Introduction - Micromixers Fundamentals Design and Fabrication - page 3

Besides surface phenomena, the laminar flow condition is another challenge for designing
micromixers. The problems in micromixers are similar to those in macroscale laminar mixers. Laminar
mixers exist in many processes of food, biotechnological, and pharmaceutical industries because of the
high viscosity and slow flow velocity involved. For many applications, the flow velocity in micro-
mixers cannot be too high. The small size of micromixers leads to an extremely large shear stress in
mixing devices, even at relatively slow flow velocities. This shear stress may damage cells and other
sensitive bioparticles. In complex fluids with large molecules and cells, the fluid properties become
non-Newtonian at high shear stress. On the one hand, the high shear compromises both the metabolic
and physical integrity of cells. On the other hand, viscoelastic effects under this condition may lead to
flow instability, which can be well utilized for improving mixing.
In this topic, micromixers are categorized as passive micromixers and active micromixers [2]
( Fig. 1.1 ). Except the kinetic energy of the flow itself, passive micromixers do not require external
energy for disturbance. The mixing process relies entirely on diffusion or chaotic advection. Passive
mixers are further categorized according to the way in which the interface between the mixed phases is
arranged: parallel lamination, serial lamination, segmentation, chaotic advection, and multiphase flow.
In active micromixers, disturbances are induced by an external field. Thus, active mixers can be
categorized according to the physical phenomenon of the disturbance as a pressure-driven flow,
electrohydrodynamics, dielectrophoretics, electrokinetics, magnetohydrodynamics, acoustics, and
heat. The designs of active micromixers are often complex because of additional components. External
power sources are needed for the operation of active micromixers. Thus, the integration of active
mixers in a microfluidic system is both challenging and expensive. In contrast, passive micromixers do
not require external actuators except those for fluid delivery. Passive micromixers are robust, stable in
operation, and easily integrated in a more complex system. Figure 1.1 depicts an overview on the
different types of micromixers discussed in this topic.
The time scale of mixing processes changes with miniaturization. Most micromixers are used as
a reaction platform for analysis or synthesis. Mixing and chemical reactions are interrelated [3] . While
reaction kinetics and reaction time do not change with miniaturization, mixing time can be signifi-
cantly affected by the mixer design as well as by the mixer type. This fact leads to two important issues
related to chemical reaction: measurement of real reaction kinetics and control over reaction products.
In macroscale, mixing time is usually much larger than reaction time. The reaction rate is therefore
mostly determined by the mixing time. In microscale, mixing time can be reduced to the same order or
even less than the reaction time. Measurement of real reaction kinetics is therefore possible inmicroscale.
Mixing time and, consequently, the reaction products can be possibly controlled in microscale. If
the reaction results in only one product, mixing time can only affect the reaction rate. If there are more
than one product, mixing time determines the product composition and distribution. The following
example shows the impact of mixing type on reaction results. Assuming a reaction between the
substrate S and reagent R:

S þ R P 1 (1.1)

/

where P 1 is the desired reaction product. However, P 1 can react with R to form an undesired product P 2 :
P 1 þ R / P 2

(1.2)

If mixing relies on the relatively slow process of molecular diffusion, as in the case of a parallel
lamination micromixer, P 1 has enough time to react with R. Therefore, the main product of the reaction