The cavitation subharmonic signal : mechanistic source and optimised detection

The cavitation subharmonic signal, emitted at frequency values sub-multiple to that of the acoustic driving, is held to be exclusive to the occurrence of driven bubbles within a host medium. Recently, detection of the subharmonic signal has seen a resurgence of interest, particularly for the prospect of cavitation-mediated therapy during the application of focused ultrasound to tissue. Remarkably, bubble-based mechanisms for the origin of the subharmonic signal - which can account for the range of experimental configurations from which it has been detected - have remained elusive since the signal was first identified, by Esche in 1952.<br/><br/>This thesis describes cavitation observations in water, driven by propagating focused ultrasound fields typical of those used for medical therapy, using ultra high-speed shadowgraphic imaging at frame rates well in excess of the fundamental driving frequency. Moreover, single nanosecond laser pulses at energies below the plasma-forming threshold, are used to nucleate acoustic cavitation such that activity may be observed from the outset.<br/><br/>Clouds of densely packed and strongly interacting bubbles are seen to rapidly develop from the nucleation event. Within a few acoustic cycles, the cloud adopts a breathing mode response, with component bubbles collectively oscillating, approximately in-phase. The frequency of cloud oscillation matches that of the fundamental driving, however, at intervals dependent on the pressure amplitude of the driving, the cloud undergoes strong collapses, coincident to emitting a shockwave. In parallel to the high-speed imaging, a number of hydrophone detectors are used to collect the acoustic emissions, and confirm that periodic shockwaves mediate the subharmonic signals.<br/><br/>Acoustic detection of broadband, impulsive pressure transients is particularly susceptible to convolution with the frequency response of the detector. Accordingly, a PVdF needle hydrophone was calibrated for magnitude and phase from 125 kHz – 20 MHz, at the National Physical Laboratory. Detector deconvolution is demonstrated for shockwaves emitted during the formation of large plasma-mediated bubbles, each generated with a laser pulse of energy above the threshold. Similarly, the needle hydrophone is deconvolved from the emissions collected from acoustic cavitation clouds, indicating peak-positive pressure amplitudes for periodic shockwaves in the order of 10 kPa, at the distance detected. The development of a single element passive cavitation detector, dedicated to the detection of low-amplitude shockwaves with high sensitivity, is subsequently described. Detector construction, specifically the selection of matching and backing layers, is guided via a Finite Element model of the device, adapted to support simulated shockwave propagation. Detector performance is characterised with plasma bubble shockwaves, and evaluated for the detection of the subharmonic signal from a cavitation cloud, against a commercially available device.

Name dc:type.qualificationname

Doctor of Philosophy

Level dc:type.qualificationlevel

Doctoral Thesis

Grantor dc:publisher.institution

University of Dundee

Year dc:date.issued

2016

Author dc:creator

• Johnston, Keith

Advisor dc:contributor.advisor

• Cuschieri, Alfred

Language dc:language

eng

Identifier

oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92

OAI identifier oai:identifier

oai:discovery.dundee.ac.uk:studenttheses/34568741-12be-4a90-8bbf-82f5900a4b92