Plasma temporal expansion dynamics in droplets by laser-induced breakdown spectroscopy assisted by acoustic levitation (LIBS-AL)

Laser-induced breakdown spectroscopy assisted by an acoustic levitation system (LIBS-AL) is a technique that has shown promising results for analyzing liquid in a droplet sample. However, previous publications have reported a strong dependency on a preconcentration process by heating the droplet using an IR laser to increase the signal of the spectral lines. This approach has shown successful results but involves additional equipment, sample pre-treatment, and time analysis. Hence, to improve the performance of the technique for liquid analysis, a deeper study of the laser-matter samples has been made. To achieve this objective, the shadowgraph and fast photo-detection techniques were employed to record the dynamics of plasma formation and expansion over time. The experiments were performed on different droplet sizes and ablation locations within the droplet. The results indicate variations in the amount of material ablated from the droplet. The amount of ablated material was found to be dependent on experimental parameters such as observation time (delay time), focal distance, and droplet size. Since the shadowgraph and fast-photo detection provide 2D information (area), the ablated material was evaluated with the intersection area of these images. The intersection area resulting is referred to as the effective ablated area . Therefore, in order to characterize the plasma plume expansion, the ablation efficiency (mass of the droplet ablated) was determined using the temporal evolution of the effective ablated area. The results show that the effective ablated area can reach up to 27% of the sample area across the different droplet sizes studied. Additionally, plasma expansion at the gas–liquid interface was studied using the Drag model to account for expansion against external pressure.