313 research outputs found
Density minimum and liquid-liquid phase transition
We present a high-resolution computer simulation study of the equation of
state of ST2 water, evaluating the liquid-state properties at 2718 state
points, and precisely locating the liquid-liquid critical point (LLCP)
occurring in this model. We are thereby able to reveal the interconnected set
of density anomalies, spinodal instabilities and response function extrema that
occur in the vicinity of a LLCP for the case of a realistic, off-lattice model
of a liquid with local tetrahedral order. In particular, we unambiguously
identify a density minimum in the liquid state, define its relationship to
other anomalies, and show that it arises due to the approach of the liquid
structure to a defect-free random tetrahedral network of hydrogen bonds.Comment: 5 pages, 4 figure
Energy landscape of a simple model for strong liquids
We calculate the statistical properties of the energy landscape of a minimal
model for strong network-forming liquids. Dynamics and thermodynamic properties
of this model can be computed with arbitrary precision even at low
temperatures. A degenerate disordered ground state and logarithmic statistics
for the energy distribution are the landscape signatures of strong liquid
behavior. Differences from fragile liquid properties are attributed to the
presence of a discrete energy scale, provided by the particle bonds, and to the
intrinsic degeneracy of topologically disordered networks.Comment: Revised versio
Non-Gaussian energy landscape of a simple model for strong network-forming liquids: accurate evaluation of the configurational entropy
We present a numerical study of the statistical properties of the potential
energy landscape of a simple model for strong network-forming liquids. The
model is a system of spherical particles interacting through a square well
potential, with an additional constraint that limits the maximum number of
bonds, , per particle. Extensive simulations have been carried out
as a function of temperature, packing fraction, and . The dynamics
of this model are characterized by Arrhenius temperature dependence of the
transport coefficients and by nearly exponential relaxation of dynamic
correlators, i.e. features defining strong glass-forming liquids. This model
has two important features: (i) landscape basins can be associated with bonding
patterns; (ii) the configurational volume of the basin can be evaluated in a
formally exact way, and numerically with arbitrary precision. These features
allow us to evaluate the number of different topologies the bonding pattern can
adopt. We find that the number of fully bonded configurations, i.e.
configurations in which all particles are bonded to neighbors, is
extensive, suggesting that the configurational entropy of the low temperature
fluid is finite. We also evaluate the energy dependence of the configurational
entropy close to the fully bonded state, and show that it follows a logarithmic
functional form, differently from the quadratic dependence characterizing
fragile liquids. We suggest that the presence of a discrete energy scale,
provided by the particle bonds, and the intrinsic degeneracy of fully bonded
disordered networks differentiates strong from fragile behavior.Comment: Final version. Journal of Chemical Physics 124, 204509 (2006
Probing the Viscoelastic Properties of Aqueous Protein Solutions using Molecular Dynamics Simulations
We performed molecular dynamics simulations to investigate the viscoelastic
properties of aqueous protein solutions containing an antifreeze protein, a
toxin protein, and bovine serum albumin. These simulations covered a
temperature range from 280 K to 340 K. Our findings demonstrate that lower
temperatures are associated with higher viscosity as well as a lower bulk
modulus and speed of sound for all the systems studied. Furthermore, we observe
an increase in the bulk modulus and speed of sound as the temperature increases
up to a weak maximum while the viscosity decreases. Moreover, we analyzed the
influence of protein concentration on the viscoelastic properties of the
antifreeze protein solution. We observed a consistent increase in the bulk
modulus, speed of sound, and viscosity as the protein concentration increased.
Remarkably, our molecular dynamics simulations results closely resemble the
trends observed in Brillouin scattering experiments on aqueous protein
solutions. The similarity thus validates the use of simulations in studying the
viscoelastic properties of protein water solutions. Ultimately, this work
provides motivation for the integration of computer simulations with
experimental data and holds potential for advancing our understanding of both
simple and complex systems.Comment: 7 pages, and 7 figure
Relation Between the Widom line and the Strong-Fragile Dynamic Crossover in Systems with a Liquid-Liquid Phase Transition
We investigate, for two water models displaying a liquid-liquid critical
point, the relation between changes in dynamic and thermodynamic anomalies
arising from the presence of the liquid-liquid critical point. We find a
correlation between the dynamic fragility transition and the locus of specific
heat maxima (``Widom line'') emanating from the critical point.
Our findings are consistent with a possible relation between the previously
hypothesized liquid-liquid phase transition and the transition in the dynamics
recently observed in neutron scattering experiments on confined water. More
generally, we argue that this connection between and dynamic
crossover is not limited to the case of water, a hydrogen bond network forming
liquid, but is a more general feature of crossing the Widom line. Specifically,
we also study the Jagla potential, a spherically-symmetric two-scale potential
known to possess a liquid-liquid critical point, in which the competition
between two liquid structures is generated by repulsive and attractive ramp
interactions.Comment: 6 pages and 5 figure
Mode-coupling theory predictions for a limited valency attractive square-well model
Recently we have studied, using numerical simulations, a limited valency
model, i.e. an attractive square well model with a constraint on the maximum
number of bonded neighbors. Studying a large region of temperatures and
packing fractions , we have estimated the location of the liquid-gas
phase separation spinodal and the loci of dynamic arrest, where the system is
trapped in a disordered non-ergodic state. Two distinct arrest lines for the
system are present in the system: a {\it (repulsive) glass} line at high
packing fraction, and a {\it gel} line at low and . The former is
essentially vertical (-controlled), while the latter is rather horizontal
(-controlled) in the plane. We here complement the molecular
dynamics results with mode coupling theory calculations, using the numerical
structure factors as input. We find that the theory predicts a repulsive glass
line -- in satisfactory agreement with the simulation results -- and an
attractive glass line which appears to be unrelated to the gel line.Comment: 12 pages, 6 figures. To appear in J. Phys. Condens. Matter, special
issue: "Topics in Application of Scattering Methods for Investigation of
Structure and Dynamics of Soft Condensed Matter", Fiesole, November 200
Effect of bond lifetime on the dynamics of a short-range attractive colloidal system
We perform molecular dynamics simulations of short-range attractive colloid
particles modeled by a narrow (3% of the hard sphere diameter) square well
potential of unit depth. We compare the dynamics of systems with the same
thermodynamics but different bond lifetimes, by adding to the square well
potential a thin barrier at the edge of the attractive well. For permanent
bonds, the relaxation time diverges as the packing fraction
approaches a threshold related to percolation, while for short-lived bonds, the
-dependence of is more typical of a glassy system. At intermediate
bond lifetimes, the -dependence of is driven by percolation at low
, but then crosses over to glassy behavior at higher . We also
study the wavevector dependence of the percolation dynamics.Comment: Revised; 9 pages, 9 figure
Free energy and configurational entropy of liquid silica: fragile-to-strong crossover and polyamorphism
Recent molecular dynamics (MD) simulations of liquid silica, using the
``BKS'' model [Van Beest, Kramer and van Santen, Phys. Rev. Lett. {\bf 64},
1955 (1990)], have demonstrated that the liquid undergoes a dynamical crossover
from super-Arrhenius, or ``fragile'' behavior, to Arrhenius, or ``strong''
behavior, as temperature is decreased. From extensive MD simulations, we
show that this fragile-to-strong crossover (FSC) can be connected to changes in
the properties of the potential energy landscape, or surface (PES), of the
liquid. To achieve this, we use thermodynamic integration to evaluate the
absolute free energy of the liquid over a wide range of density and . We use
this free energy data, along with the concept of ``inherent structures'' of the
PES, to evaluate the absolute configurational entropy of the liquid. We
find that the temperature dependence of the diffusion coefficient and of
are consistent with the prediction of Adam and Gibbs, including in the region
where we observe the FSC to occur. We find that the FSC is related to a change
in the properties of the PES explored by the liquid, specifically an inflection
in the dependence of the average inherent structure energy. In addition, we
find that the high behavior of suggests that the liquid entropy might
approach zero at finite , behavior associated with the so-called Kauzmann
paradox. However, we find that the change in the PES that underlies the FSC is
associated with a change in the dependence of that elucidates how the
Kauzmann paradox is avoided in this system. Finally, we also explore the
relation of the observed PES changes to the recently discussed possibility that
BKS silica exhibits a liquid-liquid phase transition, a behavior that has been
proposed to underlie the observed polyamorphism of amorphous solid silica.Comment: 14 pages, 18 figure
Potential Energy Landscape Equation of State
Depth, number, and shape of the basins of the potential energy landscape are
the key ingredients of the inherent structure thermodynamic formalism
introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys.
Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only
on the volume dependence of these landscape properties is derived. Vibrational
and configurational contributions to pressure are sorted out in a transparent
way. Predictions are successfully compared with data from extensive molecular
dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure
Numerical study of the glass-glass transition in short-ranged attractive colloids
We report extensive numerical simulations in the {\it glass} region for a
simple model of short-ranged attractive colloids, the square well model. We
investigate the behavior of the density autocorrelation function and of the
static structure factor in the region of temperatures and packing fractions
where a glass-glass transition is expected according to theoretical
predictions. We strengthen our observations by studying both waiting time and
history dependence of the numerical results. We provide evidence supporting the
possibility that activated bond-breaking processes destabilize the attractive
glass, preventing the full observation of a sharp glass-glass kinetic
transition.Comment: 15 pages, 9 figures; Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted to J. Phys.: Condens. Matt.
- …