Cavitating bubbles in focused fields: the acoustic emission spectrum and broadband noise clearance upon synchronisation

Metzger, Hildegard Elisabeth Jacoba (2025) Cavitating bubbles in focused fields: the acoustic emission spectrum and broadband noise clearance upon synchronisation. PhD thesis, University of Glasgow.

Abstract

This thesis presents a set of studies that contribute to the understanding of bubble cavitation dynamics. Acoustic detection is the most commonly used method to monitor cavitation in microbubblemediated therapies such as blood brain barrier opening. However, the acoustic emission spectrum has been empirically correlated with safety and efficacy, and does not offer a complete understanding of the underlying bubble dynamics responsible for therapeutic effects. The combination of acoustic detection and high-speed imaging used in this work provides new insights into cavitation dynamics by correlating the acoustic signature with direct observations.

The first study compares the acoustic behaviour of a range of ultrasound contrast agent formulations driven by ultrasound with therapeutically relevant frequency and pressure amplitude. A polycarbonate capillary was used to observe single bubbles and record their acoustic emissions. It was found that the physical properties such as gas core, shell type and size had no distinguishable effect on the acoustic emissions and these are instead determined by the driving parameters. Bubble collapse shock waves dominate the acoustic spectrum, with their periodicity determining the spectral features observed. Precisely timed shock wave pulse trains provide harmonic and subharmonic content based on their period. A rise in broadband noise represents a break in periodicity which occurs as bubbles oscillate more chaotically when transitioning from one periodic regime to another.

The second study transitions to a multi-bubble system and observes the cavitation induced in the bore of a tube transducer driven with increasing pressure amplitude. A spectrogram of the cavitation emission signal collected over the duration of the ramped sonication confirms a subharmonic route to chaos; the progression from harmonic emissions to broadband noise via a period-doubling bifurcation. Stroboscopic mapping demonstrates that the emergence of broadband signal is due to increasingly asynchronous bubble collapse across the population and a region of sudden broadband clearing is explained by phase synchronisation of the bubble oscillations.

The final study demonstrates the importance of hydrophone deconvolution when assessing the spectral content of cavitation emissions. Not knowing the full frequency response of a hydrophone may lead to incorrect assessment of the cavitation spectrum. This work characterises the frequency response of a fibre-optic hydrophone and uncovers a spectral peak at 3.4 MHz using bubble collapse shock waves from laser-induced cavitation. This method found that the spectral peak could be reduced by decreasing the length of the fibre protruding from the holder and the peak was amplified when measuring a pressure wave travelling perpendicular to the hydrophone axis. This evidence points towards a bending mode which should be mitigated or at least accounted for in calibration procedures.

Overall, thiswork demonstrates the complexity of bubble dynamics and argues that the binary classification of stable and inertial cavitation is not an accurate representation. Given that bubble collapse shock waves dominate the cavitation emission spectrum, the detection of non-linear emissions within these spectra is by definition an inertial effect. To fully harness cavitation in clinical and industrial applications, further research is needed to identify and characterise the specific bubble dynamics that drive beneficial outcomes, particularly those that might not be distinguishable in the acoustic spectrum. Therefore future efforts should aim to develop more comprehensive diagnostic tools that integrate acoustic data with mechanistic understanding to improve safety and efficacy for more precise therapy.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Prentice, Dr. Paul, Gachagan, Dr. Anthony and Hurrell, Dr. Andrew
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85569
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 06 Nov 2025 16:08
• Last Modified: 10 Nov 2025 09:55
• Thesis DOI: 10.5525/gla.thesis.85569
• URI: https://theses.gla.ac.uk/id/eprint/85569

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