4
1 Cavitation and Chemical Reactivity
tics of the acoustic wave, and expand gradually during the alternation of compressive
and tensile stresses. At one stage, they do violently and adiabatically collapse leading to very intense effects with localized liberation of high amounts of energy on the
form of high temperature and pressure accompanied by violent mechanical effects.
Cavitation can be initiated in many types of liquids (water, organic solvents, liquefied gases, molten organic and inorganic salts, supercritical fluids, etc.) over a wide
range of experimental conditions. This complex dynamic behaviour strongly relies
on properties of the liquid (surface tension, density, viscosity), on the acoustic field
(intensity and frequency) and on the bubbles themselves (gas content, diameter). The
minimum required acoustic pressure to enable cavitation is named ‘cavitation threshold’ (Lepoint and Lepoint-Mullié 1998). Typically, whereas the theoretical strength
of water is around 1000 bar, only 1 bar appears to be necessary in practice to initiate
cavitation. This is likely due to the presence of inhomogeneity acting as ‘structural
defects’ in the irradiated material by weakening the intermolecular bonding network
insuring cohesion of the liquid. Indeed, the presence of dissolved gas or even small
size suspended dust particles do act as nucleating sites to trigger cavitation activity. In addition, it is worth mentioning that only a certain percentage of incipient
microbubbles do reach the collapsing stage. Indeed, some do coalesce with neighbouring bubbles prior to eventually float and to release out from the liquid surface.
Some others undergo collapse phase during the positive cycle of the ultrasonic pressure to initiate characteristic physical and chemical effects of ultrasound. Moreover,
some have probably an unknown or random behaviour, revealing the complexity of
cavitation phenomenon intimately linked to the bubbles dynamic.
1.4 Stable and Transient Cavitation Modes
In terms of collapse intensity, two different cavitating modes, referred as ‘stable’ and
‘transient’, can be defined with separate effects although a distinct frontier between
the two states is not that obvious (Yasui 2011). In the case of the former, the cavitating
gas bubbles, when submitted to a low amplitude ultrasonic wave, enter into resonance
with the incident wave during a few hundreds to thousands of acoustic cycles. These
small amplitude vibrations induce linear low-scale variations in the bubble’s size with
small pressure variations, preventing violent collapse. It is usually admitted that this
type of cavitation is responsible for enhanced mechanical effects in the bulk solution.
On the contrary, a high amplitude ultrasonic wave will promote the rapid increase of
the bubble’s diameter from a few to few hundreds μm diameter within a very small
number of acoustic cycles resulting in a swift and violent collapsing step in a few μs,
that is, transient cavitation. This phenomenon is responsible for sonoluminescence
which is the emission of a diffuse light, observed as early as 1933, originating from
the centre of the collapsing bubble (Suslick and Flannigan 2008). High-frequency
ultrasound (>200 kHz) do favour the occurrence of this regime of cavitation, and
consequently, the physical effects brought up by ultrasound vanish and chemical
1 Cavitation and Chemical Reactivity
tics of the acoustic wave, and expand gradually during the alternation of compressive
and tensile stresses. At one stage, they do violently and adiabatically collapse leading to very intense effects with localized liberation of high amounts of energy on the
form of high temperature and pressure accompanied by violent mechanical effects.
Cavitation can be initiated in many types of liquids (water, organic solvents, liquefied gases, molten organic and inorganic salts, supercritical fluids, etc.) over a wide
range of experimental conditions. This complex dynamic behaviour strongly relies
on properties of the liquid (surface tension, density, viscosity), on the acoustic field
(intensity and frequency) and on the bubbles themselves (gas content, diameter). The
minimum required acoustic pressure to enable cavitation is named ‘cavitation threshold’ (Lepoint and Lepoint-Mullié 1998). Typically, whereas the theoretical strength
of water is around 1000 bar, only 1 bar appears to be necessary in practice to initiate
cavitation. This is likely due to the presence of inhomogeneity acting as ‘structural
defects’ in the irradiated material by weakening the intermolecular bonding network
insuring cohesion of the liquid. Indeed, the presence of dissolved gas or even small
size suspended dust particles do act as nucleating sites to trigger cavitation activity. In addition, it is worth mentioning that only a certain percentage of incipient
microbubbles do reach the collapsing stage. Indeed, some do coalesce with neighbouring bubbles prior to eventually float and to release out from the liquid surface.
Some others undergo collapse phase during the positive cycle of the ultrasonic pressure to initiate characteristic physical and chemical effects of ultrasound. Moreover,
some have probably an unknown or random behaviour, revealing the complexity of
cavitation phenomenon intimately linked to the bubbles dynamic.
1.4 Stable and Transient Cavitation Modes
In terms of collapse intensity, two different cavitating modes, referred as ‘stable’ and
‘transient’, can be defined with separate effects although a distinct frontier between
the two states is not that obvious (Yasui 2011). In the case of the former, the cavitating
gas bubbles, when submitted to a low amplitude ultrasonic wave, enter into resonance
with the incident wave during a few hundreds to thousands of acoustic cycles. These
small amplitude vibrations induce linear low-scale variations in the bubble’s size with
small pressure variations, preventing violent collapse. It is usually admitted that this
type of cavitation is responsible for enhanced mechanical effects in the bulk solution.
On the contrary, a high amplitude ultrasonic wave will promote the rapid increase of
the bubble’s diameter from a few to few hundreds μm diameter within a very small
number of acoustic cycles resulting in a swift and violent collapsing step in a few μs,
that is, transient cavitation. This phenomenon is responsible for sonoluminescence
which is the emission of a diffuse light, observed as early as 1933, originating from
the centre of the collapsing bubble (Suslick and Flannigan 2008). High-frequency
ultrasound (>200 kHz) do favour the occurrence of this regime of cavitation, and
consequently, the physical effects brought up by ultrasound vanish and chemical