Page 11 - Fall2013

SINGLE-BEAM ACOUSTIC TWEEZERS:
A NEW TOOL FOR MICROPARTICLE MANIPULATION
Ying Li, Jae Youn Hwang, and K. Kirk Shung
NIH Transducer Resource Center University of Southern California Los Angeles, CA 90089
Jungwoo Lee
Department of Electronic Engineering, Kwangwoon University,
Seoul 139-701, Republic of Korea
“\\\[Recent work\\\] has demonstrated the feasibility of \\\[tweezing micron-sized\\\] objects by a highly-focused acoustic beam.”
Introduction
It is difficult to move an object of a
large volume and mass; however, it
is even more difficult to accurately
manipulate a tiny object. Currently,
very few technologies are available for
tweezing micron-sized objects. Among
them are optical tweezers1 and
micropipettes2. The present article
reports primarily on a sequence of
studies undertaken in the NIH
(National Institutes of Health) Transducer Research Center (NIH TRC) at the University of California that have demonstrated the feasibility of using a highly focused acoustic beam for the noninvasive dynamic control of par- ticles in size ranges from a few to hundreds of micrometers. This kind of device is termed single beam acoustic tweezers or simply acoustic tweezers3. These studies4 have been ongoing for the past eight years. The article does not attempt a comprehensive survey of all the related literature, but reports on the successes of the research program at USC, with the hope that such an account may be of interest to the readers of this magazine. During this year period, several methods, namely a press-focused method, a self- focused method, and a lens-focused method were devel- oped to fabricate high frequency focused transducers for this application. With continuous adjustment and optimiza- tion of the fabrication process, the performance of these acoustic tweezers has been gradually improved. Currently, acoustic tweezers fabricated at USC are capable of noninva- sively manipulating a single red blood cell and a single micro-particle as small as 1 micron. In this article, the sta- tus of this technology as developed at USC is reviewed and its future direction is also discussed. The content covers the theoretical and experimental studies of acoustic tweezing or trapping phenomenon, the fabrication process of acoustic tweezers, and its applications.
Acoustic trapping force
It is essential to understand first the physical principle involved in acoustic trapping before the actual device is described. Experimental studies already demonstrated that single beam acoustic tweezers could trap particles of a size either greater or smaller than a wavelength, i.e., what might be termed as Mie particles (larger than a wavelength) or
Rayleigh particles (smaller than a wave-
length). The acoustic trapping in these
two cases has been analytically studied
by two different models. The ray
acoustics method was applied to calcu-
lating the trapping force on Mie parti-
3,4
cles . As shown in Fig.1, the acoustic
rays are reflected and refracted by the sphere when they impinge on a sphere. As they travel through and interact with the sphere, momentum transfer occurs
and results in a radiation force. The resultant force on the sphere is the integration of the radiation force from all rays. Simulation results show that the reflection plays a critical role in producing the scattering force in the direction of acoustic wave propagation, whereas refraction plays a more important role in producing the trapping force that pushes the particle toward the acoustic beam axis. Therefore, for acoustic trap- ping to occur it is preferred that the particle is homogeneous and has similar acoustic properties to the surrounding medi- um so that the refraction could take place more dominantly than reflection. Recently, the trapping force on Rayleigh par- ticle was also investigated analytically by calculating the
5
potential field of the incident acoustic beam . It demonstrat-
ed the possibility of manipulating spherical and irregular- shaped Rayleigh particles with different mechanical proper- ties. However, the feasibility of the acoustic trapping in Mie regime was only demonstrated in the case of spherical parti- cles. In other words, there is less restriction in trapping Rayleigh particles than Mie particles.
In both cases, the trapping force could be affected by var- ious parameters, such as frequency, shape and size of particle, beam width, axial position, and the acoustic properties of particles and medium. Although analytical methods are very useful to study the influence of these factors on trapping per- formance, it is difficult to estimate the absolute value of a trapping force, given the many factors which may affect the trapping force. Moreover, most of current analytical studies assumed ideal Gaussian ultrasonic beams at a single frequen- cy without considering the sensitivity of acoustic tweezers, the effect of medium attenuation, and the possible streaming effect generated by an acoustic beam. Therefore, experimen- tal methods were developed to calibrate the trapping force. The equipartition theorem method6, power spectrum analy- sis method7, and viscous drag force method8 are the three
10 Acoustics Today, October 2013