Page 223 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

222 CHAPTER 9 Application of microfluidics in cancer treatment

FIGURE 9.2 IDT placed on the piezoelectric substrate generates SAWs.

longitudinal forms of waves are found in solids. Rayleigh waves cause a particle in a
solid to move in an elliptical path along the surface and much energy is not dissipated
into the air because of boundary conditions of the earth’s surface. The SAWs using
a piezoelectric substrate coupled to IDT. The rapid changes in the electric field can
generate ultrasonic waves similar to Rayleigh waves [4].

9.1.2 Generation of SAW-induced streaming
A part of traveling SAW that has contacted liquid, reflect in the form of the longitu-
dinal wave into the liquid because liquid viscosity increases relative to the substrate.
When acoustic refraction occurs, the changed SAW is called leaky SAW. The IDT
excites SAW and the SAW propagates along the piezoelectric substrate surface within
the depth of a single wavelength. The refraction angle is known as a Rayleigh angle
and refracted wave moves along this direction. The refracted longitudinal waves gen-
erate a force in their propagation direction and induce flow into the limited liquid.
The boundaries of the limited liquid reflect the actuated liquid and lead to internal
streaming. Transformation of the SAW attenuation into a steady fluid flow (as a non-
linear phenomenon) is called SAW-induced acoustic streaming. Small amplitudes of
SAW is suitable for nano and micromixing. The internal streaming in the small fluid
volume is shown in Fig. 9.3. At large amplitudes of SAW, droplets transportation and
streaming of closed fluid layers at the surface of the substrate occurs [6–9].

FIGURE 9.3 A small droplet on a piezoelectric substrate exposed to the acoustic streaming.