The cavitation subharmonic signal : mechanistic source and optimised detection

• Main Author: Johnston, Keith
• Other Authors: Cuschieri, Alfred
• Published: University of Dundee 2016
• Subjects: 610.28
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694999

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. 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. 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. 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.