Modeling and Analysis of Non-spherical Vapor Bubbles Induced by Long-Pulsed Laser: 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. However, understanding the formation mechanisms of these non-spherical shapes and their relation to laser settings remains challenging. In this talk, we introduce a new computational model that couples multiphase fluid dynamics with laser radiation and phase transition. Key components 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. We then investigate the dynamics of pear-shaped and elongated bubbles through simulations of Ho:YAG and Thulium fiber laser experiments. The predicted bubble nucleation and morphology agree reasonably well with the experimental observation. The full-field results of laser irradiance, temperature, velocity, and pressure are analyzed to explain bubble geometry and energy transmission. Based on the numerical results, we propose a new hypothesis that vapor bubble morphing is determined by a race between advection and phase transition. To test this, we define the speeds of advection and phase transition using a simplified model problem and approximate their values for our simulations. The results support the hypothesis. This study indicates a possibility to improve laser energy delivery by designing vapor bubbles that serves as a channel (i.e., the Moses effect).