School of Chemistry - Theses
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ItemAcoustic cavitation in dual frequency ultrasound fields( 2010)Multiple frequency ultrasonic systems have become of interest in recent years in the context of scaling up and improving the efficiency of sonochemical reactors, having been found to offer certain advantages over single frequency systems. This thesis investigates dual frequency sound fields over a broad parameter space, addressing a large range of aspects of acoustic cavitation and the factors that influence these systems. Three different transducer types were employed: a low frequency sonotrode, a high frequency standing wave type transducer and a high intensity focused ultrasound (HIFU) transducer. The combination of two 20 kHz horns was found to be synergetic, with respect to sonoluminescence emission and sonochemical yields, only when the respective sizes of the two horns differed significantly, and the larger of the two horns was operated at low power and in a pulsed mode. The combination of a low frequency sonotrode with a HIFU field was highly synergistic under appropriate pulse and acoustic power conditions. This was attributed to cavitation fragments from the low frequency field providing a new source nuclei for cavitation in the HIFU cavitation zone. These results corroborate and extend upon those of previous reports, and are of relevance to the field of ultrasound lithotripsy. When a standing wave emitter was used in combination with the HIFU transducer, the reverse situation was observed, whereby HIFU cavitation stimulated cavitation in the standing wave field. This was only possible when the two frequencies were closely matched. A similar requirement was found for a system comprising two geometrically opposing standing wave emitters. It was proposed that this was due to the similar bubble active sizes rendering the stimulating mechanism more effective or due to more favourable superposition of the two sound waves with respect to bubble dynamics. For the combination of a sonotrode and standing wave system, it was found that considerable synergy (sonochemical and sonophysical) could be attained through pulsed, low power operation, and that this was further extended at low temperature, high viscosity and in the presence of coalescence inhibiting solutes. High-speed photographic observations demonstrated that the presence of the high frequency source stimulated cavitation in the vicinity of the low frequency sonotrode surface. This effect was more dramatic in the presence of coalescence inhibiting solutes. This can be ascribed to the existence of a much greater high frequency bubble population, which may act as cavitation nuclei under the horn, where the radial dynamics are dictated by the low frequency field. A combination of higher bubble density and the bubbles being driven relatively more asymmetrically and non-linearly in the dual frequency field led to a higher degree of fragmentation. The fragments of cavitation were able to (in the solute solutions) in turn, stimulate cavitation in the high frequency field, which was confirmed through analysis of the acoustic emission spectra. Sonochemical and sonoluminescence studies demonstrated a large synergy in these systems. Modelling of the radial bubble dynamics indicated that only at very low acoustic power can the combination of a high and low frequency source bring about a significant enhancement in collapse temperature. Experimental temperature determination, however, which represents a spatially and temporally averaged ‘chemical’ temperature, revealed that the collapse temperature was significantly lower than that measured during single frequency irradiation. This is likely due, at least in part, to bubbles being driven in a more asymmetric environment. This was substantiated by sonoluminescence spectra measured in the presence of sodium salts, which exhibited a much more prominent sodium emission line in the dual frequency system. Further, single bubble growth measurements indicated that dual frequency operation did not increase the rate of rectified diffusion, and actually suppressed it at elevated acoustic power, presumably because the bubbles were translocated away from the low frequency antinode. The bubble lifetime in the dual frequency field was calculated to be longer than those in either of the single frequency fields. It is plausible that this is due to a large number of bubbles pre-existing in the high frequency field prior to nucleation near the low frequency horn, extending their lifetime relative to single, low frequency sonication. Despite the fact that growth was retarded in the single bubble system, it is unclear to what extent rectified diffusion is affected in the multi-bubble field. Irrespective of the mechanism, the longer lifetime in the dual frequency field is consistent with the lower collapse temperature measured and with SL quenching studies. Bubble size distributions were determined using a pulsed ultrasound method and were found to be affected strongly by the driving frequency, acoustic power, pulse width and dissolved gas concentration. The inverse dependence of the bubble size on the driving frequency is consistent with linear resonance theory and the main implication of the studies with power, pulse duration and gas concentration is that bubble-bubble coalescence is the major determinant of the bubble size at a given frequency. Another important outcome of this investigation is that the coalescence inhibitive effect of simple electrolytes, a highly contentious issue, can be completely attributed to their effect on the dissolved gas concentration.