Material behavior insight comes as a nanoshock

At first, nanoshocks may seem like something to describe the millions of aftershocks of a large earthquake.

But Laboratory physicists are using an ultra-fast laser-based technique they've dubbed "nanoshocks" for something entirely different. In fact, the "nanoshocks" have such a small spatial scale that the scientists can use them to study shock behavior in tiny samples such as thin films or other systems with microscopic dimensions (a few tens of micrometers). In particular, they have used the technique to shock materials under high static pressure in a diamond anvil cell (DAC). 

Using a DAC — which probes the behavior of materials under ultra-high pressures (and which requires small samples) — the team statically compressed a sample of argon up to 78,000 atmospheres of pressure and then further shock=compressed it up a total of 280,000 atmospheres. They analyzed the propagating shock waves using an ultra-fast interferometric technique. Using this technique, they were able to achieve combinations of pressures, temperatures and time scales that are otherwise inaccessible.

In some experiments, they observed a metastable argon state that may have been superheated — a state at a pressure and temperature at which argon would normally would be liquid but because of the ultra-short time scale does not have enough time to melt.

"It can be used to study fundamental physical and chemical processes as well as improve our understanding of a wide range of real-world problems ranging from detonation phenomena to the interiors of planets," said LLNL physicist Jonathan Crowhurst, a co-author of a paper describing the research, which will appear in the July 15 edition of the Journal of Applied Physics.

The time scale is short enough to permit direct comparison of the experimental results with molecular dynamics simulations, which usually run for less than a nanosecond (one billionth of a second).

Shocked behavior in microscopic samples can consist of the behavior of shocked explosives before chemistry begins or the high-density, low-temperature states of light materials such as those that are found in giant gas planets, according to LLNL lead author Michael Armstrong.

"Essentially, this allows us to examine a very broad range of thermodynamic states, including states corresponding to planetary interiors and high-density, low-temperature states that have been predicted to exhibit unobserved exotic behavior," Armstrong said.

For decades, compression experiments have been used to determine the thermodynamic states of materials at high pressures and temperatures. The results are necessary to correctly interpret seismic data, and to understand planetary composition and the evolution of the early solar system, shock-wave-induced chemistry and fundamental issues in condensed matter physics.

Armstrong said their technique for launching and analyzing nanoshocks was so fast they were able to see behavior in microscopic samples that is inaccessible in experiments using static or single-shock compression.

Other LLNL team members include Sorin Bastea and Joseph Zaug.