Before diving into the mystery of glass, theoretical physicist Corentin Laudicina takes us back to high school physics for a moment. Although he has spent years studying exactly what happens in a material during the glass transition, he also understands that his research is not the easiest thing to explain off the cuff at the cafeteria table.

According to school textbooks, matter can exist in three different states: gas, liquid, and solid. Think of water vapor, liquid water, and ice. The molecules that make up the material are the same, but the way they can move differs greatly, Laudicina explains.

He pulls out his dissertation and shows a figure from the introductory chapter. “The higher the temperature, the more freely the molecules can move. In a solid, they are arranged in a crystal lattice – in a fixed position, at a fixed distance from one another – but in a liquid, they can move without having a fixed position relative to each other. In the gas phase, those movements are even freer.”

Liquid drinking glass

But in addition to these three familiar phases, there is also a fourth phase that a material can enter when it is cooled quickly enough: the glass phase. And there is something unusual about it, Laudicina explains.

“The strange thing about a material in the glass phase is that it behaves like a solid, while its internal structure is more like that of a liquid. There is no rigid crystal lattice, but rather a disordered structure. So, in a sense, a drinking glass is liquid.”

To better understand the glass phase, Laudicina and his colleagues from the Soft Matter & Biological Physics group therefore study specifically what happens when a liquid is cooled. They focus on a material’s viscosity, or, in other words, how thick and slow flowing it is.

“As the temperature drops, the molecules become progressively less able to move. Around the glass transition, however, the viscosity increases extremely rapidly – much faster than you would expect. And yet the material’s internal structure barely changes. That is the core of the mystery of glass.”

Complex computer simulations

On his laptop, Laudicina shows the livestream of a famous experiment conducted by the University of Queensland in Australia. We see an inverted funnel containing a motionless black substance.

“This pitch-drop experiment shows how viscous a liquid can become. Pitch – a viscous component of asphalt – is rock-hard at room temperature. But it is actually an extremely viscous liquid. It is so viscous that, on average, only one drop falls from the funnel every ten years.”

To understand why materials behave so differently during the glass transition from what you would expect based on their structure, you need to be able to track the movement of millions of particles over long periods of time. A microscope cannot do this directly. So theoretical physicists – who, according to Laudicina, are primarily “striving for simplicity” – turn to computer simulations.

“We reduced the molecular complexity of real liquids to a model liquid: a system of perfectly spherical particles that interact with one another in relatively simple ways.”

Using these simulations – which were so complex that he ran them through the TU/e Supercomputing Center – Laudicina was able to show that major changes in viscosity are related to particles moving heterogeneously.

Crowded festival

Not all particles in a supercooled liquid move at the same speed: some barely move at all, while others form groups that move together.

“A bit like at a festival, where, in the crowded areas, everyone has to move a little if you want to get a beer at the back of the field.”