Visualization of aerosol particles generated by near infrared nano- and femtosecond laser ablation

Abstract

The expansion of aerosols generated by near infrared (NIR) nanosecond (ns) and femtosecond (fs) laser ablation (LA) of metals at atmospheric pressures was explored by laser-induced scattering. In order to achieve adequate temporal and spatial resolution a pulsed laser source was utilized for illuminating a 0.5 mm-wide cross section of the expanding aerosol. It could, for instance, be shown that NIR-ns-LA under quiescent argon atmosphere provokes the formation of a dense aerosol confined within a radially propagating vortex ring. The expansion dynamics achieved under these conditions were found to be fairly slow whereas the degree of aerosol dispersion for NIR-ns-LA using helium drastically increased due to its lower viscosity. As a consequence, the maximum diameter of expansion differed by a factor of approximately four. The trajectories of aerosol particles generated by NIR-ns-LA using argon could, furthermore, be simulated on the basis of computational fluid dynamics (CFD). For this purpose, a model inspired by the thermal character of NIR-ns-LA taking into account a sudden temperature build-up of 10,000 K at the position of the laser focus was implemented. In contrast, NIR-fs-LA generally resulted in extremely dynamic expansion patterns. Initial aerosol velocities derived from corresponding expansion plots varied from 10 m/s up to 30 m/s for fs-LA using argon and helium, respectively. Our results, moreover, indicate that fs-LA carried out under helium atmosphere favours a chaotic aerosol expansion. Analytical implications concerning, e.g. dispersion phenomena or the choice of the LA protocol and physical dimensions of future ablation cell designs are discussed.