Numerical Calculation of Acoustic Radiation Force and Torque in Acoustophoresis

258 Pages·2017·3.6 MB·English

by  Felix Bob Wijaya

NUMERICAL CALCULATION OF ACOUSTIC
RADIATION FORCE AND TORQUE IN
ACOUSTOPHORESIS
FELIX BOB WIJAYA
(B.Sc. and M.Sc., Bandung Institute of Technology, Indonesia)
A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2017
Supervisor:
Associate Professor Lim Kian Meng
Examiners:
Associate Professor Leng Siew Bing, Gerard
Dr Ong Eng Teo
Professor Jurg Dual, ETH ZURICH

Declaration
I hereby declare that this thesis is my original work and it has been written
by me in its entirety. I have duly acknowledged all the sources of informa-
tion which have been used in the thesis.
This thesis has also not been submitted for any degree in any university
previously.
Felix Bob Wijaya
April 2017
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Acknowledgments
First and foremost, I have to thank my supervisor, Associate Professor
Lim Kian Meng. Without his great patience and invaluable guidance, this
thesis would have never been accomplished. I would like to thank you very
much for your support and understanding over these past four years. I
must also thank my colleagues and seniors for the joyful friendship and
fruitful discussion. Most importantly, none of this could have happened
without my family. To my mother, Sylvia Lily Simawati, thank you very
much for your unceasing encouragement and unconditional love. Every
time I was ready to quit, you did not let me and I am forever grateful. I
would also like to thank my sister and brother-in-law for their support and
guidance.
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Summary
Acoustic radiation force has been widely used to manipulate particle
movement in microfluidic devices. In acoustophoresis applications, accurate
calculation of the acoustic radiation force is essential. Many analytical
solutions of acoustic radiation force have been proposed, mainly for spheres
and spheroids in an axisymmetric configuration. In this study, a three-
dimensionalboundaryelementmodelisdevelopedtocalculatetheradiation
force and torque acting on particles of arbitrary shapes and sizes subjected
to arbitrary acoustic waves. This numerical model provides a more versatile
and accurate calculation of the radiation force and torque over the available
analytical solutions. In this model, the acoustic domain is assumed to
be infinite and the fluid inside the domain is inviscid. Due to these
assumptions, this model is only valid for the cases where the particle size is
larger than the thickness of viscous boundary layer. Moreover, for the case
of a compressible particle, the shear wave inside the particle is neglected
in this model. Hence, the particle resonance, which might be set up when
the particle size approaches the shear wavelength, is not considered in this
model.
In the first two parts of this study, the radiation force and torque acting
on non-spherical particles subjected to acoustic standing waves and Bessel
beams are calculated by using this numerical model. For the case of
acoustic standing wave, the direction of the radiation force is determined
by the acoustic contrast factor of the particle. The radiation torque rotates
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the particle to the position in which the particle has the largest projected
area in the pressure nodal plane. For the case of acoustic Bessel beam, a
negative axial force could be obtained by properly setting the beam cone
angle and the characteristic length of the particle. Analysis of the particle
stability in transverse plane shows the potential ability of the acoustic
Bessel beam for particle separation based on shape.
In the third part, the boundary element model is further used to calculate
the interparticle force and torque on a pair of spheroids subjected to an
acoustic standing wave. The numerical results show that the interparticle
force is dominant over the radiation force when the two particles are close
each other. On the other hand, the interparticle torque is negligible com-
pared to the radiation torque, even when the distance between the particles
is small. Furthermore, the result of the total force and torque calculation
providesinsightintothephysicsbehindtheparticleagglomerationobserved
in experiments.
In the fourth part of this study, an alternative way to calculate the acoustic
radiationforceandtorqueisproposed. Inthisalternativeway,theboundary
element model is combined with the multipole translation method to
provide the multipole representation of the scattered field with respect to
the center of the particle. With the multipole coefficients, the radiation
force and torque are calculated by using the general expressions of the
radiation force and torque reported in the literature. The calculation
results obtained from this alternative method match well with the results
obtained from the far-field integration.
Biological cells undergo deformation while being trapped by the acoustic
radiation force at the pressure node. Based on this phenomenon, in the last
part of this study, a numerical framework which combines the boundary
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element model and an axisymmetric shell model is developed to estimate
the stiffness of cell membrane from the cell deformation. This numerical
framework is used to estimate the membrane stiffness of algae and red
blood cells in this study. The results for the red blood cell are in a good
agreement with the values reported in the previous studies.