Micromixers based on chaotic advection - Micromixers Fundamentals Design and Fabrication - page 209

FIGURE 6.15

Micromixer based on F-shape microchannel (after [22] ).

than unity. Because of the required shallow side channels, the master mold was fabricated with two
lithography steps. The mixer was then molded in PDMS and covered by bonding to a glass slide.
Experimental results show that the required mixer length is proportional to Pe 0.25 .
Figure 6.15 shows a design with a three-dimensional channel network [22] . The micromixer consists
of 8 mixing units. Each mixing unit is constructed from two F-shape channels machined in two sides of
the device. The two F-shapes realize the splitting and recombination of mixing fluids. In contrast to
conventional sequential lamination mixers, their streams are not split and recombined by channel walls,
but by twisted streams. Similar to micromixers based on Dean vortices, the twisted streams caused by
the three-dimensional channel network also induce chaotic advection, which is effective at higher
Reynolds numbers. This design can easily be implemented for mass production using hot embossing or
injection molding. Kim et al. [22] fabricated the mold from electroplated nickel and used it as an insert
for a commercial injection molding system. The micromixer was made of cyclic olefin copolymer
(COC). The two polymeric parts are finally thermally bonded to form the mixing channels. The total
mixer's length is 10 mm. The width and the height of the main channel are 250

m m and 60 m m,

< <

respectively. The mixer works in the Reynolds number range of approximately 1 10.

Xia et al. [23] use three-dimensionally crossing channels to induce chaotic advection. Similar to the
above design, the three-dimensional microchannel network was fabricated on two sides of the device.
Figure 6.16 shows the two mixer designs investigated by Xia et al. [23] . The mixers consist of mixing
units, which are three-dimensional X-shaped crossing microchannels. The crossing channels are
perpendicular to each other and are slanted by 45 relative to the main flow axis. In the first design
( Fig. 6.16 (a)), the two-layer channels first go across each other at A. At B, a fluid enters the top layer,
while the other fluid makes a 90 turn. Both fluids join at C and enter the bottom layer. At the bottom
layer, the mixing fluids split again. While one stream remains in the bottom layer, the other stream
enters the top layer. The two streams cross each other at E and the process starts again. The second
design has simpler structures. The mixing unit is shorter with joining and splitting on the two layers
( Fig. 6.16 (b)). Simulation results show chaotic advection at a Reynolds number as low as Re ¼ 0.2.
The mixer was tested with highly viscous glycerol solution to minimize molecular diffusion.
Experimental results show that with a sufficient number of mixing units, these designs can work at
a Reynolds number on the order of 0.01.

Re