Active micromixers - Micromixers Fundamentals Design and Fabrication - page 270

FIGURE 7.22

Mixing based on electrokinetic disturbance: (a) disturbance in axial direction; (b) disturbance in transversal
direction.

adhesive bonding with epoxy. The channels have a cross section of 1 mm m. The liquid

streams are introduced by syringe pumps. High-voltage AC excitation is introduced along the mixing
channel by platinum electrode wires. The electrokinetic disturbance was introduced in both contin-
uous and stop mode. The micromixer depicted in Fig. 7.22 (b) was etched in glass. The microchannels
are 300

300 m

m deep. Transversal electrokinetic disturbance is introduced in a square
mixing chamber measuring 1 mm

m m wide and 100 m

m which corresponds to a volume of 100 nL. The
inlet streams can be either pressure-driven or electrokinetically driven. In the later case, porous
dielectric frits are used for high fluidic resistance. Thus, instability can be contained in the mixing
chamber. In both mixer types, chaotic advection was observed when electrokinetic instability
occurred.
Fu et al. [36] used transversal electrokinetic disturbance in their micromixer design. The micro-
channels were etched in glass. The mixing channel is 100

1mm 100 m

m m

m deep. The mixing
streams are introduced electrokinetically. The instability was realized by periodically switching the
electric field in the disturbance channels. At a relatively low switching frequency of 1 Hz and a driven
field of 50 V/cm, full mixing was achieved at 1 mm downstream from the inlet.
Sasaki et al. [37] used an electrode pair with a wavy design to create secondary electrokinetic
flow in the channel cross section. An AC electric field across the mixing channel will cause trans-
versal flows, which are confined in two recirculation vortices, Fig. 7.23 (a). With the wavy electrode
designs, the two vortices have different sizes, which are controlled by the position of the gap between
the two electrodes. The mixing channel was etched in glass with a width and a depth of 120

m wide and 30

m m and

40 m, respectively. The electrodes are made of platinum sputtered on the glass substrate. The

mixing streams are driven by pressure. Without applied voltage at the electrodes, no secondary flow
exists. Mixing occurred with a peak-to-peak voltage of 20 V and a frequency of 1 kHz. Electroos-
motic secondary flow causes chaotic advection similar to the case of passive micromixers based on
chaotic advection discussed in the previous chapter. Mixing was achieved at a Peclet number as high
as 2000.

m