The effects of fluid flow and surface stabilisation on sonoluminescence and sonochemistry.
Creators: name: Wood, Richard James
Publisher: name: University of Surrey
Year: 2020

description: In general, bubble surface instabilities (herein known as bubble transience), increase sonochemical (SC) processes and decrease processes, such as sonoluminescence (SL), that require higher collapse intensities. However, there is a limited understanding of how SL and SC processes are impacted by bubble transience in a practical sense. Thus, research and experimentation was based on the variation of parameters that would affect transience which, in turn, would determine ultrasound process outcomes. Firstly, the ultrasonic system was characterised using SL, and the SC processes of sonochemiluminescence (SCL) and potassium iodide (KI) dosimetry. Frequencies of 20 kHz (ultrasonic horn) and 44, 300 and 1000 kHz (ultrasonic plates) were used with applied flow rates of 0, 24, 228 and 626 mL / min. These frequency and flow settings were used throughout the work unless otherwise specified. The SCL and dosimetry showed disparities throughout, attributed to changes in the energy of collapse, fragmentation and oxygen concentration / solvated electrons. At low frequency, SCL and dosimetry increased under more transient collapse conditions, as measured by the minimisation of the more pyrolytic process of SL. Here, it was theorised that bubble fragmentation and radical transfer to solution, favoured dosimetry over SCL. Further, where SL and SCL activity overlapped they showed the expected reciprocal relationship of decrease / increase with bubble transience, but where activity differed there was poor correlation. Then, the impact of bubble transience on the intensity of collapse on SL from KI solutions (0.1, 1 and 2 M) under fluid flow and stabilised conditions were studied. At 20 kHz (horn) and 44 kHz (plate), flow reduced bubble coalescence and clustering, increasing SL intensity. However, for the 44 kHz system, at higher flow rates, bubble transience could also reduce SL. With an increase in KI concentration, at low frequency (44 kHz), localised activity could be expanded, then at higher frequencies (300 and 1000 kHz), SL activity increased towards the transducer. This indicated reduced attenuation of the sound field, attributed to a reduction in bubble size / clustering with the salt. An increase in standing wave formations (plate) or activity at the horn tip, with power (20 kHz), stabilisation (44 and 300 kHz) or flow (1000 kHz), allowed flow and salt concentration to reduce bubble coalescence / clustering in those regions. This effect could negate a decrease in SL, with increase in KI concentration at low frequency, as previously observed by other authors. To understand how bubble transience affected SC and SL processes, the degradation of phenol and SL from phenol under fluid flow (and stabilised) conditions was investigated. Flow could augment phenol degradation at all frequencies. For the 20 kHz (horn) and 300 kHz systems, phenol degradation correlated with iodide dosimetry which suggested an oxidative process, however, 44 and 1000 kHz showed poor correlation. At 44 kHz, degradation was hypothesised to occur inside the bubbles under transient (flow) bubble conditions, as indicated by SL quenching when degradation was maximised. This was theorised to occur via nanodroplet / rectified diffusion mechanisms. At 1000 kHz the disparity between phenol degradation and iodide dosimetry was attributed to a reduction in collapse intensity and fragmentation which affected the reaction schemes. Here, the fragmentation conditions with flow were not sufficient to increase the dosimetry, whereas for degradation, fragmentation was less influential. SL analysis for higher concentrations of phenol (horn and plate transducers) showed that intensity could be increased with flow / stabilisation. This was attributed a reduction in coalescence / clustering by both flow and the surface properties of the phenol. The methods of SC and SL characterisation were then applied to further understand the ultrasonic degradation of perfluorooctanesulfonic acid (PFOS). PFOS is a surfactant that will reside more readily at the bubbles surface than phenol. Therefore, effects at the bubble surface and bubble population may play more of a role. Plate transducers at frequencies of 44, 400, 500 and 1000 kHz, and a single input power (40 W) provided a range of SC and SL characteristics to measure against PFOS degradation. At higher frequencies, degradation of up to 96.9% (400 kHz) of the initial PFOS value was achieved, with similar, but slightly lower, degradations at 300 and 1000 kHz. However, at 44 kHz no degradation was observed. Therefore, an increase in the number of bubbles and bubble surface availability at the higher frequencies was favourable to the process, and further degradation did not take place inside the bubbles at low frequency. Comparison to SL and SC processes indicated that pyrolysis at the bubble surface may not be the only degradation mechanism, rather solvated electrons at the bubble surface likely contribute to PFOS degradation. Overall, bubble transience increased by fluid flow was, generally, favourable to SC processes. Also, flow could increase SL, in unison with the intrinsic properties of solutes (and stabilisation), by reducing coalescence / clustering in reinforced high pressure regions. Increases in bubble transience were also hypothesised to influence reaction pathways, bubble fragmentation or nanodroplet / diffusion mechanisms, more favourable to some SC processes than others. Then, the comparison of SC and SL to PFOS degradation suggested that solvated electrons may be contributing to bubble surface SC reactions. By the understanding of the fundamentals of ultrasound cavitation and bubble characteristics, both SL and SC processes have been augmented and new insights have been realised.