Active micromixers - Micromixers Fundamentals Design and Fabrication - page 280

7.6.3 Design examples
Bau et al. [43] fabricated their MHD mixer using co-fired ceramic tapes. The ceramic tapes consist of
oxide particles, glass frit, and an organic binder. The tape can be cast with a thickness of 40 mor

more and subsequently machined by laser, milling, or lithography. Microchannel with a depth from
10

m

m to a few millimeters can be formed. Metal electrodes can be printed of sputtered on the green
tape. After firing, the organic binder will disappear, leaving the sintered oxide particles and forming
the solid substrate. In the micromixer reported by Bau et al. [43] , electrodes were formed by printing
gold paste on the substrate. The depth, width, and length of the mixing channel are 1 mm, 4.7 mm, and
22.3 mm, respectively. A rectangular permanent magnet was positioned under the mixing channel to
generate the external magnetic field. Mixing was observed when a DC voltage of 4 V was applied at
the electrodes.
Qian and Bau [45] used electrodes on the side wall of a Y-mixer to induced MHD disturbance. The
electrode configuration is similar to the EHD-based active micromixer depicted in Fig. 7.15 (a). The
mixing channel has a cross section of 4 mm

m

2 mm.
The annular channel configuration depicted in Fig. 7.29 was implemented by West et al. [46] . The
prototype was fabricated using a brass disc positioned inside a brass ring. The radius of curvature is
R ¼ 0.5 mm is defined by the

thickness of the brass disc and the brass ring. The device is covered on both sides by two transparent
polycarbonate plates. Another version was fabricated in silicon using anisotropic etching with KOH
with the same radius of curvature, but an average channel width of 1.16 mm. The electrodes are made
of chromium and gold, which are sputtered on the channel side wall. Alternatively, the annular
microchannel can also be made of SU-8, a negative thick resist [47] . The sidewall electrodes were
sputtered over a stencil. The electrode consists of a 100 nm-thick platinum layer on a 1

5 mm and the channel width is W¼ 2 mm. The channel height h ¼

m-thick
copper layer. MHD actuation utilized AC signal to avoid electrolysis and electrode degradation. These
devices were successfully used for mixing and amplification of DNA samples.

m

7.7 ACOUSTIC DISTURBANCE
A piezoelectric bimorph disc is often the actuator of choice in microfluidic devices, due to its
simplicity in implementation and the high energy efficiency. Another key advantage of piezoelectric
actuator is the high actuating frequency.
The high frequency generates acoustic energy, which in turn induces secondary flow in a mixing
chamber. Acoustic disturbance can be induced by choosing actuating frequency at the resonant modes
of a membrane. Following, models for vibration modes of rectangular and circular membranes are
considered. These two basic shapes are the most often used for mixing chambers. The following
analytical solutions of the membrane deflections at different vibration modes can be applied as the
boundary condition for modeling the flow in a mixing chamber. The coupling of a moving wall
condition to the Navier-Stokes equation can be implemented with a numerical model.

7.7.1 Vibration of a rectangular membrane [49]
We consider here a rectangular membrane with a width W , a length L , and a constant surface tension T ,
Fig. 7.33 (a). The edges of the membrane are fixed. Fig. 7.33 (b) depicts a small element d x d y with an