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=Acoustic Tweezers= Acoustic tweezers are devices that utilize ultrasonic transducers to move microscopic particles with standing sound waves. Currently research is being done to improve the capabilities of these tweezers however their applications could include in-vivo surgery, the study of biological cells and other bio-medical fields. Currently some similar techniques include optical tweezers, Magnetic tweezers and electrical tweezers.

General description
Acoustic tweezers are able to manipulate objects on the microscopic level only using sound waves. These sound waves produce a standing wave field. In this field particles are moved from antinodes to nodes where the object is trapped at the node. These nodes are located every half wavelength inside the standing wave field. The exact location of the field is determined by the frequency of the propagated wave. When the frequency of the acoustic tweezers changes it shifts the standing wave field which in turn move the particles in a certain direction depending on the location of the nodes and antinodes. A possible set up for one dimensional and two dimensional manipulation are depicted in Figure 1. In one dimensional patterning only two transducers are used and placed parallel to each other. These two transducers produce two identical standing acoustic waves that are propagated in opposite directions. These waves then interfere with each other which results in the acoustic radiation force pushing the particles towards pressure nodes or antinodes.

History
In the early 1990’s Professor Arthur Ashkin proposed the use of optical tweezers to trap particles using laser trapping. Around the same time Professor Junru Wu proposed an alternative way to trap particles called Acoustical Tweezers. Wu used two ultrasonic beams that propagated in opposite directions to create a stable force potential well which, he observed, trapped the latex particles and frog eggs. Wu also observed that when particles of different sizes were exposed to these ultrasonic beams, the smaller ones organized themselves in “bands” which is a characteristic of standing waves. He believed that when exposed to two beams of the same frequency propagated along opposite directions a standing wave is formed parallel to the origin of the beams. In 2005 Jungwoo Lee and his team demonstrated the theoretical practicality of acoustic tweezers. Before his simulations acoustic tweezers were still only an idea but after Lee and his team's discoveries the field of acoustic tweezers would take a large step towards reality. Using computer simulations to show the feasibility of acoustic tweezers and how they are dependent on both the size of the particle being moved relative to the degree of focusing and the degree of acoustic impedance.

Acoustic Tweezers vs. Optic Tweezers
Acoustic tweezers are a relatively new discovery though the concept of moving microscopic objects is not. A currently popular technique today is optic tweezers which use laser beams to either attract or repel object. This method is able to successfully manipulate a wide range of particles varying in sizes from tens of nanometers to micrometers. Though this may seem like a very small range, in the realm of micro-particles it is not. The downside of the optic tweezers method is the fact that its penetration depth is very limited. If the intensity of the laser beam was increased the device could penetrate deeper but it would also produce more heat which could damage any object that was being targeted. Acoustic tweezers do not have these limitations. Acoustic tweezers use of sound waves produces virtually no heat and can penetrate deeper than optic tweezers while still being able to manipulate tiny objects.

Changing the frequency of the Transducers
The ability to change the frequency of the standing waves produced was recently discovered by Xiaoyun Ding and his team of researchers. Before Ding and his team, transducers had either a fixed or very limited range of frequencies with which they could produce ultrasonic waves. Ding was able to create transducers with a tunable frequency range by using slanted finger interdigital transducers instead of the more commonly used interdigital transducers.

Theoretical use of Bessel Beam with Acoustic tweezers
Being able to focus the standing sound waves into a single beam for individual particle manipulation has not been realized yet. Using Bessel beams it may be possible to develop acoustic tweezers that can manipulate a single particle without affecting its surroundings. F. Mitri theorized that by using Bessel beams and their non-diffractive qualities, it may be possible to focus the standing surface acoustic waves produced by the ultrasonic transducers into a single hollow beam to manipulate these microscopic objects.

Multi-Foci Fresnel Lenses
Youngki Choe explored the use of Multi-Foci Fresnel lenses to produce a Bessel beam with a negative region where the microparticles are trapped. This would allow for a single transducers to trap microparticles in three dimensional space. The acoustic tweezers were submerged in the water and fixed to a moving stage while the particles freely floated on and in the water. When the tweezers produced their acoustic waves the water, and the particles, stirred and began to circle the tweezers. When the particles finally hit the area where the Bessel beam was they become trapped to the spot and are held there even when other particles collide with them. These trapped particles were also still able to move over a wide range which was only limited by the actual transducers being used. The importance of this research is the ability to move particles using only a single transducer and in this case Multi-Foci Fresnel lenses in three dimensional space.