowing fluid and the inner wall surface of the microchannel greatly influences the behaviour of fluidic flow in microfluidics. Many researchers have tried to pattern the surface, introduce obstacles to flow, and include micro- or nanoprotruded structures to enhance the mixing efficiency by manipulating the microchannel flow. New and rapid advances in MEMS and micro/nanofabrication technologies have enabled researchers to experiment with increasingly complex designs, enabling rapid transformation and dissemination of new knowledge in the field of microfluidics. Here, we report the fluid flow characteristics, mixing, and associated phenomena about microfluidic systems. Microfluidic systems and components such as microreactors, micromixers, and microchannels are reviewed in this work. We review active and passive micromixers, with a primary focus on widely used passive micromixers. Various microchannel geometries and their features, mixing efficiencies, numerical analysis, and fabrication methods are reviewed. Applications as well as possible future trends and advancements in this field, are included too. It is expected to make the reader curious and more familiar with the interesting field of microfluidics.

Micromixing optimization of non-newtonian fluids with heterogeneous zeta potential

This paper aims to improve micromixing in T-shaped microchannels using heterogeneous zeta potential, which is essential for analyzing biochemical fluids in various industrial applications. We investigate the impact of different parameters, including zeta potential, applied electric field, and a number of heterogeneous surface patterns, on microchannel mixing performance. Our results demonstrate that increasing the zeta potential on the heterogeneous pair patches amplifies the recirculation zone, improving mixing performance within the microchannel. Therefore, we recommend increasing the value of heterogeneous zeta potential on the patches to enhance the mixing performance further. Additionally, we examine the non-Newtonian behavior of the fluid using the power law model and find that pseudoplastic fluids (n < 1) have lower mixing performance compared to dilatant fluids (n > 1) due to higher velocity. We evaluate the effects of various parameters on mixing performance to design an efficient micromixer that achieves rapid and homogeneous mixing of sample fluids. Finally, we optimize the micromixing efficiency using Taguchi to achieve the optimum value of mixing. The optimized value of micromixing efficiency for the corresponding non-Newtonian fluid is also determined.

Vibration-induced streaming flow near a sharp edge: Flow structure and instabilities in a large span of forcing amplitude.

The steady streaming generated near solid walls by the periodic forcing of a viscous fluid is known to be strongly enhanced near sharp structures, owing to centrifugal effects that lead to the generation of an intense jet from the sharp tip. This flow has been shown to provide efficient active mixing in microchannels, due to strong transverse velocity. The forcing is often prescribed by acoustic transducers, but it can also be generated from low-frequency time-periodic flow ensured by mechanical vibrations. In this paper, we study the flow structure generated by low-frequency forcing (typically 10Hz) around a sharp tip. Using direct numerical simulations, we extract both the time-periodic and steady responses within a large span of amplitude of vibrations. When the amplitude is smaller than the tip radius of curvature, we recover the flow structure observed at higher frequencies (>1kHz) in previous studies, namely, an intense symmetric central jet and a quadratic dependence for the characteristic streaming velocity with the oscillating velocity v_{s}∼v_{a}^{2}. At higher amplitudes, such a scaling no longer holds and the streaming flow pattern loses its left-right symmetry. We then analyze the mechanisms of the instability from the careful examination of the instationary flow fields, and we propose possible mechanisms for such a flow transition involving the coupling between the streaming jet and instationary vorticity.

Magnetic Non-Spherical Particles Inducing Vortices in Microchannel for Effective Mixing.

Mixing in microfluidic channels is dominated by diffusion owing to the absence of chaotic flow. However, high-efficiency microscale mixing over short distances is desired for the development of lab-on-chip systems. Here, enhanced mixing in microchannels achieved using magnetic nonspherical particles (MNSPs), is reported. Benefiting from the nonspherical shape of the MNSPs, secondary vortices exhibiting cyclical characteristics appear in microchannels when the MNSPs rotate under an external magnetic field. Increasing the rotation rate enlarges the secondary vortices, expanding the mixing zone and enhancing the mixing, resulting in a mixing efficiency exceeding 0.9 at Re of 0.069-0.69. Complementary micro-particle image velocimetry (µPIV) for flow field analysis clarifies the mixing mechanism. In addition, a chaotic vortex area is generated in the presence of two MNSPs, which shortens the distance required for achieving an appropriate mixing efficiency. This study demonstrates the potential of employing MNSPs as efficient mixers in lab-on-chip devices.

CFD simulation and experimental study of antisolvent precipitation through impinging jets for synthesis of nanodrug particles

The nucleation and growth kinetics of loratadine, one of the primary antihistamines among drugs, are determined experimentally in confined liquid impinging jet channels, followed by simulating the anti-solvent precipitation process to obtain ultrafine loratadine nanoparticles. The effect of supersaturation is assessed as the essential factor on the nucleation and growth rates. Based on classical nucleation theory, induction time and nucleation rate showed homogeneous and heterogeneous nucleation regions in high and low supersaturations, respectively. The interfacial free energy measured from the induction time is obtained at about 18.99 mJ/m2. The size-dependent particle growth and growth dispersion effects are assessed through the experimental data. The nonlinear curve fitting with the experimental data in log population density plot at residence time of 7.69 s reveals that the four-parameter model has a better fit, with no growth dispersion. The microchannel mixing performance is assessed by analyzing the Damkohler number under different Reynolds numbers. The loratadine particle size distribution (CSD) is obtained by combining the population balance equation with the computational fluid dynamics in following the discretization method with and without turbulence aggregation kernel. The simulated CSD with turbulence aggregation kernel are 7 % more consistent with experimental results. The effects of collision angle, outlet nozzle length, Reynolds number, and solution concentration on the average particle size are assessed. An increase in the collision angle from 60 to 180°decreases the average particle size. The ultrafine loratadine nanoparticles of 49 nm are obtained with a narrow distribution, related to the angle of 180⁰, outlet nozzle length of 0.5 cm, Reynolds number of 4875, and the solution concentration of 35 mg/ml.

Chaos and irreversibility of a flexible filament in periodically driven Stokes flow.

The flow of Newtonian fluid at low Reynolds number is, in general, regular and time-reversible due to absence of nonlinear effects. For example, if the fluid is sheared by its boundary motion that is subsequently reversed, then all the fluid elements return to their initial positions. Consequently, mixing in microchannels happens solely due to molecular diffusion and is very slow. Here, we show, numerically, that the introduction of a single, freely floating, flexible filament in a time-periodic linear shear flow can break reversibility and give rise to chaos due to elastic nonlinearities, if the bending rigidity of the filament is within a carefully chosen range. Within this range, not only the shape of the filament is spatiotemporally chaotic, but also the flow is an efficient mixer. Overall, we find five dynamical phases: the shape of a stiff filament is time-invariant-either straight or buckled; it undergoes a period-two bifurcation as the filament is made softer; becomes spatiotemporally chaotic for even softer filaments but, surprisingly, the chaos is suppressed if bending rigidity is decreased further.

Boundary condition induced passive chaotic mixing in straight microchannels

When fluids flow through straight channels sustained turbulence occurs only at high Reynolds numbers [typically Re∼O(1000)]. It is difficult to mix multiple fluids flowing through a straight channel in the low Reynolds number laminar regime [Re&amp;lt;O(100)] because in the absence of turbulence, mixing between the component fluids occurs primarily via the slow molecular diffusion process. This Letter reports a simple way to significantly enhance the low Reynolds number (in our case Re≤10) passive microfluidic flow mixing in a straight microchannel by introducing asymmetric wetting boundary conditions on the floor of the channel. We show experimentally and numerically that by creating carefully chosen two-dimensional hydrophobic slip patterns on the floor of the channels, we can introduce stretching, folding, and/or recirculation in the flowing fluid volume, the essential elements to achieve mixing in the absence of turbulence. We also show that there are two distinctive pathways to produce homogeneous mixing in microchannels induced by the inhomogeneity of the boundary conditions. It can be achieved either by (1) introducing stretching, folding and twisting of fluid volumes, i.e., via a horse-shoe type transformation map, or (2) by creating chaotic advection, achieved through manipulation of the hydrophobic boundary patterns on the floor of the channels. We have also shown that by superposing stretching and folding with chaotic advection, mixing can be optimized in terms of significantly reducing mixing length, thereby opening up new design opportunities for simple yet efficient passive microfluidic reactors.

Numerical simulation and experimental investigation of gas-liquid two-phase flow in a complex microchannel

The present investigation evaluated a microchannel with variable cross sections and internal elements by experimental and numerical methods. The numerical and experimental phase interface of gas-liquid two-phase flow is captured through the Coupled Level-Set and Volume-Of-Fraction (CLSVOF) method and the canny edge detection method, respectively. The microchannel is analysed with different baffle angles (bending angle of the baffle, BA), and the optimal BA is chosen. The optimization goal is to increase the area of high-shear region and reduce the area of other region. Experiments involving the microchannel with optimal 67.5° BA are conducted. The experimental results confirm the microchannel mixing potency that the optimal structure of the high-shear region accelerates the fluid velocity, amplifies disturbances resulting from the merging of different gas phases and promotes the expansion of the turbulent domain.

Numerical investigation on pressure-driven electro osmatic flow and mixing in a constricted micro channel by triangular obstacle

Purpose The characteristics of fluid motions in micro-channel are strong fluid-wall surface interactions, high surface to volume ratio, extremely low Reynolds number laminar flow, surface roughness and wall surface or zeta potential. Due to zeta potential, an electrical double layer (EDL) is formed in the vicinity of the wall surface, namely, the stern layer (layer of immobile ions) and diffuse layer (layer of mobile ions). Hence, its competent designs demand more efficient micro-scale mixing mechanisms. This paper aims to therefore carry out numerical investigations of electro osmotic flow and mixing in a constricted microchannel by modifying the existing immersed boundary method. Design/methodology/approach The numerical solution of electro-osmotic flow is obtained by linking Navier–Stokes equation with Poisson and Nernst–Planck equation for electric field and transportation of ion, respectively. Fluids with different concentrations enter the microchannel and its mixing along its way is simulated by solving the governing equation specified for the concentration field. Both the electro-osmotic effects and channel constriction constitute a hybrid mixing technique, a combination of passive and active methods. In microchannels, the chief factors affecting the mixing efficiency were studied efficiently from results obtained numerically. Findings The results indicate that the mixing efficiency is influenced with a change in zeta potential (ζ), number of triangular obstacles, EDL thickness (λ). Mixing efficiency decreases with an increment in external electric field strength (Ex), Peclet number (Pe) and Reynolds number (Re). Mixing efficiency is increased from 28.2 to 50.2% with an increase in the number of triangular obstacles from 1 to 5. As the value of Re and Pe is decreased, the overall percentage increase in the mixing efficiency is 56.4% for the case of a mixing micro-channel constricted with five triangular obstacles. It is also vivid that as the EDL overlaps in the micro-channel, the mixing efficiency is 52.7% for the given zeta potential, Re and Pe values. The findings of this study may be useful in biomedical, biotechnological, drug delivery applications, cooling of microchips and deoxyribonucleic acid hybridization. Originality/value The process of mixing in microchannels is widely studied due to its application in various microfluidic devices like micro electromechanical systems and lab-on-a-chip devices. Hence, its competent designs demand more efficient micro-scale mixing mechanisms. The present study carries out numerical investigations by modifying the existing immersed boundary method, on pressure-driven electro osmotic flow and mixing in a constricted microchannel using the varied number of triangular obstacles by using a modified immersed boundary method. In microchannels, the theory of EDL combined with pressure-driven flow elucidates the electro-osmotic flow.

Numerical Simulation of Flow in a Wavy Wall Microchannel Using Immersed Boundary Method

Background: Fluid flow in microchannels is restricted to low Reynolds number regimes and hence inducing chaotic mixing in such devices is a major challenge. Over the years, the Immersed Boundary Method (IBM) has proved its ability in handling complex fluid-structure interaction problems. Objectives: Inspired by recent patents in microchannel mixing devices, we study passive mixing effects by performing two-dimensional numerical simulations of wavy wall in channel flow using IBM. Methods: The continuity and Navier-Stokes equations governing the flow are solved by fractional step based finite volume method on a staggered Cartesian grid system. Fluid variables are described by Eulerian coordinates and solid boundary by Lagrangian coordinates. A four-point Dirac delta function is used to couple both the coordinate variables. A momentum forcing term is added to the governing equation in order to impose the no-slip boundary condition between the wavy wall and fluid interface. Results: Parametric study is carried out to analyze the fluid flow characteristics by varying amplitude and wavelength of wavy wall configurations for different Reynolds number. Conclusion: Configurations of wavy wall microchannels having a higher amplitude and lower wavelengths show optimum results for mixing applications.

Auto-ignition performance of an electrified microreactor with constructal geometries

Common techniques employed to minimize ignition energy of gaseous fuels in micro reactors such as the use of catalyst are being plagued with setbacks. This report demonstrates the use of electric field to sustain combustion, minimize heat loss and enhance reactant mixing in microchannels of varying geometries. We set two defining constructal parameters for serpentine and straight microchannels of 0.05 to 0.25 to investigate geometric effect on the mixing of the reactants. Inlet concentration of propane and oxygen was set at 0.15 mol/dm3 and 0.7 mol/dm3 respectively. Reynolds numbers 400, 470, 530, 600 and 670 corresponding to inlet velocities of 0.06 m/s, 0.07 m/s, 0.08 m/s, 0.09 m/s and 0.1 m/s were used in the straight channel to study the dependence of reaction rate, temperature drop and reactants diffusion on the combustion process. We report that at an inlet temperature of 500 K which was below the propane’s auto ignition temperature of 743 K, the reaction could occur in the presence of a 50 V applied voltage. Increasing constructual parameter (β) yielded an increase in reaction rate and a decrease in temperature drop. At the same constructual parameter, the serpentine geometry displayed a better result with the peak reaction rate of 894 mol/m3s. More so, increase in the Reynolds number and shape factor for the two geometries led to an increase in reaction rate and propane consumption. These findings could be suitably beneficial to provide minimal fuel requirement for miniaturized vehicles and micro-heat engines.

Numerical investigation of the effect of electrode arrangement and geometry on electrothermal fluid flow pumping and mixing in microchannel

Micro-mixers, due to their vast applications in different fields have been of great interest to the researchers in recent years. In this study, a transient two-dimensional solver was developed in the open-source code, OpenFOAM to simulate electrothermal effect in a micro mixer. Then, as the first step, the obtained mixing quality and fluid velocity of three different geometries with various electrode arrangements were compared. According to the results, the micromixer with a trapezoidal chamber and a circular cavity has the highest mixing and pumping efficiency. Then, in the selected micromixer, the effect of different parameters was studied such as voltage, electrode heating, electrical conductivity and size of electrode installed on the cavity and channel. The results showed that, in some phase differences, increasing the voltage improves the quality of mixing, and in others, it has an inverse effect. In addition, it is found that increasing size of electrodes in the cavity, decreases the fluid velocity and increases the mixing quality. Finally, by increasing voltage on different electrodes, a remarkable micro-mixer with a mixing quality of 99 % and an average fluid velocity of 64 μm/s was obtained.

An immersed boundary method for simulations of flow and mixing in micro-channels with electro kinetic effects

Immersed boundary method (IBM) is a methodology in which any real stationary or moving complex bodies in flow or heat transfer or mixing field is treated by distinct forcing functions introduced in the respective governing equations, thereby its presence is dealt with much easiness in the computational domain. This work proposes an extension of an IBM based on discrete forcing approach to simulate pressure driven electro-osmotic flow and mixing enhancement in constricted micro channels. The fractional-step based temporal discretisation and finite-volume basedspatial discretisation on a staggered mesh are used to solve the governing equations. Concentration forcing function is a parameter, which is newly defined in this model to satisfy the no-flux boundary condition for species concentration on the IB and is introduced in convection-diffusion equation. The present computational model is validated with the published experimental and numerical results in the literature and finally some sample results are also exemplified.

An immersed boundary method for simulations of flow and mixing in micro-channels with electro kinetic effects

An immersed boundary method for simulations of flow and mixing in micro-channels with electro kinetic effects

Rapid mixing in microchannel using standing bulk acoustic waves

We present a technique for mixing the fluids in a microchannel using ultrasonic waves. Acoustic mixing is driven by the acoustic body force, which depends on the density gradient and speed of the sound gradient of the inhomogeneous fluid domain. In this work, mixing of fluids in a microchannel is achieved via an alternating multinode mixing method, which employs acoustic multinode standing waves of time-varying wavelengths at regular time intervals. The proposed technique is rapid, efficient, and found to enhance the mixing of fluids significantly. It is shown that the mixing time due to acoustic mixing (2–3 s) is reduced by two orders of magnitude compared to the mixing time only due to diffusion (400 s). Furthermore, we investigate the effects of the acoustic mixing on different fluid flow configurations and sound wave propagation directions as they have a direct influence on mixing time and have rarely been addressed previously. Remarkably, it is found that mixing performance is strongly dependent on the direction of the acoustic wave propagation. The acoustic field propagated parallel to the fluid-fluid interface mixes fluids rapidly (2–3 s) as compared to the acoustic field propagated perpendicular to the fluid-fluid interface (40 s).

Novel 3-D T-Shaped Passive Micromixer Design with Helicoidal Flows

Laminar fluid flow and advection-dominant transport produce ineffective mixing conditions in micromixers. In these systems, a desirable fluid mixing over a short distance may be achieved using special geometries in which complex flow paths are generated. In this paper, a novel design, utilizing semi-circular ridges, is proposed to improve mixing in micro channels. Fluid flow and scalar transport are investigated employing Computational Fluid Dynamics (CFD) tool. Mixing dynamics are investigated in detail for alternative designs, injection, and diffusivity conditions. Results indicate that the convex alignment of semi-circular elements yields a specific, helicoidal-shaped fluid flow along the mixing channel which in turn enhances fluid mixing. In all cases examined, homogenous concentration distributions with mixing index values over 80% are obtained. When it is compared to the classical T-shaped micromixer, the novel design increases mixing index and mixing performance values by the factors of 8.7 and 3.3, respectively. It is also shown that different orientations of ridges adversely affect the mixing efficiency by disturbing the formation of helicoidal-shaped flow profile.

Studying on the Optimum Frequency of Driving Voltage of Electro-osmotic

Electro-osmotic actuation is an effective method of mixing micro-scale fluids. In this paper, an annular micro-channel mixing model, in which electro-osmosis are used to mix fluids, is established to simulate fluid mixing condition. Simulation results show that for fixed flow parameters, there is a supply frequency that can optimize the mixing efficiency. As the frequency increases, the mixing efficiency increases firstly and then decreases. The optimal mixing efficiency is about 95% at the frequency of 25 Hz. Furthermore, the influence of flow velocity, electrode position and voltage amplitude on the optimal frequency is proposed, and the physical mechanisms are discussed. Numerical analysis shows the electro-osmotic of electric field can produce a vortex, which can increase the mixing efficiency in micro-channels effectively. The mixing efficiency can be improved by increasing voltage amplitude and the electrode gap. When the velocity of fluids increases, the residence time of particles decreases, the frequency of optimization will increase accordingly.

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The present investigation evaluated a microchannel with variable cross sections and internal elements by experimental and numerical methods. The numerical and experimental phase interface of gas-liquid two-phase flow is captured through the Coupled Level-Set and Volume-Of-Fraction (CLSVOF) method and the canny edge detection method, respectively. The microchannel is analysed with different baffle angles (bending angle of the baffle, BA), and the optimal BA is chosen. The optimization goal is to increase the area of high-shear region and reduce the area of other region. Experiments involving the microchannel with optimal 67.5° BA are conducted. The experimental results confirm the microchannel mixing potency that the optimal structure of the high-shear region accelerates the fluid velocity, amplifies disturbances resulting from the merging of different gas phases and promotes the expansion of the turbulent domain.