Engineering Acoustics/Acoustic Levitation

Definition

Acoustic levitation employs sound radiation to lift objects. It mostly deals with non-linear phenomenon (since the resulting force on the object is due to non linear properties of wave motion).

Motivation behind developing an acoustic reactor

The force generated due to acoustic radiation pressure is generally much larger than force of electromagnetic radiation pressure which makes the study of these forces interesting and noteworthy.

Secondly, this phenomenon will allow successful containerless experiments. The importance of such studies is illustrated by the following:

Kinetic studies can be classified into two categories:

1. The first includes material fixed to the walls.
2. The second includes the flow of particles into and from an apparatus

The drawback of existing methods is that only one type of particle can be used. Consequently, the behavior reported isn't accurate (since the walls in the first case and the surrounding particles in the second case can have an effect on the behavior under study).

This elimination of walls can provide further insight by discarding supports in addition to reducing the interactions with other particles (e.g.: by handling a single bubble).

One way to achieve this airborne application is by employing a fascinating application of acoustics, namely acoustic levitation which involves levitating objects using sound radiation.

Applications of this phenomenon and the corresponding technology can include material processing in space without using any containers. This may be particularly useful in the study of materials that are extremely corrosive.

Moreover, sonoluminescence and acoustic cavitation encounter this acoustic force.

Other applications can include measuring densities and analyzing fluid dynamics in which surface tension plays an important role. Lastly, acoustic positioning is another potential application.

• Discovery News lists an interesting application of acoustic levitation.
• Acoustic Levitation on Mars illustrates an adventurous application of this technology.

Components of an Acoustic Reactor

Figure 1: Schematic of the set up

A simple acoustic reactor requires a:

• A transducer to generate the desired sound waves. These transducers usually generate intense sounds, with sound pressure levels greater than 150 dB.
• A reflector

In order to focus the sound, transducers and reflectors in general have concave surfaces. The reflection of longitudinal sound waves off the reflector leads to interference between the compressions and rarefactions. Perfect interference will result in a standing acoustic wave, i.e., a wave that will appear to have the same position at any time.

With this simple arrangement of transducer and reflector, one can achieve stable levitation but cannot steer the sample. To do so, Weber, Rey, Neuefeind and Benmore have described an arrangement in their paper that describes the use of two transducers. These transducers adjust the location by altering the acoustic phases (which is carried out electronically).

Single Bubble Sonoluminescence

This phenomena occurs when a single bubble encounters a non linear dynamic, namely, rapid compression of bubble preceded by an expansion which takes place slowly. When the bubble is compressed rapidly, it can get so hot that it emits a flash of light

Figure 2: Sonoluminescence - mechanism

Theory

(source: Theory of long wavelength acoustic radiation pressure by Löfstedt and Putterman)

Starting with the integral form of conservation of momentum,

${\frac {\partial \rho v_{i}}{\partial t}}+{\frac {\partial \Pi _{ij}}{\partial r_{j}}}=0$

$\int _{v}{\frac {\partial \rho v_{i}}{\partial t}}+\int _{S_{o}}\Pi _{ij}dS_{j}+\int _{S_{K}}\Pi _{ij}dS_{j}=0$

where

$\Pi _{ij}$ is the stress tensor,

$\rho ,v$ are the local fluid density and velocity,

$S_{o}$ the surface of the object (at time t),

$S_{K}$ a surface far from the object, and

V the volume bounded by these surfaces.

Using the relation,

$\int _{v}{\frac {\partial \rho v_{i}}{\partial t}}dr+{\frac {d}{dt}}\int \rho v_{i}dr-\int _{S_{o}}\rho v_{i}v_{j}dS_{j}=0$

${\frac {d}{dt}}\int \rho v_{i}dr+\int _{S_{o}}(\Pi _{ij}-\rho v_{i}v_{j})dS_{j}+\int _{S_{R}}\Pi _{ij}dS_{j}=0$

Time average of this equation gives an expression for the force on a moving sphere

$\langle F_{i}\rangle =\langle \int _{S_{o}}(\Pi _{ij}-\rho v_{i}v_{j})dS_{j}\rangle =-\int _{S_{R}}\langle \Pi _{ij}\rangle dS_{j}=0$

Assuming an ideal fluid,

The Galilean invariant contribution to the stress tensor is

$\Pi _{ij}-\rho v_{i}v_{j}=p\delta _{ij}$

and

$p=-\rho _{eq}{\frac {\partial \phi }{\partial t}}-\rho _{eq}v^{2}/2+{\frac {\rho _{eq}}{2c^{2}}}\left({\frac {\partial \phi }{\partial t}}\right)^{2}$

here, $v=\nabla \phi$

$\rho _{eq}$ represents the equilibrium density

$c$ denotes the speed of sound

Acoustic radiation force on an object in an ideal fluid is

$\langle F_{i}\rangle =-\int _{S_{R}}{[-{\frac {\rho \langle v^{2}\rangle }{2}}+{\frac {\rho }{2c^{2}}}\langle {\frac {\partial \phi }{\partial t}}^{2}\rangle +\rho \langle v_{i}v_{j}\rangle ]dS_{j}}$

Multipole Expansion of acoustic radiation force

Consider the linear wave equation,

${\frac {\partial ^{2}\phi }{\partial t^{2}}}-c^{2}\Delta ^{2}\phi =0$

where

$\phi =\phi _{i}+\phi _{s}$

here $\phi _{i}$ is given by transducer,

$\phi _{s}$ is given by the corresponding boundary condition at the object where s stands for 'scattered'

$\phi _{s}=Re\sum _{n=0}^{\infty }B_{n}h_{n}(kr)P_{n}(cos\theta )e^{-i\omega t}$

$h_{n}$ here are the outgoing spherical Hankel functions

$P_{n}$ here are the Legendre polynomials

$\omega =kc$

k: wave number of the sound field imposed on the object

As r approaches infinity,

$\lim _{r\to \infty }\phi _{s}\to Re{\frac {e^{i(kr-\omega t)}}{kr}}\sum _{n}(-i)^{n+1}B_{n}P_{n}(cos\theta )$

For standing waves,

$\phi _{i}=ReAsin(kz)e^{-i\omega t}$

Thus by computing $\phi$ and using the result in the expression for the radiation force we get

$F_{z}=2\pi \rho A[coskz_{o}(ImB_{o})-sinkz_{o}(ImB_{1})$ ............................................ (1)

Radiation Force on a spherical object

Consider a spherical body with radius $R_{o}$ and density ρ_o

The wave equation inside the sphere is given by: $:{\frac {\partial ^{2}\Phi }{\partial t^{2}}}-c_{o}^{2}{\nabla ^{2}\Phi _{o}}-\zeta _{o}\rho _{o}{\frac {\partial \nabla ^{2}\Phi _{o}}{\partial t}}=0$

where

$\Phi _{o}$ is the velocity potential

$c_{o}$ is the speed of sound in the sphere

$\zeta _{o}$ characterizes the damping in the sphere

(Effects due to thermal conductivity and shear viscosity are neglected)

The solution to this equation is given by:

$:\Phi _{o}=Re\sum _{n=0}^{\infty }C_{n}j_{n}(k_{o}r)P_{n}(cos\theta )e^{-i\omega t}$

where $j_{n}$ are spherical Bessel functions

$k_{o}$ is complex (because of dissipation)

$k_{o}=k_{o}+i\alpha _{o}$

where

$k_{o}=\omega /c_{o}$

$\alpha _{o}=k_{o}^{2}\zeta _{o}/2c_{o}\rho _{o}$

Here,$\alpha _{o}$ is the attenuation coefficient of sound in the sphere

The boundary conditions are given by

$\rho \Phi (R_{o})=\rho _{o}\Phi _{o}(R_{o})$

At r = $R_{o}$

${\frac {\partial \Phi }{\partial t}}={\frac {\partial \Phi _{o}}{\partial t}}$

To satisfy these conditions, the incident wave is expanded using spherical harmonics

$sin(kz)=sin(kz_{o})\sum _{n=0}^{\infty }a_{2n}(kr)P_{2l}cos\theta +cos(kz_{o})\sum _{n=0}^{\infty }a_{2n+1}(kr)P_{2l+1}cos\theta$

where

$a_{2l}={\frac {4l+1}{2}}.\int _{-1}^{1}coskrxP_{2l}(x)dxa_{2l+1}={\frac {4l+3}{2}}.\int _{-1}^{1}sinkrxP_{2l+1}(x)dx$

Using the above relations, one can compute $a_{o}$

$a_{o}={\frac {sinkr}{kr}}.$

When $kR_{o}<<1$

$sinkz=sinkz_{o}[1-{\frac {1}{2}}.k^{2}r^{2}cos^{2}\theta +...]+coskz_{o}[krcos\theta -...]$

The boundary condition for the case of a standing wave can be deduced as follows:

For monopole term,

$\rho Asinkz_{o}-\rho B_{o}{\frac {ie^{ikR_{o}}}{kR_{o}}}=\rho _{o}C_{o}j_{o}(x_{o})$,

and

$-A({\frac {k^{2}R_{o}}{3}}sinkz_{o}+B_{o}e^{ikR_{o}}({\frac {1}{R_{o}}}+{\frac {i}{(kR_{o})^{2}}})=C_{o}k_{o}j_{o}^{'}(x_{o})$

For dipole term

$\rho A(kR_{o})coskR_{o}-\rho B_{1}e^{ikR_{o}}({\frac {1}{kR_{o}}}+{\frac {i}{(kR_{o})^{2}}})=\rho _{o}C_{1}j_{1}(x_{o})$,

and

$Akcoskz_{o}+B_{1}e^{ikR_{o}}({\frac {-i}{R_{o}}}+{\frac {2}{kR_{o}^{2}}}+{\frac {2i}{k^{2}R_{o}^{3}}})=C_{1}k_{o}j_{1}^{'}(x_{o})$,

where

$x_{o}=k_{o}R_{o}$

$B_{1}$ and $B_{o}$ can be obtained as functions of A

Using these relations for $B_{o}$ and $B_{1}$ in the radiation force expression we get:

Thus the radiation force on a sphere is given by

$F_{z}=-\pi k^{3}R^{3}A^{2}sin2kz_{o}Re[f_{o}+f_{1}]$

where $f_{o}={\frac {(1/3)(\rho _{o}/\rho )k^{2}R_{o}^{2}b_{o}(x_{o})+1}{k^{2}R_{o}^{2}[1+(\rho _{o}/\rho )b_{o}(x_{o})+ikR_{o}]}}$

$f_{1}={\frac {(\rho _{o}/\rho )b_{1}(x_{o})-1}{2(\rho _{o}/\rho )b_{1}(x_{o})+1}}$

with

$b_{o}(x_{o})={\frac {j_{o}(x_{o})}{x_{o}j_{o}^{'}(x_{o})}}$

$b_{1}(x_{o})={\frac {j_{1}(x_{o})}{x_{o}j_{1}^{'}(x_{o})}}$

If we neglect damping, assume $x_{o}<<1$ and assume the sphere is incompressible (i.e. $c_{o}$ approaches infinity), then the radiation force simplifies to:

$F_{z}=-\pi k^{3}R_{o}^{3}A^{2}sin2kz_{o}\rho _{o}{5\rho _{o}-2\rho \over 3(2\rho _{o}+\rho )}$

This expression for radiation force (in a standing wave field) was initially derived by King. Note that the radiation force is directly proportional to the cube of the radius and directly proportional to the velocity amplitude.

Assumptions

1. kR << 1, i.e., the wavelength of the sound field is much larger than the dimension of the sphere.
2. incompressible object (used by King, although Gorkov has derived results that permits finite compressibility of the sphere)

Sphere is suspended when sum of the forces acting on it equals zero, i.e., when force due to gravity balances the upward levitation force.

As a result, the object is attracted to regions of minimum potential energy (pressure nodes). Antinodes are regions experiencing high pressures.

To ensure the generation of a standing wave, the transducer must be placed at a certain distance from the reflector and a particular frequency should be used to get satisfactory results. This distance should be a multiple of half the wavelength of the sound produced to make sure the nodes and antinodes are stable.

Secondly, the direction of the force exerted by the radiated pressure due to the sound waves must be parallel to the direction of gravity.

Since the stable areas should be large enough and able to support the object to be levitated, the object's dimensions should lie between one third and one half of the wavelength. It is important to note that the higher the frequency, the smaller the dimensions of the object one is trying to levitate (since wavelength and frequency are inversely proportional to each other)

The materials of the object is important too, since the density along with the dimensions will give the value for its mass and determine the gravitational force and consequently whether the upward force produced by the pressure radiation is suitable.

Another characteristic important when talking about material properties is the Bond number which is important when dealing with drops of fluid. It characterizes the surface tension and size of the liquid relative to the fluid surrounding it. The lower the Bond number, the greater the chances that the drop will burst.

Finally, to achieve such high pressures (that can cancel the gravitational force), linear waves are insufficient. Therefore, non-linear waves play an important role in acoustic levitation. This is easily one of the reasons why the study of acoustic levitation is challenging. Nonlinear acoustics is a field that deals with physical phenomena difficult to comprehend. Based on experimental observations, heavy spheres incline to velocity antinodes, light particles are closer to the nodes.

Other effects on levitation force

Temperature, pressure, fluid medium characteristics (density, particle velocity) affect the levitation force. It is important to remember that the medium changes as conditions change. The fluid medium consists of reactants and products that change with reaction rate.

Thus, consequently the levitation force is affected. To compensate for medium changes, resonance tracking system can be employed (which helps to maintain stable levitation of the particle under study)

Design considerations

The sphere or particle under study should experience a lateral force which will act as a positioning force (along with the more obvious vertical levitating force) Rotation of the sphere about its axis will ensure uniform heating and stability.

Using non-spherical particles

When levitating non-spherical particles, the largest cross section of the object will end up aligning itself perpendicular to the axis of the standing wave.

Traveling vs Standing waves

King discovered that the radiation pressure exerted by a standing wave is much larger than the pressure exerted by a traveling wave (which has the same amplitude as the standing wave)

This is because the pressure exerted by a standing wave is due to the interference between the incident and scattered waves. Pressure exerted by a traveling wave is due to contributions from scattered field only.

References

1. Theory of long wavelength acoustic radiation pressure by Löfstedt and Putterman
2. Development of an acoustic levitation reactor by Cao Zhuyoua, Liu Shuqina, Li Zhimina, Gong Minglia, Ma Yulongb and Wang Chenghaob
3. HowStuffWorks

Useful Sites

1. Spherical Hankel Functions
2. Legendre Polynomials
3. Multipole expansion