35 research outputs found
Connection between slow and fast dynamics of molecular liquids around the glass transition
The mean-square displacement (MSD) was measured by neutron scattering at
various temperatures and pressures for a number of molecular glass-forming
liquids. The MSD is invariant along the glass-transition line at the pressure
studied, thus establishing an ``intrinsic'' Lindemann criterion for any given
liquid. A one-to-one connection between the MSD's temperature dependence and
the liquid's fragility is found when the MSD is evaluated on a time scale of
approximately 4 nanoseconds, but does not hold when the MSD is evaluated at
shorter times. The findings are discussed in terms of the elastic model and the
role of relaxations, and the correlations between slow and fast dynamics are
addressed.Comment: accepted by Phys Rev E (2010
On the correlation between fragility and stretching in glassforming liquids
We study the pressure and temperature dependences of the dielectric
relaxation of two molecular glassforming liquids, dibutyl phtalate and
m-toluidine. We focus on two characteristics of the slowing down of relaxation,
the fragility associated with the temperature dependence and the stretching
characterizing the relaxation function. We combine our data with data from the
literature to revisit the proposed correlation between these two quantities. We
do this in light of constraints that we suggest to put on the search for
empirical correlations among properties of glassformers. In particular, argue
that a meaningful correlation is to be looked for between stretching and
isochoric fragility, as both seem to be constant under isochronic conditions
and thereby reflect the intrinsic effect of temperature
The consequence of excess configurational entropy on fragility: the case of a polymer/oligomer blend
By taking advantage of the molecular weight dependence of the glass
transition of polymers and their ability to form perfectly miscible blends, we
propose a way to modify the fragility of a system, from fragile to strong,
keeping the same glass properties, i.e. vibrational density of states,
mean-square displacement and local structure. Both slow and fast dynamics are
investigated by calorimetry and neutron scattering in an athermal
polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene
considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer
have the same heat capacity in the glass and in the liquid, their fragilities
differ strongly. This difference in fragility is related to an extra
configurational entropy created by the mixing process and acting at a scale
much larger than the interchain distance, without affecting the fast dynamics
and the structure of the glass
Colloids in light fields: particle dynamics in random and periodic energy landscapes
The dynamics of colloidal particles in potential energy landscapes have
mainly been investigated theoretically. In contrast, here we discuss the
experimental realization of potential energy landscapes with the help of light
fields and the observation of the particle dynamics by video microscopy. The
experimentally observed dynamics in periodic and random potentials are compared
to simulation and theoretical results in terms of, e.g. the mean-squared
displacement, the time-dependent diffusion coefficient or the non-Gaussian
parameter. The dynamics are initially diffusive followed by intermediate
subdiffusive behaviour which again becomes diffusive at long times. How
pronounced and extended the different regimes are, depends on the specific
conditions, in particular the shape of the potential as well as its roughness
or amplitude but also the particle concentration. Here we focus on dilute
systems, but the dynamics of interacting systems in external potentials, and
thus the interplay between particle-particle and particle-potential
interactions, is also mentioned briefly. Furthermore, the observed dynamics of
dilute systems resemble the dynamics of concentrated systems close to their
glass transition, with which it is compared. The effect of certain potential
energy landscapes on the dynamics of individual particles appears similar to
the effect of interparticle interactions in the absence of an external
potential
Dynamic heterogeneities in attractive colloids
We study the formation of a colloidal gel by means of Molecular Dynamics
simulations of a model for colloidal suspensions. A slowing down with gel-like
features is observed at low temperatures and low volume fractions, due to the
formation of persistent structures. We show that at low volume fraction the
dynamic susceptibility, which describes dynamic heterogeneities, exhibits a
large plateau, dominated by clusters of long living bonds. At higher volume
fraction, where the effect of the crowding of the particles starts to be
present, it crosses over towards a regime characterized by a peak. We introduce
a suitable mean cluster size of clusters of monomers connected by "persistent"
bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure
Non-monotonic temperature evolution of dynamic correlations in glass-forming liquids
The viscosity of glass-forming liquids increases by many orders of magnitude
if their temperature is lowered by a mere factor of 2-3 [1,2]. Recent studies
suggest that this widespread phenomenon is accompanied by spatially
heterogeneous dynamics [3,4], and a growing dynamic correlation length
quantifying the extent of correlated particle motion [5-7]. Here we use a novel
numerical method to detect and quantify spatial correlations which reveal a
surprising non-monotonic temperature evolution of spatial dynamical
correlations, accompanied by a second length scale that grows monotonically and
has a very different nature. Our results directly unveil a dramatic qualitative
change in atomic motions near the mode-coupling crossover temperature [8] which
involves no fitting or indirect theoretical interpretation. Our results impose
severe new constraints on the theoretical description of the glass transition,
and open several research perspectives, in particular for experiments, to
confirm and quantify our observations in real materials.Comment: 7 page
Temperature dependence of three point correlation functions of viscous liquids the case of glycerol
What causes the dramatic slowing down of amp; 64258;ow and relaxation that leads to glass formation in liquids as temperature decreases is hardly understood so far and sub ject of intensive research work. It is tempting to ascribe the strong temperature dependence of the dynamics, irrespective of molecular details, to a collective or cooperative behaviour characterized by a length scale that grows as one approaches the glass transition. To access this length experimentally, we use the recently introduced three point dynamic susceptibility, from which can be extracted a number of molecules dynamically correlated during the structural relaxation, Ncorr. The three point functions are related to the sensitivity of the averaged two time dynamics to external control parameters, such as temperature and density. We studied Ncorr values in an important temperature range for a large number of liquids, and found that it systematically grows when approaching the glass transition. We specially emphasize here the case of glycerol for which we combined dielectric and neutron spin echo spectroscopy to cover more than 16 decades in relaxation tim