292 research outputs found
Implicit Finite-Size Effects in Computer Simulations
The influence of periodic boundary conditions (implicit finite-size effects)
on the anisotropy of pair correlations in computer simulations is studied for a
dense classical fluid of pair-wise interacting krypton atoms near the triple
point. Molecular dynamics simulation data for the pair distribution function of
N-particle systems, as a function of radial distance, polar angle, and
azimuthal angle are compared directly with corresponding theoretical
predictions [L. R. Pratt and S. W. Haan, J. Chem. Phys. 74, 1864 (1981)]. For
relatively small systems of N=60, 80, and 108 atoms, significant angular
variation is observed, which is qualitatively, and in several cases
quantitatively, well predicted by theory. Finite-size corrections to the
spherically-averaged radial distribution function, however, are found to be
comparable to random statistical errors for runs of 10^5 time steps.Comment: plain TeX, 14 pages + 16 postscript figures, to appear Z. Phys.
Dynamics of a Rigid Rod in a Glassy Medium
We present simulations of the motion of a single rigid rod in a disordered
static 2d-array of disk-like obstacles. The rotational, , and
center-of-mass translational, , diffusion constants are calculated
for a wide range of rod length and density of obstacles . It is found
that follows the behavior predicted by kinetic theory for a hard
disk with an effective radius . A dynamic crossover is observed in
for comparable to the typical distance between neighboring
obstacles . Using arguments from kinetic theory and reptation, we
rationalize the scaling laws, dynamic exponents, and prefactors observed for
. In analogy with the enhanced translational diffusion observed in
deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for
.Comment: 8 pages, 4 figures. Major changes. To be published in Europhysics
Letter
Fractional Stokes-Einstein and Debye-Stokes-Einstein relations in a network forming liquid
We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein
(DSE) relations for translational and rotational motion in a prototypical model
of a network-forming liquid, the ST2 model of water. We find that the emergence
of ``fractional'' SE and DSE relations at low temperature is ubiquitous in this
system, with exponents that vary little over a range of distinct physical
regimes. We also show that the same fractional SE relation is obeyed by both
mobile and immobile dynamical heterogeneities of the liquid
Long wavelength structural anomalies in jammed systems
The structural properties of static, jammed packings of monodisperse spheres
in the vicinity of the jamming transition are investigated using large-scale
computer simulations. At small wavenumber , we argue that the anomalous
behavior in the static structure factor, , is consequential of an
excess of low-frequency, collective excitations seen in the vibrational
spectrum. This anomalous feature becomes more pronounced closest to the jamming
transition, such that at the transition point. We introduce an
appropriate dispersion relation that accounts for these phenomena that leads us
to relate these structural features to characteristic length scales associated
with the low-frequency vibrational modes of these systems. When the particles
are frictional, this anomalous behavior is suppressed providing yet more
evidence that jamming transitions of frictional spheres lie at lower packing
fractions that that for frictionless spheres. These results suggest that the
mechanical properties of jammed and glassy media may therefore be inferred from
measurements of both the static and dynamical structure factors.Comment: 8 pages, 6 figure captions. Completely revised version to appear in
Phys. Rev.
Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?
Recent neutron scattering experiments on aqueous salt solutions of
amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508
(2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of
t-butanol pairs, bridged by a chloride ion via
hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic
butanol-butanol contacts. Here we present a joint experimental/theoretical
study on the same system, using a combination of molecular dynamics simulations
and nuclear magnetic relaxation measurements. Both theory and experiment
clearly support the more intuitive scenario of an enhanced number of
hydrophobic contacts in the presence of the salt, as it would be expected for
purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although
our conclusions arrive at a structurally completely distinct scenario, the
molecular dynamics simulation results are within the experimental errorbars of
the Bowron and Finney work.Comment: 15 pages twocolumn revtex, 11 figure
Experimental observation of the intricate free-energy landscape for a soft glassy system
In the free energy landscape picture of glassy systems, the slow dynamics
characteristic of these systems is believed to be due to the existence of a
complicated free-energy landscape with many local minima. We show here that for
a colloidal glassy material multiple paths can be taken through the free energy
landscape, that can even lead to different 'final' non-ergodic states at the
late stages of aging. We provide clear experimental evidence for the
distinction of gel and glassy states in the system and show that for a range of
colloid concentrations, the transition to non-ergodicity can occur in either
direction (gel or glass), and may be accompanied by 'hesitations' between the
two directions. This shows that colloidal gels and glasses are merely global
free-energy minima in the same free energy landscape, and that the paths
leading to these minima can indeed be complicated.Comment: 5 pages, 5 figure
Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations
We present a study of a water-in-oil microemulsion in which surfactant coated
water nanodroplets are dispersed in the isotropic phase of the thermotropic
liquid crystal 5CB. As the temperature is lowered below the isotropic to
nematic phase transition of pure 5CB, the system displays a demixing transition
leading to a coexistence of a droplet rich isotropic phase with a droplet poor
nematic. The transition is anticipated, in the high T side, by increasing
pretransitional fluctuations in 5CB molecular orientation and in the
nanodroplet concentration. The observed phase behavior supports the notion that
the nanosized droplets, while large enough for their statistical behavior to be
probed via light scattering, are also small enough to act as impurities,
disturbing the local orientational ordering of the liquid crystal and thus
experiencing pretransitional attractive interaction mediated by paranematic
fluctuations. The pretransitional behavior, together with the topology of the
phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model
which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres
Triplet correlations in two-dimensional colloidal model liquids
Three-body distribution functions in classical fluids have been theoretically
investigated many times, but have never been measured directly. We present
experimental three-point correlation functions that are computed from particle
configurations measured by means of video-microscopy in two types of
quasi-two-dimensional colloidal model fluids: a system of charged colloidal
particles and a system of paramagnetic colloids. In the first system the
particles interact via a Yukawa potential, in the second via a potential
. We find for both systems very similar results: on increasing
the coupling between the particles one observes the gradual formation of a
crystal-like local order due to triplet correlations, even though the system is
still deep inside the fluid phase. These are mainly packing effects as is
evident from the close resemblance between the results for the two systems
having completely different pair-interaction potentials.Comment: many pages, 8 figures, contribution to the special issue in J.Phys.
Cond. Mat. of the CECAM meeting in LYON ''Many-body....'
Three-Particle Correlations in Simple Liquids
We use video microscopy to follow the phase-space trajectory of a
two-dimensional colloidal model liquid and calculate three-point correlation
functions from the measured particle configurations. Approaching the
fluid-solid transition by increasing the strength of the pair-interaction
potential, one observes the gradual formation of a crystal-like local order due
to triplet correlations, while being still deep inside the fluid phase.
Furthermore, we show that in a strongly interacting system the Born-Green
equation can be satisfied only with the full triplet correlation function but
not with three-body distribution functions obtained from superposing
pair-correlations (Kirkwood superposition approximation).Comment: 4 pages, submitted to PRL, experimental paper, 2nd version: Fig.1 and
two new paragraphs have been adde
Microscopic dynamics and relaxation processes in liquid Hydrogen Fluoride
Inelastic x-ray scattering and Brillouin light scattering measurements of the
dynamic structure factor of liquid hydrogen fluoride have been performed in the
temperature range. The data, analysed using a viscoelastic
model with a two timescale memory function, show a positive dispersion of the
sound velocity between the low frequency value and the high
frequency value . This finding confirms the existence of
a structural () relaxation directly related to the dynamical
organization of the hydrogen bonds network of the system. The activation energy
of the process has been extracted by the analysis of the temperature
behavior of the relaxation time that follows an Arrhenius law.
The obtained value for , when compared with that observed in another
hydrogen bond liquid as water, suggests that the main parameter governing the
-relaxation process is the number of the hydrogen bonds per molecule.Comment: 9 pages and 12 figure
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