648 research outputs found
Lifetime of dynamic heterogeneity in strong and fragile kinetically constrained spin models
Kinetically constrained spin models are schematic coarse-grained models for
the glass transition which represent an efficient theoretical tool to study
detailed spatio-temporal aspects of dynamic heterogeneity in supercooled
liquids. Here, we study how spatially correlated dynamic domains evolve with
time and compare our results to various experimental and numerical
investigations. We find that strong and fragile models yield different results.
In particular, the lifetime of dynamic heterogeneity remains constant and
roughly equal to the alpha relaxation time in strong models, while it increases
more rapidly in fragile models when the glass transition is approached.Comment: Submitted to the proceedings of the 6th EPS Liquid Matter Conference,
Utrecht 2-6 July 200
Coarse-grained microscopic model of glass formers
We introduce a coarse-grained model for atomic glass formers. Its elements
are physically motivated local microscopic dynamical rules parameterized by
observables. Results of the model are established and used to interpret the
measured behaviors of supercooled fluids approaching glass transitions. The
model predicts the presence of a crossover from hierarchical super-Arrhenius
dynamics at short length scales to diffusive Arrhenius dynamics at large length
scales. This prediction distinguishes our model from other theories of glass
formers and can be tested by experiment.Comment: 5 pages, 5 figure
Space-time Thermodynamics of the Glass Transition
We consider the probability distribution for fluctuations in dynamical action
and similar quantities related to dynamic heterogeneity. We argue that the
so-called "glass transition" is a manifestation of low action tails in these
distributions where the entropy of trajectory space is sub-extensive in time.
These low action tails are a consequence of dynamic heterogeneity and an
indication of phase coexistence in trajectory space. The glass transition,
where the system falls out of equilibrium, is then an order-disorder phenomenon
in space-time occurring at a temperature T_g which is a weak function of
measurement time. We illustrate our perspective ideas with facilitated lattice
models, and note how these ideas apply more generally.Comment: 5 pages, 4 figure
Two-step devitrification of ultrastable glasses
The discovery of ultrastable glasses has raised novel challenges about glassy
systems. Recent experiments studied the macroscopic devitrification of
ultrastable glasses into liquids upon heating but lacked microscopic
resolution. We use molecular dynamics simulations to analyse the kinetics of
this transformation. In the most stable systems, devitrification occurs after a
very large time, but the liquid emerges in two steps. At short times, we
observe the rare nucleation and slow growth of isolated droplets containing a
liquid maintained under pressure by the rigidity of the surrounding glass. At
large times, pressure is released after the droplets coalesce into large
domains, which accelerates devitrification. This two-step process produces
pronounced deviations from the classical Avrami kinetics and explains the
emergence of a giant lengthscale characterising the devitrification of bulk
ultrastable glasses. Our study elucidates the nonequilibrium kinetics of
glasses following a large temperature jump, which differs from both equilibrium
relaxation and aging dynamics, and will guide future experimental studies
Dynamics of glass-forming liquids. XVI. Observation of ultrastable glass transformation via dielectric spectroscopy
The transformation of vapor-deposited ultrastable glasses of indomethacin (IMC) into the supercooled liquid state near Tg is monitored by means of dielectric spectroscopy. Films with thickness between 400 and 800 nm are deposited on differential interdigitated electrode cells and their loss profiles are measured during isothermal annealing using a dual-channel impedance technique for frequencies between 0.03 and 100 Hz. All dielectric loss spectra observed during the transformation process can be explained by a volume fraction of the supercooled liquid that increases linearly with time. From the early stages of the transformation to the liquid that is formed via complete annealing of the ultrastable glass, the average dielectric relaxation time as well as the distribution of relaxation times of the liquid component are identical to those of the conventional liquid obtained by cooling the melt. The dependence of the transformation rate on the film thickness is consistent with a growth front mechanism for the direct conversion from the ultrastable glass to the equilibrium supercooled liquid. We conclude that the IMC liquid recovered from the ultrastable glass is structurally and dynamically identical to the conventional supercooled state
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
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