Micromixers based on chaotic advection - Micromixers Fundamentals Design and Fabrication - page 210

FIGURE 6.16

Micromixer based on X-shaped microchannel (after [23] ).

The two-layer concept allows the fabrication of fairly complex channel structures. Chang et al. [24]
used this concept to design mixing units that can rotate and laminate mixing fluid streams. Figure 6.17
shows the two designs for realizing the rotation of the mixing fluids either in a counterclockwise
direction ( Fig. 6.17 (a)) or in the clockwise direction ( Fig. 6.17 (b)). The arrangement of channel
sections in the two layers shown in Fig. 6.17 allows the fluids to rotate while passing through the
mixing channel. The concept is similar to the screw-like Kenics static mixer in macroscale. The
concept of sequential lamination is depicted in Fig. 6.18 (a). Combining the concepts of rotation and
lamination can result in a very complex flow pattern. Figure 6.18 (b) shows the combination of
counterclockwise and clockwise rotation in a single device, while the design depicted in Fig. 6.18 (c)
combines all three concepts of lamination, clockwise rotation and counterclockwise rotation. The
micromixers were fabricated in two layers of PDMS using an SU-8 mold. Typical channel cross
section is on the order of 200

m

m 100

m

m. Experimental results show that the mixer can work well
with 10 mixing units for a wide range of Reynolds numbers from 0.26 to 26. The combined designs of
alternate rotation ( Fig. 6.18 (b)) and lamination with alternate rotation ( Fig. 6.18 (b)) give the best
results. In terms of pressure drop, these two designs also require a higher driving pressure due to the
pressure losses in the complex channel structure. The maximum pressure required for these mixers at
Re ¼ 26 was about 7 kPa, which is reasonably low.

6.3.2 Chaotic advection with flow-guiding structures on channel walls
Secondary flow pattern similar to the Dean vortices can be generated by the axial pressure gradient if
ridges are placed at the channel wall at an angle q with respect to the flow direction [25,26] .Atlow
Reynolds numbers, these structures on the wall act as an anisotropic fluid resistance. The resistance is
minimum if the ridges are parallel to the flow direction ( q ¼ 0 ). The maximum resistance is achieved
if the ridges are perpendicular to the flow direction q ¼ 90 . The anisotropic resistance guides the flow
along the direction of ridges. This design is called the slanted groove micromixer (SGM). Figure 6.19
shows the typical stream lines of the secondary flow caused by an array of slanted ridges. As a result,