The Phenomena of Exploding Crystals

Science can progress in unexpected ways, and the recent work of NYU Abu Dhabi Associate Professor of Chemistry Panče Naumov is proof of this. Naumov has been studying a fascinating characteristic of crystals: when exposed to UV light, the crystals explode and jump.

With NYUAD Postdoctoral Associate Manas Panda, Naumov recently published a paper that investigates this "thermosalient effect" (the jumping) of the crystals.

The researchers are heading off into new territory. "It's exciting because every result is bringing something new to the field," Naumov said.

Though crystals that show similar properties have been reported by scientists in the past, the materials didn't garner much attention from researchers because it was difficult to explain their odd properties.

Moreover, in the scientific community, there can be a reluctance to study a topic that might be considered strange, Naumov said. "But we now have the instrumentations to look at the phenomena and explain them in further detail, and people are now more keen to work on things they could not explain in the past," he noted.

Naumov's team has published several papers on the topic, however, their research is still in the early stages. "At the beginning we didn't even have standard methods to observe these materials," Naumov said. "So we had to develop tools and software to look at these phenomena."

The researchers first look at the structure of the crystals and the properties that change in the process of their popping. They analyze velocity, height of the jump, and also conduct microscopic measurements. "We try to record the motion and convert the motion into numbers," Naumov said. "And then we try to figure out how these numbers correlate with properties of the materials."

Chemically, these materials share strong hydrogen bonds in one or two dimensions. They are soft, and "the softness is important because the strain that is accumulated inside the crystal is accumulated without mechanical response," Naumov said.

The response in the crystal — triggered by UV light — travels through the crystal extremely fast. The researchers use a high-speed camera attached to a microscope to record the reaction. "We see that these changes occur at about 10,000 times the speed of normal phase transitions," Naumov said. "So, we have a transition that is comparable to the velocity of sound."

As far as practical applications go, it may be possible to use these materials as actuators in a variety of devices. "We call the materials we study technomimetic and biomimetic crystals, because we can potentially mimic technical gears or biological systems that have evolved to actuate themselves," Naumov said.

There are many applications in which it would be useful to translate light or heat into mechanical motion. For instance, in sensor or printing technology. But at this stage, the fundamental research of understanding the structure and characteristics of the crystals is more important than thinking about potential applications, Naumov noted.

There is another peculiar property of these materials. As they expand along one axis, they shrink along another. Naumov said that in the research community there is a competition to see who can create a compound that adheres most closely to this principle. "If you can combine a material that has a high positive thermal expansion with a material that has a high negative thermal expansion, you'd have a material that has zero total expansion, which would be important to avoid the detrimental effects of heating on electronics," he added.