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School of Physics and Astronomy

New insights into the glass transition

What happens to a liquid when you cool it to obtain a glass? Despite the apparent simplicity of this question, understanding glass transition has defied the previous efforts of many theorists, and has become has become one of the "most interesting unsolved problem in solid state theory", according to eminent scientists. This is particularly exciting because we now claim to understand such exotic states of matter such as superfluidity and superconductivity, yet cooling a liquid to obtain glass comes across as conceptually simple.

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The main problem of glass transition has been to explain the origin of heat capacity jump at the glass transition temperature Tg. A glass is not different from a liquid in terms of structure, and is equally disordered, so a phase transition approach does not apply. Instead, our new theory views glass as a liquid that stops flowing at the experimental time scale. Importantly, the very fact that a liquid stops flowing means that liquid elastic and thermal properties change, with the accompanying jump of heat capacity. In this way, we reconcile the above long-lived controversy in physics.

Our theory says that from the physical perspective, glasses are not different from liquids. The difference is only quantitative, but this quantitative difference can be huge: the time it takes silica glass to flow at room temperature is longer than the age of the Universe. Our theory thereore implies that glasses are really exciting systems because they support the processes that are active over periods of time that exceed astronomical time scales. No other system is known where a dynamic property (relaxation time or viscosity in our case) can change by so many orders of magnitude.

Apart from glass transition, our theory pushes the boundary of what is known about the origin of sharp changes of system properties such as heat capacity. Until now, it was believed that this happens only if there is a phase transition. We, on the hand, have found that the same can happen in a new class of systems and phenomena in general when a system stops relaxing (flowing) on the experimental time scale. We are therefore excited to see how this theory can explain other phenomena that are similar to glass transition.

This research has been published as K. Trachenko and V. V. Brazhkin, Physical Review B 83, 014201 (2011)"