Micromixers based on chaotic advection - Micromixers Fundamentals Design and Fabrication - page 204

microchannels are 100 m deep ( Fig. 6.9 (c)). The mixer is based on a concept similar

to sequential lamination. A number of serial mixing units split, turn, and recombine the mixing
streams. The 90 turns can induce secondary flow and improve mixing. The micromixer was tested
with two protein solutions. Despite the relatively small molecular diffusion coefficients of the protein
( D ¼ 2 10 11 m 2 /s), effective mixing was achieved with a purely laminar flow at Reynolds numbers
ranging from 0.1 to 2. At these low Reynolds number, and correspondingly low Dean numbers, Dean
vortices are negligible. Mixing time can be improved if the same mixer works in the intermediate
Reynolds number range 10

m m wide and 70 m

< Re < 100.

6.2.2 Chaotic advection with other channel designs
Mixing at intermediate Reynolds numbers can benefit from combining Taylor dispersion with Dean
vortices. As discussed in Section 2.3 , the effective axial dispersion coefficient is proportional to the
square of the flow velocity, thus, to the square of Reynolds number. According to (6.5) , the strength of
Dean vortices is linearly proportional to the Reynolds number as well. Thus, the higher the Reynolds
number, the more effective are both effects. Taylor dispersion and Dean flow can be best utilized in
a micromixer design, which provided curved channels or turns as well as variation in velocity
distribution. A Tesla structure was generally used in micropump as a flow rectification component.
Hong et al. [15] demonstrated an in-plane micromixer with modified Tesla structures as depicted in
Fig. 6.10 (a). The channel structures provide a curved section and a diffuser section. The curved section
allows the split fluid streams to collide at angles close to 180 . Collision of two mixing streams was the
basic concept for mixing at high Reynolds number with a simple T-mixer. The micromixer was
fabricated using hot embossing. The master was fabricated using the “poor-man” LIGA technology
(see Section 4.3.2). Nickel was electroplated in an SU-8 mold, which was structured using

FIGURE 6.10

Different micromixer designs for an intermediate Reynolds number: (a) Tesla mixer with four mixing units and
(b) recycle mixer with five mixing units.