Sonochemiluminescence from a single cavitation bubble in water.

The dramatic effects of microbubble collapse in ultrasonic fields, that is, acoustic cavitation, are known and exploited throughout numerous industrial and research areas, such as the material sciences, environmental remediation, and medicine. The extremely rapid collapse process leads to localized heating of the bubble interior, which breaks molecular bonds of the gas and vapor within the bubble generating active radical species. The chemical effects of acoustic cavitation are referred to as sonochemistry. A further consequence of bubble collapse is the emission of light, sonoluminescence (SL). The exact mechanism of SL is complex and to a large extent depends on the intrabubble conditions. It is generally thought that under relatively mild conditions, radiative recombination of excited molecular species dominates, whereas under more extreme conditions, radiation emanates from a plasma in the form of bremsstrahlung (i.e., breaking radiation). Ultrasonic systems typically give rise to clusters of many bubbles, often forming elaborate structures, which are categorized as “multibubble” systems. Although useful from an application perspective, multibubble systems are inherently difficult to study on a fundamental level due to the chaotic and ephemeral nature of the bubble field. With the advent of so-called “single-bubble” cavitation, such difficulties are circumvented, in that one can isolate spatially and study a single, acoustically driven bubble under specific and welldefined experimental conditions. In recent years, researchers have developed this into a tremendously powerful tool, providing rich insight into the enigmatic nature of cavitation. However, the sonoluminescing single bubble is thought not to be representative of cavitation in a multibubble field due to the high symmetry of collapse that results from the absence of neighboring bubbles or surfaces. Initially, the repetitive collapse of a single bubble was shown to produce a broad SL spectrum devoid of emission lines in water. In contrast, discrete molecular emission lines from excited solvent molecules, their degradation products, and alkali salts are routinely observed to punctuate the multibubble SL spectrum. This fundamental difference was attributed to the nature of the collapse event itself: the symmetrical collapse of the single bubble facilitates more efficient concentration of energy and higher core temperatures, leading to the formation during single-bubble sonoluminescence (SBSL) of highly ionized plasma, in which the emission from the excited molecular states is quenched. However, it should also be noted that under certain conditions, plasma formation has also been demonstrated in an aqueous MBSL system. Interesting studies have been conducted bridging the gap between the two forms of cavitation (single and multibubble), in which a spatially stable single bubble was forced to “dance”, that is, erratic spatial translation on a microscopic scale was induced. The spectrum from the dancing bubble was found to contain sodium emission lines identical to the multibubble spectrum. The same behavior has also been reported for CN* emission in various organic solvents. In a similar vein, sonochemical reactions, such as the emission from luminol (which fluoresces upon oxidation by sonochemically produced primary OHC and oxidizing secondary radicals) and I3 oxidation, have been reported not to occur from a stable SL emitting single bubble, but were observed when a spatial instability was introduced through an increase in dissolved gas content. Lepoint et al. have reported visual evidence of sonochemical reactions (Weissler reaction) from a single bubble, noting that the conditions under which the bubble was spatially unstable favored sonochemistry and that sonochemistry was suppressed during intense sonoluminescence. In contrast to the aforementioned studies, Didenko and Suslick have detected sonochemical reaction products under SBSL conditions (albeit at a relatively high frequency of 52 kHz and with higher gas content), which form with reaction efficiencies comparable to the multibubble system. Thus, the literature in the field of single-bubble sonochemistry is seemingly contradictory, and a systematic study is required to determine the specific conditions at which a single bubble is chemically active. The apparent dichotomy of single and multibubble cavitation and the inconsistent reports of single-bubble sonochem[a] Dr. A. Brotchie, Dr. J. Schneider, Dr. D. Shchukin, Prof. H. Mchwald Department of Interfaces Max Planck Institute of Colloids and Interfaces Am M hlenberg 2 14476 Potsdam-Golm (Germany) Fax: (+49)331-567-7814 E-mail : adam.brotchie@mpikg.mpg. de [b] Dr. J. Schneider, Dr. R. Pflieger Marcoule Institute for Separation Chemistry (ICSM) UMR 5257-CEA-CNRS-UMII-ENSCM Centre de Marcoule, BP 17171 30207 Bagnols s/Ceze Cedex (France)