Active micromixers - Micromixers Fundamentals Design and Fabrication - page 256

magnitude and a low disturbance frequency. The spatiotemporal resonances are indicated by the
dashed lines. At the resonance, the interface between the two liquids is strongly distorted, but returns
straight at the intersections.
Kim and Breuer [16] utilized E. coli bacteria to create disturbance on the nanometer scale. The
motion of the bacterium's flagella increase the apparent diffusion coefficient of the solute. Up to
four times improvement of diffusion coefficient can be achieved. In their experiments, Kim and Breuer
used a conventional focusing mixer with three inlets, where the middle stream contains large mole-
cules with molecular weight on the order of 2

10 [5] . To improve the diffusion coefficient of these
large molecules, wild-type E. coli was mixed in the solvent stream. The apparent diffusion coefficient
is then proportional to the concentration of the bacteria. Introducing chemoeffectors into the side
stream can control the apparent diffusion coefficient at a fixed bacteria concentration. For instance,
chemoattractants, such as L-aspartic, can increase diffusion to the side stream, while chemorepellants,
such as nickel sulfate (NiSiO 4 ), decrease it.
As analyzed in 7.2.3, the first-in-last-out pulsed source-sink concept is easy to implement in micro
scale. Figure 7.13 shows the micromixer implemented by Evan et al. [17] . The mixing chamber and
microchannel network were etched 100

m

m deep in silicon. The mixing channel has two inlets for the
solute and solvent. The other four inlets are the two source-sink pairs. The positions of the sources and
sinks are indicated in Fig. 7.13 . These sources and sinks are connected to thermopneumatic micro-
pumps. Switching between the sink-source pairs is controlled by thermopneumatic microvalves. The
liquids are first loaded into the mixing chamber. The micropumps and microvalves are synchronized so
that the two source-sink pairs are activated alternately. While the first pair drives liquid from the
bottom to the top, the second pair drives from the top to the bottom. The microchannel network allows
the realization of the first-in-last-out concept. Thermopneumatic actuators for micropumps and

FIGURE 7.13
Active micromixer based on pulsed source-sink concept.