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Cracked: Scientists show how glass crystallises in real-time

Read time: 5 mins
7 Sep 2020
Cracked: Scientists show how glass crystallises in real-time

[Image Credits: Khamkéo Vilaysing on Unsplash]

Scientists capture devitrification in glass, which may take thousands of years otherwise, in movies.

One of the four fundamental states of matter is solid, and solids are all around us—from the phone on which you may be reading this to the table on which you have placed your laptop. In a crystalline solid, the molecules are arranged into repeated structures extending in all directions. Physicists refer to solids without such order in their structures as amorphous solids.

Glass is an amorphous solid we use every day. Much like a liquid, it exhibits no repetition in its molecular structure. Owing to its density, neither does it flow like liquids. Glass can transform from its amorphous to its crystalline state through a process physicists call ‘devitrification’. However, devitrification can introduce several problems. For example, the Tiffany glass windows in cathedrals would develop spots, ruining the stained glass art, had the glass devitrified due to its contact with the air.

In a new study, scientists from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, and the Indian Institute of Science (IISc), Bengaluru, have devised a unique way to observe the process of devitrification under a microscope, in real-time. Funded by the Department of Science and Technology (DST), Government of India, their study was published in the journal Nature Physics.

It is impossible to watch the dynamics of this transformation at a molecular level as the constituent particles are very small. Hence, the physicists carefully chose to work with a glass made of polymer particles suspended in water. Known as a colloidal suspension, it mimics the amorphous nature of glass, and has properties akin to glass. The particles in the colloidal suspension are ten thousand times bigger than an ordinary atom, which facilitates observing devitrification in real-time with an optical microscope.

Working with this suspension wasn’t easy! The colloid can crystallise because of external influences, like higher ambient temperature. If the table on which it is placed shook, it would automatically devitrify, leaving no room for the researchers to identify what controls devitrification. They had to painstakingly ensure no disturbance to their experimental setup, and perform numerous trials until they got it right.

“I had to try 30 different setups of the experiment, and finally, one worked,” shares Divya Ganapathi from IISc, the lead author of the study.

The researchers then used a special kind of microscope to obtain sharp images of the dense colloidal suspension, which is not possible with ordinary microscopes.

Scientists have previously studied devitrification through numerical simulations on the computer and predicted that it proceeds in two manners. The first resembles avalanches seen in snow-covered mountains. Periods of inactivity of molecules, that last over days, are broken by sudden spells of rearrangement into a crystalline structure. Each ‘avalanche’ lasts for a few hours and spreads across a few hundred molecules. The other manner of devitrification is slow and continuous crystallisation of a few particles at a time.

The researchers captured snapshot images of devitrification with a microscope, one every 20 seconds—an endeavour that takes anywhere between 68 and 130 hours!

“I spent the entire duration of the experiment in the laboratory, monitoring the setup,” shares Divya.

To their surprise, the team observed something not predicted by the earlier calculations. They found that devitrification simultaneously proceeded in both manners, in different parts of the suspension, equally. To investigate why, they again turned to the computers.

Although there is no order in the arrangement of the colloidal suspension, it did not deter them from trying to find a method in the madness. “Physicists have been unable to do this over 50 years,” says Rajesh Ganapathy, an associate professor at JNCASR and an author of the study. The researchers calculated a mathematical quantity akin to the softness of different regions in glass. This quantity, first proposed by a team of physicists in 2016, estimates the ease of movement of the constituents of the suspension. The mathematical quantity calculated in the current study was not only able to predict which regions of the glass were likely to crystallise but also the manner in which they would proceed.

“Crystallisation occurs in regions that are softer than the others,” says Ajay K. Sood, Professor at IISc, also an author of the study. By looking at a present snapshot, the researchers can now predict where the devitrification will occur days later. “The predictions strongly overlap what we see in the experiments,” says Rajesh. “The computer algorithm we developed is so good that we only need a snapshot to make the predictions,” adds Divya.

The study is the first-ever attempt at developing a molecular understanding of devitrification. “We have found that the structure in the glass has the hidden information of its future dynamics,” opines Ajay, talking about the findings.

Earlier attempts to stop devitrification were blind shots in the dark, but this finding offers the scientists a clear goal—they need to decrease the softness of glass. But, it is still not clear how they can achieve that.

“The road ahead is challenging, and a continuous synergy between theory and experiments will shed more light,” Ajay signs off.

This article has been run past the researchers, whose work is covered, to ensure accuracy.