Biomedical Engineering Reference
In-Depth Information
Cross-section of the
fluids in micromixer
Coanda effect
a
Fluid 1
Fluid 2
b
Liquid1_mole
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
After first
mixing unit
Before mixing
After second
mixing unit
After third
mixing unit
50 µm
200 µm
100 µm
After eight
mixing unit
After seventh
mixing unit
90°
175 µm
Mixed
fluids
Mixed fluids
FIGURE 3.100
Microluidic.homogenizer.based.on.Tesla.mixer..(From.Chien-Chong.Hong,.Jin-Woo.
Choi,.and.Chong.H..Ahn,.“A.novel.in-plane.passive.microluidic.mixer.with.modiied.Tesla.struc-
tures,”.
Lab Chip
.4,.109,.2004..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)
2002 that, in fact, you do not. Because luids are incompressible, inasmuch the same way that a
car at high speed can lip over if one of its front wheels hits a small post, a luid will also start a
rotating (helical) motion if it enters a channel that has slanted grooves on its loor (
Figure 3.101a
and
b
). More eicient swirling patterns can be achieved with an asymmetric “herringbone” or
“Chevron” groove design whose center is shited periodically (
Figure 3.101c
and
d
). In other
words, the small amount of luid that goes into the grooves transmits its momentum to the rest
of the luid in the channel, causing it to rotate; because the grooves are slanted, a recirculation
occurs. his mixer (along with one of the early serpentine micromixers) was originally dubbed
a “chaotic micromixer,” which is a big word to express how a small deviation in the position of a
particle at the inlet can result in a large deviation at the outlet.
Jing-Tang Yang and colleagues at National Tsing Hua University in Taiwan have done a
thorough study of the herringbone mixer and several design variations and have found that
by introducing a zigzag barrier on the roof of the channel(!), more recirculation (advection) is
introduced, thus enhancing mixing (
Figure 3.102
). Not surprisingly, the performance of this
design, which they call “circulation disturbance micromixer,” is enhanced at higher Re. With
specialized (tilted illumination) photolithography, it has been possible to fabricate mixers (in
SU-8) that also have grooves…on the walls!
3.10.1.5 Homogenization Induced by Surface Charge Patterns
Patterns of surface electrical charges have been successfully exploited to create local recircula-
tion of luids. George Whitesides' group at Harvard University was able to deposit stripes of thin
polymer layers with similar but reverse electrical charges, which locally modulates the sign of
electro-osmotic low in an orthogonally placed microchannel (
Figure 3.103
). Although clever,
this design is sensitive to surface contamination and surface aging, which changes the charge
density on the surface.
3.10.1.6 Homogenization Induced by Bubble-Based Acoustic Streaming
Ultrasonic waves have been investigated for a while as a means to improve mixing in micro-
channels by inducing pressure waves that disturb low. Tony Jun Huang's group from
