It's a persistent fallacy that glass already is a liquid, spread by misinformed high school teachers and tour guides. But that's not technically true – glass is an amorphous solid. Normally when a substance transitions from a liquid to a solid, the formerly free-flowing atoms line up into a rigid crystal formation. That's not the case with glass though: its atoms "freeze" in their disordered state.
Or at least, that's how it usually goes. In the new study, the researchers discovered a form of glass where the atoms exhibit a complex behavior that's never been seen in bulk glass before. Essentially, the atoms can move but aren't able to rotate.
The team made this discovery in a model system of colloidal suspensions. These mixtures are made up of large solid particles suspended in a fluid, making it easier for scientists to observe the physical behavior of atoms or molecules. Normally these particles are spheres, but for this experiment the team used elliptical ones so they could tell which direction they were pointing.
The researchers tested different concentrations of particles in the fluid, tracking how well they could move and rotate. Eventually they found that at higher concentrations, the particles blocked each other from rotating, but they could still move, forming a liquid glass state.
"At certain particle densities orientational motion froze whereas translational motion persisted, resulting in glassy states where the particles clustered to form local structures with similar orientation," says Andreas Zumbusch, lead author of the study.