Sonochemistry cavitation bubble

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Fig. 1.1 The regions for transient cavitation bubbles and stable cavitation bubbles when they are defined by the shape stability of bubbles in the parameter space of ambient bubble radius (R0) and the acoustic amplitude (p ). The ultrasonic frequency is 515 kHz. The thickest line is the border between the region for stable cavitation bubbles and that for transient ones. The type of bubble pulsation has been indicated by the frequency spectrum of acoustic cavitation noise such as nf0 (periodic pulsation with the acoustic period), nfo/2 (doubled acoustic period), nf0/4 (quadrupled acoustic period), and chaotic (non-periodic pulsation). Any transient cavitation bubbles result in the broad-band noise due to the temporal fluctuation in the number of bubbles. Reprinted from Ultrasonics Sonochemistry, vol. 17, K.Yasui, T.Tuziuti, J. Lee, T.Kozuka, A.Towata, and Y. Iida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

Hart Edwin J, Henglein A (1987) Sonochemistry of aqueous solutions H2-02 combustion in cavitation bubbles. J Phys Chem 91 3654—3656... 

Physical Chemist who specializes in Sonochemistry, teaches undergraduate and postgraduate Chemistry and is a senior academic staff member of the School of Chemistry, University of Melbourne. Ashok is a renowned sono-chemist who has developed a number of novel techniques to characterize acoustic cavitation bubbles and has made major contributions of applied sonochemistry to the Food and Dairy industry. His research team has developed a novel ultrasonic processing technology for improving the functional properties of dairy ingredients. Recent research also involves the ultrasonic synthesis of functional... 

Further investigations of chemical kinetics and transformation products will be carried out during the final phase of the project. In order to truly understand sonochemical effects, the behavior of the individual bubbles and the bubble clouds must be more finely resolved. Physical characterization of cavitation bubble clouds will also be performed. Thus, a more fundamental link will be established between bulk, observable parameters and sonochemistry, via the physics and hydrodynamics of the cavitating cloud. 

FIGURE 2.29 Development and collapse of cavitation bubbles. (Reprinted from Mason, T. J., Sonochemistry, Oxford University Press, Oxford, 1999. With permission.)... 

The formation of cavitation bubbles decreases with increasing ultrasonic frequency. A simple qualitative explanation of this effect could be that, at very high frequency the rarefaction (and compression) cycle is extremely short and the formation of a cavity in the liquid requires a hnite time to permit the molecules to puU apart. Hence, when the wavelength approaches or becomes shorter than this time, cavitation becomes more difficult to achieve. For this reason, and due to mechanical problems with transducers at high frequencies, the frequencies generally used for sonochemistry are between 20 KHz and 40 KHz. 

The effect of ultrasound on this solvolysis reaction increases as the reaction temperature is lowered. Thus the sonochemical effect increases from 1.7 at 25°C to 20 at 10°C. This inverse relationship between temperature and ultrasonic effect is a common observation in sonochemistry. In simple terms the more vapour which enters the cavitation bubble the more of a cushion it will provide against violent collapse. Hence any reduction in... 

In the above example, the decomposition ratio (for which the exact method of determination does not appear clearly in the paper) increases notably when the frequency reaches a critical value. The violent shock undergone by a surface hit by a cavitation bubble should be similar in nature if not in intensity to other mechanical shocks, with analogous chemical consequences. Such analogies of the sonochemistry of solids with mechanochemistry were suggested previously. ... 

Recent studies, especially of sonoluminescence (Ch. 1), have led to questions about the existence of a cold plasma inside the cavitation bubbles. In the usual definition, a cold plasma is an ionized state of matter in which the molecules have a temperature below a few himdred K, but the electronic temperature reaches several thousand K. The existence of ionized species in the bubbles and their implication at some stage of the reaction s initial step would make sonochemistry more or less similar to the chemistry in mass spectrometers, with consequences on the modeling and prediction of the reactivity. 

---