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Research project: Birkin: Single Bubble Sonoluminescence

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The emission of light by acoustically driven gas bubbles has been termed sonoluminescence. This phenomenon is intriguing due to the unusual properties generated within the collapsing gas bubble driven by an appropriate acoustic field.

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While acoustic energy densities are relatively low the production of light from these collapses indicates an extreme concentration of energy into a relatively small space. While this phenomenon has been known for decades the unusual properties that exist within the gas cloud has been the subject of much speculation and conjecture. Part of the difficulty in studying this phenomenon is that the emission of light is extremely short (on the sub nanosecond timescale) and relatively weak. Additionally, the chaotic nature of the environment produced by a multibubble cloud results in a ‘bubble averaged’ output from most systems used to study the process. While this is useful in the interpretation of the global effects of cavitation on chemical and physical processes, it does not always aid the understanding of the individual cavitation events. In order to alleviate this problem a number of different approaches have been adopted. These include the generation of relatively large cavitation bubbles (e.g. by the discharge of lasers into a media) and the study of single bubbles trapped within an appropriate sound field. These single bubbles have, under appropriate conditions, been chown to emit light and are termed ‘single bubble sonoluminescence’ (SBSL). These have been studied extensively in relation to the physical and chemical processes occurring within the system. This process is characterised by a clock like repetitive light emission, an emission at the blue end of the visible spectrum, a black body like spectra, the absence of species specific emission lines (e.g. OH and Na) and the short emission linewidth (duration ca. 60 - 260 ps depending on the physical conditions). One of the parameters of interest in relation to the SBSL approach is quantification of the chemical output of an individual bubble. The chemical efficiency of a SBSL event has been reported using a radical trapping procedure. These results were then related to the number of radical (e.g. OH) species produced per acoustic cycle or photon yield. While this study revealed that chemical processes could be driven within the interior of a SBSL event, several key questions remain. This project builds on initial work performed at Southampton and will extend our knowledge and understanding of this fascinating process. In order to achieve this goal, acoustoelectrochemical experiments are performed in an attempt to determine the local flux of chemically active material produced close to an SBSL event. In addition, the local environment will be altered using a variety of electrochemical techniques.

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