Laser Focus Gives Result on Two Sources of Nanoparticle Formation

To solve this case, a group of scientists turned to computational sleuthing tactics. 

This group is led by Leonid Zhigilei of the University of Virginia (UVA), the group used the Oak Ridge Leadership Computing Facility's (OLCF's) 27-petaflop Titan supercomputer view and to model the interaction that happens between short laser pulses and the metal targets that is done at the atomic scale. This is known as the laser ablation and this process involves metals that are irradiated with laser beam to remove layers of material selectively, this then changes the surface structure of the target and it generates the nanoparticles of the target. 

To broaden the research on the relationship between the nanoparticle generation and the laser ablation, Zhigilei's team spent hours through the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program as they investigated the mechanisms responsible for forming two distinct populations of nanoparticles. This research is focused exclusively on how these processes happen in liquid environments.

The UVA scientists collaborated with a research group from the University of Duisburg-Essen, Germany. In 2018, the result of their study was published in Nanoscale; the journal's back cover featured a laser ablation image OLCF computer scientist Benjamin Hernandez created using SIGHT, a customizable visualization tool he developed. The OLCF is a US Department of Energy (DOE) Office of Science User Facility located at DOE's Oak Ridge National Laboratory (ORNL).

To be able to differentiate the sources of nanoparticles from small (less than 10 nanometers) to large (10 or more nanometers), the researchers ran molecular dynamic simulations on Titan, it then modeled gold and silver targets in water that is irradiated by laser ablation. 

"These metals are stable, inert, and do not actively react with the surrounding environment," Zhigilei said. "Additionally, silver has useful antibacterial properties."

During the ablation process, the laser pulses provides heat to the metal target's surface, it then leads to decomposition that is explosive and that region becomes a mixture of small liquid droplets and vapor. This mixture is ejected from irradiated target, and it forms ablation plume. This phase is known as the "explosive boiling", this process has been researched and studied for years for laser ablation in a vacuum. 

But when ablation happens in a liquid environment, the interaction of the ablation plume with the liquid environment complicates the whole process by slowing down the ablation plume, it then leads to the formation of a hot metal layer that is pushing against the liquid.

This interaction can start a rapid succession of hydrodynamic instabilities in the molten metal layer, and it can cause it to partially or entirely disintegrate and produce massive nanoparticles. 

"When you first turn on a lava lamp, the heavy fluid sits on top of the light fluid, but then it begins to flow under the action of gravitational acceleration and creates some interesting flow patterns and particle formation," Zhigilei said. "Something similar happens with laser ablation-the heavy layer of hot metal is rapidly decelerated by water, which produces hydrodynamic instabilities at the metal-water interface that generate large nanoparticles."

The group observed the movements of individual atoms to record useful information concerning both ways to nanoparticle generation.

"We had to quickly pivot from atoms on the scale of less than one nanometer to hundreds of nanometers, which required solving equations for hundreds of millions of atoms in our simulations," Zhigilei said. "This type of work is only possible on large supercomputers like Titan."