Unveiling the Early Dynamics of Laser-Induced Plasma and Cavitation Bubble in Water

A recent study published Spectrochimica Acta Part B: Atomic Spectroscopy offers insight into the behavior of plasma and the interaction between cavitation bubbles and plasma (1). In this study, the researchers employed fast imaging and shadowgraph techniques to analyze the temporal evolution of plasma and the associated bubble formation within the first 2 μs after laser initiation (1). The results uncovered by the research team helps advance our knowledge of the underlying principles involved in underwater laser-induced breakdown spectroscopy (LIBS).

Dark blue ocean surface seen from underwater | Image Credit: © katatonia - stock.adobe.com

Laser-induced cavitation bubbles are created when a high-energy laser pulse interacts with a liquid medium, which is typically water. The intense laser energy rapidly heats and vaporizes the surrounding liquid, generating a localized region of high-pressure vapor (1). The vapor rapidly expands as the pressure is gradually increased, and this process is what creates the bubble. The expansion is driven by the release of stored energy from the vaporization process. The cavitation bubble undergoes a dynamic evolution, with its size increasing until it reaches a maximum extent and then rapidly collapsing. The collapse of the bubble produces a shockwave, resulting in a phenomenon known as cavitation (1). Cavitation is an occurrence that sees the formation of secondary bubbles and the emission of acoustic signals (1).

The research team observed that the plasma initially formed at the laser focal point and expanded toward the incident laser beam (1). Over time, the plasma expands both forward and backward, exhibiting a sub-plasma structure at low laser energies. During the laser pulse, the plasma demonstrates pulse-to-pulse repeatability, but in the period of 20 to 50 ns after the laser pulse, it experiences severe pulse-to-pulse fluctuation with an uneven emission distribution (1). Subsequently, the plasma stabilizes once again at longer times.

The researchers also sought to create a connection between the plasma characteristics and bubble dynamics. To accomplish this, the researchers compared the shadowgraph images of the bubble with plasma images (1). The morphology and evolution of the bubble were found to align well with the plasma morphology (1). The temporal variation of the bubble radius followed the equation R = at0.4, where "a" is a constant associated with the laser energy (1). Furthermore, the researchers were able to identify a transitional phase between the moving breakdown phase and the thermal expansion phase at 20–50 ns (1). They accomplished this by examining the bubble wall's evolution curve and calculating the bubble pressure based on the Gilmore model (1).

The findings highlight the critical role of the interaction between cavitation bubbles and plasma in shaping the characteristics of laser-induced plasma in water. By unraveling the early dynamics of plasma and cavitation bubbles, this research contributes to a better understanding of the underlying processes involved in underwater laser-induced breakdown spectroscopy. These insights pave the way for further advancements in laser-based analytical techniques in aqueous environments.

Reference

(1) Jia, Z.; Li, D.; Tian, Y.; Pan, H.; Zhong, Q.; Yao, Z.; Lu, Y.; Guo, J.; Zheng, R. Early dynamics of laser-induced plasma and cavitation bubble in water. Spectrochimica Acta Part B: At. Spectrosc. 2023, 206, 106713. DOI: 10.1016/j.sab.2023.106713