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 . 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 , the dielectric amplitude under pressure follows the isomorph prediction , isochronal lines in the P-T phase diagram are the same for different response functions , and the picosecond dynamics is invariant along alpha relaxation isochrones close to Tg . 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  Dyre, J.C., Hidden Scale Invariance in Condensed Matter, J. Phys. Chem. B 118, 10007 (2014)  Gundermann, D. et al. Predicting the density scaling exponent from Prigogine-Defay ratio measurements, Nature Physics 7, 816 (2011)  Wence, X. et al .Isomorph theory prediction for the dielectric loss variation along an isochrone, J. Non Chryst Solid 407, 190 (2015)  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).  Hansen, H, et al. Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers, Nature Communications 9, 518 (2018)  Niss, K. and Hecksher, T. Perspective: Searching for simplicity rather than universality in glass-forming liquids, J. Chem. Phys. 149, 230901 (2018).