Biomedical Engineering Reference
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photolithography. The nickel mold was then used to emboss the mixer structures in a cyclic olefin
copolymer (COC) substrate. After drilling the inlets and outlets, the micromixer was sealed by thermal
bonding to a blank COC substrate. Complete mixing was achieved after 10 mixing units and at
a Reynolds number of about 10.
Jeon et al. [17] reported a design that utilizes the instability caused by the Coanda effect. A mixing
unit provides two feedback channels to remix the fluids. The mixed fluids are split into two streams
before rejoining to enter the next mixing unit. Fig. 6.10 (b) shows a micromixer with five such mixing
units. The micromixer was fabricated in PDMS using soft lithography. The final device was bonded to
a glass slide with the help of oxygen plasma treatment. The microchannels have a depth of 100
m
m, the
width of the inlet of each mixing unit is 50
m. At low Reynolds numbers, no instability occurs and the
flow in the feedback arms flow in the same direction as the mean flow. At higher Reynolds numbers
(Re
m
14), recirculation appears at the entrance of the mixing unit. Feedback flow follows the direction
as shown in Fig. 6.10 (b). Good mixing can be achieved in the range of 10
>
100.
In macroscale, chaotic advection was achieved in circular pipes using partitioned walls that rotate
and split the mixing fluids [16] . Modifying the channel shapes can induce chaotic advection and
improve mixing. Figure 6.11 shows the twisted design reported by Park et al. [18] . The micromixer is
constructed based on two layers of PDMS. Each layer has mixing segments that are identical in shape.
The segments are joined at their ends, where the fluid goes up or down between the two layers. This
channel structure allows the fluid to rotate in a helical manner, which promotes chaotic advection
( Fig. 6.11 ). The microchannels are 100
<
Re
<
m in height. Park et al. also combined the
split-and-recombine concept of sequential lamination with rotation. Five rotational mixer segments are
arranged in parallel. The group of five segments are joined with the next group by a shift of half of the
channel width. In this device, the fluids are first split into five streams. While rotating in the mixer
segment, the streams are further divided into two groups at the entrance of the following group due to
m
m in width and 50
m
FIGURE 6.11
A twisted mixer for an intermediate Reynolds number.
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