Researchers at the Technical University of Darmstadt are investigating the ageing processes in materials. For the first time, they have measured the ticking of an internal clock in glass. When evaluating the data, they discovered a surprising phenomenon.
When researching the movements of molecules in glass or plastic, physicists from Darmstadt have now discovered that these movements are time-reversible if they are viewed from a certain perspective. The team led by Till Böhmer at the Institute for Condensed Matter Physics at the Technical University of Darmstadt has now published its results in Nature Physics.
Glasses or plastics consist of a tangle of molecules. The particles are in constant motion, causing them to slip into new positions again and again. They are permanently seeking a more favorable energetic state, which changes the material properties over time—the glass ages. In useful materials such as window glass, however, this can take billions of years. The ageing process can be described by what is known as the “material time.” Imagine it like this: the material has an internal clock that ticks differently to the clock on the lab wall. The material time ticks at a different speed depending on how quickly the molecules within the material reorganize.
The researchers noticed something as they directed a laser at the sample made of glass. The molecules within it scatter the light. The scattered beams overlap and form a chaotic pattern of light and dark spots on the camera’s sensor. Statistical methods can be used to calculate how the fluctuations vary over time—in other words, how fast the material’s internal clock ticks. “This requires extremely precise measurements which were only possible using state-of-the-art video cameras,” says Blochowicz.
But it was worth it. The statistical analysis of the molecular fluctuations, which researchers from Roskilde University in Denmark helped with, revealed some surprising results. In terms of material time, the fluctuations of the molecules are time-reversible. This means that they do not change if the material time is allowed to tick backwards, similar to the video of the pendulum, which looks the same when played forwards and backwards.
“However, this does not mean that the ageing of materials can be reversed,” emphasizes Böhmer. Rather, the result confirms that the concept of material time is well chosen because it expresses the entire irreversible part of the ageing of the material. Its ticking embodies the passage of time for the material in question. Everything else that moves in the material in relation to this time scale does not contribute to ageing. Just as, metaphorically speaking, children playing around in the back seat of a car do not contribute to its movement.
The Darmstadt researchers believe that this generally applies to disordered materials, as they examined two classes of material—glass and plastic—and carried out a computer simulation of a model material—with the same results.
Image – Glass building, Wilhelmstrasse 65 in Berlin-Mitte. Courtesy of Wikipedia Commons.
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