2,909 research outputs found
New class of compounds have very low vapor pressures
Magnesium hexahydrate tetrachlorometallates are 50-volume-percent water, have a high melting point and possess a low vapor pressure. These new compounds are relatively noncorrosive, thermally stable, and water soluble but not hygroscopic. They may have potential applications as cooling fluids
The viscous slowing down of supercooled liquids as a temperature-controlled superArrhenius activated process: a description in terms of frustration-limited domains
We propose that the salient feature to be explained about the glass
transition of supercooled liquids is the temperature-controlled superArrhenius
activated nature of the viscous slowing down, more strikingly seen in
weakly-bonded, fragile systems. In the light of this observation, the relevance
of simple models of spherically interacting particles and that of models based
on free-volume congested dynamics are questioned. Finally, we discuss how the
main aspects of the phenomenology of supercooled liquids, including the
crossover from Arrhenius to superArrhenius activated behavior and the
heterogeneous character of the relaxation, can be described by an
approach based on frustration-limited domains.Comment: 13 pages, 4 figures, accepted in J. Phys.: Condensed Matter,
proceedings of the Trieste workshop on "Unifying Concepts in Glass Physics
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Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
refereein
Liquid-liquid phase transition in Stillinger-Weber silicon
It was recently demonstrated that the Stillinger-Weber silicon undergoes a
liquid-liquid first-order phase transition deep into the supercooled region
(Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects
of perturbations on this phase transition. We show that the order of the
liquid-liquid transition changes with negative pressure. We also find that the
liquid-liquid transition disappears when the three-body term of the potential
is strengthened by as little as 5 %. This implies that the details of the
potential could affect strongly the nature and even the existence of the
liquid-liquid phase.Comment: 13 page
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-Gaussian excitations model for the glass transition
We develop a modified "two-state" model with Gaussian widths for the site
energies of both ground and excited states, consistent with expectations for a
disordered system. The thermodynamic properties of the system are analyzed in
configuration space and found to bridge the gap between simple two state models
("logarithmic" model in configuration space) and the random energy model
("Gaussian" model in configuration space). The Kauzmann singularity given by
the random energy model remains for very fragile liquids but is suppressed or
eliminated for stronger liquids. The sharp form of constant volume heat
capacity found by recent simulations for binary mixed Lennard Jones and soft
sphere systems is reproduced by the model, as is the excess entropy and heat
capacity of a variety of laboratory systems, strong and fragile. The ideal
glass in all cases has a narrow Gaussian, almost invariant among molecular and
atomic glassformers, while the excited state Gaussian depends on the system and
its width plays a role in the thermodynamic fragility. The model predicts the
existence of first-order phase transition for fragile liquids.Comment: 12 pages, 12 figure
Configurational Entropy and its Crisis in Metastable States: Ideal Glass Transition in a Dimer Model as a Paragidm of a Molecular Glass
We discuss the need for discretization to evaluate the configurational
entropy in a general model. We also discuss the prescription using restricted
partition function formalism to study the stationary limit of metastable
states. We introduce a lattice model of dimers as a paradigm of molecular fluid
and study metastability in it to investigate the root cause of glassy behavior.
We demonstrate the existence of the entropy crisis in metastable states, from
which it follows that the entropy crisis is the root cause underlying the ideal
glass transition in systems with particles of all sizes. The orientational
interactions in the model control the nature of the liquid-liquid transition
observed in recent years in molecular glasses.Comment: 36 pages, 9 figure
Subdiffusion and cage effect in a sheared granular material
We investigate experimentally the diffusion properties of a bidimensional
bidisperse dry granular material under quasistatic cyclic shear.The comparison
of these properties with results obtained both in computer simulations of hard
spheres systems and Lenard-Jones liquids and experiments on colloidal systems
near the glass transition demonstrates a strong analogy between the behaviour
of granular matter and these systems. More specifically, we study in detail the
cage dynamics responsible for the subdiffusion in the slow relaxation regime,
and obtain the values of relevant time and length scales.Comment: 4 pages, 6 figures, submitted to PR
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