Sonoluminescence

The intensity of sonoluminescence can easily be measured with photocells [159,163] or fiber optics [169] connected to a photomultiplier in darkened surroundings. This measurement is not invasive, and has been suggested as a standard [19]. In principle the sonoluminescence intensity could be correlated to the ultrasonic power, but at this time no direct theoretical correlation has been established. It has been used to determine the areas of maximum cavitational activity in a reactor. Any empirical correlation with power would necessitate preliminary calibration with another method, e.g. with thermal probes. Some care should be exercised when using a sonoluminescence probe for the following reasons  

The gas content of the liquid (nature, concentration), thermal conductivity, viscosity, temperature, hydrostatic pressure, frequency of the sound wave, and shape of the reactor. As a consequence the utmost care should be taken in monitoring experimental conditions to obtain good repeatability. 

Sonoluminescence mainly occurs inside the bubbles (or in their immediate surroundings) and thus cannot be representative of what happens in the liquid phase where most of the events used in ultrasound application occur. In particular the temperature and frequency dependence of sonoluminescence is quite different from that of other effects, thus luminescence decreases with temperature, while liquid-phase effects usually increase to a maximum, and then decrease [19]. 

These problems are illustrated by the following example from the work of Pettier et al. [169], Sonoluminescence and thermal probes measurements were compared at 500 kHz in a cylindrical cup-horn cell. The influence of liquid height and of 

This study highlights the fact that the dosimeter is probably responding to some selected effects of ultrasound, and not always of all the transmitted power. This point will be explored in more detail later. 

The principle shown in Fig. 5.141 ean be applied to a number of similar experiments with randomly emitted light pulses. An early application was the measurement of seintillator deeay times by radioaetive decay [167] and the use of seintilla-tion pulses for fluoreseenee excitation. 

A more reeent applieation is time-resolved recording of sonoluminescence. Sonolumineseenee is generated if a gas bubble in a liquid is exeited by ultrasound [20, 72, 139, 190, 233]. The emitted light eonsists of extremely short flashes with a duration of 100 ps and less. 

The efficiency of the measurement can be increased by multiwavelength detection. The monochromator is replaced with a polychromator, and a multianode PMT with routing electronics is used to detect the full spectrum. However, despite its obvious benefits, no application of multiwavelength TCSPC to sonoluminescence has yet been published. 

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The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kH2 to 1 GH2. With sound velocities in Hquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to lO " 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. 

Fig. 1. Transient acoustic cavitation the origin of sonochemistry and sonoluminescence.

Multiple-Bubble Sonoluminescence. The sonoluminescence of aqueous solutions has been often examined over the past thirty years. The spectmm of MBSL in water consists of a peak at 310 nm and a broad continuum throughout the visible region. An intensive study of aqueous MBSL was conducted by VerraH and Sehgal (35). The emission at 310 nm is from excited-state OH, but the continuum is difficult to interpret. MBSL from aqueous and alcohol solutions of many metal salts have been reported and are characterized by emission from metal atom excited states (36). 

Sonoluminescence from nonaqueous Hquids has only recentiy been examined. Flint and SusHck reported the first MBSL spectra of organic Hquids (37). With various hydrocarbons, the observed emission is from excited states of (d Ilg — 11, the Swan lines), the same emission seen in flames. 

Single-Bubble Sonoluminescence. The spectra of MBSL and SBSL are dramatically different. MBSL is generally dominated by atomic and molecular emission lines, but SBSL is an essentially featureless emission that iacreases with decreasiag wavelength. For example, an aqueous solution of NaCl shows evidence of excited states of both OH- and Na ia the MBSL spectmm however, the SBSL spectmm of an identical solution shows no evidence of either of these peaks (30). Similady, the MBSL spectmm falls off at low wavelengths, while the SBSL spectmm continues to rise, at least for bubbles containing most noble gases (38). 

Fig. 7. Sonoluminescence of excited state Emission from the At = +1 manifold of the d Hg — H transition (Swan band) of Reproduced with...

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. 

SONOLUMINESCENCE SPECTROSCOPY. POSSIBILITIES AND PROSPECTS OF ITS USE IN ANALYTICAL CHEMISTRY... 

There are two types in acoustic cavitation. One is transient cavitation and the other is stable cavitation [14, 15]. There are two definitions in transient cavitation. One is that the lifetime of a bubble is relatively short such as one or a few acoustic cycles as a bubble is fragmented into daughter bubbles due to its shape instability. The other is that bubbles are active in light emission (sonoluminescence (SL)) or chemical reactions (sonochemical reactions). Accordingly, there are two definitions in stable cavitation. One is that bubbles are shape stable and have a long lifetime. The other is that bubbles are inactive in SL and chemical reactions. There exist... 

Experimentally, Brotchie et al. [55] have shown that the range of ambient radius of sonoluminescing (SL) bubbles in which the temperature is relatively high... 

In some literature, there is a description that a bubble with linear resonance radius is active in sonoluminescence and sonochemical reactions. However, as already noted, bubble pulsation is intrinsically nonlinear for active bubbles. Thus, the concept of the linear resonance is not applicable to active bubbles (That is only applicable to a linearly pulsating bubble under very weak ultrasound such as 0.1 bar in pressure amplitude). Furthermore, a bubble with the linear resonance radius can be inactive in sonoluminescence and sonochemical reactions [39]. In Fig. 1.8, the calculated expansion ratio (/ max / Rq, where f max is the maximum radius and R0 is the ambient radius of a bubble) is shown as a function of the ambient radius (Ro) for various acoustic amplitudes at 300 kHz [39]. It is seen that the ambient radius for the peak in the expansion ratio decreases as the acoustic pressure amplitude increases. While the linear resonance radius is 11 pm at 300 kHz, the ambient radius for the peak at 3 bar in pressure amplitude is about 0.4 pm. Even at the pressure amplitude of 0.5 bar, it is about 5 pm, which is much smaller than the linear resonance radius. 

Fig. 1.9 The calculated results as a function of ambient radius at 300 kHz and 3 bar in ultrasonic frequency and pressure amplitude, respectively. The horizontal axis is in logarithmic scale, (a) The peak temperature (solid) and the molar fraction of water vapor (dash dotted) inside a bubble at the end of the bubble collapse, (b) The rate of production of oxidants with the logarithmic vertical axis. Reprinted with permission from Yasui K, Tuziuti T, Lee J, Kozuka T, Towata A, Iida Y (2008) The range of ambient radius for an active bubble in sonoluminescence and sonochemical reactions. J Chem Phys 128 184705. Copyright 2008, American Institute of Physics...

Yasui K, Tuziuti T, Sivakumar M, Iida Y (2004) Sonoluminescence. Appl Spectrosc Rev 39 399 136... 

Suslick KS, Flannigan DJ (2008) Inside a collapsing bubble Sonoluminescence and the conditions during cavitation. Ann Rev Phys Chem 59 659-683... 

Holzfuss J, Riiggeberg M, Billo A (1998) Shock wave emissions of a sonoluminescing bubble. Phys Rev Lett 81 5434-5437... 

Weninger KR, Camara CG, Putterman SJ (2001) Observation of bubble dynamics within luminescent cavitation clouds sonoluminescence at the nano-scale. Phys Rev E 63 016310... 

Yasui K (2002) Influence of ultrasonic frequency on multibubble sonoluminescence. J Acoust Soc Am 112 1405-1413... 

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... 

Guan J, Matula TJ (2003) Time scales for quenching single-bubble sonoluminescence in the presence of alcohols. J Phys Chem 107 8917-8921... 

Matula TJ, Cordry SM, Roy RA, Crum LA (1997) Bjerknes force and bubble levitation under single-bubble sonoluminescence conditions. J Acoust Soc Am 102 1522-1527... 

Yasui K (2001) Temperature in multibubble sonoluminescence. J Chem Phys 115 2893-2896... 

Yasui K, Tuziuti T, Lee J, Kozuka T, Towata A, Iida Y (2008) The range of ambient radius for an active bubble in sonoluminescence and sonochemical reactions. J Chem Phys 128 184705... 

Brenner MP, Hilgenfeldt S, Lohse D (2002) Single-bubble sonoluminescence. Rev Mod Phys 74 425 184... 

Yasui K (2001) Effect of liquid temperature on sonoluminescence. Phys Rev E 64 016310... 

Hilgenfeldt S, Grossmann S, Lohse D (1999) A simple explanation of light emission in sonoluminescence. Nature (London) 398 402-405... 

Yasui K (1999) Mechanism of single-bubble sonoluminescence. Phys Rev E 60 1754—1758... 

Yasui K (1996) Variation of liquid temperature at bubble wall near the sonoluminescence threshold. J Phys Soc Jpn 65 2830-2840... 

Storey BD, Szeri AJ (2000) Water vapour, sonoluminescence and sonochemistry. Proc R Soc Lond A 456 1685-1709... 

Sostaric JZ (1999) Interfacial effects on aqueous sonochemistry and sonoluminescence. PhD thesis, University of Melbourne, Australia... 

Yasui K (2002) Effect of volatile solutes on sonoluminescence. J Chem Phys 116 2945-2954... 

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