Active micromixers - Micromixers Fundamentals Design and Fabrication - page 262

FIGURE 7.16

Active micromixer based on dielectrophoretic disturbance.

Deval et al. [32] used polystyrene spheres as the stirring particles for mixing in an aqueous solution
with a conductivity of 10-20 mS/m. The mixing concept utilizes the frequency-dependency of the
DEP force. At high frequencies, the DEP force is negative. The stirring particles are repelled from the
electrodes. At low frequencies, the particles move toward the electrodes due to the positive DEP force.
With a critical frequency on the order of a few megahertz, stirring motion was achieved by using an AC
signal with a magnitude of 10 V and a frequency changing alternately between 700 kHz (attracting
mode) and 15 MHz (repelling mode). These two modes are switched with a frequency of 1 Hz. The
attracting and repelling actuation modes cause stretching and folding of the fluid, thus improve mixing.
The electrodes were made of a sputtered chromium/gold layer on a silicon substrate. The electrodes
were insulated from the fluid by a silicon nitride layer. The mixing channel was formed by a 25

m

m-

thick SU-8 layer. The mixing channel measures 50 m m 25 m m in cross section. A mixing chamber

was designed as 100 m square walls. The two inlet streams are driven by syringe pumps.

Figure 7.16 shows the concept of this active micromixer.
Lee et al. [33] reported another design of the above micromixer. The mixing chamber is located
symmetrically at the center. The mixing chamber measures 200

m m 100 m

m. The driving
voltage was switched between 100 kHz and 10 MHz to achieve transversal disturbance in the mixing
chamber.

m m 200 m m 25 m

7.5 ELECTROKINETIC DISTURBANCE
The key parameters affecting an electrokinetic flow are the electrohydrodynamic properties of the
fluid, such as conductivity and permittivity, the electric field and the zeta potential at the channel walls.