Laser induced cavitation: Plasma generation and breakdown shockwave

Abstract

Laser induced cavitation is one of the effective techniques to generate controlled cavitation bubbles, both for basic study and for applications in different fields of engineering and medicine. Unfortunately, control of bubble formation and symmetry is hardly achieved due to a series of concurrent causes. In particular, the need to focus the laser beam at the bubble formation spot leads, in general, to a conical region proximal to the light source where conditions are met for plasma breakdown. A finite sized region then exists where the electric field may fluctuate depending on several disturbing agents, leading to possible plasma fragmentation and plasma intensity variation. Such irregularities may induce asymmetry in the successive bubble dynamics, a mostly undesired effect if reproducible conditions are sought for. In the present paper, the structure of the breakdown plasma and the ensuing bubble dynamics are analyzed by means of high speed imaging and intensity measurements of the shockwave system launched at breakdown. It is found that the parameters of the system can be tuned to optimize repeatability and sphericity. In particular, symmetric rebound dynamics is achieved almost deterministically when a pointlike plasma is generated at the breakdown threshold energy. Spherical symmetry is also favored by a large focusing angle combined with a relatively large pulse energy, a process which, however, retains a significant level of stochasticity. Outside these special conditions, the elongated and often fragmented conical plasma shape is found to be correlated with anisotropic and multiple breakdown shockwave emission.

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

“…Therefore, in the ideal case of a spherical collapse, the generated shock pressure is sufficient to break the crystal; however, in our case where multiple collapses are observed due to collapsing asymmetry (elongated plasma breakdown) [52] , [56] , the distributed shock pressure is expected to be comparatively less. Moreover, this exaggeration of the maximum pressure generated from a single shock wave compared to multiple pressure shocks is also in good agreement with experimental measurements as discussed elsewhere [26] , [51] .…”

“…So, it is fair to say that the whole crystal fragmentation is essentially a reaction to cumulative shock waves (~5 shock fronts typically monitored after the final collapse) impact from a single laser bubble. Similar response was observed in all other cases of single laser bubble experiments where fragmentation essentially occurred from the split cavitation and asymmetric rebound producing multiple shock waves caused by the residual asymmetry during implosion as achieving a perfect spherical shape of the bubble is barely possible [51] . It is also evident from Fig.…”

“…It is to be noted that these predicted pressure values are for an ideal case of a symmetric bubble collapse that produces a single shock wave. It has been reported that the pressure amplitude of the propagating shock waves emitted from an asymmetric bubble collapse at a distance of 3 mm from the plasma centre is in the range of 4–11 MPa depending upon the focussing angle of the laser beam [51] . This variation is also indicative of the fact that the emitted shock wave is formed through superimposition of signals originating from diverse locations across the elongated plasma region.…”

An analysis of surface breakup induced by laser-generated cavitation bubbles in a turbulent liquid jet

“…The distortion might result in a transient larger or smaller focusing angle. The focusing angle could affect the shape of the laser-induced plasma (Tian et al 2016;Vogel et al 1996), and the shape and dynamics of the cavitation bubble (Sinibaldi et al 2019). The wrinkled surface also affected the interaction between the bubble and the surface, as it is known that shape of the surface affects the bubble and breakup dynamics (Obreschkow et al 2006).…”

Irradiation of successive pulsed and continuous-wave lasers on a transparent polyethylene film

“…With the development of highspeed imaging instrumentation, the dynamic process of laser-induced plasma was photographed. Sinibaldi et al 5 analyzed the plasma generated by a laser in liquid, and the structure of the breakdown plasma and the ensuing bubble dynamics in the liquid were recorded. Brieschenk et al 6,7 recorded pictures of the early evolution of a laser-induced plasma explosion in air using highspeed time-resolved Schlieren images.…”