Associate Professor:

Kristine Niss


Do simple liquids exist?

"Isochronal jumps in the phase diagram studied by neutron scattering"

Experimental studies of the dynamics in molecular liquids from pico- to kilosecond as a function of temperature and pressure.

A project funded by DFF Sapere Aude : Starting Grant

The aim of our research

This project is a part of the Glass and Time.

The glass transition is a universal phenomenon which in its essence is independent of the interactions between the atoms or molecules of the liquid forming the glass. The universal character of the transition has led physicists to believe that there is one underlying simple explanation of the phenomenon. More than 15 years ago the Nobel laureate P.W. Anderson stated that understanding the glass-transition is deepest unsolved problem in solid state physics [1], but a satisfactory explanation is still lacking. Understanding the glass-transition implies understanding the dynamics (eg. diffusion, vibration, relaxation) and the physics in general of the viscous liquid forming the glass. Gasses can to a great extend be understood via the ideal gas model where the interactions between particles are ignored. In crystalline solids the physics can be understood by exploding the periodic nature of the structure. No similar simplification can be made for liquids. Here we need to consider the interaction between the molecules, the kinetic energy and the amorphous structure. This is a challenge to a model systems or a theory explaining the glass transition and the physics of liquids in general.
The "Glass and Time" group on RUC has during the last five years succeeded in identifying a class of simple liquids and a theory for describing these has been developed [2]. This theory, called the isomorph theory, has proven very effective in describing computer simulated liquids with simple interactions (e.g. Lennard Jones liquids). But how useful is the theory when it comes to understanding the physics the liquids we deal with in the laboratory? In 2011 we published the first experimental results supporting the isomorph theory [3]. Here we showed how the relative effect of density and temperature on the glass transition can be understood in terms of the isomorph theory. There are several other predictions of the theory, many of these are directly related to the glass transition. Our current and future experimental work aims at testing these predictions.
The tools used are mainly dielectric spectroscopy and neutron scattering. The control of temperature as well as pressure plays an important role in all our studies.

[1] Science 267, 1609 (1995)
[2] J. Chem. Phys. 131, 234504 (2009)
[3] Nature Physics 7, 816 (2011)