Laser-induced shock-wave-expanded nanobubbles in spherical geometry

• Published in: Ultrasonics sonochemistry Vol. 89; p. 106160
• Main Authors: Horvat, Darja, Agrež, Vid, Požar, Tomaž, Starman, Bojan, Halilovič, Miroslav, Petkovšek, Rok
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2022; Elsevier
• Subjects: Bubble; Cavitation; Pre-illumination; Secondary cavitation; Shock wave; Short Communication; Bubble; Secondary cavitation; Cavitation; Shock wave; Pre-illumination
• ISSN: 1350-4177; 1873-2828
• DOI: 10.1016/j.ultsonch.2022.106160

•Reflected spherical shock wave generates strong negative pressure transient.•Secondary cavitation cloud is generated at the location of negative pressure.•Pre-illumination and insonification are key to secondary cavitation threshold. The secondary cavitation generation following laser-induced breakdown in aqueous media in spherical geometry, mimicking the geometry of the frontal part of the human eye, was studied. A numerical simulation of the shock wave propagation was performed, yielding peak-pressure maps, correctly predicting the location of the secondary cavitation onset for different shock wave source positions. The comparison between the simulation results and the experiments, performed with a high-precision, multiple-illumination technique, supports the suggested description of the nature of the secondary cavitation onset. It is shown that large transient negative pressures are created at the location of the acoustic image of the shock wave source, which is different from the optical focus. After the passage of the shock wave, abundant secondary cavitation is generated there. Additionally, the existence of an important contributing factor to the reduction of the secondary cavitation threshold is supported by the experimental results, namely the pre-illumination of the water by the breakdown-generating laser pulse, playing a crucial role in conditioning the medium. There is strong experimental evidence of the existence of another mechanism of pre-conditioning the water for the secondary cavitation onset, namely in the form of repetitive negative pressure pulse passage through the same volume, an indication of a possible two- or multiple-stage process.