Active micromixers - Micromixers Fundamentals Design and Fabrication - page 288

FIGURE 7.39

Acoustic micromixers: (a) acoustic-streaming based micromixer (after [52] ); (b) acoustic micromixer with an
actuating membrane (after [53,54] ).

streaming. Mixing is more effective, if the actuating frequency is switched between different resonant
modes. The resulting chaotic mixing pattern of this “mode-hopping” operation leads to faster
mixing compared to the single-mode operation. Full mixing was generally achieved after less than
30 seconds.
Liu et al. utilized acoustic streaming induced around an air bubble for mixing [55] . In an acoustic
field, the air/liquid interface of an air bubble vibrates and generates its own resonant acoustic field. The
behavior of bubbles in sound fields is determined largely by their resonance characteristics. The
resonant frequency of a bubble is estimated as:

s

3 kp 0
r

1
2 pa

f res ¼ (7.109)

where a is the bubble radius, k c p / c y is the ratio of specific heats for the gas, p 0 is the hydrostatic

pressure, and r is the density of the liquid. Acoustic streaming is generated in the fluid surrounding an
air bubble in an acoustic field. If the bubble is excited with a frequency close to its resonant frequency,
acoustic streaming of recirculation flow around the bubble. This recirculation flow can be used to
improve mixing. Figure 7.40 shows the basic concept of the mixing chamber. The mixing chamber is
300

¼

m deep and has a diameter of 15 mm. The mixing chamber and the air pockets are machined in
a polycarbonate (PC) substrate. Since PC is a hydrophobic material, air will be trapped in the pockets
when the mixing chamber is filled with liquid. The piezoelectric disc is made of PZT and has
a diameter of 15 mm diameter. The disc was driven by a sinusoidal 5 V peak-to-peak signal at
a frequency of 2 kHz.
Yaralioglu et al. utilized acoustic streaming to disturb the flow in a conventional Y-mixer [56] .
While the channel is made of PDMS, the piezoelectric actuator is integrated into the cover quartz
wafer. The master for the PDMS channel was fabricated in silicon using deep reactive ion etching
(DRIE). The piezoelectric actuators were fabricated on a quartz wafer. A 300 nm-thick gold layer was
first deposited and patterned on the quartz wafer, Fig. 7.41 .A8-

m

m m-thick zinc oxide was deposited on