Mar 20 – 22, 2019
Nordita, Stockholm
Europe/Stockholm timezone

Experimental support of the isomorph theory – and beyond

Mar 21, 2019, 9:00 AM
132:028 (Nordita, Stockholm)


Nordita, Stockholm


Kristine Niss (Roskilde University)


It is an old and open question what governs the dynamics of liquids. Particularly understanding the super-cooled liquids as they approach the glass transition and the characteristic time scales exceed experimentally accessible time scales is a challenge. The isomorph theory is an approximative theory, which has been shown to predict the dynamics of simple computer simulated liquids (e.g. LJ-systems) with surprisingly high precision, while it does not hold for complex systems with directional bonds or competing interactions [1]. In order to test the isomorph theory experimentally we have focused on van der Waals bonded glass-forming liquids. We have experimentally verified several predictions of the isomorph theory; the density scaling exponent can be found from single state-point thermo-mechanical measurements [2], the dielectric amplitude under pressure follows the isomorph prediction [3], isochronal lines in the P-T phase diagram are the same for different response functions [4], and the picosecond dynamics is invariant along alpha relaxation isochrones close to Tg [5]. Moreover, we have found that the dynamics of van der Waals bonded liquids with no visible beta relaxation is even simpler than what can be predicted from isomorph theory [4,6]: 1) the spectral shape of the alpha relaxation is independent of both temperature and pressure in a dynamical range of at least 10 decades, and 2) the alpha-relaxation time of different response functions, which probe different dynamical properties all follow the same temperature and pressure dependence. Based on this we propose that a basic (ideal-gas type) model of the dynamics of glass-forming liquids should encompass this simplicity in a natural way, while still exhibiting the dynamical hall-mark features; non-exponential spectral shape and non-Arrhenius temperature dependence of the alpha-relaxation time. REFERENCES [1] Dyre, J.C., Hidden Scale Invariance in Condensed Matter, J. Phys. Chem. B 118, 10007 (2014) [2] Gundermann, D. et al. Predicting the density scaling exponent from Prigogine-Defay ratio measurements, Nature Physics 7, 816 (2011) [3] Wence, X. et al .Isomorph theory prediction for the dielectric loss variation along an isochrone, J. Non Chryst Solid 407, 190 (2015) [4] Roed, L, Niss, K., Jacobsen, B. Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior glass forming molecular liquid, J. Chem. Phys. 143, 221101 (2015). [5] Hansen, H, et al. Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers, Nature Communications 9, 518 (2018) [6] Niss, K. and Hecksher, T. Perspective: Searching for simplicity rather than universality in glass-forming liquids, J. Chem. Phys. 149, 230901 (2018).

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