One micron length scale controls kinetic stability of low energy glasses

Matter usually exists in one of two familiar states: a solid, where atoms are locked in place, or a liquid, where they flow freely. But there is a third, puzzling state called a glass. When a liquid cools down too quickly to form a crystal, its molecules slow to a halt while still jumbled in a disordered arrangement, much like a crowd of people frozen mid-step in a busy street. This state is not truly stable; it is a supercooled liquid trapped in a temporary holding pattern, slowly trying to find a more comfortable, lower-energy arrangement. For decades, scientists have believed that the behavior of these materials is governed by tiny, nanometer-scale movements, where small groups of molecules rearrange themselves in unison. Understanding how these glasses eventually relax into a more stable state is crucial, not just for basic physics, but for industries that rely on the long-term stability of amorphous materials, such as pharmaceuticals and advanced electronics.

A team of researchers from the United States and Germany has now discovered that the rules governing this relaxation are not just about tiny molecular clusters. Instead, they found that for a specific type of highly stable glass, the entire transformation process is controlled by a much larger distance: one micrometer. This is a thousand times larger than the nanometer scales previously thought to be the limit of such influence. By studying thin films of a drug called indomethacin, the scientists observed that the time it takes for the glass to turn back into a flowing liquid depends entirely on the thickness of the film, but only up to a certain point. Once the film becomes thicker than one micrometer, the thickness no longer matters. This unexpected crossover reveals that the glass is not a uniform block of material, but rather a landscape dotted with rare, hidden spots where the transformation begins, spaced roughly one micrometer apart.

To see this hidden structure, the researchers created films of indomethacin with thicknesses ranging from 75 nanometers to 30. They made these films by depositing the material as a vapor onto a very cold surface, a technique that packs the molecules more tightly and efficiently than standard cooling methods. This process creates a "low energy" glass, one that is exceptionally stable and sits deep in its potential energy landscape, far below the usual state of a typical glass. The researchers then placed these films on tiny, sensitive sensors capable of measuring heat capacity with extreme precision. They heated the samples to a temperature just above the point where the glass normally turns into a liquid and held them there, watching to see how long it took for the entire film to transform.

The results were striking. For the thinnest films, those under 600 nanometers, the time required to transform increased directly with the thickness. This behavior suggested a simple mechanism: the change started at the surface exposed to the air and moved steadily inward, like a wave washing over a beach. As the film got thicker, the wave had to travel further, taking more time to reach the bottom. This surface-initiated growth happened at a constant speed, moving about 0.04 nanometers every second. However, as the films grew thicker than 600 nanometers, the relationship changed. The transformation time began to level off, and for films between 1.4 and 30, the time it took to transform became completely independent of the film's thickness.

This shift in behavior points to a different mechanism taking over in the thicker samples. In these larger films, the transformation does not wait for the surface wave to travel all the way through. Instead, the change begins simultaneously at multiple locations deep inside the bulk of the material. The researchers calculated that these starting points, or "initiation sites," are spaced approximately one micrometer apart. When the film is thinner than this distance, the surface wave reaches the bottom before any internal spots can start the process. But once the film is thicker than one micrometer, these internal spots begin to grow into the surrounding glass, creating pockets of liquid that expand and eventually merge. Because these spots are so far apart, the time it takes for them to fill the entire volume depends on how fast they grow, not on how thick the film is.

The evidence for this internal spacing comes from several angles. The researchers noted that the transformation kinetics for the thickest films followed a mathematical pattern often used to describe how crystals form, suggesting that the liquid regions are growing from distinct, scattered points. Furthermore, other experiments on similar materials had already hinted at irregularities in the glass structure on a scale larger than 500 nanometers. The consistency of the one-micrometer distance across different temperatures and film thicknesses suggests that this is a fundamental structural feature of these highly stable glasses, rather than a random occurrence. It implies that even in a glass that appears perfectly uniform, there are rare, slightly less stable regions separated by a significant distance, waiting for the right conditions to trigger a change.

This discovery challenges the long-held view that glass behavior is dominated solely by nanometer-scale processes. While the initial movement of molecules happens on a tiny scale, the overall stability and transformation of these special glasses are dictated by a much larger, micrometer-scale architecture. The researchers propose that these initiation sites could be tiny defects in the molecular packing, or perhaps regions where the molecules are slightly more mobile than their neighbors. Although they cannot yet pinpoint exactly what these sites are, the clear measurement of their spacing provides a new way to think about the internal structure of glasses. It suggests that the stability of a glass is not just a matter of how tightly its molecules are packed, but also of how far apart the weak points are distributed.

The implications of this finding extend beyond the specific drug studied. The researchers believe that this one-micrometer length scale might be a general feature of highly stable glasses, regardless of how they are made. If true, it means that the durability and shelf-life of many amorphous materials could be influenced by the distance between these hidden initiation sites. For materials that are even more stable than the ones tested here, this distance could potentially be even larger, perhaps reaching ten micrometers or more. The study opens a new window into the hidden geography of glasses, showing that even in a material that looks the same everywhere, there is a larger-scale structure waiting to be discovered.