Hydrodynamic simulation of laser ablation of gold into water

Nanoparticles (NPs) of different materials have found numerous applications due to their unique properties. For instance, particular attention has been attracted to several very promising medical applications, such as cancer and antibacterial treatment, imaging, censors, etc. Therefore, new methods are required to prepare liquid solutions with NPs using ultra-short and long laser pulses. Laser ablation (LA) is one such simple and versatile technique. The major advantage of the LA method is in its ''green'' character, which means that NPs are almost unaffected chemically. For simulation of multi-stage processes of femtosecond laser ablation of gold target into ambient water we develop the two-temperature wide-range hydrodynamic model [1]. It consistently describes the laser energy absorption, thermal conduction, electron-phonon/ion coupling, material expansion and compression. For the description of thermodynamic functions of gold we use thermodynamically complete multi-phase equation of state (EOS) with stable and metastable phases. Usage of this EOS enables us to describe phase transitions as well as introduce kinetics for the evolution of the metastable liquid state. It is shown in numerical simulations that the large-size particles are the result of liquid layer ejection and fragmentation. This regime is observed for low laser intensities when the thermodynamic trajectories enter the metastable liquid region far below the critical point. For higher laser intensities, the near-critical and supercritical trajectories appear for front target layers and the mass fraction of the liquid-gas mixture increases. Formation of small NPs can be observed for these trajectories and estimation of the NPs size matches quite well with the experimental findings. The calculation results thus explain bimodal size distributions of NPs frequently observed in experiment. [1] M. Povarnitsyn et al. Phys. Chem. Chem. Phys., 2013, V. 15, P. 3108