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A breakthrough in decoding the mystery of glass formation

A team of Bangalore-based researchers may well have made big strides in answering an age old question in physics: how is glass formed? In a carefully crafted experiment the researchers have gained deep insights into the formation of glass which were in speculation stage since late 1960s. Their findings are published in Nature Physics.

“Our work constitutes the first experimental study that distinguishes between two prominent competing theories of glass formation”, says Prof Ajay Sood of Department of Physics, Indian Institute of Science. He is a senior member of the group which carried out the study.

The four member team included Prof Ganapathy of the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Mr Shreyas Gokhale, and Ms Hima Nagamanasa. Mr Shreyas and Ms Hima Nagamanasa are graduate students working under the guidance of Prof Sood and Prof Ganapathy respectively.

Glass, though known for its transparency, is also famous for not revealing its inner secrets. It is a solid on the outside, and resembles a 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, 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. This bizarre state of affairs is glass.

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.

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.

The IISc-JNCASR team have experimentally shown, 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.

“Our findings show that the glass transition is fundamentally thermodynamic in origin, a fact hitherto unestablished”, says Prof Ganapathy on the significance of their findings.

However, pinning the ever-moving colloidal particles is a challenge. “The main challenge lies in realizing a wall of frozen particles and sustaining it over long periods of time to study its influence on glassy dynamics. This involves pinning hundreds of particles in a particular configuration simultaneously for several hours. We overcame this difficulty through a marriage of holographic optical tweezers and video microscopy”, say Hima Nagamanasa and Shreyas Gokhale.

The group wants to further understand the physics of glass formation. Prof Sood says, “We would like to go further. We are on the right track to unravel the deep mystery of glass formation”.

 

About the authors:

Ajay Sood is a Professor at the Department of Physics, Indian Institute of Science, Bangalore. Rajesh Ganapathy is a Professor at the International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore. Shreyas Gokhale and Hima Nagamanasa are PhD students working under the guidance of Ajay Sood and Rajesh Ganapathy respectively.

 

Contact:

Prof Ajay Sood. Email: asood@physics.iisc.ernet.in, Phone: 080-2360 2238

Prof Rajesh Ganapathy. Email: rajeshg@jncasr.ac.in, Phone: 080-2208 2572

 

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physics