On the origins and outcomes of laser-nucleated bubble collapse events at high ambient pressures

Thesis (Ph.D.)--Boston UniversityThis thesis covers a range of topics related to the effects of fluid pressure on the laser nucleation of bubbles and bubble clouds in water, the collapse characteristics of those bubbles, and the outcomes of those collapses, including single and multi bubble sonoluminescence and the formation of a high pressure phase of water in the vicinity of the collapse. The disparate nature of these phenomena obscure the purposes they served in relation to a bigger project seeking to optimize the collapse of bubble clouds in connection with recent interest in acoustic inertial confinement fusion. The laser breakdown studies sought to explain anomalous nucleation characteristics of bubble clouds at different ambient pressures. It was shown in these studies that the laser induced dielectric breakdown threshold in water is a function of pressure, and that while this was problematic insofar as it made it difficult to repeatably nucleate identical bubbles and bubble clouds, it could be utilized as a non-contact method for measuring pressures in the fluid. The multi bubble sonoluminescence studies were initially designed to use MBSL events as markers for the collapse strength of bubble clouds in the resonators. However, when it was observed via imaging that events produced were bright, large, and long-lived, with radii and lifetimes on the order of ~ 300 µm and ~ 70 ns), respectively, the study was repeated for single bubbles. SBSL studies showed comparably large and long-lived events, with radii and lifetimes on the order of 300 µm and ~ 70 ns, respectively. SBSL studies consistently showed the formation of two ring-like structures in the vicinity of collapsing bubbles, with the radii of these rings being on the orders of 100 µm and 250 µm. Further analysis revealed that the rings formed at the location in the fluid where the pressures first exceeded 1.6 and 18 GPa, respectively. While these pressures are sufficient to generate a number of water's crystalline phases, observations suggest they are instead the result of a liquid or amorphous transition