Inside the obscure world of glass

Did you know the rigid glass covering your window is liquid on the inside? This
mysterious property of glass – its internal structure resembling a liquid – has puzzled scientists for more than half a century.

Glass has a variety of uses: it allows light into our homes, makes up for defects in our eyesight, is ubiquitous on our phones and makes beautiful cutlery. You can have crack proof ‘gorilla glass’ on your smartphone on the one hand, and the windshield glass of your car cracking in a spider web pattern without producing shards, on the other.

“Knowing the secret behind the formation of glass can help us design glass more scientifically,” said Ajay Sood, professor of Physics, Indian Institute of Science (IISc), Bengaluru. Ajay, along with Rajesh Ganapathy, professor of Physics at the Jawaharlal Nehru Centre for Advanced Scientific Research (JANCASR), and their PhD students Shreyas Gokhale, Hima Nagamanasa and Chandan Misra have achieved breakthroughs in solving the mystery of glass formation.

Dual nature
Glass, though known for its transparency, is also famous for not revealing its inner secrets. It is solid on the outside, and resembles liquid on the inside. Of course, scientists have proposed different theories to explain this ‘split personality’, but the debate is still on. Without sufficient experimental observation to back up a single theory, the mysterious state of glass continues to evade our understanding.

Liquids become solids when cooled below, certain temperature. During this transition, the jiggling liquid molecules settle down to a structure, just like school children who settle down at their respective places when a teacher appears. Most of the solids, from metals to table salt, have a well-defined, repetitive structure. However, when some liquids are cooled rapidly, they end up in a mixed state: they become solids at the macro level, but their molecules get frozen randomly, without forming repetitive structures, which is what happens to glass.

This is a peculiar state: the molecules inside are still like those in a liquid, but the material as a whole is more like a solid. Interestingly, given enough time, even wine glasses will flow, though they are not as fluent as the beautiful red wine they contain.

Over the years, two broad approaches have emerged to explain this strange phenomenon. One approach claims that the glass formation is associated with a kind of thermodynamic phase transition. In physics jargon, this approach is called Random First Order Transition (RFOT). The other approach, called the Dynamical Facilitation, is built on the premise that glassy dynamics is governed by the concerted motion of small mobile defects, and there is no thermodynamic phase transition.

Both the propositions are very difficult to test for a simple reason: we can not zoom into the internal world of glasses and see what is really happening at the level of individual molecules. That world is too small to be seen with our devices.

Testing theories
Ajay and his team conducted two experiments to test the assumptions of both the theories. In one experiment, they suspended rugby ball-like objects, which are about one thousandth of a millimetre long, in water, and studied their motion. “Each of these tiny rugby balls can be thought of as a substitute for a molecule, and a concentrated suspension of these tiny particles behaves very much like glass,” reveals Shreyas Gokhale.
By making some careful measurements, the researchers were able to show that dynamic facilitation indeed plays a significant role in glass formation. “There are two different things here – things that can rotate, and things that can move. Things that rotate (like a rod) will have a significant effect on the formation of glass,” Prof Ajay Sood explains. “This is an important result. It’s not just that particles freeze in place; what we have shown is, the orientation (rotational angle) of these particles is equally important.” After testing the validity of the dynamical facilitation theory, researchers conducted an experiment to test RFOT. According to RFOT, molecules of a glass-forming liquid rearrange collectively in the form of clusters.

The shape as well as size of these clusters, known as cooperatively rearranging regions (CRRs), change with temperature. At high temperatures CRRs are small and look like strings, whereas at low temperatures close to the glass transition, they are large and look like balls. Also, two lengths scales, point-to-set length and dynamic correlation length, vary on approaching the glass transition. While the former grows continuously, the latter grows initially, but then shows a dip when CRRs go from being stringy to spherical.

Great results
The IISc-JNCASR team experimentally showed, for the first time, that these predictions are indeed true. They managed this feat by simultaneously pinning a large number of particles of a colloidal glass-forming liquid by manipulating light fields.

Of course, the most challenging part of the experiment was to pin the ever moving colloidal particles into one place.  “Our findings show that the glass transition is fundamentally thermodynamic in origin, a fact hitherto unestablished,” says Rajesh.

With two breakthrough experiments relating to the formation of glass, and a few more in the pipeline, the IISc – JANCASR team is well poised to gain a few more deep insights into glass formation.  “We would like to go further. We are on the right track to unravel the deep mystery of glass formation,” says Ajay.