Micromixing is a crucial process within microfluidic systems such as micro total analysis systems (μTAS). A state-of-art review on microstructured mixing devices and their mixing phenomena is given. The review first presents an overview of the characteristics of fluidic behavior at the microscale and their implications in microfluidic mixing processes. According to the two basic principles exploited to induce mixing at the microscale, micromixers are generally classified as being passive or active. Passive mixers solely rely on pumping energy, whereas active mixers rely on an external energy source to achieve mixing. Typical types of passive micromixers are discussed, including T- or Y-shaped, parallel lamination, sequential, focusing enhanced mixers, and droplet micromixers. Examples of active mixers using external forces such as pressure field, electrokinetic, dielectrophoretic, electrowetting, magneto-hydrodynamic, and ultrasound to assist mixing are presented. Finally, the advantages and disadvantages of mixing in a microfluidic environment are discussed.

Micromixing Within Microfluidic Devices

In this paper, micro T-mixers are fabricated and tested to investigate their feasibility as a rapid mixing micromixer. The micro T-mixers are fabricated out of a silicon substrate and bonded to a Pyrex glass plate to enable their mixing performances be observed and characterized. The mixing is characterized using a blue dye and a colourless liquid, the results are further verified by the hydrolysis reaction of dichloroacetyl phenol red. Different pressures are applied onto the inlets of the micro T-mixers and their corresponding mixing performances are observed with an optical microscope. Liquid streams break up into striations at progressively higher Reynolds number of flow and there exists a Reynolds number, between 400 and 500, when these striations disappear into uniform concentration across the mixing channel. The observations are further supported by computer simulations, which enable the fast mixing to be explained by the asymmetrical flow conditions at the inlets, in addition to the generation of vortices and secondary flow at the junction. It is shown that for a micro T-mixer with a mixing channel having a hydraulic diameter of 67 μm, an applied pressure of 5.5 bar is sufficient to cause complete mixing within less than a millisecond after the two liquids make contact.

Micro T-mixer as a rapid mixing micromixer

Experimental investigation of liquid–liquid mixing in T-shaped micro-mixers using μ-LIF and μ-PIV

Single-phase fluid flow and mixing in microchannels

A State-of-the-Art Review of Mixing in Microfluidic Mixers

Fluid mixing in a T-shaped micro-mixer

Computation of scales and quality of mixing in a T-shaped microreactor

Theoretische und experimentelle Untersuchungen der Mischvorgänge in T-förmigen Mikroreaktoren: Teil 1: Numerische Simulation und Beurteilung des Strömungsmischens

Since the evaluation of the homogeneity of a distribution can be subdivided into two parts (magnitude and spatial distribution), a good key value describing the quality of the distribution has to consider both. The value of the coefficient of variation (CoV) covers the effects of the fluctuation's magnitude, but any spatial information is lost. Therefore, a complementary key value (Coefficient of Distribution—CoD) was developed to consider both, magnitude and spatial distribution together. The new method is successfully applied to evaluate the vapour velocity distribution at the entrance to the packed bed above the vapour feed of an industrial distillation column with a large diameter.

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Characterization and quantification of the quality of gas flow distributions

Publisher Summary This chapter describes the way of mixing in a T-shaped microreactor, as well as the scales and quality of mixing. In case of a T-shaped micro-mixer, the secondary flow acts mainly in cross directions, i.e. perpendicular to the axial direction, and can be used to mix the two feed streams. To assess and optimize the mixing process, this qualitative picture has to be understood more thoroughly and significant quantitative information has to be added. Reactive mixing is a multiscale process per se, since educts are usually initially segregated, while mixing on the molecular level is necessary for any chemical reaction to occur. Whatever transport processes are involved on the macro- or meso-scale, the final step to obtain homogeneity on the molecular scale—the so-called micro-mixing—can solely be achieved by molecular diffusion. To avoid adverse effects of imperfect mixing on conversion and selectivity, the total mixing time should be well below the time-scale of chemical reaction.

Carsten Stemich

Papers

Fluid mixing in a T-shaped micro-mixer

Computation of scales and quality of mixing in a T-shaped microreactor

Theoretische und experimentelle Untersuchungen der Mischvorgänge in T-förmigen Mikroreaktoren: Teil 1: Numerische Simulation und Beurteilung des Strömungsmischens

Characterization and quantification of the quality of gas flow distributions

Mixing in a T-shaped microreactor: scales and quality of mixing