107 research outputs found
Shearing a Glassy Material: Numerical Tests of Nonequilibrium Mode-Coupling Approaches and Experimental Proposals
The predictions of a nonequilibrium schematic mode-coupling theory developed
to describe the nonlinear rheology of soft glassy materials have been
numerically challenged in a sheared binary Lennard-Jones mixture. The theory
gives an excellent description of the stress/temperature `jamming phase
diagram' of the system. In the present paper, we focus on the issue of an
effective temperature Teff for the slow modes of the fluid, as defined from a
generalized fluctuation-dissipation theorem. As predicted theoretically, many
different observables are found to lead to the same value of Teff, suggesting
several experimental procedures to measure Teff. New, simple experimental
protocols to access Teff from a generalized equipartition theorem are also
proposed, and one such experiment is numerically performed. These results give
strong support to the thermodynamic interpretation of Teff and make it
experimentally accessible in a very direct way.Comment: Version accepted for publication - Physical Review Letter
Characterizing dynamic length scales in glass-forming liquids
Reply to Comment by Flenner and Szamel on our paper in Nature Physics 8, 164
(2012).Comment: 1 pag
Fluctuation-dissipation relation in a sheared fluid
In a fluid out of equilibrium, the fluctuation dissipation theorem (FDT) is
usually violated. Using molecular dynamics simulations, we study in detail the
relationship between correlation and response functions in a fluid driven into
a stationary non-equilibrium state. Both the high temperature fluid state and
the low temperature glassy state are investigated. In the glassy state, the
violation of the FDT is quantitatively identical to the one observed previously
in an aging system in the absence of external drive. In the fluid state,
violations of the FDT appear only when the fluid is driven beyond the linear
response regime, and are then similar to those observed in the glassy state.
These results are consistent with the picture obtained earlier from theoretical
studies of driven mean-field disordered models, confirming the similarity
between these models and real glasses.Comment: 4 pages, latex, 3 ps figure
Shear yielding of amorphous glassy solids: Effect of temperature and strain rate
We study shear yielding and steady state flow of glassy materials with
molecular dynamics simulations of two standard models: amorphous polymers and
bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear
yield stress and the steady state flow stress in simple shear both drop
linearly with increasing temperature. The dependence on strain rate can be
described by a either a logarithm or a power-law added to a constant. In marked
contrast to predictions of traditional thermal activation models, the rate
dependence is nearly independent of temperature. The relation to more recent
models of plastic deformation and glassy rheology is discussed, and the
dynamics of particles and stress in small regions is examined in light of these
findings
Spontaneous and induced dynamic correlations in glass-formers II: Model calculations and comparison to numerical simulations
We study in detail the predictions of various theoretical approaches, in
particular mode-coupling theory (MCT) and kinetically constrained models
(KCMs), concerning the time, temperature, and wavevector dependence of
multi-point correlation functions that quantify the strength of both induced
and spontaneous dynamical fluctuations. We also discuss the precise predictions
of MCT concerning the statistical ensemble and microscopic dynamics dependence
of these multi-point correlation functions. These predictions are compared to
simulations of model fragile and strong glass-forming liquids. Overall, MCT
fares quite well in the fragile case, in particular explaining the observed
crucial role of the statistical ensemble and microscopic dynamics, while MCT
predictions do not seem to hold in the strong case. KCMs provide a simplified
framework for understanding how these multi-point correlation functions may
encode dynamic correlations in glassy materials. However, our analysis
highlights important unresolved questions concerning the application of KCMs to
supercooled liquids.Comment: 23 pages, 12 fig
Glassy dynamics of partially pinned fluids: an alternative mode-coupling approach
We use a simple mode-coupling approach to investigate glassy dynamics of
partially pinned fluid systems. Our approach is different from the
mode-coupling theory developed by Krakoviack [Phys. Rev. Lett. 94, 065703
(2005), Phys. Rev. E 84, 050501(R) (2011)]. In contrast to Krakoviack's theory,
our approach predicts a random pinning glass transition scenario that is
qualitatively the same as the scenario obtained using a mean-field analysis of
the spherical p-spin model and a mean-field version of the random first-order
transition theory. We use our approach to calculate quantities which are often
considered to be indicators of growing dynamic correlations and static
point-to-set correlations. We find that the so-called static overlap is
dominated by the simple, low pinning fraction contribution. Thus, at least for
randomly pinned fluid systems, only a careful quantitative analysis of
simulation results can reveal genuine, many-body point-to-set correlations
Slow dynamics of a confined supercooled binary mixture II: Q space analysis
We report the analysis in the wavevector space of the density correlator of a
Lennard Jones binary mixture confined in a disordered matrix of soft spheres
upon supercooling. In spite of the strong confining medium the behavior of the
mixture is consistent with the Mode Coupling Theory predictions for bulk
supercooled liquids. The relaxation times extracted from the fit of the density
correlator to the stretched exponential function follow a unique power law
behavior as a function of wavevector and temperature. The von Schweidler
scaling properties are valid for an extended wavevector range around the peak
of the structure factor. The parameters extracted in the present work are
compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys.
Rev.
Static and dynamic properties of a reversible gel
We study a microscopically realistic model of a physical gel and use computer
simulations to investigate its static and dynamic properties at thermal
equilibrium. The phase diagram comprises a sol phase, a coexistence region
ending at a critical point, a gelation line, and an equilibrium gel phase
unrelated to phase separation. The global structure of the gel is homogeneous,
but the stress is supported by a fractal network. Gelation results in a
dramatic slowing down of the dynamics, which can be used to locate the
transition, which otherwise shows no structural signatures. Moreover, the
equilibrium gel dynamics is highly heterogeneous as a result of the presence of
particle families with different mobilities. An analysis of gel dynamics in
terms of mobile and arrested particles allows us to elucidate several
differences between the dynamics of equilibrium gels and that of glass-formers.Comment: 9 pages, 7 figures, paper presented at the 10th Granada Seminar on
Computational and Statistical Physic
Intermittent origin of the large violations of the fluctuation dissipation relations in an aging polymer glass
The fluctuation-dissipation relation (FDR) is measured on the dielectric
properties of a polymer glass (polycarbonate)in the range . It
is found that after a quench below the glass transition temperature the
fluctuation dissipation theorem is strongly violated. The amplitude and the
persistence time of this violation are decreasing functions of frequency. At
frequencies larger than 1Hz it persists for about . The origin of this
violation is a highly intermittent dynamics characterized by large
fluctuations. The relevance of these results for recent models of aging
dynamics are discussed.Comment: to be published in Europhysics Letter
Critical fluctuations and breakdown of Stokes-Einstein relation in the Mode-Coupling Theory of glasses
We argue that the critical dynamical fluctuations predicted by the
mode-coupling theory (MCT) of glasses provide a natural mechanism to explain
the breakdown of the Stokes-Einstein relation. This breakdown, observed
numerically and experimentally in a region where MCT should hold, is one of the
major difficulty of the theory, for which we propose a natural resolution based
on the recent interpretation of the MCT transition as a bona fide critical
point with a diverging length scale. We also show that the upper critical
dimension of MCT is d_c=8.Comment: Proceedings of the workshop on non-equilibrium phenomena in
supercooled fluids, glasses and amorphous materials (17-22 September, 2006,
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