Materials scientists typically rely on their eyes to analyze data, but soon they could employ their ears as well. Setting the motions of molecules to music can help scientists identify hidden patterns in their data that might otherwise be too small, or occur over such short time scales that they’re easily missed by the human eye.
That’s the hope of Asegun Henry, a mechanical engineer at Georgia Tech. He has applied for a National Science Foundation grant to create an educational app that catalogues unique musical signatures for every element in the periodic table. He’s even setting them to music. No longer must we rely solely on words or diagrams to comprehend the differences between the molecular structure for solids, liquids, and gases. We can listen to them, too.
“My hope is that it will be an interesting tool to teach the periodic table, but also to give people some notion about the idea that the entire universe is moving around and making noise,” Henry told Gizmodo. “You just can’t hear it.”
There are lots of folks doing interesting things with sonification, including interdisciplinary collaborations between scientists and artists. Several years ago, a project called LHCSound built a library of the “sounds” of a top quark jet and the Higgs boson, among others. That project, headed by CERN physicist Lily Asquith, had a broader aim: to develop sonfiication as a technique for analyzing the data from particle collisions, so that physicists could “detect” subatomic particles by ear. Henry is doing something similar. His use of sonification in his research has already led to a recent paper in the Journal of Applied Physics.
Materials get their properties from molecular structure. But those component molecules aren’t static objects; they are constantly vibrating as the bonds between the atoms that make up molecules move about. The simplest toy model is that of two (or three) round balls attached by a flexible spring that can move in various ways. They can rock back and forth like a pendulum, stretch, twist, and wag. And those vibrations interact with each other like waves.
The more atoms in a molecule, the more combinations between the various “vibrational modes” (different kinds of waves) are possible. “How the energy of the interaction changes with respect to the distance between the molecules dictates a lot of the physics,” said Henry, such as how well a given material conducts heat (a property known as thermal conductivity).
Different kinds of elements will have specific kinds of acoustic signatures. This, for example, is the sound of crystalline silicon:
“We have to slow down the vibrations of the atoms so you can hear them, because they’re too fast, and at too high frequencies,” Henry said. “But you’ll be able to hear the difference between something low on the periodic table and something like carbon that’s very high. One will sound high-pitched, and one will sound low.”
It’s more than just a fun exercise. Henry and his graduate student, Wei Lv, were interested in a peculiar feature of polymers, long chains of molecules all strung together, with thousands upon thousands of different modes of vibration that interact with each other. Polymers are much more complicated than the simple toy models, so it’s harder to describe their interactions mathematically. Scientists must rely on computer simulations to study the vibrations.
But when Henry and Lv ran their computer simulations, they noticed that some of the polymers they were modeling didn’t behave as expected. Tweak the starting parameters a little, and the system will evolve normally up to a point, but then diverge into what amounts to a patterned series of vibrations—not random. The simulated polymer becomes thermally superconductive—that is, capable of transporting heat with no resistance, much like the existing class of superconducting materials that conduct electricity without resistance (albeit at very low temperatures).
Enrico Fermi had noticed this odd effect in early computer simulations of chains of particles 50 years ago, dubbing it a “shocking little discovery.” But this was the first time it had been observed in an actual (non-idealized) polymer. “Toy models are fictitious and designed to be really simple and plain so that you can analyze them easily,” said Henry. “We did this with a real system, and the [effect] actually persisted.”
Henry and Lv successfully identified three vibrational modes out of all those thousands that were responsible for the phenomenon. But the usual analysis techniques—like plotting the amplitudes of the modes over time in a visual graph—didn’t reveal anything noteworthy. It wasn’t until they decided to sonify the data that they pinpointed what was going on. This involved mapping pitch, timbre, and amplitude onto the data to translate it into a kind of molecular music.
They discovered that the three modes would fade in and out over time, and eventually they would synchronize with each other. This created a kind of sonic feedback loop until the simulated material became thermally superconductive. Those so-called “divergent” polymers sound like this:
In contrast, this is the more typical sound of “convergent” polymers:
Put them together, and they sound like this:
Granted, it sounds less like a musical tone, and more like sandpaper on wood. But there’s useful information contained within those rasping sounds that would not be apparent if you were just looking at the visual graph. “As soon as you play it, your ears pick up on it immediately,” said Henry. So it’s solid proof-of-principle of sonification as an analytical tool for materials science.
Henry is still working on finding the underlying mechanism behind the phenomenon: why does it manifest in some polymer systems, but not others? If he succeeds, it may one day be possible to physically make thermal superconducting materials for real, thereby opening up all kinds of practical applications. “It would change the world,” said Henry. “Conceptually you’d be able to run a thermal superconducting pipe from the Sahara desert and provide heat to the rest of the world.”
In the meantime, he has an app to build, so we can all compose our own molecular music.