## Abstract

The overall theme of this work has been to experimentally test the shoving model and isomorph theory related to the dynamics of glass-forming liquids, both of which, rather than being universal explanations, are expected to work in the simplest case. We test the connection between fast and slow dynamics in light of the shoving model from the temperature dependence of the mean-squared displacement from neutron scattering at nanosecond timescale and the elastic modulus from shear mechanics. We find the fast dynamics to correlate with the alpha relaxation time and fragility in agreement with predictions from the shoving model. The shoving model is tested on three liquids with simple dynamic behaviour in two versions, one formulated in terms of the nstantaneous elastic modulus and one expressed in terms of the mean-squared displacement. We also test the underlying assumption connecting the two versions, directly relating the temperature dependence of the mean-squared displacement and that of the shear modulus. In the viscous liquid, we find this to hold. We interpret the discrepancy at higher temperatures where the mean-squared displacement has a stronger temperature dependence than the shear modulus, as the alpha relaxation entering the neutron instrument window. In the view of the shoving model, the short-time properties govern the viscous slowing down.

We have developed a new sample cell for doing simultaneous dielectric and neutron spectroscopy at elevated pressure. This new high-pressure cell allows us to do experiments with high accuracy. From the dielectric signal, we can determine the alpha relaxation time fast and with high precision in a large area of the temperaturepressure phase diagram while studying nano- and picosecond dynamics from neutron spectroscopy.

We use the new sample cell to locate isochrones, i.e. lines of constant alpha

relaxation time in temperature and pressure with the purpose of testing isomorph theory on three systems, two simple van der Waals and a hydrogen bonded liquid.

We find density scaling and isochronal superpositioning to hold for all three systems on alpha relaxation dynamics, and for the two van der Waals liquids, also when we have separation of timescales, i.e. the alpha relaxation is not contributing to the picosecond dynamics. The concept of isomorphs is observed to break down in two cases for the hydrogen bonding system: in density scaling of intramolecular motion and in isochronal superposition of the picosecond dynamics when there is separation of timescales. We show for one of the van der Waals liquids how the picosecond dynamics can be expressed as a function of the alpha relaxation time in agreement with the prediction of the existence of a one-dimensional phase diagram from isomorph theory, where one parameter is believed to control all dynamics.

We have developed a new sample cell for doing simultaneous dielectric and neutron spectroscopy at elevated pressure. This new high-pressure cell allows us to do experiments with high accuracy. From the dielectric signal, we can determine the alpha relaxation time fast and with high precision in a large area of the temperaturepressure phase diagram while studying nano- and picosecond dynamics from neutron spectroscopy.

We use the new sample cell to locate isochrones, i.e. lines of constant alpha

relaxation time in temperature and pressure with the purpose of testing isomorph theory on three systems, two simple van der Waals and a hydrogen bonded liquid.

We find density scaling and isochronal superpositioning to hold for all three systems on alpha relaxation dynamics, and for the two van der Waals liquids, also when we have separation of timescales, i.e. the alpha relaxation is not contributing to the picosecond dynamics. The concept of isomorphs is observed to break down in two cases for the hydrogen bonding system: in density scaling of intramolecular motion and in isochronal superposition of the picosecond dynamics when there is separation of timescales. We show for one of the van der Waals liquids how the picosecond dynamics can be expressed as a function of the alpha relaxation time in agreement with the prediction of the existence of a one-dimensional phase diagram from isomorph theory, where one parameter is believed to control all dynamics.

Translated title of the contribution | Dynamik i glasdannende væsker: Vil teori og eksperiment nogensinde mødes? |
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Original language | English |

Place of Publication | Roskilde |
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Publisher | Roskilde Universitet |

Number of pages | 189 |

Publication status | Published - 11 Jan 2018 |

Series | Tekster fra IMFUFA |
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Number | 507 |

ISSN | 0106-6242 |