Microfluidics - Introduction to BioMEMS - page 194

3.10.1.2 Homogenization by 3-D Serpentines
Homogenization by pulsatile low is very fast but produces a nonuniform low (unless it is sinu-
soidally driven) and requires extra energy consumption and hardware. Several groups have
researched designs that passively homogenize the solutions without any energy input. Because
solutions only mix by difusion when lowing in parallel low lines, a successful strategy has
been to force the low lines to cross each other using “serpentine” designs with 3-D turns, a strat-
egy inspired on macroscale “twisted-pipe” conigurations. his strategy was irst implemented
on the microscale in 2000 by a collaborative efort between the groups of Hassan Aref (who
had shown chaotic advection in twisted pipes) and David Beebe, then both at the University of
Illinois at Urbana-Champaign ( Figure 3.95 ). Characteristically of these serpentine mixers, the
mixing efect is enhanced as the Re increases ( Figure 3.95c , triangles). If the device is built with
planar (2-D) turns, then mixing is not enhanced as much ( Figure 3.95c , squares). It should be
emphasized that, whereas the low lines cross by an efect of the geometry of the channel, the
regime is still laminar—there is no turbulence ( Figure 3.95d ).
here has been an intense debate about how to build the best possible serpentine—what
should each unit (each turn) look like? he earliest implementation, by David Beebe's group,
was limited by the use of silicon micromachining in the range of geometries and turns that
could be implemented—only overlapping “U”-shaped turns were possible (see Figure 3.95a
inset). With the advent of PDMS micromolding, it has been possible to investigate various other
types of turns, including overlapping “C” and “L” turns ( Figure 3.96 ). In Figure 3.96d , we see

a b

Re = 12 Re = 70

Outlet

300
µm

300
µm

900
µm

600
µm

#10

#4

600 µm
300 µm

#3

#2
Viewing
window
#1

Stream 1
Stream 2

c

1

0.8

d

FEMLAB simulation

0.6

Straight channel

0.4

Square wave
3D serpentine

0.2

0

0 25 50 75 100 125 150

Velocity (µm/s)

Reynolds number

0 1500 3000 4500 6000

FIGURE 3.95 Passive. micromixer. based. on. chaotic. advection.. (a-c,. from. Robin. H.. Liu,. Mark.
A. Stremler,. Kendra. V.. Sharp,. Michael. G.. Olsen,. Juan. G.. Santiago,. Ronald. J.. Adrian,. Hassan.
Aref,. and. David. J.. Beebe,. “Passive. mixing. in. a. three-dimensional. serpentine. microchannel,”. J.
Microelectromech. Syst. . 9,. 190,. 2000;. d,. FEMLAB. simulation. of. low. in. 3-D. serpentine. turns,.
courtesy.of.Paul.Yager..Figure.contributed.by.David.Beebe.)