Bubble acoustic field

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. 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

Similar spatial distribution of active bubbles has been observed in partially degassed water and in pure water irradiated with pulsed ultrasound [67]. For both the cases, the number of large inactive bubbles is smaller than that in pure water saturated with air under continuous ultrasound, which is similar to the case of a surfactant solution. As a result, enhancement in sonochemical reaction rate (rate of oxidants production) in partially degassed water and in pure water irradiated with pulsed ultrasound has been experimentally observed [70, 71]. With regard to the enhancement by pulsed ultrasound, a residual acoustic field during the pulse-off time is also important [71]. 

As ultrasonic frequency increases, the acoustic field is more restricted above an ultrasonic transducer. Roughly speaking, when the wavelength of ultrasound (2 = c/f, where c is the sound velocity in the liquid and/is the ultrasonic frequency) is much smaller than the radius of the transducer, the acoustic field is restricted above the transducer. It should be noted that the sound velocity in a bubbly liquid is smaller or occasionally larger than that in liquid without bubbles [87, 88]. 

Lee J, Ashokkumar M, Kentish S, Grieser F (2005) Determination of the size distribution of sonoluminescence bubbles in apulsed acoustic field. J Am Chem Soc 127 16810-16811... 

Mettin R (2005) Bubble structures in acoustic cavitation. In Doinikov AA (ed) Bubble and particle dynamics in acoustic fields modem trends and applications, pp. 1-36. Research Signpost, Trivandrum... 

Mettin R, Akhatov I, Parlitz U, Ohl CD, Lauterbom W (1997) Bjerknes forces between small cavitation bubbles in a strong acoustic field. Phys Rev E 56 2924—2931... 

An issue as interesting as it is contentious is that of electrolyte inhibition of bubble coalescence. Recently, a number of studies have reported the ion-specific nature of electrolyte inhibition of bubble coalescence, albeit in static (non-acoustic) fields [43 -9]. Some electrolytes appear to be highly efficacious whereas others almost completely ineffectual in inhibiting coalescence and ion combination rules have been devised to predict the behavior of various ion pairs. Various explanations have been proposed, most implying a gas-liquid interfacial mechanism. Christenson and Yaminsky [44] have reported a correlation between the inverse Marangoni factor, (dy/ dc]) 2, and coalescence inhibition ability for several different... 

The acoustic bubble size, determined through a pulsed MBSL method developed by Lee at al. [30], was also found to obey a similar dependence on gas concentration as did the coalescence in the same electrolyte solutions [41], as can be seen in Fig. 14.8. It can be inferred from these results that gas concentration controls the extent of coalescence, which itself is the main determinant of the bubble size in an acoustic field. 

It is the subsequent fate of some of these bubbles, as they oscillate in the applied sinusoidal acoustic field, which is the origin of sonochemistry. However, before embarking upon a discussion of bubble dynamics let us consider what other factors apart from degassing, pressurising and filtration affect the onset of cavitation. 

Eq. 2.25 is useful in that it allows an estimate of the point in the compression cycle where total collapse is likely to occur. For example, the collapse time of a bubble of radius 10 cm (R ), in water at an ambient pressure (Pq) of 1 atm, is approximately 1 ps. Since an applied acoustic field of 20 kHz has a compression cycle of 25 ps, it is expected that total collapse will occur in the first 4 % of the cycle. 

In any cavitation field most of the visible bubbles will be oscillating in a stable manner and it is perhaps pertinent that we concentrate our discussions first on the fate of such bubbles in the acoustic field. If we assume that we have a bubble with an equilibrium radius, R, existing in a liquid at atmospheric pressure Pjj, then the oscillation of the bubble and in particular the motion of the bubble wall, under the influence of the applied sinusoidal acoustic pressure (P ) is a simple dynamical problem, akin to simple harmonic motion for a spring. 

It has been argued (Appendix 3, Eq. A.21) that the collapse time for a bubble, initially of radius R, is considerably shorter than the time period of the compression cyde. Thus the external pressure Pj (= P + Pjj), in the presence of an acoustic field, maybe assumed to remain effectively constant (Pj ) during the collapse period. Neglecting surface tension, assuming adiabatic compression (i. e. very short compression time), and replacing R, by R, the size of the bubble at the start of collapse, the motion of the bubble wall becomes... 

Chirone, R., Massimilla, L. and Russo, S. (1992). Bubbling Fluidization of a Cohesive Powder in an Acoustic Field. In Fluidization VII. Ed. Potter and Nicklin. New York Engineering Foundation. 

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