Laser ablation in liquid: bridge from a plasma stage to bubble formation

Abstract

Laser ablation through liquid is an important process that have to be studied for applications which use laser ablation in liquid (LAL) and laser shock peening (LSP). LAL is employed for production of suspensions of nanoparticles, while LSP is applied to increase hardness and fatique/corrosion resistance properties of a surface layer. A bubble appears in liquid around the laser spot focused at a target surface after strong enough laser pulse. In the paper we connect the early quasi-plane heated layer created by a pulse in liquid and the bubble forming at much later stages. In the previous works these early stage from one side and the late stage from another side existed mainly as independent entities. At least, quantitative links between them were unknown. We consider how the quasi-plane heated layer of liquid forms thank to thermal conduction, how gradually conduction becomes weaker, and how the heated layer of liquid nearly adiabatically expands to few orders of magnitude in volume during the drop of pressure. Our molecular dynamics simulations show that the heated layer is filled by the diffusive atomic metal-liquid mixture. Metal atoms began to condense into nanoparticles (NP) when they meet cold liquid outside the edge of a mixing zone. This process limits diffusive expansion of metal atoms, because diffusive ability of NP is less than this ability for individual atoms. Thus the mixture expands together with hot liquid, and the NPs approximately homogeneously fill an interior of a bubble.