Numerical analysis of a rapid magnetic microfluidic mixer

Computational Fluid-Structure Interaction in Microfluidics

Micro elastofluidics is a transformative branch of microfluidics, leveraging the fluid-structure interaction (FSI) at the microscale to enhance the functionality and efficiency of various microdevices. This review paper elucidates the critical role of advanced computational FSI methods in the field of micro elastofluidics. By focusing on the interplay between fluid mechanics and structural responses, these computational methods facilitate the intricate design and optimisation of microdevices such as microvalves, micropumps, and micromixers, which rely on the precise control of fluidic and structural dynamics. In addition, these computational tools extend to the development of biomedical devices, enabling precise particle manipulation and enhancing therapeutic outcomes in cardiovascular applications. Furthermore, this paper addresses the current challenges in computational FSI and highlights the necessity for further development of tools to tackle complex, time-dependent models under microfluidic environments and varying conditions. Our review highlights the expanding potential of FSI in micro elastofluidics, offering a roadmap for future research and development in this promising area.

Mixing Mechanism of Microfluidic Mixer with Staggered Virtual Electrode Based on Light-Actuated AC Electroosmosis

In this paper, we present a novel microfluidic mixer with staggered virtual electrode based on light-actuated AC electroosmosis (LACE). We solve the coupled system of the flow field described by Navier–Stokes equations, the described electric field by a Laplace equation, and the concentration field described by a convection–diffusion equation via a finite-element method (FEM). Moreover, we study the distribution of the flow, electric, and concentration fields in the microchannel, and reveal the generating mechanism of the rotating vortex on the cross-section of the microchannel and the mixing mechanism of the fluid sample. We also explore the influence of several key geometric parameters such as the length, width, and spacing of the virtual electrode, and the height of the microchannel on mixing performance; the relatively optimal mixer structure is thus obtained. The current micromixer provides a favorable fluid-mixing method based on an optical virtual electrode, and could promote the comprehensive integration of functions in modern microfluidic-analysis systems.

Joule heating effects on electrokinetic flows with conductivity gradients

Instability occurs in the electrokinetic flow of fluids with conductivity and/or permittivity gradients if the applied electric field is beyond a critical value. Understanding such an electrokinetic instability is significant for both improved transport (via the suppressed instability) and enhanced mixing (via the promoted instability) of liquid samples in microfluidic applications. This work presents the first study of Joule heating effects on electrokinetic microchannel flows with conductivity gradients using a combined experimental and numerical method. The experimentally observed flow patterns and measured critical electric fields under Joule heating effects to different extents are reasonably predicted by a depth-averaged numerical model. It is found that Joule heating increases the critical electric field for the onset of electrokinetic instability because the induced fluid temperature rise and in turn the fluid property change (primarily the decreased permittivity) lead to a smaller electric Rayleigh number.

Numerical study on the effect of planar normal and Halbach magnet arrays on micromixing

The effective mixing process is critical in biological and chemical processes. The main objective of the present study is to investigate the influence of normal and Halbach magnet arrays on the mixing performance of a three-inlet micromixer numerically. In this microdevice, ferrofluid is injected into the center inlet, and water is injected into two other inlets. The influence of Remanent Flux Density Norm (RFDN), number of magnets, magnet distance from the main microchannel entrance, and inlet flow rate is considered. It is revealed that the micromixer with magnets exhibits a 165% improvement in the mixing efficiency compared to the one with no magnetic field. The results show that increasing the magnetic field does not always increase the mixing quality. Even in some cases, it has a negative effect. It is demonstrated that the mixing efficiency is strongly influenced by the magnet arrangement. An optimal position is found for the magnet arrangement to achieve the maximum mixing efficiency of 91%. Contrary to the normal configuration, Halbach magnet array creates a parabolic profile for flux density. Halbach array can improve the mixing performance, depending on all magnets’ RFDN. The proposed microchannel can be used as a useful device for biological applications.

Integrated Immunomagnetic Bead-Based Microfluidic Chip for Exosomes Isolation

Exosomes are essential early biomarkers for health monitoring and cancer diagnosis. A prerequisite for further investigation of exosomes is the isolation, which is technically challenging due to the complexity of body fluids. This paper presents the development of an integrated microfluidic chip for exosomes isolation, which combines the traditional immunomagnetic bead-based protocol and the recently emerging microfluidic approach, resulting in benefits from both the high-purity of the former and the automated continuous superiority of the latter. The chip was designed based on an S-shaped micromixer with embedded baffle. The excellent mixing efficiency of this micromixer compared with Y-shaped and S-shaped micromixers was verified by simulation and experiments. The photolithography technique was employed to fabricate the integrated microfluidic chip, and the manufacturing process was elucidated. We finally established an experimental platform for exosomes isolation with the fabricated microfluidic chip built in. Exosomes isolation experiments were conducted using this platform. The distribution and morphology of the isolated exosomes were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Quantitative size analyses based on transmission electron micrographs indicated that most of the obtained particles were between 30 and 150 nm. Western blot analyses of the isolated exosomes and the serum were conducted to verify the platform’s capability of isolating a certain subpopulation of exosomes corresponding to specified protein markers (CD63). The complete time for isolation of 150 μL serum samples was approximately 50 min, which was highly competitive with the reported existing protocols. Experimental results proved the capacity of the established integrated microfluidic chip for exosomes isolation with high purity, high integrity, and excellent efficiency. The platform can be further developed to make it possible for practical use in clinical applications as a universal exosomes isolation and characterization tool.

Rapid Microfluidic Mixer Based on Ferrofluid and Integrated Microscale NdFeB-PDMS Magnet

Ferrofluid-based micromixers have been widely used for a myriad of microfluidic industrial applications in biochemical engineering, food processing, and detection/analytical processes. However, complete mixing in micromixers is extremely time-consuming and requires very long microchannels due to laminar flow. In this paper, we developed an effective and low-cost microfluidic device integrated with microscale magnets manufactured with neodymium (NdFeB) powders and polydimethylsiloxane (PDMS) to achieve rapid micromixing between ferrofluid and buffer flow. Experiments were conducted systematically to investigate the effect of flow rate, concentration of the ferrofluid, and micromagnet NdFeB:PDMS mass ratio on the mixing performance. It was found that mixing is more efficient with lower total flow rates and higher ferrofluid concentration, which generate greater magnetic forces acting on both streamwise and lateral directions to increase the intermixing of the fluids within a longer residence time. Numerical models were also developed to simulate the mixing process in the microchannel under the same conditions and the simulation results indicated excellent agreements with the experimental data on mixing performance. Combining experimental measurements and numerical simulations, this study demonstrates a simple yet effective method to realize rapid mixing for lab-on-chip systems.

Recent Advances in Continuous-Flow Particle Manipulations Using Magnetic Fluids

Magnetic field-induced particle manipulation is simple and economic as compared to other techniques (e.g., electric, acoustic, and optical) for lab-on-a-chip applications. However, traditional magnetic controls require the particles to be manipulated being magnetizable, which renders it necessary to magnetically label particles that are almost exclusively diamagnetic in nature. In the past decade, magnetic fluids including paramagnetic solutions and ferrofluids have been increasingly used in microfluidic devices to implement label-free manipulations of various types of particles (both synthetic and biological). We review herein the recent advances in this field with focus upon the continuous-flow particle manipulations. Specifically, we review the reported studies on the negative magnetophoresis-induced deflection, focusing, enrichment, separation, and medium exchange of diamagnetic particles in the continuous flow of magnetic fluids through microchannels.

Electrokinetic instability in microchannel ferrofluid/water co-flows

Electrokinetic instability refers to unstable electric field-driven disturbance to fluid flows, which can be harnessed to promote mixing for various electrokinetic microfluidic applications. This work presents a combined numerical and experimental study of electrokinetic ferrofluid/water co-flows in microchannels of various depths. Instability waves are observed at the ferrofluid and water interface when the applied DC electric field is beyond a threshold value. They are generated by the electric body force that acts on the free charge induced by the mismatch of ferrofluid and water electric conductivities. A nonlinear depth-averaged numerical model is developed to understand and simulate the interfacial electrokinetic behaviors. It considers the top and bottom channel walls’ stabilizing effects on electrokinetic flow through the depth averaging of three-dimensional transport equations in a second-order asymptotic analysis. This model is found accurate to predict both the observed electrokinetic instability patterns and the measured threshold electric fields for ferrofluids of different concentrations in shallow microchannels.

A Rapid Magnetofluidic Micromixer Using Diluted Ferrofluid

Effective and rapid mixing is essential for various chemical and biological assays. The present work describes a simple and low-cost micromixer based on magnetofluidic actuation. The device takes advantage of magnetoconvective secondary flow, a bulk flow induced by an external magnetic field, for mixing. A superparamagnetic stream of diluted ferrofluid and a non-magnetic stream are introduced to a straight microchannel. A permanent magnet placed next to the microchannel induced a non-uniform magnetic field. The magnetic field gradient and the mismatch in magnetic susceptibility between the two streams create a body force, which leads to rapid and efficient mixing. The micromixer reported here could achieve a high throughput and a high mixing efficiency of 88% in a relatively short microchannel.

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.

Prospects of Magnetic Nanoparticles for Magnetic Field-Assisted Mixing of Fluids with Relevance to Chemical Engineering

Utilization of efficient, safe and controllable alternative energization approaches towards green and sustainable processes is vigorously explored in the field of process intensification. In this contribution, magnetic fields are specifically discussed and possible mechanisms to exploit this form of energy excitation for fluid-phase mixing in confined spaces are introduced. Magnetic nanofluids are par excellence the most suitable media for transmission of magnetic energy into a target fluid. In addition, their benign nature makes them suitable candidates for biological applications in microfluidics. The interaction of magnetic fluids with magnetic fields, as governed by the equations of motion in ferrohydrodynamics, can generate different mechanisms for fluidic actuations. These mechanisms are mainly the result of the type of magnetic field enabled, e.g., non-uniform static, oscillating or rotating magnetic fields, their strength or the magnetization of polar fluids, in addition to the momentum exchange induced between the rotating magnetic nanoparticles and the carrier fluid in rotating magnetic fields. With an emphasis on applications in microfluidic devices, the review of recent advances in the present contribution shows how such a variety of magnetic fields can be taken advantage of to mix fluids. Mixing in electrically conducting fluids in the framework of magnetohydrodynamics, as another class of magnetic field-assisted mixing is also another subject of this review. This latter category benefits from the absence of magnetic nanoparticles but on the other hand requires complex structuring of mixing devices as imposed by indispensable and appropriate interactions between electric and magnetic fields. The reviewed research findings in this category show how the generation of complex fluid motions is attainable specifically in micron-sized conduits.

Mass transport improvement in microscale using diluted ferrofluid and a non-uniform magnetic field

We investigate the mass transport enhancement of a non-magnetic fluorescent dye with the help of diluted ferrofluid and a non-uniform magnetic field.

Rapid magnetofluidic mixing in a uniform magnetic field

This paper reports the investigation of mixing phenomena caused by the interaction between a uniform magnetic field and a magnetic fluid in a microfluidic chamber. The flow system consists of a water-based ferrofluid and a mixture of DI water and glycerol. Under a uniform magnetic field, the mismatch in magnetization of the fluids leads to instability at the interface and subsequent rapid mixing. The mismatch of magnetization is determined by concentration of magnetic nanoparticles. Full mixing at a relatively low magnetic flux density up to 10 mT can be achieved. The paper discusses the impact of key parameters such as magnetic flux density, flow rate ratio and viscosity ratio on the mixing efficiency. Two main mixing regimes are observed. In the improved diffusive mixing regime under low field strength, magnetic particles of the ferrofluid migrate into the diamagnetic fluid. In the bulk transport regime under high field strength, the fluid system is mixed rapidly by magnetically induced secondary flow in the chamber. The mixing concept potentially provides a wireless solution for a lab-on-a-chip system that is low-cost, robust, free of induced heat and independent of pH level or ion concentration.

Convenient quantification of methanol concentration detection utilizing an integrated microfluidic chip

A rapid and simple technique is proposed for methanol concentration detection using a PMMA (Polymethyl-Methacrylate) microfluidic chip patterned using a commercially available CO2 laser scriber. In the proposed device, methanol and methanol oxidase (MOX) are injected into a three-dimensional circular chamber and are mixed via a vortex stirring effect. The mixture is heated to prompt the formation of formaldehyde and is flowed into a rectangular chamber, to which fuchsin-sulphurous acid is then added. Finally, the microchip is transferred to a UV spectrophotometer for methanol detection purposes. The experimental results show that a correlation coefficient of R(2) = 0.9940 is obtained when plotting the optical density against the methanol concentration for samples and an accuracy as high as 93.1% are compared with the determined by the high quality gas chromatography with concentrations in the range of 2 ∼ 100 ppm. The methanol concentrations of four commercial red wines are successfully detected using the developed device. Overall, the results show that the proposed device provides a rapid and accurate means of detecting the methanol concentration for a variety of applications in the alcoholic beverage inspection and control field.

High-performance microfluidic rectifier based on sudden expansion channel with embedded block structure

A high-performance microfluidic rectifier incorporating a microchannel and a sudden expansion channel is proposed. In the proposed device, a block structure embedded within the expansion channel is used to induce two vortex structures at the end of the microchannel under reverse flow conditions. The vortices reduce the hydraulic diameter of the microchannel and, therefore, increase the flow resistance. The rectification performance of the proposed device is evaluated by both experimentally and numerically. The experimental and numerical values of the rectification performance index (i.e., the diodicity, Di) are found to be 1.54 and 1.76, respectively. Significantly, flow rectification is achieved without the need for moving parts. Thus, the proposed device is ideally suited to the high pressure environment characteristic of most micro-electro-mechanical-systems (MEMS)-based devices. Moreover, the rectification performance of the proposed device is superior to that of existing valveless rectifiers based on Tesla valves, simple nozzle/diffuser structures, or cascaded nozzle/diffuser structures.