[Paper Review] Long-Pulse Laser-Induced Cavitation: A Race Between Advection and Phase Transition

Xuning Zhao, Wentao Ma+4 more|arXiv (Cornell University)|Aug 23, 2023

Ultrasound and Cavitation PhenomenaMaterials Science3 citations

TL;DR

This study proposes a computational model coupling compressible multiphase flow, laser radiation, and phase transition to explain the shape evolution of long-pulse laser-induced vapor bubbles. It identifies a race between advection and sustained vaporization as the key mechanism: when vaporization outpaces advection, elongated conical bubbles form; when advection dominates, spherical pear-shaped bubbles emerge—supported by simulations matching experimental bubble morphologies and revealing vaporization durations of <1 μs (pear-shaped) vs. >50 μs (elongated).

ABSTRACT

Vapor bubbles generated by long-pulsed laser often have complex non-spherical shapes that reflect some characteristics (e.g., direction, width) of the laser beam. The transition between two commonly observed shapes - namely, a rounded pear-like shape and an elongated conical shape - is studied using a new computational model that combines compressible multiphase fluid dynamics with laser radiation and phase transition. Two laboratory experiments are simulated, in which Holmium:YAG and Thulium fiber lasers are used separately to generate bubbles of different shapes. In both cases, the bubble morphology predicted by the simulation agrees reasonably well with the experimental measurement. The simulated laser radiance, temperature, velocity, and pressure fields are analyzed to explain bubble dynamics and energy transmission. It is found that due to the lasting energy input (i.e. long-pulsed laser), the vapor bubble's dynamics is driven not only by advection, but also by the continuation of vaporization. Notably, vaporization lasts less than 1 microsecond in the case of the pear-shaped bubble, versus more than 50 microseconds for the elongated bubble. It is hypothesized that the bubble's shape is the result of a competition. When the speed of advection is higher than that of vaporization, the bubble tends to grow spherically. Otherwise, it elongates along the laser beam direction. To clarify and test this hypothesis, the two speeds are defined analytically using a simplified model, then estimated for the experiments using simulation results. The results support the hypothesis. They also suggest that a higher laser absorption coefficient and a narrower beam facilitate bubble elongation.

Motivation & Objective

• To understand the physical mechanisms behind non-spherical vapor bubble morphologies in long-pulsed laser-induced cavitation.
• To resolve the unresolved causal link between laser parameters and bubble shape (e.g., pear-like vs. conical).
• To investigate whether phase transition persists throughout the laser pulse, challenging the assumption of instantaneous vaporization.
• To develop a predictive framework for controlling bubble shape through laser settings such as absorption coefficient and beam width.

Proposed method

• Developed a computational model integrating compressible multiphase fluid dynamics with laser energy deposition and phase transition.
• Simulated two experimental setups using Ho:YAG and Thulium fiber lasers with measured laser power profiles as inputs.
• Employed a fine computational mesh resolving the laser fiber diameter with over 240 elements for high spatial resolution.
• Used a simplified analytical model to define and estimate the speeds of bubble growth via advection and vaporization.
• Tracked laser radiance, temperature, velocity, and pressure fields to analyze energy transmission and bubble dynamics.
• Compared simulation results with experimental bubble shapes to validate the model and test the advection-vaporization competition hypothesis.

Experimental results

Research questions

• RQ1 What physical processes govern the transition between pear-shaped and elongated conical bubbles in long-pulsed laser cavitation?
• RQ2 Does phase transition (vaporization) persist for a significant duration during long-pulse laser irradiation, or is it instantaneous?
• RQ3 How do the relative speeds of advection and vaporization determine bubble morphology?
• RQ4 What laser parameters (e.g., absorption coefficient, beam width) influence the dominance of one growth mechanism over the other?
• RQ5 Can the bubble shape be predicted and controlled based on the competition between advection and sustained vaporization?

Key findings

• The simulation accurately reproduced both pear-shaped and elongated bubble morphologies observed in experiments with Ho:YAG and Thulium fiber lasers.
• Vaporization duration was less than 1 μs for the pear-shaped bubble, but exceeded 50 μs for the elongated bubble, indicating sustained phase transition in the latter.
• The speed of bubble growth via vaporization was approximately two orders of magnitude higher in the elongated bubble case compared to the pear-shaped case.
• The speed of advection remained relatively constant across both cases, indicating that differences in bubble shape arise primarily from differences in vaporization speed.
• The hypothesis that bubble shape results from a race between advection and vaporization was validated by simulation-based speed estimates.
• Higher laser absorption coefficients and narrower beam widths were predicted to enhance vaporization speed and promote elongated bubble formation.