2,381 research outputs found
Spinodal of supercooled polarizable water
We develop a series of molecular dynamics computer simulations of liquid
water, performed with a polarizable potential model, to calculate the spinodal
line and the curve of maximum density inside the metastable supercooled region.
After analysing the structural properties,the liquid spinodal line is followed
down to T=210 K. A monotonic decrease is found in the explored region. The
curve of maximum density bends on approaching the spinodal line. These results,
in agreement with similar studies on non polarizable models of water, are
consistent with the existence of a second critical point for water.Comment: 8 pages, 5 figures, 2 tables. To be published in Phys. Re
Variation of the glass transition temperature with rigidity and chemical composition
The effects of flexibility and chemical composition in the variation of the
glass transition temperature are obtained by using the Lindemann criteria, that
relates melting temperature with atomic vibrations. Using this criteria and
that floppy modes at low frequencies enhance in a considerable way the average
cuadratic displacement, we show that the consequence is a modified glass
transition temperature. This approach allows to obtain in a simple way the
empirically modified Gibbs-DiMarzio law, which has been widely used in
chalcogenide glasses to fit the changes in the glass transition temperature
with the chemical composition . The method predicts that the constant that
appears in the law depends upon the ratio of two characteristic frequencies (or
temperatures). Then, the constant for the Se-Ge-As glass is estimated by using
the experimental density of vibrational states, and the result shows a very
good agreement with the experimental fit from glass transition temperature
variation
A simple solvable energy landscape model that shows a thermodynamic phase transition and a glass transition
When a liquid melt is cooled, a glass or phase transition can be obtained
depending on the cooling rate. Yet, this behavior has not been clearly captured
in energy landscape models. Here a model is provided in which two key
ingredients are considered based in the landscape, metastable states and their
multiplicity. Metastable states are considered as in two level system models.
However, their multiplicity and topology allows a phase transition in the
thermodynamic limit, while a transition to the glass is obtained for fast
cooling. By solving the corresponding master equation, the minimal speed of
cooling required to produce the glass is obtained as a function of the
distribution of metastable and stable states. This allows to understand cooling
trends due to rigidity considerations in chalcogenide glasses.Comment: 4 pages (letter), 2 figure
Energy landscape and rigidity
The effects of floppy modes in the thermodynamical properties of a system are
studied. From thermodynamical arguments, we deduce that floppy modes are not at
zero frequency and thus a modified Debye model is used to take into account
this effect. The model predicts a deviation from the Debye law at low
temperatures. Then, the connection between the topography of the energy
landscape, the topology of the phase space and the rigidity of a glass is
explored. As a result, we relate the number of constraints and floppy modes
with the statistics of the landscape. We apply these ideas to a simple model
for which we provide an approximate expression for the number of energy basins
as a function of the rigidity. This allows to understand certains features of
the glass transition, like the jump in the specific heat or the reversible
window observed in chalcogenide glasses.Comment: 1 text+3 eps figure
An improved Monte Carlo method for direct calculation of the density of states
We present an efficient Monte Carlo algorithm for determining the density of
states which is based on the statistics of transition probabilities between
states. By measuring the infinite temperature transition probabilities--that
is, the probabilities associated with move proposal only--we are able to
extract excellent estimates of the density of states. When this estimator is
used in conjunction with a Wang-Landau sampling scheme [F. Wang and D. P.
Landau, Phys. Rev. Lett. 86, 2050 (2001)], we quickly achieve uniform sampling
of macrostates (e.g., energies) and systematically refine the calculated
density of states. This approach requires only potential energy evaluations,
continues to improve the statistical quality of its results as the simulation
time is extended, and is applicable to both lattice and continuum systems. We
test the algorithm on the Lennard-Jones liquid and demonstrate good statistical
convergence properties.Comment: 7 pages, 4 figures. to appear in Journal of Chemical Physic
Structural Order in Glassy Water
We investigate structural order in glassy water by performing classical
molecular dynamics simulations using the extended simple point charge (SPC/E)
model of water. We perform isochoric cooling simulations across the glass
transition temperature at different cooling rates and densities. We quantify
structural order by orientational and translational order metrics. Upon cooling
the liquid into the glassy state, both the orientational order parameter
and translational order parameter increase. At T=0 K, the glasses fall
on a line in the - plane or {\it order map}.
The position of this line depends only on density and coincides with the
location in the order map of the inherent structures (IS) sampled upon cooling.
We evaluate the energy of the IS, , and find that both order
parameters for the IS are proportional to . We also study the
structural order during the transformation of low-density amorphous ice (LDA)
to high-density amorphous ice (HDA) upon isothermal compression and are able to
identify distinct regions in the order map corresponding to these glasses.
Comparison of the order parameters for LDA and HDA with those obtained upon
isochoric cooling indicates major structural differences between glasses
obtained by cooling and glasses obtained by compression. These structural
differences are only weakly reflected in the pair correlation function. We also
characterize the evolution of structural order upon isobaric annealing, leading
at high pressure to very-high density amorphous ice (VHDA).Comment: submitte
Nonequilibrium static growing length scales in supercooled liquids on approaching the glass transition
The small wavenumber behavior of the structure factor of
overcompressed amorphous hard-sphere configurations was previously studied for
a wide range of densities up to the maximally random jammed state, which can be
viewed as a prototypical glassy state [A. Hopkins, F. H. Stillinger and S.
Torquato, Phys. Rev. E, 86, 021505 (2012)]. It was found that a precursor to
the glassy jammed state was evident long before the jamming density was reached
as measured by a growing nonequilibrium length scale extracted from the volume
integral of the direct correlation function , which becomes long-ranged
as the critical jammed state is reached. The present study extends that work by
investigating via computer simulations two different atomic models: the
single-component Z2 Dzugutov potential in three dimensions and the
binary-mixture Kob-Andersen potential in two dimensions. Consistent with the
aforementioned hard-sphere study, we demonstrate that for both models a
signature of the glass transition is apparent well before the transition
temperature is reached as measured by the length scale determined from from the
volume integral of the direct correlation function in the single-component case
and a generalized direct correlation function in the binary-mixture case. The
latter quantity is obtained from a generalized Orstein-Zernike integral
equation for a certain decoration of the atomic point configuration. We also
show that these growing length scales, which are a consequence of the
long-range nature of the direct correlation functions, are intrinsically
nonequilibrium in nature as determined by an index that is a measure of
deviation from thermal equilibrium. It is also demonstrated that this
nonequilibrium index, which increases upon supercooling, is correlated with a
characteristic relaxation time scale.Comment: 26 pages, 14 figure
Supersaturated dispersions of rod-like viruses with added attraction
The kinetics of isotropic-nematic (I-N) and nematic-isotropic (N-I) phase
transitions in dispersions of rod-like {\it fd}-viruses are studied.
Concentration quenches were applied using pressure jumps in combination with
polarization microscopy, birefringence and turbidity measurements. The full
biphasic region could be accessed, resulting in the construction of a first
experimental analogue of the bifurcation diagram. The N-I spinodal points for
dispersions of rods with varying concentrations of depletion agents (dextran)
were obtained from orientation quenches, using cessation of shear flow in
combination with small angle light scattering. We found that the location of
the N-I spinodal point is independent of the attraction, which was confirmed by
theoretical calculations. Surprisingly, the experiments showed that also the
absolute induction time, the critical nucleus and the growth rate are
insensitive of the attraction, when the concentration is scaled to the distance
to the phase boundaries.Comment: 13 pages, 14 figures. accepted in Phsical Review
Scaling properties of critical bubble of homogeneous nucleation in stretched fluid of square-gradient density-functional model with triple-parabolic free energy
The square-gradient density-functional model with triple-parabolic free
energy is used to study homogeneous bubble nucleation in a stretched liquid to
check the scaling rule for the work of formation of the critical bubble as a
function of scaled undersaturation , the
difference in chemical potential between the bulk undersaturated
and saturated liquid divided by between the liquid
spinodal and saturated liquid. In contrast to our study, a similar
density-functional study for a Lennard-Jones liquid by Shen and Debenedetti [J.
Chem. Phys. {\bf 114}, 4149 (2001)] found that not only the work of formation
but other various quantities related to the critical bubble show the scaling
rule, however, we found virtually no scaling relationships in our model near
the coexistence. Although some quantities show almost perfect scaling relations
near the spinodal, the work of formation divided by the value deduced from the
classical nucleation theory shows no scaling in this model even though it
correctly vanishes at the spinodal. Furthermore, the critical bubble does not
show any anomaly near the spinodal as predicted many years ago. In particular,
our model does not show diverging interfacial width at the spinodal, which is
due to the fact that compressibility remains finite until the spinodal is
reached in our parabolic models.Comment: 10 pages, 10 figures, Journal of Chemical Physics accepted for
publicatio
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