1.4 Stable and Transient Cavitation Modes
5
ones become predominant. However, this renders burdensome to operate this range
of frequency in heterogeneous conditions unless adequate stirring is enabled with
the possible risk of altering ultrasonic field and bubbles’ dynamic.
1.5 ‘Hotspots’ Theory and Reacting Zones
Several theories aiming at identifying and modelling direct and indirect effects
brought up by collapsing bubbles on chemical systems, namely, plasma discharge,
electrical, supercritical and hotspot theories, have been explored and critically confronted during the 1990s and 2000s. The ‘hotspot’ theory remains nowadays the
generally admitted theory but it is of probable sight that the truth lies in the middle
of these three theories (Chatel 2016). Each collapsing bubble is considered as a single microreactor, i.e. a local ‘hotspot’, where chemical and physical modifications
do take origin. Numerous experimental and theoretical calculations have been conducted to estimate the temperature and pressure reached during the collapsing phases
and values as high as 10
3 to 2 × 10
3 bar and 5 × 10
3 to 6 × 10
3 K, respectively, have
been found. A considerable amount of work has been done to support this theory,
and it is now conceded that reactions take place in all likelihood in three distinct
zones of the hotspots:
• The centre of the cavitation bubble, where the highest temperature and pressure are
reached. Trapped gaseous materials (solvents vapours, volatile organic molecules,
etc.) in the collapsing bubbles undergo homolytic cleavage to afford free radical species. These then react with molecules located within the vicinity of the
liquid/gas interface of the bubble.
• The interfacial zone between the bubble shell and the bulk solution, where the
temperature reaches up to 2000 K after collapsing step. The physical state of
this interface is still the cradle of nourished discussions notably how is dissipated/transformed the high amount of energy released during the adiabatic collapsing phase. In that region are localized most of chemical reactions involving
radical species as their lifetime is generally too short to reach the bulk solution
region in their native state. These radicals may either recombine or react with
non-volatile molecules unable to fill the bubbles which may accumulate in that
zone.
• Unreacted radicals formed in the cavities of the bubbles or at the interfacial zone
recombine to form in situ stable molecules able then to react in the bulk solution
region. Mechanical effects such as acoustic streaming, micro-jets, shock wave and
shear forces do occur, leading to subsequent physical changes on solid materials
or to emulsification in the case of a liquid/liquid system.
5
ones become predominant. However, this renders burdensome to operate this range
of frequency in heterogeneous conditions unless adequate stirring is enabled with
the possible risk of altering ultrasonic field and bubbles’ dynamic.
1.5 ‘Hotspots’ Theory and Reacting Zones
Several theories aiming at identifying and modelling direct and indirect effects
brought up by collapsing bubbles on chemical systems, namely, plasma discharge,
electrical, supercritical and hotspot theories, have been explored and critically confronted during the 1990s and 2000s. The ‘hotspot’ theory remains nowadays the
generally admitted theory but it is of probable sight that the truth lies in the middle
of these three theories (Chatel 2016). Each collapsing bubble is considered as a single microreactor, i.e. a local ‘hotspot’, where chemical and physical modifications
do take origin. Numerous experimental and theoretical calculations have been conducted to estimate the temperature and pressure reached during the collapsing phases
and values as high as 10
3 to 2 × 10
3 bar and 5 × 10
3 to 6 × 10
3 K, respectively, have
been found. A considerable amount of work has been done to support this theory,
and it is now conceded that reactions take place in all likelihood in three distinct
zones of the hotspots:
• The centre of the cavitation bubble, where the highest temperature and pressure are
reached. Trapped gaseous materials (solvents vapours, volatile organic molecules,
etc.) in the collapsing bubbles undergo homolytic cleavage to afford free radical species. These then react with molecules located within the vicinity of the
liquid/gas interface of the bubble.
• The interfacial zone between the bubble shell and the bulk solution, where the
temperature reaches up to 2000 K after collapsing step. The physical state of
this interface is still the cradle of nourished discussions notably how is dissipated/transformed the high amount of energy released during the adiabatic collapsing phase. In that region are localized most of chemical reactions involving
radical species as their lifetime is generally too short to reach the bulk solution
region in their native state. These radicals may either recombine or react with
non-volatile molecules unable to fill the bubbles which may accumulate in that
zone.
• Unreacted radicals formed in the cavities of the bubbles or at the interfacial zone
recombine to form in situ stable molecules able then to react in the bulk solution
region. Mechanical effects such as acoustic streaming, micro-jets, shock wave and
shear forces do occur, leading to subsequent physical changes on solid materials
or to emulsification in the case of a liquid/liquid system.