Computational study of laser fragmentation in liquid: Phase explosion, inverse Leidenfrost effect at the nanoscale, and evaporation in a nanobubble

Laser fragmentation in liquid is an effective and environment-friendly processing technique capable of yielding colloidal nanoparticles and atomic clusters with a narrow size distribution. The advancement of this technique can be facilitated by an improved understanding of processes that control the sizes, shapes, and structures of the produced nanoparticles. In this work, the dependence of the fragmentation mechanisms on the energy density deposited by the laser pulse is investigated in atomistic simulations performed for 20 nm Au nanoparticles irradiated in water by 10 ps laser pulses. The simulations reveal that the decrease in the absorbed laser energy leads to sequential transitions from the regime of “strong” phase explosion, when all products of an explosive phase decomposition of the irradiated nanoparticle are promptly injected into the water surrounding a nanobubble formed around the nanoparticle, to two distinct regimes of nanoparticle fragmentation leading to the formation of a large central nanoparticle surrounded by smaller satellite fragments. First, in the regime of “mild” phase explosion, the central nanoparticle is produced by the reflection of some of the hot metal droplets generated by the explosive decomposition of the nanoparticle from the boundary of the nanobubble. This reflection is attributed to the inverse Leidenfrost effect acting at the nanoscale. The reflected droplets converge in the center of the nanobubble and coalesce into a single droplet that solidifies shortly after the collapse of the nanobubble. Further decrease in the absorbed laser energy brings the irradiation conditions below the threshold for the phase explosion and results in the formation of a core-satellite structure of the fragmentation products through an interplay of the intense evaporation from the surface of the irradiated nanoparticle, evolution of the nanobubble, and condensation of the metal vapor into clusters and small satellite nanoparticles. The computational predictions are related to the experimental observations, and the connections between the fragmentation mechanisms, the nanoparticle size distribution, and the generation of internal crystal defects are discussed.