Laser Ablation of Gold into Water: near Critical Point Phenomena and Hydrodynamic Instability

Laser ablation of gold irradiated through the transparent water is studied. We follow dynamics of gold expansion into the water along very long (up to 200 ns) time interval. This is significant because namely at these late times pressure at a contact boundary between gold (Au) and water decreases down to the saturation pressure of gold. Thus the saturation pressure begins to influence dynamics near the contact. The inertia of displaced water decelerates the contact. In the reference frame connected with the contact, the deceleration is equivalent to the free fall acceleration in a gravity field. Such conditions are favorable for the development of Rayleigh-Taylor instability (RTI) because heavy fluid (gold) 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. Atomistic simulation with Molecular Dynamics method combined with Monte-Carlo method (MD-MC) for large electron heat conduction in gold is performed to gain a clear insight into the underlying mechanisms. MD-MC 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 gold droplets fall into the water.