759 research outputs found
Simulation-based equation of state of the hard disk fluid and prediction of higher-order virial coefficients
We present new molecular dynamics results for the pressure of the pure hard
disk fluid up to the hexatic transition (about reduced density 0.9). The data
combined with the known virial coefficients (up to ) are used to build
an equation of state, to estimate higher-order virial coefficients, and also to
obtain a better value of . Finite size effects are discussed in detail.
The ``van der Waals-like'' loop reported in literature in the vicinity of the
fluid/hexatic transition is explained by suppressed density fluctuations in the
canonical ensemble. The inflection point on the pressure-density dependence is
predicted by the equation of state even if the hexatic phase simulation data
are not considered.Comment: 9 pages, 3 figures, presented at The Seventh Liblice Conference on
the Statistical Mechanics of Liquids (Lednice, Czech Republic, June 11--16,
2006
Comment on `Universal relation between the Kolmogorov-Sinai entropy and the thermodynamic entropy in simple liquids'
The intriguing relations between Kolmogorov-Sinai entropy and self diffusion
coefficients and the excess (thermodynamic) entropy found by Dzugutov and
collaborators do not appear to hold for hard sphere and hard disks systems.Comment: 1 page revte
Mode-coupling theory of the stress-tensor autocorrelation function of a dense binary fluid mixture
We present a generalized mode-coupling theory for a dense binary fluid
mixture. The theory is used to calculate molecular-scale renormalizations to
the stress-tensor autocorrelation function (STAF) and to the long-wavelength
zero-frequency shear viscosity. As in the case of a dense simple fluid, we find
that the STAF appears to decay as over an intermediate range of
time. The coefficient of this long-time tail is more than two orders of
magnitude larger than that obtained from conventional mode-coupling theory. Our
study focuses on the effect of compositional disorder on the decay of the STAF
in a dense mixture.Comment: Published; withdrawn since ordering in the archive gives misleading
impression of new publicatio
Computing the local pressure in molecular dynamics simulations
Computer simulations of inhomogeneous soft matter systems often require
accurate methods for computing the local pressure. We present a simple
derivation, based on the virial relation, of two equivalent expressions for the
local (atomistic) pressure in a molecular dynamics simulation. One of these
expressions, previously derived by other authors via a different route,
involves summation over interactions between particles within the region of
interest; the other involves summation over interactions across the boundary of
the region of interest. We illustrate our derivation using simulations of a
simple osmotic system; both expressions produce accurate results even when the
region of interest over which the pressure is measured is very small.Comment: 11 pages, 4 figure
Transport coefficients for dense hard-disk systems
A study of the transport coefficients of a system of elastic hard disks,
based on the use of Helfand-Einstein expressions is reported. The
self-diffusion, the viscosity, and the heat conductivity are examined with
averaging techniques especially appropriate for the use in event-driven
molecular dynamics algorithms with periodic boundary conditions. The density
and size dependence of the results is analyzed, and comparison with the
predictions from Enskog's theory is carried out. In particular, the behavior of
the transport coefficients in the vicinity of the fluid-solid transition is
investigated and a striking power law divergence of the viscosity in this
region is obtained, while all other examined transport coefficients show a drop
in that density range.Comment: submitted to PR
Long Wavelength Instability for Uniform Shear Flow
Uniform Shear Flow is a prototype nonequilibrium state admitting detailed
study at both the macroscopic and microscopic levels via theory and computer
simulation. It is shown that the hydrodynamic equations for this state have a
long wavelength instability. This result is obtained first from the
Navier-Stokes equations and shown to apply at both low and high densities.
Next, higher order rheological effects are included using a model kinetic
theory. The results are compared favorably to those from Monte Carlo
simulation.Comment: 12 pages, including 2 figure
Applicability of dynamic facilitation theory to binary hard disk systems
We numerically investigate the applicability of dynamic facilitation (DF) theory for glass-forming binary hard disk systems where supercompression is controlled by pressure. By using novel efficient algorithms for hard disks, we are able to generate equilibrium supercompressed states in an additive nonequimolar binary mixture, where microcrystallization and size segregation do not emerge at high average packing fractions. Above an onset pressure where collective heterogeneous relaxation sets in, we find that relaxation times are well described by a “parabolic law” with pressure. We identify excitations, or soft spots, that give rise to structural relaxation and find that they are spatially localized, their average concentration decays exponentially with pressure, and their associated energy scale is logarithmic in the excitation size. These observations are consistent with the predictions of DF generalized to systems controlled by pressure rather than temperature
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