An investigation of electrochemical single bubble sonoluminescence

Abstract

A technique for stable and reproducible electrochemically generated SBSL events (eSBSL) is presented. This approach is shown to have some advantages over conventional seeding methods. These include control of the initial gas flux and control of the initial SBSL bubble composition within the same media (e.g. the ability to seed with H₂ and O₂ for example). Comparison of different cell types shows that a cell setup based on conventional round bottomed flasks is the most appropriate for eSBSL generation. Typically a critical current (-0.11 mA) is required to generate a hydrogen seeded eSBSL event from a 50 µm diameter Pt electrode while oxygen seeding requires +0.15 mA under the same conditions.

Two types of bubble dynamics are observed when high-speed imaging is employed to study the eSBSL event behaviour. Most of the bubbles oscillate repetitively in a non-linear fashion as expected. However, some bubbles fragmented into clusters of smaller bubbles before coalescing in a repetitive fashion. The eSBSL event is shown to be extremely sensitive to experimental parameters such as the acoustic pressure amplitude, solution temperature, solution composition, applied frequency and local environment. In particular trace noble gase (Ar) is shown to affect the eSBSL event characteristics. At Ar concentrations below 0.3 µmol dm^-3 (determined by a residual gas analyser) short-lived (10-40 s) and fading eSBSL events are observed. The emission spectra showed a red centred ‘black body like’ character in disagreement with the available literature. In addition some evidence for OH* emission is presented for these short lived events. It is proposed that these eSBSL events contain primarily gas they were seeded with. At higher Ar concentrations (0.3-1.5 µmol dm-3), long-lived SBSL events are observed. These tend to produce two kinds of emission spectra. One is a blue centred spectrum and second is a double-peaked red-blue spectrum. It is postulated that these eSBSL bubbles initially contain the gas they were seeded with but are gradually converted to an Ar rich noble gas SBSL event from the solution. At higher solution gas concentrations no stable SBSL could be observed. Emission lifetime is measured using time correlated single photon counting (TCSPC). The FWHM values for hydrogen/oxygen seeded eSBSL events are in the range of 290-315 ps under the conditions employed.

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ORCID for Peter R. Birkin: orcid.org/0000-0002-6656-4074

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