Cavitation in focused ultrasound

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.

Date of Award	2013
Original language	English
Supervisor	Paul Prentice (Supervisor)