Page 245 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

244 CHAPTER 9 Application of microfluidics in cancer treatment

FIGURE 9.14 The acoustic intensity magnitude.

FIGURE 9.15 The size and direction of the force.

9.5.5 Force
Due to the nonlinear behavior of acoustic field, induced pressure is observed. This
pressure field will create a force on particles. The size and direction of the force are
shown in Fig. 9.15.
The maximum force value is located at a maximum intensity. Because the poly-
styrene acoustic contrast factor is positive, the radiation force direction for particles
is from pressure antinode to node. That’s why particles go toward channel wall and
center.

9.5.6 Motion of particles
The effect of the acoustic field on the particle motion is depicted in Fig. 9.16.
In t = 0 (before applying the acoustic field), particles distributed regularly in the
channel. As soon as the acoustic field applied, particles start to move toward pres-
sure nodes. After 8 s they are almost aggregated in pressure nodes. The velocity of
particles is under influence of acoustic streaming and ARF. Also the weakness of
acoustic streaming comparing to ARF is obvious, the total particle movement pattern
is similar to radiation force direction.
Due to nonlinear acoustic behavior in the fluid, the pressure known as acoustic
radiation pressure and the velocity known as acoustic streaming is induced. The car-
rying momentum and energy by the acoustic wave will move particles. The deter-
mination of the exact desired particle motion takes experimental and numerical
investigation. The radiation force is dominant for larger particles whereas acoustic
streaming is dominant for small particles. In working frequency for separation and
sorting applications, SAW devices do not have a considerable thermal effect. Acoustic