ABSTRACT
The mechanical effects of ultrasound have been found to be useful in a variety of potential applications, communication and imaging are some that can be mentioned. When ultrasound is passed through a liquid medium, physical and chemical effects are generated due to the interaction of the sound energy with dissolved microbubbles in the liquid (Ashokkumar and Mason, 2007). The readers, after reading various Chapters presented in this book, should be in a position to relate these physical and chemical effects to acoustic cavitation. The physical forces such as turbulence, shock wave, and microstreaming are helpful in enhancing mass transfer leading to increased reaction rates and emulsification processes. The chemical effects relate to the generation of a variety of reducing and oxidizing radicals that can be used for specific chemical reactions. In this book, a number of leading researchers have summarized their research activities and reviewed specific research work in the area of sonochemical synthesis of nanomaterials. The key issues focused by these researchers are developing a simple and energy efficient synthetic methodology, synthesizing nanomaterials with high catalytic activity, developing nanomaterials with controlled physical and chemical properties, and the use of sonochemically synthesized nanomaterials for specific applications. For sustained development of the sonochemical technique for synthesizing nanomaterials, other aspects that control the efficiency of sonochemical reactions in general need to be addressed in addition to the above-mentioned key issues. Hence, the focus of this chapter is to provide some key criteria that need to be kept in mind when ultrasonics and sonochemistry are used for synthesizing nanomaterials. For some reactions, the physical forces are as important as the chemical effects. For example, in emulsion polymerization reactions, the shear forces are crucial to generate the emulsion droplets (Price, 2003). It is well known that the shear forces generated by a 20 kHz horn system are much stronger due to the high energy density at the tip of the horn. Despite a very low radical yield at this frequency, emulsion polymerization reactions have been successfully achieved at 20 kHz only. Of course, dual-frequency systems involving 20 kHz and higher frequencies have been used to generate nanoemulsions (Nakabayashi et al., 2011). On the contrary, substantial amounts of primary radicals are generated at very high frequencies (>200 kHz to <1000 kHz). For chemical reactions in which the shear forces
are not that important, higher frequencies are suitable (Okitsu et al., 2005). In fact, the yield of radicals as a function of frequency is something that has not been clearly dealt with in the literature. For chemical reactions that primarily depend on the radical yield, a clear understanding of the factors that affect the amount of radicals produced is important. For a given volume of sonicated solution at a certain acoustic power level (above the cavitation threshold for all frequencies), the number of cavitation bubbles may increase with an increase in frequency. A decrease in the resonance size of the cavitation bubble with an increase in frequency has been predicted theoretically (Leighton, 1994) and proven to be the case experimentally (Brotchie et al., 2009). This leads to two opposing effects. With an increase in frequency, the maximum bubble temperature reached upon bubble collapse decreases; this would lead to a decrease in radical yield per bubble. However, as the number of bubbles increases with an increase in frequency (provided other experimental parameters remain constant), the total radical yield increases with an increase in frequency. A schematic representation of an increase in the number of standing waves as a function of frequency that results in an increase in the number of bubbles is shown in Fig. 13.1.
