title: Periodic shock-emission from acoustically driven cavitation clouds: a source of the subharmonic signal.
abstract: Single clouds of cavitation bubbles, driven by 254kHz focused ultrasound at pressure amplitudes in the range of 0.48-1.22MPa, have been observed via high-speed shadowgraphic imaging at 1×10(6) frames per second. Clouds underwent repetitive growth, oscillation and collapse (GOC) cycles, with shock-waves emitted periodically at the instant of collapse during each cycle. The frequency of cloud collapse, and coincident shock-emission, was primarily dependent on the intensity of the focused ultrasound driving the activity. The lowest peak-to-peak pressure amplitude of 0.48MPa generated shock-waves with an average period of 7.9±0.5μs, corresponding to a frequency of f0/2, half-harmonic to the fundamental driving. Increasing the intensity gave rise to GOC cycles and shock-emission periods of 11.8±0.3, 15.8±0.3, 19.8±0.2μs, at pressure amplitudes of 0.64, 0.92 and 1.22MPa, corresponding to the higher-order subharmonics of f0/3, f0/4 and f0/5, respectively. Parallel passive acoustic detection, filtered for the fundamental driving, revealed features that correlated temporally to the shock-emissions observed via high-speed imaging, p(two-tailed) < 0.01 (r=0.996, taken over all data). Subtracting the isolated acoustic shock profiles from the raw signal collected from the detector, demonstrated the removal of subharmonic spectral peaks, in the frequency domain. The larger cavitation clouds (>200μm diameter, at maximum inflation), that developed under insonations of peak-to-peak pressure amplitudes >1.0MPa, emitted shock-waves with two or more fronts suggesting non-uniform collapse of the cloud. The observations indicate that periodic shock-emissions from acoustically driven cavitation clouds provide a source for the cavitation subharmonic signal, and that shock structure may be used to study intra-cloud dynamics at sub-microsecond timescales.
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK.
authors.forename: Keith
authors.initials: K
authors.lastname: Johnston
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Tapia-Siles
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK; Diagnostic Sonar Ltd., Livingston EH54 7BX, UK.
authors.forename: Bjoern
authors.initials: B
authors.lastname: Gerold
authors.affiliation: Department of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway.
authors.forename: Michiel
authors.initials: M
authors.lastname: Postema
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK.
authors.forename: Sandy
authors.initials: S
authors.lastname: Cochran
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK.
authors.forename: Alfred
authors.initials: A
authors.lastname: Cuschieri
authors.affiliation: Institute for Medical Science and Technology, Division of Imaging and Technology, University of Dundee, Dundee DD2 1FD, UK; Diagnostic Sonar Ltd., Livingston EH54 7BX, UK. Electronic address: p.prentice@dundee.ac.uk.
authors.forename: Paul
authors.initials: P
authors.lastname: Prentice
keywords: Acoustic cavitation; Cloud dynamics; Collapse; Shock-wave; Subharmonic