The fundamentals of power ultrasound - A review

Abstract

The principal method behind applications of power ultrasound is that of acoustic cavitation. This paper aims to provide an overview of bubble behaviour during acoustic cavitation, including phenomena such as transient and stable cavitation, rectified diffusion, coalescence and sonoluminescence. Application of these effects to processes such as nanomaterial synthesis, emulsion formation and waste water treatment is then described.

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54 - Vol. 39 August (2011) No. 2 Acoustics Australia
THE FUNDAMENTALS OF POWER ULTRASOUND
– A REVIEW
Thomas Leong, Muthupandian Ashokkumar and Sandra Kentish*
Department of Chemical & Biomolecular Engineering and School of Chemistry,
University of Melbourne, VIC 3010
sandraek@unimelb.edu.au
INTRODUCTION
Power ultrasound refers to the section of the sound spectrum
from 20 kHz through to around 1MHz. The basis of many
applications of ultrasound in this frequency range is acoustic
cavitation, which is the formation, growth and collapse of
microbubbles within an aqueous solution [1] resultant from
pressure fl uctuations that occur in the applied sound fi eld. The
event of a collapsing bubble is a microscopic implosion that
generates high local turbulence and the release of heat energy.
The consequence is a signifi cant increase of temperature and
pressure of up to several thousand degrees Kelvin and several
hundred Bar. These physical phenomena are the same as those
reported in hydrodynamic cavitation which results in damage
of mechanical items such as pumps and propellers [2].
These effects can be exploited in a vast array of benefi cial
applications [3]. Elevated temperatures [4] in the vicinity of
collapsing bubble “hot spots” can be utilised to enhance the
chemical reaction rates of some processes, due to the increased
heat and the formation of free radicals. Strong disturbances
of pressure resultant from shockwave emissions lead to
mechanical effects such as mixing and shearing which, for a
chemical reaction, can serve to increase encounters between
reactants, accelerate dissolution or aid the renewal at the surface
of a solid reactant. These conditions, generated by the collapse
of bubbles, are the basis for most aspects of sonoprocessing
and sonochemistry. Examples of signifi cant applications of
acoustic cavitation developed for commercial use include
wastewater treatment [5], food and beverages processing [6],
and the formation of protein microbubbles which can be used
for image contrast agents [7] or drug delivery vehicles [8].
The current review briefl y covers the main concepts which
are vital to the understanding of the cavitation phenomena
followed by an overview of some of the current applications of
ultrasound induced cavitation and some thoughts on what will
be in store for the future. On the subject of acoustic cavitation,
Neppiras [9] has written an excellent review that covers
the important physics of cavitation in sound fi elds. Other
invaluable sources of information can be found in the books
by Young [1], Brennen [10] and Leighton [11] which detail
the mathematical derivations of the basic theories of cavitation
and bubble dynamics along with experimental data for these
theories. A more recent review by Lauterborn [12] is another
excellent reference for those wishing to gain an insight to the
fundamental behaviour of bubbles in an acoustic fi eld.
HISTORY
Cavitation was fi rst reported in 1895 by Thornycroft and
Barnaby [2] when they observed that the propeller of their
submarine became pitted and eroded over a relatively short
operation period. Their observation was the consequence
of collapsing bubbles due to hydrodynamic cavitation that
generated intense pressure and temperature gradients in the
local vicinity. In 1917, Lord Rayleigh [13] published the
fi rst mathematical model describing a cavitation event in an
incompressible fl uid. It was not until 1927 when Loomis [14]
reported the fi rst chemical and biological effects of ultrasound,
that workers recognised that cavitation could be a useful tool
in chemical reaction processes. One of the fi rst applications
reported in the literature was the use of ultrasound induced
cavitation to degrade a biological polymer [15]. Since then,
applications of ultrasound induced cavitation have increased
in popularity, particularly as novel alternatives to processes
such as the production of polymer [16], for the enhancement
of chemical reactions [17], emulsifi cation of oils [18] and
degradation of chemical or biological pollutants [19]. The
advantage of using acoustic cavitation for these applications
is that much more mild operating conditions are utilised in
comparison to conventional techniques and many reactions
which may require toxic reagents or solvents are not necessary.
ACOUSTIC CAVITATION AND BUBBLE
FORMATION
In acoustic cavitation, a sound wave imposes a sinusoidally
varying pressure upon existing cavities in solution [1] (see
Figure 1). During the negative pressure cycle, the liquid is
pulled apart at sites containing such a gaseous impurity, which
are known as “weak spots” in the fl uid. The number of bubbles
that are produced during this rarefaction cycle is proportional
to the density of such weak spots present in the fl uid [10].
The principal method behind applications of power ultrasound is that of acoustic cavitation. This paper aims to provide
an overview of bubble behaviour during acoustic cavitation, including phenomena such as transient and stable cavitation,
rectified diffusion, coalescence and sonoluminescence. Application of these effects to processes such as nanomaterial
synthesis, emulsion formation and waste water treatment is then described.

Acoustics Australia Vol. 39 August (2011) No. 2 - 55
There are two known mechanisms for cavity or bubble
formation [1]. One mechanism involves pre-existing bubbles
in the liquid which are stabilised against dissolution because
the surface is coated with contaminants such as a skin of
organic impurity. A second mechanism relies on the existence
of solid particles (motes) in the liquid with gas trapped in these
particles, where nucleation takes place. There can also be tiny
crevices in the walls of the vessel or container where gas is
trapped. The pressure inside a gas crevice is lower than the
outside liquid pressure. Consequently, gas diffuses into the
gas pocket, causing it to grow. A bubble is then created as the
gas pocket departs from the crevice under the infl uence of a
radiation force.
As can be seen in Figure 1, a bubble formed in one of these
ways may then grow until it reaches a critical size known as
its resonance size. The resonance size of a bubble depends
on the applied frequency of the sound fi eld. When bubbles
reach their resonance size due to growth by processes called
rectifi ed diffusion or coalescence, two possible events may
occur. The bubble may become unstable and collapse, often
violently, within a single acoustic cycle or over a small number
of cycles. This is termed transient cavitation. The other
possibility is that the bubble oscillates for many cycles at, or
near, the linear resonance size. This is termed stable cavitation.
The terms transient and stable cavitation are also used to
defi ne whether or not the bubbles are active in light emission
(sonoluminescence) or chemical reactions [20].
A simple relationship that can relate the resonance size of
the bubble with the frequency is given by equation (1):
F x R ≈ 3 (1)
where F is the frequency in Hz and R is the bubble radius
in m. Note that this equation gives only a very approximate
theoretical resonance size [1, 11].
A more accurate version of equation (1) is the linear
resonance radius which can be calculated using the following
equation [1]:
Rr =
3γp∞
ρω2
(2)
where γ is the specifi c heat ratio of the gas inside the bubble,
p∞ is the ambient liquid pressure, ρ is the liquid density and
ω is the angular frequency of ultrasound (all in SI units). In
practice, the size for an active bubble is usually smaller than
this radius due to the nonlinear nature of the bubble pulsation
[21].
At 20 kHz ultrasound frequency, the bubbles generated in the
sound fi eld are relatively large and their collapse results in strong
shockwaves which can be useful for mechanical shearing applications
such as emulsifi cation [18]. Between 100 to 1000 kHz, the bubbles
generated are much smaller. However, their collapse induces
a higher increase in temperature which can be more useful
for sonochemical purposes [22]. At above 1 MHz frequency,
cavitational effects are much weaker. However, there are
some industrial applications in this frequency range such as
the gentle cleaning of electronic parts and the nebulisation of
liquids to create fi ne sprays. This higher frequency range is also
commonly used for medical and industrial imaging purposes.
BUBBLE BEHAVIOUR IN AN ACOUSTIC
FIELD
Gas bubbles in liquids under the infl uence of a sound fi eld
can do several things, as can be seen in Figure 2. A bubble can
meet another bubble in solution, combining to form a larger
bubble. This is termed coalescence. In a gas saturated solution
such as water above a certain threshold pressure, individual
bubbles can also grow with time over several acoustic cycles.
This is termed rectifi ed diffusion. If gas bubbles grow large
enough, they can leave the system entirely due to buoyancy. This
is termed degassing. Bubbles of a certain size can also become
unstable and collapse, often violently. The range of bubble radii
at which this occurs is very wide, and is usually much lower than
the linear resonance radius [21]. Bubble collapse can sometimes
be accompanied by fragmentation into smaller bubbles. Under
suitable conditions, light emission can be observed, and this is
Figure 1: Graphical summary of the event of bubble formation, bubble growth and subsequent collapse over several acoustic cycles. A bubble
oscillates in phase with the applied sound wave, contracting during compression and expanding during rarefactions.

56 - Vol. 39 August (2011) No. 2 Acoustics Australia
termed sonoluminescence. Bubbles below the threshold pressure
for rectifi ed diffusion can dissolve into solution.
It is possible to predict the behaviour of a single gas
bubble provided knowledge is known about the radius, the
driving frequency, the driving pressure and the dissolved gas
concentration. Each of these phenomena is described in more
detail in the following sections.
The onset of stable and transient cavitation
Apfel [23, 24] has used equations for bubble growth
thresholds developed by Safar [25] to produce a series of
cavitation prediction charts. An example of these charts, for a
10 kHz frequency system is shown in Figure 3 and illustrates
the areas of different cavitation activity:
Region A – The bubbles are under inertial control and
bubble growth only occurs via rectifi ed diffusion. Upon
reaching resonance (R/Rr = 1) the bubbles undergo more
violent behaviour and collapse.
Region B – Growth by rectifi ed diffusion and/or by
mechanical means may occur although the bubble is not
initially transient. Upon fragmentation, the microbubbles
formed may exist in region C.
Region C – This region is the transient region for cavitation
and the border with region B indicates the transient threshold,
also known as the Blake threshold [26].
Safar’s equation enables the prediction of the rectifi ed diffusion
pressure threshold PD for a bubble of radius RD (Equation (3)) and
indicates the threshold between Regions A and B:
(3)
Figure 3: Cavitation prediction chart for a 10 kHz system in a 100%
gas saturated system taken from Apfel [24]. Region A is for a bubble
under inertial control, B the region for growth by rectified diffusion
and C the region for transient cavitation.
Figure 2: Bubble nuclei under the influence of an acoustic sound field can grow via either coalescence or rectified diffusion. Upon reaching
an unstable size, the bubble will collapse, possibly fragmenting to form smaller bubbles accompanied by an emission of light if conditions are
suitable. Bubbles that become bigger than the resonance size will leave the system by buoyancy.

Acoustics Australia Vol. 39 August (2011) No. 2 - 57
Here, η is the solution viscosity, σ is the surface tension, ω
and ωr the driving and resonance frequency respectively, P0
the ambient pressure, and Ci and C0 are the concentrations of
dissolved gas in the liquid far from the bubble and at saturation,
respectively. The Blake threshold pressure PB is defi ned as:
(4)
Neppiras [27] developed similar predictions for the transient
thresholds based on Apfel’s criterion for a bubble at the radius
of the transient threshold and Blake’s threshold pressure. He
used another expression from Safar [25] which included a
multiplying factor that extended the formula so that it applied
for bubbles through to resonance, not just R0 < Rr. More
recently, computer simulations performed by Yasui [28] for
various acoustic frequencies have been used to show different
regions of bubble behaviour, namely dissolving bubbles, stable
and unstable bubbles which may emit SL under the correct
conditions, and degas bubbles, which oscillate radially at a low
amplitude and do not emit SL.
In a multibubble system, the behaviour of bubbles is more
complex due to the multiple pathways in which a bubble can
enter or leave the system and also different pathways in which it
can grow or collapse. It is both diffi cult to predict theoretically
and monitor experimentally the precise bubble behaviour in
such systems. In order to understand bubble behaviour in an
acoustic fi eld, it is prudent to begin from the simplest case of a
single bubble that is oscillating in an acoustic fi eld.
Dynamics of a single bubble
The Rayleigh-Plesset equation is commonly used to model
the fundamental motion of a bubble in an acoustic fi eld. Those
looking for an in depth derivation of the equation can fi nd it in
the book by Young [1].
For motion of the bubble wall we have the result derived
by Besant [29]:
(5)
where R is the radius of the bubble wall at any time, is the
wall velocity, P∞ is the pressure in the liquid at infi nity, PL is
the pressure in the liquid at the bubble wall and ρ the liquid
density.
Noltingk and Neppiras [30, 31] extended this fundamental
equation to include the effects of surface tension due to the
Laplace pressure of the bubble (Equation (6)). To take this into
account, at R = R0 the gas pressure in the bubble is P0 + 2σ/ R0
where P0 is the ambient pressure in the liquid and σ is the
surface tension. Adiabatic heat transfer is assumed with γ being
the ratio of specifi c heats of the gas.
(6)
A viscosity term for the liquid was later added by Poritsky [32] and
he showed that this term arises only in the boundary conditions rather
than through the Navier-Stokes equation. The equation then becomes
(7)
where η is the viscosity of the liquid. Equations (5), (6) and (7)
are often referred to as the Rayleigh-Plesset equation and are
fundamental in the analysis of bubble behaviour.
We usually subject the bubble to a sound fi eld such that the
pressure P varies as
P = P0 - P
A sinωt (8)
where P0 is the steady state pressure (usually atmospheric
pressure), ω is the angular frequency and P
A is the amplitude
of the driving pressure. When we add this pressure term into
Equation (6), we obtain
32σ2σR0
13γ
2R0RRρ
RR+ R2 =-- (P0 - P
A sinωt)P0 + (9)
which is the fundamental equation of a single gas bubble under
the infl uence of an oscillating sound wave.
Rectifi ed diffusion – growth of a single bubble
The fundamental equations from the previous section
have been applied to model the process of bubble growth or
dissolution known as rectifi ed diffusion. This phenomenon
relates to an unequal transfer of mass across the bubble
interface during the rarefaction and compression of the
sound wave cycle. Above the threshold pressure for rectifi ed
diffusion, this unequal mass transfer causes the bubbles to
slowly grow. Below this pressure, the bubble dissolves due to
the greater infl uence of the Laplace pressure exerted on the
bubble wall by surface tension. Those looking for an elegant
review of the history of the developments of the theory behind
rectifi ed diffusion should refer to the work by Crum [33].
Eller and Flynn [34] developed a theory to account for
this uneven mass transfer by two main effects. These are
known as the area and shell effects. The area effect refers to
the fact that diffusion of gas into the bubble occurs when the
bubble is larger during the expansion phase, whilst diffusion
out of a bubble occurs when the bubble is smaller during the
compression phase. As the rate of diffusion across an interface
is proportional to the surface area available for mass transfer,
more gas diffuses into the bubble than out. Over a number of
acoustic cycles, a net infl ow of gas into the bubble results,
leading to bubble growth.
The shell effect refers to the mass transfer boundary layer
through which mass transfer occurs. As the bubble shrinks
in the compression phase, this shell thickness increases. In
contrast, as the bubble expands, the shell thins as depicted in
Figure 4. The concentration gradient is thus lower when the
bubble is in compression, thereby resulting in a lower driving
force for mass transfer.
The solution to the rectifi ed diffusion problem is an interesting
one, as it is complicated by a moving boundary layer as the bubble
oscillates. This problem has been solved in two different ways
by both Eller and Flynn [34] and Hsieh and Plesset [35,36], and
details of their approach can be obtained in their papers.

58 - Vol. 39 August (2011) No. 2 Acoustics Australia
Figure 4: A depiction of the change in mass transfer boundary layer
(shell) thickness of a bubble during the expansion and compression
cycles of an acoustic wave. The concentration gradient is thus
enhanced during bubble expansion.
In the case of Eller and Flynn, they showed that the change
in the number of moles n of a gas in a bubble is given by
= 4πDR0C0 +R0 H
dn 1/2
R(R/R0)4
dt R0πDt (10)
where H is defi ned by
44
RRPg
R0R0
P0
H = -
Ci
C0
(11)
The pointed brackets in this case imply time average, where t
is the time. Ci is the concentration of dissolved gas in the liquid
far from the bubble, C0 is the saturation concentration of gas in
the liquid and D is the diffusivity of the gas.
Crum [33] later used Eller’s derivation and extended it
by taking into account the thermodynamics of the process.
The end result for the change in bubble radius for a spherical
bubble as a function of time is
(12)
where d = Rg T C0 / P0. Here Rg is the universal gas constant
and T is the temperature.
An alternative mathematical analysis was presented by
Fyrillas and Szeri [37-39] in a series of papers to analyse the
phenomena of rectifi ed diffusion. Their derivation utilised
Lagrangian coordinates rather than spherical coordinates
in order to account for the moving boundary condition.
The Henry’s Law boundary condition describing the gas
concentration at the surface of the bubble wall was also split
into a smooth and oscillatory solution to the problem.
Bubble coalescence
In a single bubble system, the only growth pathway
possible is via rectifi ed diffusion. To investigate the behaviour
of a multibubble system, we must also consider the process of
bubble coalescence. The bubble coalescence process can be
described in three steps [40,41]:
1. The bubbles come into contact to form a fi lm of
thickness between 1 to 10 μm
2. This fi lm reduces in thickness
3. When the fi lm becomes suffi ciently thin, rupture
occurs and the bubbles coalesce.
Studies of coalescence behaviour in the absence of
ultrasound have been performed by various workers [40-
43]. The review by Chaudhari and Hofmann [44] provides a
comprehensive overview of the coalescence behaviour of gas
bubbles in liquids.
Studies by Lee [45] and Sunartio [46] have found that
coalescence behaviour is similar to that reported in the absence
of ultrasound. An improved understanding of coalescence
behaviour in ultrasound systems will ultimately improve the
effi ciency of a range of sonoprocessing applications where
bubble population and sizes are important.
Sonoluminescence
The violence of a transient collapse can sometimes
be characterised by the emission of light, termed
sonoluminescence. Sonoluminescence was fi rst observed
in the 1930s by two different groups of workers; Marinesco
and Trillat [47] in 1933 and Frenzel and Schultes [48] in
1934. Paounoff [49] validated these observations in 1947 by
showing that the exposure of photographic plates occurred at
locations of pressure maxima (antinodes) of the standing wave.
It became clear to workers in the fi eld that the gas bubbles
generated during acoustic cavitation were responsible for the
emission of light.
The high temperatures and pressures generated with the
onset of inertial cavitation also serves to induce a range of
chemical reactions within and surrounding the bubble [50].
The extreme conditions enable the transduction of acoustic
energy into light energy that has a very short emission lifetime.
Sonoluminescence can be produced in the case of a
single bubble (see Figure 5) undergoing extremely nonlinear
pulsations, termed single-bubble sonoluminescence (SBSL)
and also in the case of a fi eld of bubbles undergoing cavitation,
termed multibubble sonoluminescence (MBSL). In the former
case, a single intensely bright dot suspended in a standing
wave can be observed [22]. The intensity of the emitted light
is dependent on various factors that include the amount and
type of dissolved gases in the liquid [51], the frequency of the
applied ultrasound [52], the applied sound pressure amplitude,
hydrostatic pressure and addition of particular solutes [53-56].
Multibubble sonoluminescence can often be used as a probe
for cavitation activity in a solution.
There are a number of theories as to the mechanism for
sonoluminescence. These are discussed by Finch [57] and
Jarman [58]. During the compression phase of the oscillating
bubble, the contents of the bubble are heated [1]. This causes
excitation of the gas in the bubble, promoting the formation
and recombination of excited species. Recent numerical
simulations by Yasui et al. [21] showed that the main mechanism
of the light emission in sonoluminescence is actually electron-
atom bremsstrahlung that occurs in the weakly ionised plasma
formed inside the heated bubble. Bremsstrahlung radiation
is light that results from an electron being accelerated by the
collision with an ion or a neutral atom.

Acoustics Australia Vol. 39 August (2011) No. 2 - 59
Figure 5: Sonoluminescence response of a single bubble in degassed
water. The curve corresponds to the scattered laser light intensity of
a levitated bubble undergoing non-linear oscillation in a standing
wave. Still images show the corresponding bubble expanding from
approximately 5 μm radius to 60 μm radius during one acoustic cycle
at a frequency of 20 kHz. The sharp peak corresponding to the point
of bubble collapse, with a lifetime of several picoseconds, is the
sonoluminescence emitted by the bubble and is visible by the naked
eye as a bright glow.
The relative intensities of the sonoluminescence from many
different gases dissolved in water have been studied [51]. The
general trend found was that as the thermal conductivity of the
gas increases, the sonoluminescence was found to decrease
and this correlated (for the series of noble gases) with the
size of the atom. If sonoluminescence is due to an adiabatic
compression during rapid collapse of the cavitation bubble,
then energy loss due to thermal conduction will indeed lower
the fi nal temperature. Other factors that infl uence the bubble
temperature include the amount of bubble vapour that becomes
trapped inside the bubble [59] and the concentration of gas as
the sonoluminescence intensity is related to the number of
bubbles [60].
Figure 6: Experimental data taken from Young [51] showing the fit
of experimental sonoluminescence intensity data with that of the
theoretically determined cavitation temperature for the noble gases.
Hickling [61] explained that if bubbles are suffi ciently
small, loss of heat from the bubble into the liquid can
signifi cantly reduce the temperature of the collapse, resulting in
lower sonoluminescence intensity. He demonstrated his theory
analytically by means of solution of the equation of motion
of gas in the collapsing bubble and good agreement between
theory and experimental measurements were confi rmed.
Figure 6 (taken from Young [51]) compares the
experimental data points for the noble gases of the SL intensity
as a function of the theoretical temperature of the gas. This
correlation between the predicted temperatures and the SL
intensity supports the hot-spot theory for SL.
The intensity of the emitted light can be approximated by
the following equation from Yasui et al. [21]:
PBr,atom = 4.6 x 10-44nenT/V (13)
where ne is the number of free electrons inside the bubble, n
is the total number of neutral atoms inside a bubble, T is the
temperature inside the bubble, and V is the bubble volume.
It is possible to determine the temperature of the gases
inside the bubble during cavitation, by correlation with the
sonoluminescence emission spectra. This is quite readily
achieved in multibubble systems, where the emission
spectra consist of detailed line structures. Temperatures of
approximately 5000 K were determined by Flint and Suslick [4].
However, in the case of a single bubble, free from disturbances
of other bubbles, the emission spectra normally obtained are
featureless, as shown from the comparison by Matula et al.
[62]. Work by Suslick [63, 64] has recently overcome this
hurdle, by using xenon and argon fi lled bubbles in sulfuric acid.
They were able to obtain good spectral details, from which a
temperature of 15,000 K was deduced – a temperature as high
as that found on the surface of bright stars [65].
SONOCHEMISTRY
Ultrasound induced cavitation is an extremely useful and
versatile tool to carry out chemical reactions. Sonochemistry
refers to the area of chemistry where chemical reactions are
induced by sound. The range of ultrasound frequency commonly
used in sonochemistry range from 20 kHz to ~ 1 MHz. A recent
review for sonochemistry has been written by Ashokkumar and
Mason and forms the basis of the following section [66].
Radical formation
The extreme temperature conditions generated by a
collapsing bubble can also lead to the formation of radical
chemical species. Ultrasonic waves in water have been shown
to form radicals by the following reaction due to homolytic
cleavage:
H2O → H. + OH. (14)
The hydroxy and hydroxyl radical formed in this reaction
are highly reactive and rapidly interact with other radical or
chemical species in solution. H. atoms are highly reducing
in nature and OH. radicals are highly oxidizing. A common
product of this reaction in water is hydrogen peroxide.

60 - Vol. 39 August (2011) No. 2 Acoustics Australia
The generation of H. and OH. radicals, commonly referred
to as primary radicals, has been confi rmed and quantifi ed by a
number of experimental techniques. Common methods include
the use of ESR spin traps and dosimeters [67] or the reaction
with chemicals such as teraphthalic acid [68] that will lead to
the formation of hydroxyterephthalate which can be assayed
with spectroscopy. One of the more simple methods to quantify
the amount of OH. radicals formed is by use of the “Weissler”
method, which involves the oxidation of iodide ions [69].
In this technique, iodine is added to water which has been
sonicated which reacts with the hydrogen peroxide formed.
The reaction scheme for this method is:
2OH. → H2O2
H2O2 + 2I- → 2OH- + I2 (15)
I2 + I- → I3-
The quantity of I3
-
can be measured by ultraviolet
spectrophotometry at 353 nm.
Suslick [70] has suggested that sonochemical reactions
can occur at two sites of a bubble. The fi rst is the bubble’s
interior gas phase, and is suggested as the dominant site for
sonochemical reaction due to the intense temperatures attained
during collapse (~5000 K). The second is the liquid shell
surrounding the bubble, which can reach temperatures of up to
1900 K. In addition to these two primary reaction sites on the
bubble, solutes in bulk solution beyond the bubble itself can
react with the radicals formed inside or on the surface of the
collapsing bubble.
Nanomaterial synthesis
Due to the reducing and oxidizing potential of the primary
radicals generated during acoustic cavitation, sonochemistry
has been used extensively in materials synthesis. The radicals
are useful in initiating certain chemical reactions in organic and
organometallic chemistry, as well as initiating polymerization
[71]. They can also be used as a means to cross link proteins
and this is an important mechanism for the synthesis of protein
microspheres. A comprehensive review by Suslick and Price
[72] gives examples of many applications of material synthesis
using ultrasound.
A simple example of metal synthesis in an aqueous medium
is that of the reduction of gold from Au(III) nanoparticles to Au
(0) [73]. Under an inert atmosphere, the only reducing species
in water are H. atoms and these act to reduce the Au (III) ions
to produce Au(0) by the following reaction:
AuCl -
4
+ 3H. → Au+ 3H+ + 4Cl- (16)
Ultrasound processing has generally been found to provide
selectivity in the reaction product. For example, under
ultrasonic irradiation, switching from an ionic pathway to a
radical pathway is often observed. An example is the reaction
of styrene with lead tetra-acetate in acetic acid [74].
In recent years, ultrasound generated radicals have been
widely used as a novel technique for polymer synthesis [71].
The advantage of ultrasound induced polymerization has
several advantages, namely that no chemical initiators or
costabilisers are required, reaction temperatures are lower,
polymerization rates are faster, conversions of reactant are
greater and larger molecular weights can be produced. Teo et
al. [75] for example, has used ultrasound to initiate emulsion
polymerization of methacrylate monomers. Their results
show that the mechanism involved is very similar to that of
conventional polymerization processes.
Shear and mechanical mixing
Ultrasound-driven growth and collapse of bubbles is
accompanied by the generation of shock waves, microstreamers,
and microjets which lead to increased turbulence and shear
forces which can facilitate mass transport. These physical
phenomena can be used for a range of shearing and mixing
applications. Cavitation can be used to induce emulsifi cation
of two immisible liquids such as oil in water. The jetting
behaviour from transient collapse can be used to disperse fi ne
drops of one liquid into the other when cavitation occurs at the
interface between the two liquids. Leong et al. [18] showed
that oil emulsions with mean particle sizes as low as 40 nm
could be generated using a 20 kHz ultrasound frequency and
an appropriate surfactant/co-surfactant system. These results
were comparable to those generated from homogenization with
a Microfl uidizerTM [76], with the advantage that the ultrasonic
horn would be easier to clean and more effi cient due to lower
equipment wear rates.
Mason [77] provides an example of the use of ultrasound as
an alternative to phase transfer catalysts, which are compounds
that enable the transfer of a water-soluble reagent into an
organic phase. The use of ultrasound produces fi ne emulsions
that can be used to disperse the aqueous phase into the organic
phase. An example is the formation of dichlorocarbene which
has been reported to achieve a much higher conversion in the
presence of ultrasound in comparison with the case where the
reagents were simply stirred.
Medical applications
More recent advances of sonochemistry are in the fi eld of
microbubble synthesis. Grinstaff and Suslick [78] developed
an ultrasonic technique whereby cavitation induced radical
formation and emulsifi cation were used to synthesise air or
liquid fi lled proteinaceous microspheres. In this procedure,
proteins dissolved in a liquid are irradiated with ultrasound
to form a stable foam-emulsion. The stability of this foam is
caused by cavitation induced radicals that cause the protein
molecules to cross-link, generating a spherical protein shell.
The principal cross-linking agent is the superoxide ion created
by the extremely high temperatures produced during acoustic
cavitation. The mechanism proposed by Suslick suggests that
disulfi de cross-linkages form, although other workers [79]
suggest that the presence of SH groups is not necessary for
successful microsphere formation.
Air-fi lled microbubbles are in clinical use as echo-contrast
agents for sonographic applications. Many articles and review
articles are available in the literature for those interested
in the medical applications available [7, 8]. Commercial
microbubbles such as Defi nityTM and OptisonTM are marketed
for in vivo use where these spheres are injected into the
bloodstream of a patient. The application of ultrasound to

Acoustics Australia Vol. 39 August (2011) No. 2 - 61
the affected area causes bubbles to vibrate in response to the
pressure changes of the sound wave. Bubbles of an appropriate
size range will vibrate very strongly at resonance, making
them several thousand times more refl ective than normal body
tissues. The result is improved image resolution of tissues and
organs. In some cases, a liquid can be encapsulated inside these
protein spheres. This ability to encapsulate liquids can be used
for targeted or time released drug delivery [80]. Figure 7 taken
from Zhou et al. [81] shows lysozyme protein microbubbles
fi lled with various types of organic oils. Care must be taken
however when generating sonicated protein products intended
for in vivo use or as food ingredients. Stathopulos et al. [82]
have recently reported that the sonication of proteins can lead
to the formation of amyloid aggregates. It is possible that such
aggregates can give rise to immunogenicity, toxicity or even
disease responses in the subject.
Figure 7: (a) Optical microscopic image and SEM images of (b)
tetradecane, (c) dodecane, (d) sunflower oil and (e) perfluorohexane-
filled lysozyme microspheres taken from Zhou et al. [81].
Sonochemical degradation of pollutants
Another useful application of sonochemistry is in the fi eld
of wastewater treatement. Articles by Colarusso and Serpone
[83] and Adewuyi [84] provide a comprehensive overview of
sonochemistry for use in environmental applications such as
the degradation of pollutants. The hydroxyl radicals generated
during cavitation can be used for the oxidative degradation of
organic pollutants in an aqueous system. The heat produced
in the cavitation process can also be used to remove volatile
pollutants by pyrolytic decomposition. Singla et al. [19]
recently reported that sonochemical degradation of various
organic pollutants could be achieved by both oxidative and
pyrolytic mechanisms.
Destruction of microorganisms by power ultrasound is
also possible [85]. However, ultrasound has been shown to
have no direct impact on spores or gram positive organisms
and so is often used in conjunction with more conventional
techniques such as chlorination or treatment with heat and
pressure, such that increased effectiveness is achieved with
lower requirements for chemical or energy usage.
CONCLUSIONS, FUTURE DEVELOPMENTS
AND EXPECTED ADVANCEMENTS
Ultrasound induced cavitation is an effective tool to
induce mechanical shear and to perform a variety of chemical
reactions. This approach has been available for some decades
but deployment is now rapidly increasing due to the recent
availability of industrial scale ultrasonic horns and ultrasonic
reactors.
Ultrasound can be energy intensive and hence emerging
applications are likely to be where high value can be added.
There exists a diverse range of applications that are well
established or have exciting future potential that meet this
criterion. The development of effective drugs for treatments
of disease by ultrasound is one of the areas which could have
widespread health benefi ts. Another emerging area is the use of
ultrasound to provide novel food ingredients and our work is
focused in particular on the production of novel dairy products.
The need for more sustainable processing of consumer goods
will be satisfi ed in part by such continuing developments in
ultrasound processing and sonochemistry.
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