3 research outputs found
Universal Structural Relaxation in Supercooled Liquids
One of the hallmarks of molecular dynamics in deeply supercooled liquids is
the non-exponential character of the relaxation functions. It has been a long
standing issue if 'universal' features govern the lineshape of glassy dynamics
independent of any particular molecular structure or interaction. In the paper,
we elucidate this matter by giving a comprehensive comparison of the spectral
shape of depolarized light scattering and dielectric data of deeply supercooled
liquids. The light scattering spectra of very different systems, e.g. hydrogen
bonding and van der Waals liquids but also ionic systems, almost perfectly
superimpose and show a generic lineshape of the structural relaxation,
approximately following a high frequency power law . However,
the dielectric loss peak shows a more individual shape. In systems with low
dipole moment generic behavior is also observed in the dielectric spectra,
while in strongly dipolar liquids additional crosscorrelation contributions
mask the generic structural relaxation
Intermolecular Cross-Correlations in the Dielectric Response of Glycerol
We suggest a way to disentangle self- from cross-correlation contributions in
the dielectric spectra of glycerol. Recently it was demonstrated for
monohydroxy alcohols that a detailed comparison of the dynamic susceptibilities
of photon correlation and broadband dielectric spectroscopy allows to
unambiguously disentangle a collective relaxation mode known as the Debye
process, which could arises due to supramolecular structures, and the
-relaxation, which proves to be identical in both methods. In the
present paper, we apply the same idea and analysis to the paradigmatic glass
former glycerol. For that purpose we present new light scattering data from
photon correlation spectroscopy measurements and combine these with literature
data to obtain a data set covering a dynamic range from Hz.
Then we apply the above mentioned analysis by comparing this data set with a
corresponding set of broadband dielectric data. Our finding is that even in a
polyalcohol self- and cross-correlation contributions can approximately be
disentangled in that way and that the emerging picture is very similar to that
in monohydroxy alcohols. This is further supported by comparing the data with
fast field cycling NMR measurements and dynamic shear relaxation data from the
literature, and it turns out that, within the described approach, the
-process appears very similar in all methods, while the pronounced
differences observed in the spectral density are due to a different expression
of the slow collective relaxational contribution. In the dielectric spectra the
strength of this peak is reasonably well estimated by the Kirkwood correlation
factor, which supports the view that it arises due to dynamic
cross-correlations, which were previously often assumed to be negligible in
dielectric measurements