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Cavitation in focused ultrasound

Cavitation in focused ultrasound

Student thesis: Doctoral ThesisDoctor of Philosophy

Bjoern Gerold

Original languageEnglish
Awarding Institution
Award date2013

Abstract

A novel experimental configuration is developed combining a high intensity
focused ultrasound source and a pulsed-laser, for the study
of cavitation in a field typical of those used for therapeutic ultrasound.
The sonoptic chamber is specifically designed to avoid the formation
of acoustic standing waves, known to have a critical influence
on cavitation behaviour. A new technique of laser-nucleated acous-
tic cavitation is presented, whereby a laser-pulse of energy below the
breakdown threshold for the host medium, acts to nucleate acoustic
cavitation in a pre-established field. This facilitates the incorporation
of high-speed cameras for interrogation at unprecedented temporal
and spatial resolution, combined with acoustic detection directly correlated
to the observed cavitation activity. A number of cavitation
phenomena are investigated, including bubble-ensemble oscillations at
a very early stage of development, in response to the acoustic driving.
The frequency of oscillation, which bifurcates with increasing intensity,
is also detected in the acoustic emissions. The application of
a single-bubble model predicts a source for the acoustic emissions of
quiescent radius equivalent to the bubble-ensemble observed, for each
intensity investigated. The physical translation of the ensemble, due
to the radiation force imposed by the primary field, is also analysed.
For laser-pulses of energy above the breakdown threshold, applying
focused ultrasound to the cavity promotes and actuates jet-formation.
The characteristics of the so formed jets depend on the intensity and
location of the cavity relative to the ultrasound focus.

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