81 research outputs found
Polymer Brushes in Cylindrical Pores: Simulation versus Scaling Theory
The structure of flexible polymers endgrafted in cylindrical pores of
diameter D is studied as a function of chain length N and grafting density
\sigma, assuming good solvent conditions. A phenomenological scaling theory,
describing the variation of the linear dimensions of the chains with \sigma, is
developed and tested by Molecular Dynamics simulations of a bead-spring model.Comment: 35 pages, 38 figure
Surface Transitions for Confined Associating Mixtures
Thin films of binary mixtures that interact through isotropic forces and
directionally specific "hydrogen bonding" are considered through Monte Carlo
simulations. We show, in good agreement with experiment, that the single phase
of these mixtures can be stabilized or destabilized on confinement. These
results resolve a long standing controversy, since previous theories suggest
that confinement only stabilizes the single phase of fluid mixtures.Comment: LaTeX document, documentstyle[aps,preprint]{revtex}, psfig.sty,
bibtex, 13 pages, 4 figure
Coarse-grained models for fluids and their mixtures: Comparison of Monte Carlo studies of their phase behavior with perturbation theory and experiment
The prediction of the equation of state and the phase behavior of simple
fluids (noble gases, carbon dioxide, benzene, methane, short alkane chains) and
their mixtures by Monte Carlo computer simulation and analytic approximations
based on thermodynamic perturbation theory is discussed. Molecules are
described by coarse grained (CG) models, where either the whole molecule
(carbon dioxide, benzene, methane) or a group of a few successive CH_2 groups
(in the case of alkanes) are lumped into an effective point particle.
Interactions among these point particles are fitted by Lennard-Jones (LJ)
potentials such that the vapor-liquid critical point of the fluid is reproduced
in agreement with experiment; in the case of quadrupolar molecules a
quadrupole-quadrupole interaction is included. These models are shown to
provide a satisfactory description of the liquid-vapour phase diagram of these
pure fluids. Investigations of mixtures, using the Lorentz-Berthelot (LB)
combining rule, also produce satisfactory results if compared with experiment,
while in some previous attempts (in which polar solvents were modelled without
explicitly taking into account quadrupolar interaction), strong violations of
the LB rules were required. For this reason, the present investigation is a
step towards predictive modelling of polar mixtures at low computational cost.
These very simple coarse-grained models of small molecules developed here
should be useful e.g. for simulations of polymer solutions with such molecules
as solvent.Comment: J. Chem. Phys., accepte
Dynamics of a stretched nonlinear polymer chain
We study the relaxation dynamics of a coarse-grained polymer chain at
different degrees of stretching by both analytical means and numerical
simulations. The macromolecule is modelled as a string of beads, connected by
anharmonic springs, subject to a tensile force applied at the end monomer of
the chain while the other end is fixed at the origin of coordinates. The impact
of bond non-linearity on the relaxation dynamics of the polymer at different
degrees of stretching is treated analytically within the Gaussian
self-consistent approach (GSC) and then compared to simulation results derived
from two different methods: Monte-Carlo (MC) and Molecular Dynamics (MD).
At low and medium degrees of chain elongation we find good agreement between
GSC predictions and the Monte-Carlo simulations. However, for strongly
stretched chains the MD method, which takes into account inertial effects,
reveals two important aspects of the nonlinear interaction between monomers:
(i) a coupling and energy transfer between the damped, oscillatory normal modes
of the chain, and (ii) the appearance of non-vanishing contributions of a
continuum of frequencies around the characteristic modes in the power spectrum
of the normal mode correlation functions.Comment: 17 pages, 9 figure
Optical identification of hybrid magnetic and electric excitations in Dy3Fe5O12 garnet
Far-infrared spectra of magneto-dielectric Dy3Fe5O12 garnet were studied
between 13 and 100 cm-1 and at low temperatures between 5 and 80 K. A
combination of transmission, reflectivity, and rotating analyzer ellipsometry
was used to unambiguously identify the type of the dipole activity of the
infrared modes. In addition to purely dielectric and magnetic modes, we
observed several hybrid modes with a mixed magnetic and electric dipole
activity. These modes originate from the superexchange between magnetic moments
of Fe and Dy ions. Using 4x4 matrix formalism for materials with Mu=/=1, we
modeled the experimental optical spectra and determined the far-infrared
dielectric and magnetic permeability functions. The matching condition
Mu(Wh)*Se=Eps(Wh)*Sm for the oscillator strengths Se(m) explains the observed
vanishing of certain hybrid modes at Wh in reflectivity.Comment: paper and supplement appendi
Surface and capillary transitions in an associating binary mixture model
We investigate the phase diagram of a two-component associating fluid mixture
in the presence of selectively adsorbing substrates. The mixture is
characterized by a bulk phase diagram which displays peculiar features such as
closed loops of immiscibility. The presence of the substrates may interfere the
physical mechanism involved in the appearance of these phase diagrams, leading
to an enhanced tendency to phase separate below the lower critical solution
point. Three different cases are considered: a planar solid surface in contact
with a bulk fluid, while the other two represent two models of porous systems,
namely a slit and an array on infinitely long parallel cylinders. We confirm
that surface transitions, as well as capillary transitions for a large
area/volume ratio, are stabilized in the one-phase region. Applicability of our
results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical
Review E; corrected versio
Phase diagrams of classical spin fluids: the influence of an external magnetic field on the liquid-gas transition
The influence of an external magnetic field on the liquid-gas phase
transition in Ising, XY, and Heisenberg spin fluid models is studied using a
modified mean field theory and Gibbs ensemble Monte Carlo simulations. It is
demonstrated that the theory is able to reproduce quantitatively all
characteristic features of the field dependence of the critical temperature
T_c(H) for all the three models. These features include a monotonic decrease of
T_c with rising H in the case of the Ising fluid as well as a more complicated
nonmonotonic behavior for the XY and Heisenberg models. The nonmonotonicity
consists in a decrease of T_c with increasing H at weak external fields, an
increase of T_c with rising H in the strong field regime, and the existence of
a minimum in T_c(H) at intermediate values of H. Analytical expressions for
T_c(H) in the large field limit are presented as well. The magnetic para-ferro
phase transition is also considered in simulations and described within the
mean field theory.Comment: 14 pages, 12 figures (to be submitted to Phys. Rev. E
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