451 research outputs found
Deformation-induced accelerated dynamics in polymer glasses
Molecular dynamics simulations are used to investigate the effects of
deformation on the segmental dynamics in an aging polymer glass. Individual
particle trajectories are decomposed into a series of discontinuous hops, from
which we obtain the full distribution of relaxation times and displacements
under three deformation protocols: step stress (creep), step strain, and
constant strain rate deformation. As in experiments, the dynamics can be
accelerated by several orders of magnitude during deformation, and the history
dependence is entirely erased during yield (mechanical rejuvenation). Aging can
be explained as a result of the long tails in the relaxation time distribution
of the glass, and similarly, mechanical rejuvenation is understood through the
observed narrowing of this distribution during yield. Although the relaxation
time distributions under deformation are highly protocol specific, in each case
they may be described by a universal acceleration factor that depends only on
the strain.Comment: 15 pages, 15 figure
Simulations of aging and plastic deformation in polymer glasses
We study the effect of physical aging on the mechanical properties of a model
polymer glass using molecular dynamics simulations. The creep compliance is
determined simultaneously with the structural relaxation under a constant
uniaxial load below yield at constant temperature. The model successfully
captures universal features found experimentally in polymer glasses, including
signatures of mechanical rejuvenation. We analyze microscopic relaxation
timescales and show that they exhibit the same aging characteristics as the
macroscopic creep compliance. In addition, our model indicates that the entire
distribution of relaxation times scales identically with age. Despite large
changes in mobility, we observe comparatively little structural change except
for a weak logarithmic increase in the degree of short-range order that may be
correlated to an observed decrease in aging with increasing load.Comment: 9 pages, 12 figure
Aging Effects Across the Metal-Insulator Transition in Two Dimensions
Aging effects in the relaxations of conductivity of a two-dimensional
electron system in Si have been studied as a function of carrier density. They
reveal an abrupt change in the nature of the glassy phase at the
metal-insulator transition (MIT): (a) while full aging is observed in the
insulating regime, there are significant departures from full aging on the
metallic side of the MIT, before the glassy phase disappears completely at a
higher density ; (b) the amplitude of the relaxations peaks just below the
MIT, and it is strongly suppressed in the insulating phase. Other aspects of
aging, including large non-Gaussian noise and similarities to spin glasses,
also have been discussed.Comment: 4+ pages, 5 figures; minor changes, accepted for publication in PR
Compressed correlation functions and fast aging dynamics in metallic glasses
We present x-ray photon correlation spectroscopy measurements of the atomic
dynamics in a Zr67Ni33 metallic glass, well below its glass transition
temperature. We find that the decay of the density fluctuations can be well
described by compressed, thus faster than exponential, correlation functions
which can be modeled by the well-known Kohlrausch-Williams-Watts function with
a shape exponent {\beta} larger than one. This parameter is furthermore found
to be independent of both waiting time and wave-vector, leading to the
possibility to rescale all the correlation functions to a single master curve.
The dynamics in the glassy state is additionally characterized by different
aging regimes which persist in the deep glassy state. These features seem to be
universal in metallic glasses and suggest a non diffusive nature of the
dynamics. This universality is supported by the possibility of describing the
fast increase of the structural relaxation time with waiting time using a
unique model function, independently of the microscopic details of the system.Comment: 7 pages, 4 figures. To be published in J. Chem. Phy
Interplay between shear loading and structural aging in a physical gel
We show that the aging of the mechanical relaxation of a gelatin gel exhibits
the same scaling phenomenology as polymer and colloidal glasses. Besides,
gelatin is known to exhibit logarithmic structural aging (stiffening). We find
that stress accelerates this process. However, this effect is definitely
irreducible to a mere age shift with respect to natural aging. We suggest that
it is interpretable in terms of elastically-aided elementary (coilhelix)
local events whose dynamics gradually slows down as aging increases geometric
frustration
Prediction of long and short time rheological behavior in soft glassy materials
We present an effective time approach to predict long and short time
rheological behavior of soft glassy materials from experiments carried out over
practical time scales. Effective time approach takes advantage of relaxation
time dependence on aging time that allows time-aging time superposition even
when aging occurs over the experimental timescales. Interestingly experiments
on variety of soft materials demonstrate that the effective time approach
successfully predicts superposition for diverse aging regimes ranging from
sub-aging to hyper-aging behaviors. This approach can also be used to predict
behavior of any response function in molecular as well as spin glasses.Comment: 13 pages, 4 figure
Subdiffusion and intermittent dynamic fluctuations in the aging regime of concentrated hard spheres
We study the nonequilibrium aging dynamics in a system of quasi-hard spheres
at large density by means of computer simulations. We find that, after a sudden
quench to large density, the relaxation time initially increases exponentially
with the age of the system. After a surprisingly large crossover time, the
system enters the asymptotic aging regime characterized by a linear increase of
the relaxation time with age. In this aging regime, single particle motion is
strongly non-Fickian, with a mean-squared displacement increasing
subdiffusively, associated to broad, non-Gaussian tails in the distribution of
particle displacements. We find that the system ages through temporally
intermittent relaxation events, and a detailed finite size analysis of these
collective dynamic fluctuations reveals that these events are not spanning the
entire system, but remain spatially localized.Comment: 11 pages; 10 fig
Nonequilibrium Relaxations and Aging Effects in a Two-Dimensional Coulomb Glass
The relaxations of conductivity have been studied in the glassy regime of a
strongly disordered two-dimensional electron system in Si after a temporary
change of carrier density during the waiting time t_w. Two types of response
have been observed: a) monotonic, where relaxations exhibit aging, i.e.
dependence on history, determined by t_w and temperature; b) nonmonotonic,
where a memory of the sample history is lost. The conditions that separate the
two regimes have been also determined.Comment: 4 pages; published versioi
Off-equilibrium dynamics of the two-dimensional Coulomb glass
The dynamics of the 2D Coulomb glass model is investigated by kinetic Monte
Carlo simulation. An exponential divergence of the relaxation time signals a
zero-temperature freezing transition. At low temperatures the dynamics of the
system is glassy. The local charge correlations and the response to
perturbations of the local potential show aging. The dynamics of formation of
the Coulomb gap is slow and the density of states at the Fermi level decays in
time as a power law. The relevance of these findings for recent transport
experiments in Anderson-insulating films is pointed out.Comment: 7 pages, 7 figure
Huge (but finite) time scales in slow relaxations: beyond simple aging
Experiments performed in the last years demonstrated slow relaxations and
aging in the conductance of a large variety of materials. Here, we present
experimental and theoretical results for conductance relaxation and aging for
the case-study example of porous silicon. The relaxations are experimentally
observed even at room temperature over timescales of hours, and when a strong
electric field is applied for a time , the ensuing relaxation depends on
. We derive a theoretical curve and show that all experimental data
collapse onto it with a single timescale as a fitting parameter. This timescale
is found to be of the order of thousands of seconds at room temperature. The
generic theory suggested is not fine-tuned to porous silicon, and thus we
believe the results should be universal, and the presented method should be
applicable for many other systems manifesting memory and other glassy effects.Comment: 4+ pages, 4 figure
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