“Liquid glass” may sound like a flashy new lipgloss or a slick decorative material, but it’s actually something much more substantive: a new state of matter.
Twenty years after the existence of liquid glass was first predicted, scientists have now created and observed this novel form of matter, a discovery that will open new windows into the bizarre properties of glass.
A team co-led by Matthias Fuchs and Andreas Zumbusch, professors at the University of Konstanz in Germany, were able to observe “hitherto unknown structures” that “give insight into the glass transition and reveal an additional state of matter,” according to a new paper published in Proceedings of the National Academy of Sciences.
“When we started our experiments, we did not plan to find the liquid glass state,” said Zumbusch in an email. “Initially, our plan was to study the behaviour of small (~1-10 micrometers in diameter) plastic particles in a liquid.”
Scientists often use these mixed substances, known as “colloids,” to shed light on complicated phenomena inside materials, such as glass. Colloids contain molecules in different phrases of matter: For instance, jams and jellies are colloids in which solid particles of fruit are suspended in gelatinous pectin and liquid water.
Because particles inside colloids are visible to specialized microscopes, they are used as stand-ins for the atomic processes that might be occurring inside a material on much smaller scales.
“Systems of this type have long been investigated by many research groups for two reasons,” Zumbusch said. “Many important natural systems are similar in composition: think of milk, blood, or clay.”
“Also, optical microscopy of such well-defined colloidal particles gives us important insights into complicated phenomena,” he added. “Arguably, the most intriguing of these is the glass transition.”
It’s difficult to imagine a world without glass, given that it is a key component in everything from common cookware to sophisticated telescopes. But despite the abundance of glass in nature, and the myriad glass-making techniques pioneered over millennia, there are still many unanswered questions about this versatile material’s core properties.
One of the characteristics that distinguishes glass from other substances is that it does not neatly form crystalline structures as it hardens into a solid. Instead, glass preserves particles in a disordered mix of orientations due to processes that have yet to be fully explained.
“Despite intense research efforts over many decades, little is still known about what is going on at the glass transition,” Zumbusch said. “The systems of colloids in a liquid are exciting, because they also form glasses. In contrast to other systems, however, in these we can directly watch and analyze the motion of thousands of individual particles.”
Past experiments with glass have used colloids containing spherical solid particles, which don’t have any clear orientation because of their symmetrical shape. Fuchs, Zumbusch, and their colleagues changed this formula by manufacturing plastic particles with ellipsoid forms that more closely align with the asymmetric molecules found in nature.
“Dense suspensions of spherical particles have therefore been studied in detail over the last twenty years,” explained Zumbusch. “Perfect spheres, however, are rarely found in nature. The starting point of our experiments was pure curiosity: what would change in the systems’ structure and dynamics if we deformed the particles to ellipsoids?”
With the help of confocal microscopes, the team observed the behavior of the particles inside a solvent. At a certain density limit, the asymmetric particles stopped rotating, though they were still able to move around and cluster together.
“For a certain concentration range, the particles can change their positions freely, but cannot change their orientation: a liquid glass,” Zumbusch said. “The particles behave like a swarm of fish in which each fish can swim back and forth but cannot not turn left or right. This intriguing type of correlation of the particles’ motion is striking: whereas the particle orientations are locked to each other, their positions are independent.”
“Intriguingly, global liquid crystalline order is prevented by the intertwined and ramified clusters of aligned particles,” he continued. “One can suspect that a behaviour of this type lies at the heart of the glass transition in many systems where irregular shape prevents crystalline order.”
This capacity to travel, but not spin, is the hallmark of this new liquid glass state of matter, which is “expected to also have implications in liquid crystal formation,” according to the study.
The team was surprised that their colloid revealed such unexpected details about the glass, which inspired them to scour past studies for more context. That’s how the researchers discovered a study published in 2000 that laid out a theoretical prediction of liquid glass that “did not get the attention it deserved, simply because suitable particles for experiments had not been available up to our work,” Zumbusch said.
Still, he noted that the specific structures in the colloids, in which similarly-oriented particles formed clusters that were intersected by randomly-oriented particles, were neither predicted nor observed before the team’s study. Likewise, these alignments could not be reproduced by the team’s computer simulations.
“The type of behavior observed for the [micrometer] sized particles might shed light on phenomena on vastly differing length scales, ranging from the ordering of molecules in liquid crystal devices to structure formation on astronomic length scales,” Zumbusch said.
“Our own immediate interest is now directed at trying to understand whether similar phenomena occur when we do not observe particles which are thermally agitated but which are actively driven in a flow,” he concluded. “This would be of tremendous importance for anything flowing in tubes, from blood flow, over chocolate bar production, to pumping concrete.”
Update: This article has been updated to include comments from Andreas Zumbusch.