Glass is, by nature, random. It is created by melting several minerals together at unfathomably high temperatures. Glass has a haphazard, disorganized structure, like a liquid frozen in time. However, by some happy accident, some scientists have created glass with a regular molecular pattern.
The first inkling the scientists had that something was different about the glass they'd created came from some consistent peaks in their spectroscopic data. The researchers measured how much the glass polarized – changed the orientation of – a beam of laser light. The beam's interaction with glass should produce a featureless graph where all the results are random. But instead, regular peaks appeared. This might sound unremarkable, but is a reliable indicator of a periodic arrangement of molecules. You can see the research in the Proceedings of the National Academy of Sciences.
"This is a big surprise," commented Juan de Pablo, from the University of Chicago. "Randomness is almost the defining feature of glasses. At least we used to think so. What we have done is to demonstrate that one can create glasses where there is some well-defined organization. And now that we understand the origin of such effects, we can try to control that organization by manipulating the way we prepare these glasses."
The secret to this molecular order seems to be the way that this special glass was created. Instead of cooling flowing liquid glass to a solid, this glass was formed using a glass vapor. The technique gently builds up layers of solid glass by depositing a vapor of organic molecules onto a surface, all regulated in a vacuum environment. Growing glass layer by layer is precise. It effectively traps the molecules in their "true" orientation.
The process is highly temperature dependent. The temperature has to be within the narrow range where liquid vapor can solidify. The order can be tuned by changing the temperature while the glass is precipitated, but once the glass has set it will retain molecular order even during temperature changes.
"Glasses are one of the least understood classes of materials," remarked de Pablo. "They have the structure of a liquid – disorder – but they're solids. And that's a concept that has mystified people for many decades. So the fact that we can now control the orientation of these disordered materials is something that could have profound theoretical and technological implications. We don't know what they are yet – this is a new field of research and a class of materials that didn't exist before. So we're just at the beginning."
Even though only a small number of molecules were oriented differently in this new glass, the effects are significant. The next step for de Pablo and the team will be to try to enhance this regularity and test different materials.