Researchers report new state of affairs described as ‘liquid glass’

Discovery of liquid glass sheds light on the old scientific problem of the glass transition: An interdisciplinary team of researchers from the University of Konstanz discovered a new state of matter, liquid glass, with previously unknown structural elements – new insights into the nature of glass and its transitions.

Although glass is a truly ubiquitous material that we use daily, it is an important scientific mystery. Contrary to what one might expect, the true nature of glass remains something of a mystery, with scientific research into its chemical and physical properties. In chemistry and physics, the term glass itself is a variable term: it contains the substance we know as window glass, but it can also refer to a range of other materials with properties that can be explained by glassy behavior, including for example metals, plastics, proteins and even biological cells.

While it may give the impression, glass is anything but solid. Usually, when a material transitions from a liquid to a solid state, the molecules line up to form a crystal pattern. In glass it does not happen. Instead, the molecules are effectively frozen before they crystallize. This strange and disordered condition is characteristic of a spectacle in different systems and scientists are still trying to understand exactly how this metastable condition forms.

A new state of matter: liquid glass

Research led by professors Andreas Zumbusch (Department of Chemistry) and Matthias Fuchs (Department of Physics), both based at the University of Konstanz, added just one more layer of complexity to the glass problem. Using a model system that includes suspensions of custom ellipsoidal colloids, the researchers discovered a new state of matter, liquid glass, where individual particles are able to move but are still unable to rotate – complex behavior not previously observed in large glasses. The results are presented in the Proceedings of the National Academy of Sciences.

Colloidal suspensions are mixtures or liquids containing solid particles, which are one micrometer (one millionth of a meter) or larger in size than atoms or molecules and are therefore well suited for examination by optical microscopy. It is popular among scientists studying glass transitions because it contains many of the phenomena that also occur in other glass-forming materials.

Custom ellipsoidal colloids

To date, most experiments with colloidal suspensions have been dependent on spherical colloids. However, most natural and technical systems consist of non-spherical particles. Using polymer chemistry, the team led by Andreas Zumbusch produced, stretched and cooled small plastic particles until they reached their ellipsoid shapes and then placed them in a suitable solvent. “Because of their different shapes, our particles have an orientation – as opposed to spherical particles – that gives rise to completely new and previously unexplored types of complex behaviors,” explains Zumbusch, a professor of physical chemistry and senior author of the study.

The researchers then changed particle concentrations in the suspensions and tracked the translational and rotational motion of the particles using confocal microscopy. Zumbusch says, “At certain particle densities, orientation motion froze while translational motion continued, leading to glassy conditions where the particles clustered to form local structures with the same orientation.” What the researchers call liquid glass is the result of these groups interfering with each other and mediating characteristic spatial correlations over long distances. This prevents the formation of a liquid crystal that would be the globally ordered state of matter expected from thermodynamics.

Two competing glass transitions

What the researchers observed were, in fact, two competing glass transitions – a regular phase transformation and a non-equilibrium phase transformation – that interact. “It’s incredibly interesting from a theoretical perspective,” says Matthias Fuchs, professor of soft condensation theory at the University of Konstanz and the other senior writer at the paper. “Our experiments provide the kind of evidence for the interaction between critical fluctuations and glassy arrest that has plagued the scientific community for some time.” A prediction of liquid glass remained a theoretical conjecture for twenty years.

The results further suggest that similar dynamics may be at work in other glass-forming systems and thus may help to shed the behavior of complex systems and molecules, ranging from the very small (biological) to the very large (cosmological). It can also affect the development of liquid crystal devices.