After being shaken for about a week, thousands of millimetre-sized metal beads arranged themselves into an exotic structure called a quasicrystal 鈥� and it was the biggest one yet. The creation also helped the researcher behind it win a bet against a colleague.
For something to be a crystal, its building blocks must be arranged in a repeating pattern, like the perfect grids of atoms in salt crystals. Within quasicrystals, some arrangements do repeat but never in a uniform or predictable way.
Quasicrystals were first hypothesised and created in the early 1980s. Since then, hundreds have been made in labs, and a handful found in nature. Researchers already knew that nanometre or micrometre-sized particles can be coaxed into quasicrystalline shapes, but at Paris-Saclay University and his colleagues wanted to use objects that were at least a thousand times bigger.
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They poured almost 4000 steel spheres, either 2.4 or 1.2 millimetres in size, into a shallow box where they formed a flat, nearly two-dimensional configuration. The researchers decided how many of each to include based on lengthy computer simulations of how the spheres would move if the box was shaken up and down. In the experiment, they implemented this shaking by attaching the box to a device that vibrated at a frequency of 120 hertz. While constantly shaking the beads so they never settled into equilibrium, the researchers recorded the box with a camera for about 170 hours.
鈥淭his was born as a bet with a colleague who said it would not work, but I said why not try it, it could be interesting,鈥� says Foffi. And he won the bet 鈥� the beads arranged themselves into a thin quasicrystal.
Its pattern had three basic elements: triangles made of large beads with a smaller one between them or large beads arranged in a square with either one or four smaller beads in its centre. Through a careful mathematical analysis of recordings and images from the experiment, the researchers determined that these shapes covered the surface of the table like tiles that never repeat.
at Tel Aviv University in Israel says that large quasicrystal patterns have been recorded on the surface of vibrating liquids, where they emerge from clashes between waves, but they have never been seen in a collection of such large particles or grains. 鈥淰ibrated granular matter, like sand, often behaves like a fluid, and so it was expected to eventually observe quasicrystalline patterns. This [experiment] is very exciting,鈥� he says.
Quasicrystals have been engineered in many systems, ranging from tiny, squishy particles to metal alloys, but devising an exact recipe for making a specific quasicrystal remains a challenge, says at the University of Leeds. This is, in part, because researchers need to figure out when a system will be most stable as a quasicrystal, a question that is complicated for collections of particles that are always in motion or have lots of collisions, like large and hard beads. Foffi says his team鈥檚 setup could help make quasicrystals with other kinds of building blocks, too, but they may take an even longer time to self-assemble.
鈥淭here are some very interesting quasicrystals, like the one in a system of water and a few chemicals, not that different from salad dressing, and metal alloys. But in all cases, we only connect the dots and see the quasicrystal at the end of the experiment. The next step is to go the other way: start with a quasicrystal then construct a system that鈥檚 going to make it,鈥� says Rucklidge.
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