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A team led by University of Virginia professor Leonid Zhigilei has made a virtual laboratory by using computing resources at the Oak Ridge Leadership Computing Facility to gain deeper insights into laser interactions with metal surfaces.

“Rapid expansion of practical applications of ultrashort pulse laser processing, including engineering of new materials, requires understanding of fundamental mechanisms of laser induced structural and phase transformations,” Zhigilei said in a statement.

“Experimental probing of these transformations, which take place on the picosecond time scale (one-trillionth of a second), is difficult, expensive, and often not even feasible. Performing ‘virtual experiments’ on a supercomputer provides an attractive alternative,” he added. “Moreover, computational results may guide focused experimental exploration of the most promising irradiation regimes or interesting phenomena predicted in the simulations.”

In the experiment, the researchers used a combination of virtual and real-world experiments to gain a fundamental understanding of the mechanisms for material interactions induced by lasers.

The research team used supercomputers to simulate the phase transformations at atomic scales, but to create meaningful simulations the team would need to simulate millions or billions of atoms and then watch how atoms move over a sequence of very brief moments in time called time steps.

By running long simulations consisting of millions of time steps, researchers are able to observe all of the processes happening during a laser-metal interaction during a total time of several nanoseconds.

The researchers ran a 2.8 billion-atom simulation of silver for 3.2 nanoseconds, allowing it to compare for the first time the frozen surface’s morphology—its surface structure—to experimental data.

Lasers can imbue metals with many properties by laser ablation—the process of selectively removing small amounts of material to change the surface morphology and microstructure.

While primarily invisible to the human eye, this process can make major changes to a metal’s characteristics. Laser ablation irradiates the surface of a metal in a quick, violent interaction, creating 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. The researchers also created black surfaces on metals without using paint or other synthetic materials, while short laser pulses can also locally modify the hardness of metals.

The engineers can also make a hard outer shell of a metal sample while keeping the inside softer for increased flexibility.

In many cases, metal processing occurs in a vacuum which allows engineers to prevent contaminants from getting into the processed material.

“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,” team member and University of Virginia graduate student Cheng-Yu Shih, said in a statement.

“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, Shih added.

“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.”

The team now plans on focusing 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.

  

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