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Optical Pinning of Molecules for the Study of Glass Transition – COMET

Submission summary

Understanding the true nature of the glass state remains one of the most challenging problems in condensed matter physics. Although we use glasses in a multitude of applications, at present it is not even clear whether or not a glass is just an amorphous structure with relaxation times that are so large that for all practical purposes the material can be considered as a solid, or whether a glass is a real thermodynamic equilibrium state. Some theories predict that this equilibrium state will be reached via a first order transition at the so-called Kauzmann temperature TK at which the liquid transforms into a so-called ideal glass state, but other theories challenge this. Finding the answer to this seemingly simple question is hampered by the fact that with decreasing temperature the relaxation times of the glass-forming liquid becomes so large that the system falls out of equilibrium, hence making it impossible to probe the equilibrium behavior of the system at low temperatures.

In recent years theoretical advances have suggested a possibility how this falling out of equilibirum problem might be avoided: The idea is to immobilize in the liquid a concentration c of the particles – a procedure referred to as “pinning” – and to study how the relaxation times and other thermodynamic quantities depend on this concentration. Pinning unveils features otherwise not accessible in usual glasses, constrained at c = 0. In particular, the thermodynamic transition – if any –becomes a Kauzmann line TK(c) with specific and discriminating signatures in the relaxation time map tau(T,c), which become measurable before the system falls out-of-equilibrium.

The goal of the COMET project is to test experimentally these theoretical predictions on a molecular glass-former, and thus to achieve the first pinning experiment on an atomic system. Our results will thus help to understand whether or not the slow dynamics of glass-forming systems is related to an underlying thermodynamic phase transition. In our approach, triazine-based glass-forming organic molecules will be mixed with a fraction c of “switch” molecules, obtained from the glass formers by means of a small chemical modification which makes them optically active. The orientational degree of freedom of the switch molecules will be frozen by an external light source, thus allowing the experimental realization of the molecular pinning experiment. Using dielectric spectroscopy we will be able to measure the relaxation dynamics of the system in the T-c plane. The experimental data, supported by computer simulations, will allow us to infer whether or not there is indeed a TK(c) line.

Combining an original design of a model molecular glass-former, a novel methodology and state-of-the art theory and simulations, the COMET project will thus provide the first realization of the ideal experiment proposed in the literature to probe the existence of a thermodynamic glass transition.




Project coordination

Francois LADIEU (CEA-SPEC(Service de Physique de l'Etat Condensé))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CEA/Iramis/SPEC/SPHYNX CEA-SPEC(Service de Physique de l'Etat Condensé)
Université de Montpellier Laboratoire Charles Coulomb, Université Montpellier
CEA/Iramis/NIMBE/LIONS CEA-Laboratoire NIMBE (Nanoscience et Innovation pour les Matériaux , la Biomédecine et l'Energie

Help of the ANR 380,817 euros
Beginning and duration of the scientific project: November 2015 - 48 Months

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