Biomedical Engineering Reference
In-Depth Information
mechanical robustness and a simpler process control compared with the membrane-based
structures. Also, cheap substrates, such as glass or plastics can be used, thus, the cost can
be reduced. Table 8.1 compares the properties of the microsensors fabricated from differ-
ent acoustic wave devices.
AcousticWave-BasedMicrofluidics
Microfluidic (liquid samples and reagents) manipulation, mixing, and biochemical reac-
tion at the microscale are extremely difficult because of the low Reynolds number low
conditions (Nguyan and Wu, 2005). Acoustic wave technologies are particularly well
suited to mixing and as a result are an attractive option for microfluidics applications (Luo
et al., 2009). Taking the SAW device as one example, Rayleigh-based SAW waves have a
longitudinal component that can be coupled with a medium in contact with the surface of
the device. This coupling- or friction-driven effect can transport the media, for example,
a solid slider on the surface during wave propagation as shown in Figure 8.3a, although
the displacement of the traveling Rayleigh wave is only about 20 nm or less (Kuribayashi
and Kurosawa, 2000). When liquid (either in bulk or droplet form) exists on the surface of
a SAW device, the energy and momentum of the acoustic wave are coupled into the fluid
with a Rayleigh angle, following Snell's law of refraction (see Figure 8.3b) (Wixforth, 2004;
Shiokawa
et al., 1989). The Rayleigh angle,
θ
, is defined by
v
v
sin
1
−
(8.2)
θ
=
l
s
where
v
l
and
v
s
are the velocities of the longitudinal wave in solid and liquid. The energy
and the momentum of the longitudinal wave radiated into the liquid can be harnessed for
liquid pumping and mixing. A net pressure gradient,
P
, forms in the direction of the prop-
agation of the acoustic wave and efficiently drives the liquid (Rotter et al., 1999), according
to the relation:
(b)
(a)
Pre-load
Friction drive
Droplet
Longitudinal
SAW wave
Internal
streaming
Slider
θ
R
10 nm
SAW IDT
Leaky wave
Particle motion
Wave
Elastic material
Piezoelectric materials
400 µm
FIGURE 8.3
(a) Principle of SAW motor. (From Kuribayashi, M., and Kurosawa, M.,
Ultrasonics
, 38, 15-19, 2000. With per-
mission.) (b) Interaction between propagating SAW and a liquid droplet causing acoustic streaming inside
droplet.
