Chemistry Reference
In-Depth Information
There exists a critical Dean number De
cr
¼
150 where the secondary flow pattern changes
[53]. For De
150, the
centrifugal force is dominant, leading to the formation of two additional vortices at the
outer channel wall.
The most recent micromixer designs and corresponding experimental results for single-
phase fluid flow applications have been presented and compared by Kumar et al. [34]. The
sequential presentation of micromixers here is the same as that followed by Kumar et al.
and Capretto et al. [34,54]. The fundamentals and logic behind most of the micromixer
designs were successfully brought down to engineering and physics basics by Nguyen [40].
Furthermore, the mathematical description is accompanied by an explanation of the
importance and nature of the molecular diffusion, Taylor dispersion, chaotic advection
and viscoelastic, electrokinetic, magnetic and electromagnetic effects. The variety of
available designs and their fabrications, characterizations and applications have been
reviewed by Hessel et al. [51]. Particular focus is given here to scale-up and practical
applications.
150, there is only a pair of counter-rotating vortices. At De
<
>
4.2.3 Micromixers
Micromixers can be divided into two categories, depending on the means for the
facilitation of mixing: passive and active. Passive micromixers facilitate mixing by
utilizing the flow energy to promote molecular diffusion and chaotic advection via either
special channel geometry or the introduction of a secondary flow [55]. This brings about a
requirement for efficient mixing, involving short diffusion paths and flow fragmentation
(stretching, folding, etc.). Passive mixers do not need an additional energy source other
than that provided from the pump and are thus easier to fabricate than active mixers. Active
micromixers, on the other hand, demand an external disturbance in addition to the flow
energy, such as electrical, pumping or ultrasound energy. The biggest advantage of active
micromixers is that they can provide complete mixing at extremely low flow rates and low
Re. This is more typical for lab-chip systems, which also often have stop-flow motion
modes, than for microstructured reactors. However, the need for the integration of an
external power source into the actively mixed microdevice, as well as for sometimes
complex and costly fabrication processes, is the biggest disadvantage of their employment.
4.2.3.1 Lamination-based Micromixers
Lamination-based micromixers rely purely on diffusion in promoting mixing. Thus, the
aim is to maximize the diffusive flux by maximizing the interfacial area and concentration
gradient.
T- and Y-shaped Micromixers.
The simplest of all the mixers with respect to fabrication
and design is theT- orY-shapedmixer [56-59]. Twofluids flowthrough two inlets tomerge in
the common mixing channel, increasing the mixing region at the interface by 20-30% [40].
Mixing after merging of the two streams depends solely on the diffusion of the species at
the interface of the two liquids. Even then, the interface provided is usually not sufficient
for rapid mixing, so long mixing channels are required. However, obstacles and roughening
of the walls can be implemented within the channels in order to induce vortices, enhancing
convection and thus mixing [46,57,60]. Another even simpler method is to largely increase
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