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.

/pdf/microfluidic-mixing-a-review-3egj61w5ox.pdf

Microfluidic Mixing: A Review

Keywords: Spin-Valve Sensors ; Cell Tracking Velocimetry ; On-A-Chip ; Polymerase-Chain-Reaction ; Total Analysis Systems ; Iron-Oxide Nanoparticles ; Field-Flow Fractionation ; Cross-Coupling Reactions ; Circulating Tumor-Cells ; Mode Magnetophoretic Microseparator Reference LMIS2-ARTICLE-2010-004doi:10.1021/cr9001929View record in Web of Science Record created on 2010-01-20, modified on 2016-08-08

Microfluidic applications of magnetic particles for biological analysis and catalysis.

Several gallium-based liquid metal alloys are liquid at room temperature. As 'liquid', such alloys have a low viscosity and a high surface tension while as 'metal', they have high thermal and electrical conductivities, similar to mercury. However, unlike mercury, these liquid metal alloys have low toxicity and a negligible vapor pressure, rendering them much safer. In comparison to mercury, the distinguishing feature of these alloys is the rapid formation of a self-limiting atomically thin layer of gallium oxide over their surface when exposed to oxygen. This oxide layer changes many physical and chemical properties of gallium alloys, including their interfacial and rheological properties, which can be employed and modulated for various applications in microfluidics. Injecting liquid metal into microfluidic structures has been extensively used to pattern and encapsulate highly deformable and reconfigurable electronic devices including electrodes, sensors, antennas, and interconnects. Likewise, the unique features of liquid metals have been employed for fabricating miniaturized microfluidic components including pumps, valves, heaters, and electrodes. In this review, we discuss liquid metal enabled microfluidic components, and highlight their desirable attributes including simple fabrication, facile integration, stretchability, reconfigurability, and low power consumption, with promising applications for highly integrated microfluidic systems.

Liquid metal enabled microfluidics

The demand for easy to use and cost effective medical technologies inspires scientists to develop innovative lab-on-chip technologies for point-of-care in vitro diagnostic testing. To fulfill medical needs, the tests should be rapid, sensitive, quantitative, and miniaturizable, and need to integrate all steps from sample-in to result-out. Here, we review the use of magnetic particles actuated by magnetic fields to perform the different process steps that are required for integrated lab-on-chip diagnostic assays. We discuss the use of magnetic particles to mix fluids, to capture specific analytes, to concentrate analytes, to transfer analytes from one solution to another, to label analytes, to perform stringency and washing steps, and to probe biophysical properties of the analytes, distinguishing methodologies with fluid flow and without fluid flow (stationary microfluidics). Our review focuses on efforts to combine and integrate different magnetically actuated assay steps, with the vision that it will become possible in the future to realize integrated lab-on-chip biosensing assays in which all assay process steps are controlled and optimized by magnetic forces.

/pdf/integrated-lab-on-chip-biosensing-systems-based-on-magnetic-256i86hvca.pdf

Integrated lab-on-chip biosensing systems based on magnetic particle actuation--a comprehensive review.

Performances of a magnetic particle driven micromixer are predicted numerically. This micromixer takes advantages of mixing enhancements induced by alternating actuation of magnetic particles suspended in the fluid. Effects of magnetic actuation force, switching frequency and channel’s lateral dimension have been investigated. Numerical results show that the magnetic particle actuation at an appropriate frequency causes effective mixing and the optimum switching frequency depends on the channel’s lateral dimension and the applied magnetic force. The maximum efficiency is obtained at a relatively high operating frequency for large magnetic actuation forces and narrow microchannels. If the magnetic particles are actuated with a much higher or lower frequency than the optimum switching frequency, they tend to add limited agitation to the fluid flow and do not enhance the mixing significantly. The optimum switching frequency obtained from the present numerical prediction is in good agreement with the theoretical analysis. The proposed mixing scheme not only provides an excellent mixing, even in simple microchannel, but also can be easily applied to lab-on-a-chip applications with a pair of external electromagnets.

http://www.mstf.wsu.edu/articles/pros_MFNF2.pdf

A rapid magnetic particle driven micromixer

Performances of a hybrid electrokinetic-passive micromixer are predicted numerically. An h/p-type spectral element method is used to simulate the mixing behavior in microdevices. The numerical algorithm employs modal spectral expansion in quadrilateral and unstructured triangular meshes and provides high-order numerical accuracy. A second-order accurate, stiffly stable integration scheme is used for temporal integration. In the numerical technique, the electric double layer is not resolved to avoid expensive computation, rather a slip velocity is assigned at the channel surface based on the electric field and the electroosmotic mobility. The presented hybrid mixing scheme takes advantages of mixing enhancements induced by asymmetric flow geometries and electrokinetic relay actuation. Effects of relay frequency, applied electric potential, channel width, and channel geometry on micromixing have been conducted. Numerical results show that electrokinetic relay at an appropriate frequency causes effective mixing. Moreover, asymmetric flow geometries and narrow channel width are critical for ultraeffective mixing. The proposed hybrid mixing scheme not only provides excellent mixing within very short time, but also can easily be integrated with microdevices for “lab-on-a-chip” applications because there is no need of any external mechanical pumps.

A Microfluidic Mixer Utilizing Electrokinetic Relay Switching and Asymmetric Flow Geometries

Numerical study on a rapid micromixer based on hybrid electrokinetic relay and asymmetric serpentine structures is presented. Effective mixing of liquids is essential in many applications such as drug delivery, DNA analysis/sequencing, pheromone synthesis in micro bioreactors, and biological/chemical agent detections. Rapid mixing can reduce the analysis time and permit high throughput in lab-on-a-chip or micro total analysis systems (μTAS). The proposed hybrid mixing takes advantages of both mixing enhancements induced by asymmetric flow geometries and the electrokinetic relay actuating. Simulation results show that the micro mixer is able to achieve high mixing efficiencies (94.3%) in short time (1.2s). Effects of relay frequency, electric field and channel geometry on micro-mixing have been conducted. Numerical results show that electrokinetic relay at an appropriate frequency causes effective micromixing. Moreover asymmetric flow geometries are critical for ultra effective mixing.Copyright © 2005 by ASME

A Hybrid Rapid Microfluidic Mixer Utilizing Electrokinetic Relay and Asymmetric Flow Geometries for Lab-on-a-Chip Applications

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A rapid magnetic particle driven micromixer

A Microfluidic Mixer Utilizing Electrokinetic Relay Switching and Asymmetric Flow Geometries

A Hybrid Rapid Microfluidic Mixer Utilizing Electrokinetic Relay and Asymmetric Flow Geometries for Lab-on-a-Chip Applications