Long-Pulse Laser-Induced Cavitation: A Race Between Advection and Phase Transition

Non-spherical vapor bubbles of complex geometry (e.g., elongated cone, "pear-like" shape, etc.) are often observed in applications that operate long-pulsed laser in a liquid environment. In this scenario, the causal relation between bubble dynamics and laser radiation is still unclear, complicated by the overlapping of their domains both in space and in time. In this talk, we first present a new computational model that couples multiphase fluid dynamics with laser radiation and phase transition. Key components of the model include an embedded boundary method for solving the laser radiation equation on the same "fluid mesh", a method of latent heat reservoir for predicting laser-induced vaporization, a local level set method for interface tracking, and the FIVER (FInite Volume method with Exact multi-material Riemann solvers) method for enforcing interface conditions. Next, we explore the dynamics of pear-shaped and elongated bubbles through simulations of Ho:YAG and Thulium fiber laser experiments. The numerical results are compared with high-speed images obtained from the experiments, which yields a reasonable agreement. The temperature, pressure, and velocity fields from the simulations are analyzed to explain the bubble geometry and its impact on the delivery of laser energy. Based on the numerical results, we propose a new hypothesis that the morphing of the vapor bubble is determined by a race between advection and phase transition. To test this hypothesis, we define the speeds of advection and phase transition using a simplified model problem, and approximate their values for our simulations. This study indicates a possibility to improve laser energy delivery by designing a vapor bubble that serves as a channel (i.e., the Moses effect).