What do a frying pan, an LED light, and the most cutting edge camouflage in the world have in common? Well, that largely depends on who you ask. Most people would struggle to find the link, but for University of Michigan chemical engineers Sharon Glotzer and Michael Engel, there is a substantial connection, indeed one that has flipped the world of materials science on its head since its discovery over 30 years ago.
The magic ingredient common to all three items is the quasiperiodic crystal, the “impossible” atomic arrangement discovered by Dan Shechtman in 1982. Basically, a quasicrystal is a crystalline structure that breaks the periodicity (meaning it has translational symmetry, or the ability to shift the crystal one unit cell without changing the pattern) of a normal crystal for an ordered, yet aperiodic arrangement. This means that quasicrystalline patterns will fill all available space, but in such a way that the pattern of its atomic arrangement never repeats. Glotzer and Engel recently managed to simulate the most complex quasicrystal ever, a discovery which may revolutionize the field of crystallography by blowing open the door for a whole host of applications that were previously inconceivable outside of science-fiction, like making yourself invisible or shape-shifting robots.
While most of the current applications of quasicrystals are rather mundane, such as the coating for frying pans or surgical utensils, Glotzer and Engel’s simulation of a self-assembling icosahedral quasicrystal opens up exciting new avenues for research and development, such as improved camouflage.
“Camouflage is all about redirecting light to change the appearance of something,” said Glotzer. “Making camouflage materials or any kind of transformation optics materials is all about controlling the structure of the material, controlling the spacing of the building blocks to control the way light is absorbed and reflected.”
Icosahedral quasicrystals (IQCs) are one of the several unique structures which have something called a photonic band gap, which dictates the range of photon frequencies which are permitted to pass through the material. Photonic band gaps are determined by the spatial arrangement of an atomic lattice. In other words, whether or not a photon becomes “trapped” in the lattice depends on the photonic frequency (measured as a wavelength) in relation to the space between atoms and the way these atoms are arranged (periodically, aperiodically, etc). If the wavelength falls within the range of the photonic band gap for the specific material, then the photons will not be able to propagate through the structure.
Thus, being able to manipulate photonic band gaps means that one can manipulate atomic structures in such a way that the material will only be visible within determined photonic frequencies, a critical advancement for those concerned with making people invisible, which probably at least partly accounts for why the US Department of Defense and the US Army both helped fund Glotzer and Engel’s study.
While the existence of photonic bandgaps is nothing new, being able to manipulate solid-state matter in such a way that allows one to fully exploit these bandgaps has remained elusive. In this sense, Glotzer and Engel’s simulated quasicrystal represents a return to the fundamentals of crystallography, rather than something entirely novel.
According to the team, before their simulation, scientists knew that mixing certain metals in the right thermodynamic conditions (pressure, temperature) would result in the formation of a quasicrystal. They also knew that given the correct environmental conditions, it was possible for quasicrystals to form in nature (two natural quasicrystals have been discovered to date: the first in 2009 and the second was reported on March 13, coming from a 4.5-billion year old meteorite in Russia).
What scientists didn’t understand, said Engel, was what was happening in the reaction to make these quasicrystals form. There was an input and output, but what went on inside the blackbox remained a mystery. Glotzer and Engel’s experiment was a first step in solving this a-list conundrum in materials science.
“For a long time people have looked for methods to actually model [how icosahedral quasicrystals form],” said Engel. “This is more of a fundamental importance, it doesn’t necessarily make [IQCs] have better properties or applications, but it allows us to study how these crystals form.”
Understanding how these quasicrystals form is the first step in manipulating them toward desired ends. While this ability to manipulate quasicrystals is still in a very young phase, increasing technical sophistication could conceivably lead to some pretty wild developments in the future, like Terminator-style shape-shifting robots.
Part of the reason robots modeled after T-1000 don’t roam the Earth already is because our understanding of matter and our ability to find useful applications for the staggering variety of metals found in nature is still relatively rudimentary. Understanding how quasicrystals form will fill in a huge gap in our knowledge of solid-state physics and chemistry. Increasing this knowledge in all of its forms is essential to future physical manipulation, whether or not this manipulation is directly linked to quasicrystals.
“It’s not that the icosahedral quasicrystal itself would necessarily be the structure you would shoot for [in shapeshifting materials], but it represents the kind of complexity and control that one would like to have over the building blocks of matter,” said Glotzer. “If you understand what is required to get a certain structure, than you could imagine that we could change conditions and change the structure that we get. Everything about a material depends on its structure.”
T-1000 Shapeshifter in Terminator
The quasicrystalline structure was discovered by Dan Shechtman, a professor of materials science at the Technion-Israel Institute of Technology, in 1982 while he was observing an alloy of rapidly cooled aluminum and manganese with an electron microscope.
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