Physicists identify a surprising phenomenon of aging of materials over time

Physicists from Darmstadt are studying the aging processes of materials. For the first time, they measured the ticking of an internal glass clock. As they evaluated the data, they discovered a surprising phenomenon.

We experience time as having only one direction. Who has ever seen a cup break on the ground, only to spontaneously rise again? For physicists, this is not self-evident, because the formulas that describe movements apply regardless of the direction of time.

For example, a video showing a pendulum swinging unhindered would look the same if it were running in reverse. The daily irreversibility we experience only comes into play through another law of nature, the second law of thermodynamics. This means that the disorder in a system is constantly increasing. But if the broken cup could be restored, the disorder would diminish.

One might think that the aging of materials is just as irreversible as the shattering of glass. However, by studying the movements of molecules in glass or plastic, physicists from Darmstadt discovered that these movements are reversible over time if observed from a certain angle.

The team led by Till Böhmer from the Institute for Condensed Matter Physics at the Technical University of Darmstadt published their results in Natural physics.

Glasses or plastics are made up of a tangle of molecules. The particles are in constant motion, causing them to slide into new positions again and again. They are constantly looking for a more favorable energy state, which modifies the properties of the material over time: glass ages.

However, in the case of useful materials such as window glass, this can take billions of years. The aging process can be described by what is called “material time”. Think of it this way: the material has an internal clock that ticks differently than the one displayed on the lab wall. The material’s time passes at a different rate depending on how quickly the molecules in the material rearrange themselves.

But since the discovery of the concept around fifty years ago, no one has succeeded in measuring material time. Today, Darmstadt researchers led by Professor Thomas Blochowicz have done this for the first time.

“It was a huge experimental challenge,” says Böhmer. The tiny fluctuations in the molecules had to be documented using a highly sensitive video camera. “You can’t just watch the molecules move,” adds Blochowicz.

However, the researchers noticed something. They aimed a laser at the glass sample. The molecules it contains diffuse light. The scattered beams overlap and form a chaotic pattern of light and dark spots on the camera sensor. Statistical methods can be used to calculate how fluctuations vary over time, in other words, how fast the material’s internal clock is ticking. “This requires extremely precise measurements that were only possible with state-of-the-art video cameras,” explains Blochowicz.

But it was worth it. Statistical analysis of molecular fluctuations, to which researchers from the University of Roskilde in Denmark contributed, revealed surprising results. In terms of material time, the fluctuations of molecules are reversible over time. This means they don’t change if material time is allowed to flow backwards, like the pendulum video, which looks the same when played forward and backward.

“However, this does not mean that the aging of materials can be reversed,” emphasizes Böhmer. The result rather confirms that the notion of material time is well chosen because it expresses the entire irreversible part of the aging of matter. Its ticking embodies the passage of time for the material in question.

Anything that moves in the material relative to this time scale does not contribute to aging. Just like, metaphorically speaking, children playing in the back seat of a car do not contribute to its movement.

The Darmstadt researchers believe this generally applies to disordered materials, because they looked at two classes of materials – glass and plastic – and performed a computer simulation of a model material – with the same results.

The success of physicists is only the beginning. “This leaves us with a mountain of unanswered questions,” Blochowicz says. For example, it remains to be clarified to what extent the observed reversibility in terms of material time is due to the reversibility of the physical laws of nature, or how the ticking of the internal clock differs across materials.

The researchers want to investigate further, so more exciting discoveries could be ahead.