Laser ablation of metal into liquid: Near critical point phenomena and hydrodynamic instability

Abstract

We consider the problem of dynamics of gold illuminated through water by ultrashort laser pulse. This problem is interesting itself due to its complexity and it is important for nanotechnological applications connected with a clean way (without chemistry) of nanoparticles production and also for creation of functional surfaces (e.g. for enhancing of surface Raman scattering) which differ from the functional surfaces produced by illumination through vacuum or gas. We begin with short presentation of the two-temperature phenomena inevitable when the ultrashort laser pulse is used. We present results of two-temperature (2T) one-dimensional hydrodynamic (2T-HD) simulations covering very long (up to 0.2µsec) time interval. This is significant because namely at these late times pressure at a contact boundary between gold (Au) and glass decreases down to saturation pressure of gold. And the saturation pressure begins to influence dynamics near the contact. Inertia of water is the next main actor. It decelerates the contact. In the reference frame connected with the contact the deceleration is equivalent to the free fall acceleration in a gravity field. This follows from the Einstein’s principle of the gravity/inertia equivalence. This is exact the conditions favorable for development of Rayleigh-Taylor (RT) instability (RTI) because heavy fluid (Au) is placed above the light one (water) in a gravity field. We extract the increment of RTI from 2T-HD 1D runs. Surface tension and especially viscosity significantly dump the RTI gain during deceleration. We use large scale molecular dynamics (MD) simulations to do the situation clear. MD runs show that significant amplification of surface perturbations takes place. These perturbations start just from thermal fluctuations and the noise produced by bombardment of the atmosphere by fragments of foam. The perturbations achieve amplification enough to separate the droplets from the RTI jets of gold. Thus the droplets fall into water. There is a quasi-hydrostatic equilibrium near the contact in gold. Therefore we use the word “atmosphere”. Laser action should be strong to produce nanoparticles. It is significantly higher than the nucleation threshold Fabl for gold thermo-mechanically ablated into vacuum. Absorbed energy Fabs is of the order of or higher than the evaporation (ev) threshold Fabs|ev above which the spallation plate cannot form during expansion of gold to vacuum. In this case very wide foamy zone is created. Expansion of foam doesn’t “know” about water. Foam expands freely. Thus its expansion velocities begin overcome velocity of a contact decelerated by water. This causes accretion of membranes of foam onto atmosphere created thanks to deceleration. The MD simulations beautifully illustrate this flow with shock in water, atmosphere “sitting on water”, vast foam, RTI of the contact, and accretion of foam onto atmosphere.