Page 40 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 21

A wide variety of mixers have been developed. They can be
broadly classified as active (involving input of external energy) or
passive (making use of the fluid dynamics in specific geometry of the
channel in the absence of external forces). Passive mixers are usually
easier to fabricate than active mixers, and are more suitable for appli-
cations involving sensitive species as they do not impose electrical,
mechanical, or thermal agitation [46].
One of the passive mixers developed involves a staggered herring-
bone structure to generate chaotic advection in a microchannel [47]
(Fig. 2-6). This mixer uses asymmetric grooves on the floor of the chan-
nel (the “staggered herringbone” design) to generate a transverse com-
ponent to the flow when an axial pressure gradient is applied. Because
of this transverse component, the fluid elements are stretched and
folded into one another; this process increases the contact area between
the flowing streams and facilitates mixing by diffusion. Channels with
the staggered herringbone design thus have a higher efficiency of mix-
ing laminar streams of fluid than channels with smooth walls.
Another type of passive mixer involves the use of serpentine chan-
nels [42,46]. Fluids flowing through curved channels experience both
inertial forces and centrifugal forces. Under suitable conditions, these
effects establish a radial pressure gradient whose magnitude can

3 cm
200 μm

FIGURE 2-6 Continuous-fl ow staggered herringbone mixer, in which grooved
channel walls drive alternating, asymmetric helical secondary fl ows that
chaotically stir the fl uid. Each cycle cuts the distance between stripes in half,
so that the distance between stripes decreases exponentially with the
number of cycles. Diffusive mixing occurs when the tracer can diffuse from
one stripe to the next before another cycle has occurred, giving a mixing time
that depends logarithmically on Pe. Thus the channel cross section is rapidly
mixed. (From A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezit, H. A.
Stone, and G. M. Whitesides, “Chaotic mixer for microchannels,” Science,
295, (2002), 647–651. Reprinted with permission from AAAS.)