Microfluidics - Introduction to BioMEMS - page 176

Oil phase
+ dye inlet

(a)

Reference
channel

(b)

200 µm

Comparator

Test channel

(c)

Outlet

Oil phase
inlet

∆Y

Aqueous phase
inlet

W

(d)

FIGURE 3.71 Microluidic.low.comparator..(From.Siva.A..Vanapalli,.Arun.G..Banpurkar,.Dirk.van.
den.Ende,.Michel.H..G..Duits,.and.Frieder.Mugele,.“Hydrodynamic.resistance.of.single.conined.
moving.drops.in.rectangular.microchannels,”. Lab Chip .9,.982-990,.2009..Reproduced.with.per-
mission.from.The.Royal.Society.of.Chemistry.)

3.9 Micromixers
In microluidics, the word “micromixer” is loosely used to mean three distinct types of devices:
devices in which the luids are mixed to generate dilutions of one or more compounds (“ dilu-
tion generators ”), others in which the goal is to generate a gradient over a surface (“ gradient
generators ”), and others in which the goal is to homogenize the mixture of a number of inlets
as eiciently as possible (“ homogenizers ”). he terminology can be confusing because many
gradient generators incorporate dilution generators (which are covered in Section 3.9.2), but not
all. here are also a number of devices that make good use of microscale mixing phenomena for
speciic (e.g., cellular) applications, but their design is usually also application-speciic, and thus
they are covered elsewhere in this text. A newer generation of micromixers that use microvalves
and micropumps to achieve mixing is described in Section 3.10.2.
he ield of micromixers is perhaps where microluidics has reached its tallest heights in
inventiveness. here is a simple, very fundamental reason for that: luids do not mix well on a
microscale, so scientists have had to put all their creativity at work. In your bathtub or in your
cofee mug, mixing of luids occurs by difusion, advection (i.e., by low-assisted transport), or
turbulence. In a microchannel, turbulence is ruled out because of the “imprisonment” of luids
within the channel walls that only allow for laminar low motions. Hence, all the existing micro-
mixer designs have been conceived to exploit either difusion or advection, or both, to make up
for the absence of turbulence in the microscale.

3.9.1 T- or Y-Mixer
Let us irst understand in detail what happens when two streams (in the simplest case,
two aqueous solutions) meet at an intersection between two channels in the shape of a “T”
or a “Y”—the simplest possible mixer (also known as the “ T-mixer ” or “ Y-mixer ”). Both
streams have a parabolic low proile and homogeneous concentrations before merging, and
some distance ater merging the merged low will also have a parabolic low proile but the
concentration will display an approximate step proile. How about further downstream?
Understanding the downstream evolution of the step proile in the T-mixer is a great exercise
for the microluidics student, even if done qualitatively. It is actually not straightforward,
indeed, it was not fully explained until the year 2000, when a team led by George Whitesides
and Howard Stone at Harvard University went through the trouble of imaging what was going
on. As shown in Figure 3.72 , they used a luorogenic reaction, the binding of the calcium
indicator luo-3 (introduced in one inlet) to CaCl 2 (introduced in the other inlet), to visualize