Lasers can imbue metals with many novel properties. One way to do this is to use laser ablation, or the process of selectively removing small amounts of material, thus changing the surface morphology and microstructure. While often invisible to the human eye, this process can make major changes to a metal's characteristics. Laser ablation irradiates the surface of metal in a quick, violent interaction, creating very tiny explosions of particles being removed from the material. As the metal cools, it exhibits new properties, depending on the process.
Engineers can use lasers to influence how a metal surface interacts with water--forcing water to roll off the surface in a certain direction, for instance. Researchers can create black surfaces on metals without using paint or other synthetic materials. Short laser pulses can also locally modify the hardness of metals; for increased flexibility, engineers can make a hard outer shell of a metal sample while keeping the inside softer.
In many cases, metal processing occurs in a vacuum, thus allowing engineers to prevent contaminants from getting into the processed material. Though the Zhigilei team focused primarily on simulating metal-laser interactions in a vacuum, the computing time awarded through the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program allowed the team to simulate these processes in more complex scenarios, as well. "Laser ablation in liquids, in particular, is actively used for generation of clean colloidal nanoparticles [nanoparticles that are insoluble and evenly dispersed in a solvent] with unique shapes and functionalities suitable for applications in various fields, including biomedicine, chemical catalysis, and plasmonics," said team member and University of Virginia graduate student Cheng-Yu Shih.
"While, experimentally, the liquid environment has been demonstrated to strongly affect the nanoparticle size distributions and microstructure of laser-modified surfaces, the physical mechanisms of laser surface modification and ablation in liquids are still poorly understood. The interaction of the ablation plume [a cloud of metal vapor and small droplets ejected from the irradiated target] with the liquid environment adds an additional layer of complexity to the laser ablation. Atomistic simulations help shed light on the initial, very critical stage of ablation plume and liquid interaction and predict the subsequent nanoparticle formation mechanisms at the atomic level. With access to the INCITE resources, it becomes possible to address the challenging problem of atomistic modeling of nanoparticle generation by laser ablation in liquids," Shih continued.
The team's ability to expand its simulations came from equipping its code to use accelerators like Titan's GPUs. During the course of its INCITE project, the team worked with OLCF scientific computing liaison Mark Berrill and OLCF user support staff to improve hybrid code performance.
As a result, the team was able to achieve a sevenfold speedup over CPU-only methods. These speedups helped the team run larger, more complex simulations and expand the study into the simulations of metal processing outside of a vacuum. In addition, OLCF staff helped the team optimize its codes' I/O performance by implementing the Adaptive I/O System (ADIOS) middleware into the code.
The team also worked with OLCF computer scientist Benjamin Hernandez to help with visualization of atomic configurations that consist of billions of atoms.
The team attributes a variety of computational resources to its success. "With a highly optimized computer code that runs in parallel on thousands of computer nodes and fully utilizes the capabilities of modern computing technology, including low latency and high bandwidth interconnects between the nodes and high performance GPU accelerators, it is now possible to address the most ambitious and incredibly challenging computational problems in our field," said team member and University of Virginia graduate student Maxim Shugaev.
Moving into the next year of its INCITE award, the team plans to focus on laser-metal interactions in liquids to gain a complete picture of how surface tension, critical temperature, pressure, and differing environments control metal surface morphology and microstructure.