Rapid magnetic microfluidic mixer utilizing AC electromagnetic field

Prednisolone Nanoprecipitation with Dean Instability Microfluidics Mixer

Dean flow and Dean instability play an important role in inertial microfluidics, with a wide application in mixing and sorting. However, most studies are limited to Dean flow in the microscale. This work first reports the application of Dean instability on organic nanoparticles synthesis at De up to 198. The channel geometry (the tortuous channel) is optimized by simulation, in which the mixing efficiency is considered. With the optimized design, prednisolone nanoparticles are synthesized, and the size of the most abundant prednisolone nanoparticles is down to 100 nm with an increase in the Re and De and smallest size down to 46 nm. This work serves as an ice-breaker to the real application of Dean instability by demonstrating its ability in mixing and nanomaterials like nanoparticle synthesis.

Bubble-Enhanced Mixing Induced by Standing Surface Acoustic Waves (SSAWs) in Microchannel

BAW-based micromixers usually achieve mixing enhancement with acoustic-induced bubbles, while SAW-based micromixers usually enhance mixing efficiency by varying the configuration of IDTs and microchannels. In this paper, bubble-enhanced acoustic mixing induced by standing surface acoustic waves (SSAWs) in a microchannel is proposed and experimentally demonstrated. Significant enhancement in the mixing efficiency was achieved after the bubbles were stimulated in our acoustofluidic microdevice. With an applied voltage of 5 V, 50 times amplified, the proposed mixing microdevice could achieve 90.8% mixing efficiency within 60 s at a flow rate of 240 μL/h. The bubbles were generated from acoustic cavitation assisted by the temperature increase resulting from the viscous absorption of acoustic energy. Our results also suggest that a temperature increase is harmful to microfluidic devices and temperature monitoring. Regulation is essential, especially in chemical and biological applications.

Active micromixer platform based on Lorentz force for lab-on-a-chip application

Mixing in an active micromixer was achieved using Lorentz force-assisted actuation of an enameled copper wire. A single-step template-assisted soft lithography technique was used to construct the mixing chamber. The chamber had a volume of 1.86 µl. The application of a square wave alternating current in tandem with tension in the wire provided the necessary conditions for the resonant oscillation frequency of the wire. The repeatability of the ratio of higher harmonics to the fundamental frequencies of the oscillating wire conforms to standardization of the device fabrication, assembly, and functionality. Simulations and experiments were performed to validate uniform temperature distribution in the mixing chamber. Real-time optical detection of the sample assisted in sensing the completion of chemical reactions in the chamber. Mixing of various aqueous based chemical reactions was performed. It was found that mixing efficiency was greater than 95 percent. Multiple devices were fabricated to show the usability and reproducibility of the system.

Understanding Interdependencies between Mechanical Velocity and Electrical Voltage in Electromagnetic Micromixers

Micromixers are critical components in the lab-on-a-chip or micro total analysis systems technology found in micro-electro-mechanical systems. In general, the mixing performance of the micromixers is determined by characterising the mixing time of a system, for example the time or number of circulations and vibrations guided by tracers (i.e., fluorescent dyes). Our previous study showed that the mixing performance could be detected solely from the electrical measurement. In this paper, we employ electromagnetic micromixers to investigate the correlation between electrical and mechanical behaviours in the mixer system. This work contemplates the “anti-reciprocity” concept by providing a theoretical insight into the measurement of the mixer system; the work explains the data interdependence between the electrical point impedance (voltage per unit current) and the mechanical velocity. This study puts the electromagnetic micromixer theory on a firm theoretical and empirical basis.

Ultrafast star-shaped acoustic micromixer for high throughput nanoparticle synthesis

We present an acoustically actuated microfluidic mixer, which can operate at flowrates reaching 8 ml min^-1, providing a 50-fold improvement in throughput compared to previously demonstrated acoustofluidic approaches. The device consists of a robust silicon based micro-mechanical oscillator, sandwiched between two polymeric channels which guide the fluids in and out of the system. The chip is actuated by application of an oscillatory electrical signal onto a piezoelectric disk coupled to the substrate by adhesive. At the optimal frequency, this acoustofluidic system can homogenise two fluids with a relative mixing efficiency of 91%, within 4.1 ms from first contact. The micromixer has been used to synthesize two different systems: Budesonide nanodrugs with an average diameter of 80 ± 22 nm, and DNA nanoparticles with an average diameter of 63.3 ± 24.7 nm.

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.

Magnetic fiber headspace solid-phase microextraction coupled to GC-MS for the extraction and quantitation of polycyclic aromatic hydrocarbons

A technique was developed for magnetic fiber headspace-solid phase microextraction (MF-HS-SPME) of polycyclic aromatic hydrocarbons (PAHs). The efficiency of the extraction of a steel SPME fiber coated with an aminoethyl-functionalized SBA-15 (Santa Barbara Amorphous 15; a nanoporous sorbent) is substantially improved after its magnetization during HS-SPME. The effects of magnetic field strength, extraction temperature, extraction time, moisture content of the sample, desorption time and desorption temperature were optimized using a simplex method. The application of a moderately strong magnetic field to the fiber results in up to 135% increase in the extraction efficiency and wider linear dynamic ranges. The PAHs (specifically naphthalene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene and pyrene) were then quantified by GC-MS analysis. Comparison of an electromagnet and a permanent magnet indicated the superior effect of the permanent magnet for the target analytes due to the Ohmic heating of the magnetic coil and its negative effect on the extraction of some of the PAHs. The limits of detections of the PAHs are between 0.17 to 0.57 ng g^-1 by using the electromagnet, and between 0.10 and 0.32 ng g^-1 for the permanent magnet. Relative standard deviations of 2.9 to 7.6% were obtained for six replicated analyses of the analytes. The method was applied to some polluted soil samples, and satisfactory results were obtained. Graphical abstract Schematic representation of the designed magnetic fiber headspace solid-phase microextraction (MF-HS-SPME) system using (a) an electromagnet, (b) a pair of permanent disc magnets.

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.

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.

Magnetophoresis of diamagnetic microparticles in a weak magnetic field

Magnetic manipulation is a promising technique for lab-on-a-chip platforms. The magnetic approach can avoid problems associated with heat, surface charge, ionic concentration and pH level. The present paper investigates the migration of diamagnetic particles in a ferrofluid core stream that is sandwiched between two diamagnetic streams in a uniform magnetic field. The three-layer flow is expanded in a circular chamber for characterisation based on imaging of magnetic nanoparticles and fluorescent microparticles. A custom-made electromagnet generates a uniform magnetic field across the chamber. In a relatively weak uniform magnetic field, the diamagnetic particles in the ferrofluid move and spread across the chamber. Due to the magnetization gradient formed by the ferrofluid, diamagnetic particles undergo negative magnetophoresis and move towards the diamagnetic streams. The effects of magnetic field strength and the concentration of diamagnetic particles are studied in detail.

Configurations and control of magnetic fields for manipulating magnetic particles in microfluidic applications: magnet systems and manipulation mechanisms

The use of a magnetic field for manipulating the motion of magnetic particles in microchannels has attracted increasing attention in microfluidic applications. Generation of a flexible and controllable magnetic field plays a crucial role in making better use of the particle manipulation technology. Recent advances in the development of magnet systems and magnetic field control methods have shown that it has great potential for effective and accurate manipulation of particles in microfluidic systems. Starting with the analysis of magnetic forces acting on the particles, this review gives the configurations and evaluations of three main types of magnet system proposed in microfluidic applications. The interaction mechanisms of magnetic particles with magnetic fields are also discussed.

Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application

This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5-15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (ζ) potential analysis. Bulk-heterojunction organic photovoltaic cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had good preservability and stability in working solution and the synthetic microfluidic system provided a low-cost, environmentally friendly approach to the continuous production of ZnO NPs.

Ferrofluid-based reconfigurable optofluidic switches for integrated sensing and digital data storage

We present a low-cost, reconfigurable, parallel optofluidic switch that exploits the optical and magnetic properties of water-based ferrofluid. Each switch is composed of an integrated waveguide orthogonally crossing a microfluidic channel containing high-index oil and a ferrofluid plug. The switch is turned ON or OFF by movement of the ferrofluid plug. In contrast to conventional integrated switches, ferrofluid plugs act as switching mechanisms that are portable and reconfigurable. Switches are demonstrated in parallel geometries for single and multimode waveguides. Possible applications include optofluidic memory, multiplexed sensing for lab-on-chip, or frequency-encoded laser excitation.

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.

Integrated microfluidic chip for rapid DNA digestion and time-resolved capillary electrophoresis analysis

An integrated microfluidic chip is proposed for rapid DNA digestion and time-resolved capillary electrophoresis (CE) analysis. The chip comprises two gel-filled chambers for DNA enrichment and purification, respectively, a T-form micromixer for DNA/restriction enzyme mixing, a serpentine channel for DNA digestion reaction, and a CE channel for on-line capillary electrophoresis analysis. The DNA and restriction enzyme are mixed electroomostically using a pinched-switching DC field. The experimental and numerical results show that a mixing performance of 97% is achieved within a distance of 1 mm from the T-junction when a driving voltage of 90 V/cm and a switching frequency of 4 Hz are applied. Successive mixing digestion and capillary electrophoresis operation clearly present the changes on digesting φx-174 DNA in different CE runs. The time-resolved electropherograms show that the proposed device enables a φx-174 DNA sample comprising 11 fragments to be concentrated and analyzed within 24 min. Overall, the results presented in this study show that the proposed microfluidic chip provides a rapid and effective tool for DNA digestion and CE analysis applications.

Distillation and detection of SO2 using a microfluidic chip

A miniaturized distillation system is presented for separating sulfurous acid (H(2)SO(3)) into sulfur dioxide (SO(2)) and water (H(2)O). The major components of the proposed system include a microfluidic distillation chip, a power control module, and a carrier gas pressure control module. The microfluidic chip is patterned using a commercial CO(2) laser and comprises a serpentine channel, a heating zone, a buffer zone, a cooling zone, and a collection tank. In the proposed device, the H(2)SO(3) solution is injected into the microfluidic chip and is separated into SO(2) and H(2)O via an appropriate control of the distillation time and temperature. The gaseous SO(2) is then transported into the collection chamber by the carrier gas and is mixed with DI water. Finally, the SO(2) concentration is deduced from the absorbance measurements obtained using a spectrophotometer. The experimental results show that a correlation coefficient of R(2) = 0.9981 and a distillation efficiency as high as 94.6% are obtained for H(2)SO(3) solutions with SO(2) concentrations in the range of 100-500 ppm. The SO(2) concentrations of two commercial red wines are successfully detected using the developed device. Overall, the results presented in this study show that the proposed system provides a compact and reliable tool for SO(2) concentration measurement purposes.

Miniaturized protein microarray with internal calibration as point-of-care device for diagnosis of neonatal sepsis

Neonatal sepsis is still a leading cause of death among newborns. Therefore a protein-microarray for point-of-care testing that simultaneously quantifies the sepsis associated serum proteins IL-6, IL-8, IL-10, TNF alpha, S-100, PCT, E-Selectin, CRP and Neopterin has been developed. The chip works with only a 4 μL patient serum sample and hence minimizes excessive blood withdrawal from newborns. The 4 μL patient samples are diluted with 36 μL assay buffer and distributed to four slides for repetitive measurements. Streptavidin coated magnetic particles that act as distinct stirring detection components are added, not only to stir the sample, but also to detect antibody antigen binding events. We demonstrate that the test is complete within 2.5 h using a single step assay. S-100 conjugated to BSA is spotted in increasing concentrations to create an internal calibration. The presented low volume protein-chip fulfills the requirements of point-of-care testing for accurate and repeatable (CV < 14%) quantification of serum proteins for the diagnosis of neonatal sepsis.

A Planar Microfluidic Mixer Based on Logarithmic Spirals

A passive, planar micromixer design based on logarithmic spirals is presented. The device was fabricated using polydimethylsiloxane soft photolithography techniques, and mixing performance was characterized via numerical simulation and fluorescent microscopy. Mixing efficiency initially declined as Reynolds number increased, and this trend continued until a Reynolds number of 15 where a minimum was reached at 53%. Mixing efficiency then began to increase reaching a maximum mixing efficiency of 86% at Re = 67. Three-dimensional simulations of fluid mixing in this design were compared to other planar geometries such as the Archimedes spiral and Meandering-S mixers. The implementation of logarithmic curvature offers several unique advantages that enhance mixing, namely a variable cross-sectional area and a logarithmically varying radius of curvature that creates 3-D Dean vortices. These flow phenomena were observed in simulations with multilayered fluid folding and validated with confocal microscopy. This design provides improved mixing performance over a broader range of Reynolds numbers than other reported planar mixers, all while avoiding external force fields, more complicated fabrication processes, and the introduction of flow obstructions or cavities that may unintentionally affect sensitive or particulate-containing samples. Due to the planar design requiring only single-step lithographic features, this compact geometry could be easily implemented into existing micro-total analysis systems requiring effective rapid mixing.

Experimental and numerical analysis of high-resolution injection technique for capillary electrophoresis microchip

This study presents an experimental and numerical investigation on the use of high-resolution injection techniques to deliver sample plugs within a capillary electrophoresis (CE) microchip. The CE microfluidic device was integrated into a U-shaped injection system and an expansion chamber located at the inlet of the separation channel, which can miniize the sample leakage effect and deliver a high-quality sample plug into the separation channel so that the detection performance of the device is enhanced. The proposed 45° U-shaped injection system was investigated using a sample of Rhodamine B dye. Meanwhile, the analysis of the current CE microfluidic chip was studied by considering the separation of Hae III digested ϕx-174 DNA samples. The experimental and numerical results indicate that the included 45° U-shaped injector completely eliminates the sample leakage and an expansion separation channel with an expansion ratio of 2.5 delivers a sample plug with a perfect detection shape and highest concentration intensity, hence enabling an optimal injection and separation performance.

Microfluidic mixing: a review

The aim of microfluidic mixing is to achieve a thorough and rapid mixing of multiple samples in microscale devices. In such devices, sample mixing is essentially achieved by enhancing the diffusion effect between the different species flows. Broadly speaking, microfluidic mixing schemes can be categorized as either “active”, where an external energy force is applied to perturb the sample species, or “passive”, where the contact area and contact time of the species samples are increased through specially-designed microchannel configurations. Many mixers have been proposed to facilitate this task over the past 10 years. Accordingly, this paper commences by providing a high level overview of the field of microfluidic mixing devices before describing some of the more significant proposals for active and passive mixers.