437 research outputs found
Lifetime of dynamical heterogeneity in a highly supercooled liquid
We numerically examine dynamical heterogeneity in a highly supercooled
three-dimensional liquid via molecular-dynamics simulations. To define the
local dynamics, we consider two time intervals, and
. is the relaxation time, and
is the time at which non-Gaussian parameter of the van Hove
self-correlation function is maximized. We determine the lifetimes of the
heterogeneous dynamics in these two different time intervals,
and
, by calculating the time correlation
function of the particle dynamics, i.e., the four-point correlation function.
We find that the difference between and
increases with decreasing
temperature. At low temperatures, is
considerably larger than , while
remains comparable to
. Thus, the lifetime of the heterogeneous dynamics depends
strongly on the time interval.Comment: 4pages, 6figure
Measuring Spatial Distribution of Local Elastic Modulus in Glasses
Glasses exhibit spatially inhomogeneous elastic properties, which can be
investigated by measuring their elastic moduli at a local scale. Various
methods to evaluate the local elastic modulus have been proposed in the
literature. A first possibility is to measure the local stress-local strain
curve and to obtain the local elastic modulus from the slope of the curve, or
equivalently to use a local fluctuation formula. Another possible route is to
assume an affine strain and to use the applied global strain instead of the
local strain for the calculation of the local modulus. Most recently a third
technique has been introduced, which is easy to be implemented and has the
advantage of low computational cost. In this contribution, we compare these
three approaches by using the same model glass and reveal the differences among
them caused by the non-affine deformations
Dynamical heterogeneity in a highly supercooled liquid: Consistent calculations of correlation length, intensity, and lifetime
We have investigated dynamical heterogeneity in a highly supercooled liquid
using molecular-dynamics simulations in three dimensions. Dynamical
heterogeneity can be characterized by three quantities: correlation length
, intensity , and lifetime . We evaluated
all three quantities consistently from a single order parameter. In a previous
study (H. Mizuno and R. Yamamoto, Phys. Rev. E {\bf 82}, 030501(R) (2010)), we
examined the lifetime in two time intervals
and , where is the
-relaxation time and is the time at which the
non-Gaussian parameter of the Van Hove self-correlation function is maximized.
In the present study, in addition to the lifetime , we
evaluated the correlation length and the intensity from
the same order parameter used for the lifetime . We
found that as the temperature decreases, the lifetime
grows dramatically, whereas the correlation length and the intensity
increase slowly compared to or plateaus.
Furthermore, we investigated the lifetime in more
detail. We examined the time-interval dependence of the lifetime
and found that as the time interval increases,
monotonically becomes longer and plateaus at the
relaxation time of the two-point density correlation function. At the large
time intervals for which plateaus, the heterogeneous
dynamics migrate in space with a diffusion mechanism, such as the particle
density.Comment: 12pages, 13figures, to appear in Physical Review
Elastic heterogeneity, vibrational states, and thermal conductivity across an amorphisation transition
Disordered solids exhibit unusual properties of their vibrational states and
thermal conductivities. Recent progresses have well established the concept of
"elastic heterogeneity", i.e., disordered materials show spatially
inhomogeneous elastic moduli. In this study, by using molecular-dynamics
simulations, we gradually introduce "disorder" into a numerical system to
control its modulus heterogeneity. The system starts from a perfect crystalline
state, progressively transforms into an increasingly disordered crystalline
state, and finally undergoes structural amorphisation. We monitor independently
the elastic heterogeneity, the vibrational states, and the thermal conductivity
across this transition, and show that the heterogeneity in elastic moduli is
well correlated to vibrational and thermal anomalies of the disordered system
Elastic Moduli and Vibrational Modes in Jammed Particulate Packings
When we elastically impose a homogeneous, affine deformation on amorphous
solids, they also undergo an inhomogeneous, non-affine deformation, which can
have a crucial impact on the overall elastic response. To correctly understand
the elastic modulus , it is therefore necessary to take into account not
only the affine modulus , but also the non-affine modulus that
arises from the non-affine deformation. In the present work, we study the bulk
() and shear () moduli in static jammed particulate packings over a
range of packing fractions . One novelty of this work is to elucidate
the contribution of each vibrational mode to the non-affine through a
modal decomposition of the displacement and force fields. In the vicinity of
the (un)jamming transition, , the vibrational density of states,
, shows a plateau in the intermediate frequency regime above a
characteristic frequency . We illustrate that this unusual feature
apparent in is reflected in the behavior of : As , where , those modes for
contribute less and less, while contributions from those
for approach a constant value which results in to
approach a critical value , as . At
itself, the bulk modulus attains a finite value , such that has a value that remains below . In contrast,
for the critical shear modulus , and approach the same
value so that the total value becomes exactly zero, .
We explore what features of the configurational and vibrational properties
cause such the distinction between and , allowing us to validate
analytical expressions for their critical values.Comment: 23 pages, 13 figure
Acoustic excitations and elastic heterogeneities in disordered solids
In the recent years, much attention has been devoted to the inhomogeneous
nature of the mechanical response at the nano-scale in disordered solids.
Clearly, the elastic heterogeneities that have been characterized in this
context are expected to strongly impact the nature of the sound waves which, in
contrast to the case of perfect crystals, cannot be completely rationalized in
terms of phonons. Building on previous work on a toy model showing an
amorphisation transition [Mizuno H, Mossa S, Barrat JL (2013) EPL {\bf
104}:56001], we investigate the relationship between sound waves and elastic
heterogeneities in a unified framework, by continuously interpolating from the
perfect crystal, through increasingly defective phases, to fully developed
glasses. We provide strong evidence of a direct correlation between sound waves
features and the extent of the heterogeneous mechanical response at the
nano-scale
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