Speaker
Kristine Niss
(Roskilde University)
Description
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).
