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2.2 Fluidics and microfluidics 11
2.2.1 Mixing in microfluidics
Controlling the behaviour of the fluid in microfluidic systems includes
controlling mixing of different parts of the fluid. If the mixing of a given
system is not understood or well controlled, concentration measurements
might be erroneous, or separation of e.g. particles in a flow might not
work as expected.
As shown above, microfluidics have low Reynolds numbers, so that
turbulence in the system is excluded. The absence of turbulence, like
when e.g. a cocktail is shaken, makes mixing in the system difficult,
since the fluctuations in the fluid particle velocities, which are needed
for turbulent mixing, are largely missing.
As shown in Fig. 1.1, diffusion alone will only be sufficient for systems
with dimensions below a certain scale, typically some 10-100 µm, if a
complete mixing of the liquid is to take place within a reasonable time
frame, that is in the order of seconds. For larger systems, with channel
widths of e.g. one millimeter, diffusion time rises to many minutes,
which would be too long for many point-of-care systems, and longer
than when using existing, macroscopic methods.
In order to mix a system efficiently, the different parts of the fluid
must be rearranged. If turbulence is not achievable, as in the case of
microfluidics, the fluid parts can be moved by methods like chaotic
advection [6], where a liquid is mixed by repeatedly laminating and
folding layers of e.g. high and low concentration. In this manner, the
diffusion distance is radically reduced to the order of the single layer
thickness.
The stirring mechanisms used in macro-scale lamination processes can
often only be downscaled with great difficulty to be used in microfluidic
systems. Therefore, other ways of moving liquids on this scale have been
found. Mixing in the systems can be made either actively or passively. A
number of elegant and effective solutions have emerged in the last decade,
like the passive stacked herring-bone mixer [7, 8], where ripples split
and force the flow to spiral, yielding the laminated structure. Recently,
Hardt et al. have made a review of passive micromixers [9].
Active devices like the cross-channel mixer [10, 11] have appeared
in microfluidics, too. Here, the interface of a two-layer fluid stream is
chaotically folded and stretched by a perpendicular oscillatory motion.
The benefit of active mixers is the ability to continually tune the
system to achieve optimal mixing for a variety of flow velocities. For
passive mixers, the mixing efficiency is often constrained to a limited