1,116 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
Noro-Frenkel scaling in short-range square well: A Potential Energy Landscape study
We study the statistical properties of the potential energy landscape of a
system of particles interacting via a very short-range square-well potential
(of depth ), as a function of the range of attraction to provide
thermodynamic insights of the Noro and Frenkel [ M.G. Noro and D. Frenkel,
J.Chem.Phys. {\bf 113}, 2941 (2000)] scaling. We exactly evaluate the basin
free energy and show that it can be separated into a {\it vibrational}
(-dependent) and a {\it floppy} (-independent) component. We
also show that the partition function is a function of ,
explaining the equivalence of the thermodynamics for systems characterized by
the same second virial coefficient. An outcome of our approach is the
possibility of counting the number of floppy modes (and their entropy).Comment: 4 pages, 4 figures accepted for publication on PR
Saddles and softness in simple model liquids
We report a numerical study of saddles properties of the potential energy
landscape for soft spheres with different softness, i.e. different power n of
the interparticle repulsive potential. We find that saddle-based quantities
rescale into master curves once energies and temperatures are scaled by
mode-coupling temperature T_MCT, confirming and generalizing previous findings
obtained for Lennard-Jones like models.Comment: 2 pages, 2 figure
Diffusion Anomaly in an Associating Lattice Gas Model
We investigate the relation between thermodynamic and dynamic properties of
an associating lattice gas (ALG) model. The ALG combines a two dimensional
lattice gas with particles interacting through a soft core potential and
orientational degrees of freedom. From the competition between the directional
attractive forces and the soft core potential results two liquid phases, double
criticality and density anomaly. We study the mobility of the molecules in this
model by calculating the diffusion constant at a constant temperature, . We
show that has a maximum at a density and a minimum at a
density . Between these densities the diffusivity
differs from the one expected for normal liquids. We also show that in the
pressure-temperature phase-diagram the line of extrema in diffusivity is close
to the liquid-liquid critical point and it is inside the temperature of maximum
density (TMD) line.Comment: 12 pages, 9 figure
Structure and phase behavior of colloidal dumbbells with tunable attractive interactions
We investigate thermodynamic and structural properties of colloidal dumbbells
in the framework provided by the Reference Interaction Site Model (RISM) theory
of molecular fluids and Monte Carlo simulations. We consider two different
models: in the first one we set identical square-well attractions on the two
tangent spheres composing the molecule (SW-SW model); in the second scheme, one
of square-well interactions is switched off (HS-SW model). Appreciable
differences emerge between the physical properties of the two models.
Specifically, the behavior of SW-SW structure factors points
to the presence of a gas-liquid coexistence, as confirmed by subsequent fluid
phase equilibria calculations. Conversely, the HS-SW develops a low-
peak, signaling the presence of aggregates; such a process destabilizes the
gas-liquid phase separation, promoting at low temperatures the formation of a
cluster phase, whose structure depends on the system density. We further
investigate such differences by studying the phase behavior of a series of
intermediate models, obtained from the original SW-SW by progressively reducing
the depth of one square-well interaction. RISM structural predictions
positively reproduce the simulation data, including the rise of ) in
the SW-SW model and the low- peak in the HS-SW structure factor. As for the
phase behavior, RISM agrees with Monte Carlo simulations in predicting a
gas-liquid coexistence for the SW-SW model (though the critical parameters
appears overestimated by the theory) and its progressive disappearance moving
toward the HS-SW model.Comment: 12 pages, 13 figures, 1 table, 78 reference
Thermodynamic and structural aspects of the potential energy surface of simulated water
Relations between the thermodynamics and dynamics of supercooled liquids
approaching a glass transition have been proposed over many years. The
potential energy surface of model liquids has been increasingly studied since
it provides a connection between the configurational component of the partition
function on one hand, and the system dynamics on the other. This connection is
most obvious at low temperatures, where the motion of the system can be
partitioned into vibrations within a basin of attraction and infrequent
inter-basin transitions. In this work, we present a description of the
potential energy surface properties of supercooled liquid water. The dynamics
of this model has been studied in great details in the last years.
Specifically, we locate the minima sampled by the liquid by ``quenches'' from
equilibrium configurations generated via molecular dynamics simulations. We
calculate the temperature and density dependence of the basin energy,
degeneracy, and shape. The temperature dependence of the energy of the minima
is qualitatively similar to simple liquids, but has anomalous density
dependence. The unusual density dependence is also reflected in the
configurational entropy, the thermodynamic measure of degeneracy. Finally, we
study the structure of simulated water at the minima, which provides insight on
the progressive tetrahedral ordering of the liquid on cooling
Discovery of X-ray emission from the proto-stellar jet L1551 IRS5 (HH 154)
We have for the first time detected X-ray emission associated with a
proto-stellar jet, on the jet emanating from L1551 IRS5. The IRS5 proto-star is
hidden beyond a very large absorbing column density, making the direct
observation of the jet's emission possible. The observed X-ray emission is
likely associated with the shock ``working surface'', i.e. the interface
between the jet and the circumstellar medium. The X-ray luminosity emanating
from the jet is moderate, at LX ~ 3 times 10^29 erg/s, a significant fraction
of the luminosity normally associated with the coronal emission from young
stars. The spectrum of the X-ray emission is compatible with thermal emission
from a hot plasma, with T ~ 0.5 MK, fully compatible with the temperature
expected (on the basis of the jet's velocity) for the shock front produced by
the jet hitting the circumstellar medium.Comment: To appear in "Stellar Coronae in the Chandra and XMM Era", ASP
Conference Series in press, F. Favata & J. Drake ed
Dynamics in a supercooled molecular liquid: Theory and Simulations
We report extensive simulations of liquid supercooled states for a simple
three-sites molecular model, introduced by Lewis and Wahnstr"om [L. J. Lewis
and G. Wahnstr"om, Phys. Rev. E 50, 3865 (1994)] to mimic the behavior of
ortho-terphenyl. The large system size and the long simulation length allow to
calculate very precisely --- in a large q-vector range --- self and collective
correlation functions, providing a clean and simple reference model for
theoretical descriptions of molecular liquids in supercooled states. The time
and wavevector dependence of the site-site correlation functions are compared
with detailed predictions based on ideal mode-coupling theory, neglecting the
molecular constraints. Except for the wavevector region where the dynamics is
controlled by the center of mass (around 9 nm-1), the theoretical predictions
compare very well with the simulation data.
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
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