5,002 research outputs found
Replica theory of the rigidity of structural glasses
We present a first principle scheme to compute the rigidity, i. e. the
shear-modulus of structural glasses at finite temperatures using the cloned
liquid theory, which combines the replica theory and the liquid theory. With
the aid of the replica method which enables disentanglement of thermal
fluctuations in liquids into intra-state and inter-state fluctuations, we
extract the rigidity of metastable amorphous solid states in the supercooled
liquid and glass phases. The result can be understood intuitively without
replicas. As a test case, we apply the scheme to the supercooled and glassy
state of a binary mixture of soft-spheres. The result compares well with the
shear-modulus obtained by a previous molecular dynamic simulation. The rigidity
of metastable states is significantly reduced with respect to the instantaneous
rigidity, namely the Born term, due to non-affine responses caused by
displacements of particles inside cages at all temperatures down to T=0. It
becomes nearly independent of temperature below the Kauzmann temperature T_K.
At higher temperatures in the supercooled liquid state, the non-affine
correction to the rigidity becomes stronger suggesting melting of the
metastable solid state. Inter-state part of the static response implies jerky,
intermittent stress-strain curves with static analogue of yielding at
mesoscopic scales.Comment: 52 pages, 10 figure
A geometrical model for Mixed cyanide crystals
A model of diluted random field sustained by quenched volume deformations is
shown to reproduce puzzling physical features found in X(CN)_{x}Y_{1-x} mixed
cyanide crystals. X is an alkali metal (K, Na or Rb) and Y is a spherical
halogen ion (Br, Cl or I). Critical thresholds x_c at which associated first
order ferroelastic transitions disappear are calculated exactly. The diluted
random field is shown to compete with compressibility in making the transition
first order. Transitions are then found to remain first order down to x_c
except in the case of bromine dilution where they become continuous. All the
results are in excellent agreement with available experimental data.Comment: 10 pages, late
Molecular dynamics simulations of glassy polymers
We review recent results from computer simulation studies of polymer glasses,
from chain dynamics around the glass transition temperature Tg to the
mechanical behaviour below Tg. These results clearly show that modern computer
simulations are able to address and give clear answers to some important issues
in the field, in spite of the obvious limitations in terms of length and time
scales. In the present review we discuss the cooling rate effects, and dynamic
slowing down of different relaxation processes when approaching Tg for both
model and chemistry-specific polymer glasses. The impact of geometric
confinement on the glass transition is discussed in detail. We also show that
computer simulations are very useful tools to study structure and mechanical
response of glassy polymers. The influence of large deformations on mechanical
behaviour of polymer glasses in general, and strain hardening effect in
particular are reviewed. Finally, we suggest some directions for future
research, which we believe will be soon within the capabilities of state of the
art computer simulations, and correspond to problems of fundamental interest.Comment: To apear in "Soft Matter
A study of the static yield stress in a binary Lennard-Jones glass
The stress-strain relations and the yield behavior of model glass (a 80:20
binary Lennard-Jones mixture) is studied by means of MD simulations. First, a
thorough analysis of the static yield stress is presented via simulations under
imposed stress. Furthermore, using steady shear simulations, the effect of
physical aging, shear rate and temperature on the stress-strain relation is
investigated. In particular, we find that the stress at the yield point (the
``peak''-value of the stress-strain curve) exhibits a logarithmic dependence
both on the imposed shear rate and on the ``age'' of the system in qualitative
agreement with experiments on amorphous polymers and on metallic glasses. In
addition to the very observation of the yield stress which is an important
feature seen in experiments on complex systems like pastes, dense colloidal
suspensions and foams, further links between our model and soft glassy
materials are found. An example are hysteresis loops in the system response to
a varying imposed stress. Finally, we measure the static yield stress for our
model and study its dependence on temperature. We find that for temperatures
far below the mode coupling critical temperature of the model (),
\sigmay decreases slowly upon heating followed by a stronger decrease as
\Tc is approached. We discuss the reliability of results on the static yield
stress and give a criterion for its validity in terms of the time scales
relevant to the problem.Comment: 14 pages, 18 figure
Origin of the slow dynamics and the aging of a soft glass
We study by light microscopy a soft glass consisting of a compact arrangement
of polydisperse elastic spheres. We show that its slow and non-stationary
dynamics results from the unavoidable small fluctuations of temperature, which
induce intermittent local mechanical shear in the sample, because of thermal
expansion and contraction. Temperature-induced shear provokes both reversible
and irreversible rearrangements whose amplitude decreases with time, leading to
an exponential slowing down of the dynamics with sample age.Comment: published in PRL 97, 238301, 200
Unwind: Interactive Fish Straightening
The ScanAllFish project is a large-scale effort to scan all the world's
33,100 known species of fishes. It has already generated thousands of
volumetric CT scans of fish species which are available on open access
platforms such as the Open Science Framework. To achieve a scanning rate
required for a project of this magnitude, many specimens are grouped together
into a single tube and scanned all at once. The resulting data contain many
fish which are often bent and twisted to fit into the scanner. Our system,
Unwind, is a novel interactive visualization and processing tool which
extracts, unbends, and untwists volumetric images of fish with minimal user
interaction. Our approach enables scientists to interactively unwarp these
volumes to remove the undesired torque and bending using a piecewise-linear
skeleton extracted by averaging isosurfaces of a harmonic function connecting
the head and tail of each fish. The result is a volumetric dataset of a
individual, straight fish in a canonical pose defined by the marine biologist
expert user. We have developed Unwind in collaboration with a team of marine
biologists: Our system has been deployed in their labs, and is presently being
used for dataset construction, biomechanical analysis, and the generation of
figures for scientific publication
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