19,803 research outputs found
Two-dimensional Riemannian and Lorentzian geometries from second order ODEs
In this note we give an alternative geometrical derivation of the results
recently presented by Garcia-Godinez, Newman and Silva-Ortigoza in [1] on the
class of all two-dimensional riemannian and lorentzian metrics from 2nd order
ODEs which are in duality with the two dimensional Hamilton-Jacobi equation. We
show that, as it happens in the Null Surface Formulation of General Relativity,
the Wuschmann-like condition can be obtained as a requirement of a vanishing
torsion tensor. Furthermore, from these second order ODEs we obtain the
associated Cartan connections.Comment: 9 pages, final version to appear in J. Math. Phy
Molecular dynamics study of the glass transition in confined water
A molecular dynamics simulation of SPC/E water confined in a Silica pore is
presented. The pore has been constructed to reproduce the average properties of
a pore of Vycor glass. Due to the confinement and to the presence of a strong
hydrophilic surface, the dynamic behaviour of the liquid appears to be strongly
dependent on the hydration level. The approach to the glass transition of
confined water is investigated on lowering hydration and on supercooling in the
framework of Mode Coupling Theories. At higher hydrations two quite distinct
subsets of water molecules are detectable. Those belonging to the first layer
close to the substrate suffer a severe slowing down, while the remaining ones
display a scenario typical of supercooled liquids approaching the kinetic glass
transition.Comment: Talk presented at "International Workshop on Dynamics in
Confinement", Grenoble January 2000, to be appear in J. Phys. Franc
Double dynamical regime of confined water
The Van Hove self correlation function of water confined in a silica pore is
calculated from Molecular Dynamics trajectories upon supercooling. At long time
in the relaxation region we found that the behaviour of the real space
time dependent correlators can be decomposed in a very slow, almost frozen,
dynamics due to the bound water close to the substrate and a faster dynamics of
the free water which resides far from the confining surface. For free water we
confirm the evidences of an approach to a crossover mode coupling transition,
previously found in Q space. In the short time region we found that the two
dynamical regimes are overimposed and cannot be distinguished. This shows that
the interplay between the slower and the faster dynamics emerges in going from
early times to the relaxation region, where a layer analysis of the
dynamical properties can be performed.Comment: 6 pages with 9 figures. RevTeX. Accepted for pulbication in J. Phys.
Cond. Mat
Slow dynamics of a confined supercooled binary mixture II: Q space analysis
We report the analysis in the wavevector space of the density correlator of a
Lennard Jones binary mixture confined in a disordered matrix of soft spheres
upon supercooling. In spite of the strong confining medium the behavior of the
mixture is consistent with the Mode Coupling Theory predictions for bulk
supercooled liquids. The relaxation times extracted from the fit of the density
correlator to the stretched exponential function follow a unique power law
behavior as a function of wavevector and temperature. The von Schweidler
scaling properties are valid for an extended wavevector range around the peak
of the structure factor. The parameters extracted in the present work are
compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys.
Rev.
Glass transition and layering effects in confined water: a computer simulation study
Single particle dynamics of water confined in a nanopore is studied through
Computer Molecular Dynamics. The pore is modeled to represent the average
properties of a pore of Vycor glass. Dynamics is analyzed at different
hydration levels and upon supercooling. At all hydration levels and all
temperatures investigated a layering effect is observed due to the strong
hydrophilicity of the substrate. The time density correlators show, already at
ambient temperature, strong deviations from the Debye and the stretched
exponential behavior. Both on decreasing hydration level and upon supercooling
we find features that can be related to the cage effect typical of a
supercooled liquid undergoing a kinetic glass transition. Nonetheless the
behavior predicted by Mode Coupling Theory can be observed only by carrying out
a proper shell analysis of the density correlators. Water molecules within the
first two layers from the substrate are in a glassy state already at ambient
temperature (bound water). The remaining subset of molecules (free water)
undergoes a kinetic glass transition; the relaxation of the density correlators
agree with the main predictions of the theory. From our data we can predict the
temperature of structural arrest of free water.Comment: 14 pages, 15 figures inserted in the text, to be published in J.
Chem. Phys. (2000
A route to explain water anomalies from results on an aqueous solution of salt
In this paper we investigate the possibility to detect the hypothesized
liquid-liquid critical point of water in supercooled aqueous solutions of
salts. Molecular dynamics computer simulations are conducted on bulk TIP4P
water and on an aqueous solution of sodium chloride in TIP4P water, with
concentration c = 0.67 mol/kg. The liquid-liquid critical point is found both
in the bulk and in the solution. Its position in the thermodynamic plane shifts
to higher temperature and lower pressure for the solution. Comparison with
available experimental data allowed us to produce the phase diagrams of both
bulk water and the aqueous solution as measurable in experiments. Given the
position of the liquid-liquid critical point in the solution as obtained from
our simulations, the experimental determination of the hypothesized
liquid-liquid critical point of water in aqueous solutions of salts appears
possible.Comment: 5 pages, 6 figures. Accepted for publication on the Journal of
Chemical Physics (2010)
Structural Properties of High and Low Density Water in a Supercooled Aqueous Solution of Salt
We consider and compare the structural properties of bulk TIP4P water and of
a sodium chloride aqueous solution in TIP4P water with concentration c = 0.67
mol/kg, in the metastable supercooled region. In a previous paper [D.
Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)] we found
in both systems the presence of a liquid-liquid critical point (LLCP). The LLCP
is believed to be the end point of the coexistence line between a high density
liquid (HDL) and a low density liquid (LDL) phase of water. In the present
paper we study the different features of water-water structure in HDL and LDL
both in bulk water and in the solution. We find that the ions are able to
modify the bulk LDL structure, rendering water-water structure more similar to
the bulk HDL case. By the study of the hydration structure in HDL and LDL, a
possible mechanism for the modification of the bulk LDL structure in the
solution is identified in the substitution of the oxygen by the chloride ion in
oxygen coordination shells.Comment: 10 pages, 10 figures, 2 tables. Accepted for publication on J. Phys.
Chem
Mode Coupling relaxation scenario in a confined glass former
Molecular dynamics simulations of a Lennard-Jones binary mixture confined in
a disordered array of soft spheres are presented. The single particle dynamical
behavior of the glass former is examined upon supercooling. Predictions of mode
coupling theory are satisfied by the confined liquid. Estimates of the
crossover temperature are obtained by power law fit to the diffusion
coefficients and relaxation times of the late region. The exponent
of the von Schweidler law is also evaluated. Similarly to the bulk, different
values of the exponent are extracted from the power law fit to the
diffusion coefficients and relaxation times.Comment: 5 pages, 4 figures, changes in the text, accepted for publication on
Europhysics Letter
Computer simulation of the phase diagram for a fluid confined in a fractal and disordered porous material
We present a grand canonical Monte Carlo simulation study of the phase
diagram of a Lennard-Jones fluid adsorbed in a fractal and highly porous
aerogel. The gel environment is generated from an off-lattice diffusion limited
cluster-cluster aggregation process. Simulations have been performed with the
multicanonical ensemble sampling technique. The biased sampling function has
been obtained by histogram reweighting calculations. Comparing the confined and
the bulk system liquid-vapor coexistence curves we observe a decrease of both
the critical temperature and density in qualitative agreement with experiments
and other Monte Carlo studies on Lennard-Jones fluids confined in random
matrices of spheres. At variance with these numerical studies we do not observe
upon confinement a peak on the liquid side of the coexistence curve associated
with a liquid-liquid phase coexistence. In our case only a shouldering of the
coexistence curve appears upon confinement. This shoulder can be associated
with high density fluctuations in the liquid phase. The coexisting vapor and
liquid phases in our system show a high degree of spatial disorder and
inhomogeneity.Comment: 8 pages, 8 figures, to be published in Phys. Rev.
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