Micromixers based on molecular diffusion - Micromixers Fundamentals Design and Fabrication - page 162

CHAPTER

5

Micromixers based on molecular
diffusion

CHAPTER OUTLINE

5.1 Parallel Lamination ...................................................................................................................... 163
5.1.1 Mixers based on pure molecular diffusion ....................................................................163
5.1.2 Mixers based on inertial and viscoelastic instabilities....................................................169
5.2 Sequential Lamination .................................................................................................................. 171
5.3 Sequential Segmentation .............................................................................................................. 173
5.4 Segmentation Based on Injection .................................................................................................. 175
5.5 Focusing of Mixing Streams .......................................................................................................... 177
5.5.1 Streams with the same viscosity..................................................................................177
5.5.2 Streams with different viscosities................................................................................179
5.5.3 Combination of hydrodynamic focusing and sequential segmentation .............................181
5.6 Gradient Generator Based on Diffusive Mixing ................................................................................ 187
5.6.1 Parallel lamination gradient generator .........................................................................187
5.6.2 Free-diffusion gradient generator.................................................................................191
References ......................................................................................................................................... 193

The final stage in all micromixer types is molecular diffusion. This chapter discusses micromixers that
rely entirely on diffusive transport. As pointed out in Chapter 2, diffusive mixing can be improved by
increasing the interfacial area between the solute and solvent or by decreasing the striation thickness of
these two phases. Based on Fick's law, a large interfacial area, a large gradient, and a large diffusion
coefficient can lead to a high diffusive flux. The small mixing length in microscale actually leads to
a higher concentration gradient and is thus advantageous for diffusive mixing. Because diffusion
coefficient is a material constant, larger diffusion coefficients can only be achieved by a higher
temperature and a lower viscosity. However, the resulting improvement in diffusive flux based on
temperature and viscosity is not significant. Thus, mixing in micromixers based on molecular diffusion
can only be optimized by geometrical designs for decreasing the striation thickness. In this chapter, the
basic concepts for decreasing the striation thickness include parallel lamination, sequential lamination,
sequential segmentation, segmentation based on injection, and focusing.

5.1 PARALLEL LAMINATION
5.1.1 Mixers based on pure molecular diffusion
Parallel lamination increases the interfacial area and decreases the striation thickness by splitting the
solute and the solvent each into n substreams and rejoining them later in a single stream. Compared to