2
1 Cavitation and Chemical Reactivity
aspects. Several chemical or physical ‘dosimeters’ have been subsequently designed
in order to rationalize these effects but also to authorize absolute comparison of
results obtained with different equipment.
1.2 The Sound
The sound range is very large, from 20 Hz to more than 100 GHz. Commonly
accepted subcategories are infrasound (below 20 Hz), audible sound (from 20 to
20 kHz), ultrasound (from 20 kHz to 100 MHz) and hypersound above (>100 MHz).
The sound is a mechanical vibrating wave transmitted by elasticity of the considered
medium as a longitudinal wave. The periodical succession of compressive and tensile
stresses induces fluctuating pressures alongside the direction of the wave propagation.
The variation of the acoustic pressure with time can be expressed as follows:
P A P max sin 2π f t
(1.1)
where P max is the maximum pressure amplitude and f the frequency of the sound
waves.
The physical state of the considered medium is of key importance as it is closely
related to its couple elasticity/attenuation propagating waves. Wave attenuation
depends on the absorption coefficient of the medium and of the distance from the
source. Thus, the attenuation is stronger in gases than in liquids and smaller in solids
than in liquids. As a representative example, the speed of sound is around 343 m s
−1
in air and around 1480 m s
−1 in water, and reaches values as high as 3850–5130 m s
−1
in an iron matrix.
The sonochemistry is the science of using sound waves for operating chemical/physical changes induced on matter by mechanical and chemical actions at any
frequency (Suslick 1988; Leighton 1994). However, among all usable sound frequencies, ultrasound offers the best possible combination in between sufficient energy to
enable subsequent effects on chemical systems, ease of generation with piezoelectricity. In addition, users do not suffer any noticeable discomfort since ultrasound is
inaudible for humans (Lepoint and Lepoint-Mullié 1998) (Fig. 1.1).
Two defined windows of frequency are of particular interest for sonochemists:
low-frequency ultrasound known as ‘power’ ultrasound (ca. 20–100 kHz), where
effects of physical nature are predominate; high-frequency ultrasound ranging from
200 to 2000 kHz, where effects of chemical nature are predominate, highlighting
the crucial choice of the incident ultrasonic frequency (Mason and Lorimer 2002)
(Fig. 1.2).
In any case, ultrasound cannot directly interact with vibrational/rotational frequencies of chemical bonds. Ultrasonic irradiation causes enhanced molecular motion
creating an indirect phenomenon responsible for the consequential and unusual reactivity which is called cavitation.
1 Cavitation and Chemical Reactivity
aspects. Several chemical or physical ‘dosimeters’ have been subsequently designed
in order to rationalize these effects but also to authorize absolute comparison of
results obtained with different equipment.
1.2 The Sound
The sound range is very large, from 20 Hz to more than 100 GHz. Commonly
accepted subcategories are infrasound (below 20 Hz), audible sound (from 20 to
20 kHz), ultrasound (from 20 kHz to 100 MHz) and hypersound above (>100 MHz).
The sound is a mechanical vibrating wave transmitted by elasticity of the considered
medium as a longitudinal wave. The periodical succession of compressive and tensile
stresses induces fluctuating pressures alongside the direction of the wave propagation.
The variation of the acoustic pressure with time can be expressed as follows:
P A P max sin 2π f t
(1.1)
where P max is the maximum pressure amplitude and f the frequency of the sound
waves.
The physical state of the considered medium is of key importance as it is closely
related to its couple elasticity/attenuation propagating waves. Wave attenuation
depends on the absorption coefficient of the medium and of the distance from the
source. Thus, the attenuation is stronger in gases than in liquids and smaller in solids
than in liquids. As a representative example, the speed of sound is around 343 m s
−1
in air and around 1480 m s
−1 in water, and reaches values as high as 3850–5130 m s
−1
in an iron matrix.
The sonochemistry is the science of using sound waves for operating chemical/physical changes induced on matter by mechanical and chemical actions at any
frequency (Suslick 1988; Leighton 1994). However, among all usable sound frequencies, ultrasound offers the best possible combination in between sufficient energy to
enable subsequent effects on chemical systems, ease of generation with piezoelectricity. In addition, users do not suffer any noticeable discomfort since ultrasound is
inaudible for humans (Lepoint and Lepoint-Mullié 1998) (Fig. 1.1).
Two defined windows of frequency are of particular interest for sonochemists:
low-frequency ultrasound known as ‘power’ ultrasound (ca. 20–100 kHz), where
effects of physical nature are predominate; high-frequency ultrasound ranging from
200 to 2000 kHz, where effects of chemical nature are predominate, highlighting
the crucial choice of the incident ultrasonic frequency (Mason and Lorimer 2002)
(Fig. 1.2).
In any case, ultrasound cannot directly interact with vibrational/rotational frequencies of chemical bonds. Ultrasonic irradiation causes enhanced molecular motion
creating an indirect phenomenon responsible for the consequential and unusual reactivity which is called cavitation.