1,703 research outputs found
On the Adam-Gibbs-Wolynes scenario for the viscosity increase in glasses
We reformulate the interpretation of the mean-field glass transition scenario
for finite dimensional systems, proposed by Wolynes and collaborators.
This allows us to establish clearly a temperature dependent length xi* above
which the mean-field glass transition picture has to be modified. We argue in
favor of the mosaic state introduced by Wolynes and collaborators, which leads
to the Adam-Gibbs relation between the viscosity and configurational entropy of
glass forming liquids.
Our argument is a mixture of thermodynamics and kinetics, partly inspired by
the Random Energy
Model: small clusters of particles are thermodynamically frozen in low energy
states, whereas large clusters are kinetically frozen by large activation
energies. The relevant relaxation time is that of the smallest `liquid'
clusters. Some physical consequences are discussed.Comment: 8 page
Numerical study of the temperature and porosity effects on the fracture propagation in a 2D network of elastic bonds
This article reports results concerning the fracture of a 2d triangular
lattice of atoms linked by springs. The lattice is submitted to controlled
strain tests and the influence of both porosity and temperature on failure is
investigated. The porosity is found on one hand to decrease the stiffness of
the material but on the other hand it increases the deformation sustained prior
to failure. Temperature is shown to control the ductility due to the presence
of cavities that grow and merge. The rough surfaces resulting from the
propagation of the crack exhibit self-affine properties with a roughness
exponent over a range of length scales which increases
with temperature. Large cavities also have rough walls which are found to be
fractal with a dimension, , which evolves with the distance from the crack
tip. For large distances, is found to be close to 1.5, and close to 1.0 for
cavities just before their coalescence with the main crack
Anomalous dynamical light scattering in soft glassy gels
We compute the dynamical structure factor S(q,tau) of an elastic medium where
force dipoles appear at random in space and in time, due to `micro-collapses'
of the structure. Various regimes are found, depending on the wave vector q and
the collapse time. In an early time regime, the logarithm of the structure
factor behaves as (q tau)^{3/2}, as predicted by Cipelletti et al. [1] using
heuristic arguments. However, in an intermediate time regime we rather obtain a
q tau)^{5/4} behaviour. Finally, the asymptotic long time regime is found to
behave as q^{3/2} tau. We also give a plausible scenario for aging, in terms of
a strain dependent energy barrier for micro-collapses. The relaxation time is
found to grow with the age t_w, quasi-exponentially at first, and then as
t_w^{4/5} with logarithmic corrections.Comment: 15 pages, 1 .eps figure. Submitted to EPJ-
Glassy effects in the swelling/collapse dynamics of homogeneous polymers
We investigate, using numerical simulations and analytical arguments, a
simple one dimensional model for the swelling or the collapse of a closed
polymer chain of size N, representing the dynamical evolution of a polymer in a
\Theta-solvent that is rapidly changed into a good solvent (swelling) or a bad
solvent (collapse). In the case of swelling, the density profile for
intermediate times is parabolic and expands in space as t^{1/3}, as predicted
by a Flory-like continuum theory. The dynamics slows down after a time \propto
N^2 when the chain becomes stretched, and the polymer gets stuck in metastable
`zig-zag' configurations, from which it escapes through thermal activation. The
size of the polymer in the final stages is found to grow as \sqrt{\ln t}. In
the case of collapse, the chain very quickly (after a time of order unity)
breaks up into clusters of monomers (`pearls'). The evolution of the chain then
proceeds through a slow growth of the size of these metastable clusters, again
evolving as the logarithm of time. We enumerate the total number of metastable
states as a function of the extension of the chain, and deduce from this
computation that the radius of the chain should decrease as 1/\ln(\ln t). We
compute the total number of metastable states with a given value of the energy,
and find that the complexity is non zero for arbitrary low energies. We also
obtain the distribution of cluster sizes, that we compare to simple
`cut-in-two' coalescence models. Finally, we determine the aging properties of
the dynamical structure. The subaging behaviour that we find is attributed to
the tail of the distribution at small cluster sizes, corresponding to
anomalously `fast' clusters (as compared to the average). We argue that this
mechanism for subaging might hold in other slowly coarsening systems.Comment: 35 pages, 12 .ps figures. Submitted to EPJ
Can crack front waves explain the roughness of cracks ?
We review recent theoretical progress on the dynamics of brittle crack fronts
and its relationship to the roughness of fracture surfaces. We discuss the
possibility that the intermediate scale roughness of cracks, which is
characterized by a roughness exponent approximately equal to 0.5, could be
caused by the generation, during local instabilities by depinning, of
diffusively broadened corrugation waves, which have recently been observed to
propagate elastically along moving crack fronts. We find that the theory agrees
plausibly with the orders of magnitude observed. Various consequences and
limitations, as well as alternative explanations, are discussed. We argue that
another mechanism, possibly related to damage cavity coalescence, is needed to
account for the observed large scale roughness of cracks that is characterized
by a roughness exponent approximately equal to 0.8Comment: 26 pages, 3 .eps figure. Submitted to J. Mech. Phys. Solid
Evidence of Deep Water Penetration in Silica during Stress Corrosion Fracture
We measure the thickness of the heavy water layer trapped under the stress corrosion fracture surface of silica using neutron reflectivity experiments. We show that the penetration depth is 65–85 Å, suggesting the presence of a damaged zone of ~100 Å extending ahead of the crack tip during its propagation. This estimate of the size of the damaged zone is compatible with other recent results
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
Rejuvenation in the Random Energy Model
We show that the Random Energy Model has interesting rejuvenation properties
in its frozen phase. Different `susceptibilities' to temperature changes, for
the free-energy and for other (`magnetic') observables, can be computed
exactly. These susceptibilities diverge at the transition temperature, as
(1-T/T_c)^-3 for the free-energy.Comment: 9 pages, 1 eps figur
Multiple scaling regimes in simple aging models
We investigate aging in glassy systems based on a simple model, where a point
in configuration space performs thermally activated jumps between the minima of
a random energy landscape. The model allows us to show explicitly a subaging
behavior and multiple scaling regimes for the correlation function. Both the
exponents characterizing the scaling of the different relaxation times with the
waiting time and those characterizing the asymptotic decay of the scaling
functions are obtained analytically by invoking a `partial equilibrium'
concept.Comment: 4 pages, 3 figure
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