Acoustic tweezers (or acoustical tweezers) are used to manipulate the position and movement of very small objects with
sound waves
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
. Strictly speaking, only a single-beam based configuration can be called acoustical tweezers. However, the broad concept of acoustical tweezers involves two configurations of beams: single beam and standing waves. The technology works by controlling the position of acoustic pressure nodes
[Gorkov, L. P.; Soviet Physics- Doklady, 1962, 6(9), 773-775.] that draw objects to specific locations of a standing acoustic field.
The target object must be considerably smaller than the wavelength of sound used, and the technology is typically used to manipulate microscopic particles.
Acoustic waves have been proven safe for
biological objects, making them ideal for
biomedical applications. Recently, applications for acoustic tweezers have been found in manipulating sub-millimetre objects, such as
flow cytometry, cell separation, cell trapping, single-cell manipulation, and nanomaterial manipulation. The use of one-dimensional
standing waves to manipulate small particles was first reported in the 1982 research article "Ultrasonic Inspection of Fiber Suspensions".
Method
In a
standing
Standing, also referred to as orthostasis, is a position in which the body is held in an ''erect'' ("orthostatic") position and supported only by the feet. Although seemingly static, the body rocks slightly back and forth from the ankle in the s ...
acoustic field, objects experience an acoustic-radiation force that moves them to specific regions of the field.
Depending on an object's properties, such as density and
compressibility
In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
, it can be induced to move to either acoustic pressure nodes (minimum pressure regions) or pressure antinodes (maximum pressure regions).
As a result, by controlling the position of these nodes, the precise movement of objects using sound waves is feasible. Acoustic tweezers do not require expensive equipment or complex experimental setups.
Fundamental theory
Particles in an acoustic field can be moved by forces originating from the interaction among the acoustic waves, fluid, and particles. These forces (including acoustic radiation force, secondary field force between particles, and
Stokes drag force) create the phenomena of
acoustophoresis, which is the foundation of the acoustic tweezers technology.
Acoustic radiation force
When a particle is suspended in the field of a sound wave, an acoustic radiation force that has risen from the scattering of the acoustic waves is exerted on the particle. This was first modeled and analyzed for incompressible particles in an ideal fluid by Louis King in 1934. Yosioka and Kawasima calculated the acoustic radiation force on compressible particles in a plane wave field in 1955. Gorkov summarized the previous work and proposed equations to determine the average force acting on a particle in an arbitrary acoustical field when its size is much smaller than the wavelength of the sound.
Recently, Bruus revisited the problem and gave detailed derivation for the acoustic radiation force.
As shown in Figure 1, the acoustic radiation force on a small particle results from a non-uniform flux of momentum in the near-field region around the particle,
, which is caused by the incoming acoustic waves and the scattering on the surface of the particle when acoustic waves propagate through it. For a compressible spherical particle with a diameter much smaller than the wavelength of acoustic waves in an ideal fluid, the acoustic radiation force can be calculated by
, where
is a given quantity, also called acoustic potential energy.
The acoustic potential energy is expressed as:
where
*
is the particle volume,
*
is the acoustic pressure,
*
is the velocity of acoustic particles,
*
is the fluid mass density,
*
is the speed of sound of the fluid,
*
is the time-average term,
The coefficients
and
can be calculated by
and
where
*
is the mass density of the particle,
*
is the speed of sound of the particle.
Acoustic radiation force in standing waves
The standing waves can form a stable acoustic potential energy field, so they are able to create stable acoustic radiation force distribution, which is desirable for many acoustic tweezers applications. For one-dimension planar standing waves, the acoustic fields are given by:
,
,
,
where
*
is the displacement of acoustic particle,
*
is the acoustic pressure amplitude,
*
is the angular velocity,
*
is the wave number.
With these fields, the time-average terms can be obtained. These are:
,
,
Thus, the acoustic potential energy is: