Cavitation on the tip of a fibre optic : from a single bubble to a bubble cloud

• Main Author: Milad Mohammadzadeh
• Other Authors: Ohl Claus-Dieter
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Physics
• Online Access: http://hdl.handle.net/10356/73514
• Institution: Nanyang Technological University

Minimally-invasive surgeries rely heavily on the use of fibre optics to steer through small incisions on the body and deliver laser pulses to remove unwanted tissue. Laser-induced heating at the fibre tip leads to vaporisation and formation of cavitation bubbles. These bubbles play two crucial roles in laser surgery: they act as transmission channels that connect the fibre tip to the ablation target and generate mechanical effects and motion in their surroundings. This thesis focuses on the dynamics of laser-generated bubbles, especially on the tip fibre optics. The thesis begins with investigating the dynamics of elongated, pear-shaped vapour bubbles in ablation with free-running lasers. A common clinical complication is the movement of the ablation target, which we experimentally link to the bubble-induced flow. We then provide a model to estimate the bubble shape based on the laser pulse profile. Controlling the shape of the vapour transmission channel with this model helps in reducing the resulting liquid motion and mechanical trauma due to cavitation. We then investigate the liquid motion induced by cavitation in a confined geometry, focusing on axisymmetric flow close to an oscillating bubble. Numerical simulations reveal that the bubble induces vortical structures that generate strong shear stress near the boundaries. The shearing is probed experimentally by observing the deformation of elastic biological cells in the vicinity of the bubble. Another cavitation-induced flow investigated in this thesis is generated by delivering a pulsed laser at a high repetition rate through a fibre optic. We observe a fast and localised liquid stream directed away from the fibre tip. Using particle image velocimetry, the stream is characterised as a synthetic jet driven by vortices generated during bubble collapse. High-speed recordings reveal the formation of double-bubbles on the fibre tip at certain laser settings, which we predict with a model that links the bubble size to the laser energy. Finally, the formation of bubble clouds due to photoacoustic shock waves emitted from a fibre optic is studied. Interestingly, we observe that modifying the shape of the fibre tip allows us to steer the shock waves and the resulting cavitation cloud. We end with providing accurate numerical solutions to optimise the fibre tip structure for achieving a desirable acoustic field and region of ablation.