Acoustic cavitation, chemical effects

A. A. Atchley and L. A. Crum, Acoustic cavitation and bubble dynamics, in Ultrasound, its Chemical, Physical and Biological Effect, K. S. Suslick, ed, VCH, New York (1988). 

Acoustic cavitation In this case, the pressure variations in the liquid are effected using the sound waves usually ultrasound (16 kHz to 100 MHz). The chemical changes taking place due to the cavitation induced by the passage of sound waves are commonly known as sonochemistry. 

In this chapter we will deal with those parts of acoustic wave theory which are relevant to chemists in the understanding of how they may best apply ultrasound to their reaction system. Such discussions tvill of necessity involve the use of mathematical concepts to support the qualitative arguments. Wherever possible the rigour necessary for the derivation of the basic mathematical equations has been kept to a minimum within the text. An expanded treatment of some of the derivations of key equations is provided in the appendices. For those readers who would like to delve more deeply into the physics and mathematics of acoustic cavitation numerous texts are available dealing with bubble dynamics [1-3]. Others have combined an extensive treatment of theory with the chemical and physical effects of cavitation [4-6]. 

The first requirement for the level of ultrasonic power required to cause chemical effects in a reaction is that sufficient acoustic energy must be supplied to overcome the cavitation threshold of the medium. Once this has been exceeded then the region of... 

Oxidation at the benzylic position of indane -with potassium permanganate (Eq. 3.30) gives indanone in good yields and no PTC is necessary [133]. In a two-phase system consisting of an aqueous solution of KMn04 and indane in benzene an 80 % yield can be obtained under a reduced pressure of ca. 450 Torr. The authors explain this effect by the size of the cavitation bubbles, which is dictated to some extent by the over pressure. An optimal energy transformation, from acoustic to chemical, can thus take place. 

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kHz to 1 GHz. With sound velocities in liquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to 10 4 cm. These are not molecular dimensions. Consequently, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. Instead, sonochemistry and sonoluminescence derive principally from acoustic cavitation, which serves as an effective means of concentrating the diffuse energy of sound. 

The effects of ultrasonic irradiation on photochemical reactions have been also reported. In those papers, effects of cavitation were demonstrated. Cavitation means the process in which micro bubbles, which are formed within a liquid during the rarefaction cycle of the acoustic wave, undergo violent collapse during the compression cycle of the wave.5) The dissociation of water to radicals is an example of these effects. Since activated chemical species such as free radicals have high reactivity, chemical reactions proceed. In other words, this phenomenon is a chemical effect of ultrasonic waves. 

The paper gives an overview of effects occurring or acoustic treatment of dissolved and molten polymers. Emphasis is made on acoustic cavitation discovered recently not only in low-viscous fluids but also in molten polymers. Major guidelines have been specified for practical utilization of acoustic treatment of flowable polymers in molding intensification of extrusion processes, reduction in thickness of produced films, directed mechanical destruction, chemical activation of melts, etc. Efficiency of overlapping high-frequency vibrations in molding of molten thermoplastics is discussed in terms of power consumption. 

Peshkovsky SL (1986) Cavitation of fluid in acoustic wave In Physical and chemical effects upon manufacturing processes, Metallurgiya, Moscow, p 93... 

Sonochemistry (chemical events induced by exposure to ultrasound) occupies an important place in organic chemistry. The chemical effects of high-intensity ultrasound were extensively smdied in aqueous solutions for many years, but is now applied to a variety of organic solvents. The origin of sonochemistry is acoustic cavitation the creation, growth, and implosive collapse of gas vacuoles in solution by the sound field. Acoustic cavitation is the phenomenon by which intense ultrasonic waves induce the formation, oscillation, and implosion of gas... 

In Great Britain, Crowford [2], Notlingk and Neppiras [24], Chalmers [25], and a number of other scientists have devoted their work to the ultrasound effect on metallurgical processes. Recently the investigations on intensification of various chemical and metallurgical processes under acoustic cavitation field are centered at Coventry University (Mason [26]). 

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