909 research outputs found
Consistent and transferrable coarse-grained model for semidilute polymer solutions in good solvent
We present a coarse-grained model for linear polymers with a tunable number
of effective atoms (blobs) per chain interacting by intra- and inter-molecular
potentials obtained at zero density. We show how this model is able to
accurately reproduce the universal properties of the underlying solution of
athermal linear chains at various levels of coarse-graining and in a range of
chain densities which can be widened by increasing the spatial resolution of
the multiblob representation, i.e., the number of blobs per chain. The present
model is unique in its ability to quantitatively predict thermodynamic and
large scale structural properties of polymer solutions deep in the semidilute
regime with a very limited computational effort, overcoming most of the
problems related to the simulations of semidilute polymer solutions in good
solvent conditions.Comment: 19 pages, 15 figures, 3 table
Trial wave functions for High-Pressure Metallic Hydrogen
Many body trial wave functions are the key ingredient for accurate Quantum
Monte Carlo estimates of total electronic energies in many electron systems. In
the Coupled Electron-Ion Monte Carlo method, the accuracy of the trial function
must be conjugated with the efficiency of its evaluation. We report recent
progress in trial wave functions for metallic hydrogen implemented in the
Coupled Electron-Ion Monte Carlo method. We describe and characterize several
types of trial functions of increasing complexity in the range of the coupling
parameter . We report wave function comparisons for
disordered protonic configurations and preliminary results for thermal
averages.Comment: 11 pages, 6 figures, submitted to Computer Physics Communication
Depletion effects in colloid-polymer solutions
The surface tension, the adsorption, and the depletion thickness of polymers
close to a single nonadsorbing colloidal sphere are computed by means of Monte
Carlo simulations. We consider polymers under good-solvent conditions and in
the thermal crossover region between good-solvent and behavior. In the
dilute regime we consider a wide range of values of , from (planar
surface) up to -50, while in the semidilute regime, for
( is the polymer concentration and is
its value at overlap), we only consider and 2. The results are
compared with the available theoretical predictions, verifying the existing
scaling arguments. Field-theoretical results, both in the dilute and in the
semidilute regime, are in good agreement with the numerical estimates for
polymers under good-solvent conditions.Comment: 26 pages, 12 figure
Excluded volume effects on the structure of a linear polymer under shear flow
The effect of excluded volume interactions on the structure of a polymer in
shear flow is investigated by Brownian Dynamics simulations for chains with
size . The main results concern the structure factor of chains of N=300 Kuhn segments, observed at a reduced shear rate
, where is the bare shear rate and
is the longest relaxation time of the chain. At low q, where anisotropic
global deformation is probed, the chain form factor is shown to match the form
factor of the continuous Rouse model under shear at the same reduced shear
rate, computed here for the first time in a wide range of wave vectors. At high
q, the chain structure factor evolves towards the isotropic equilibrium power
law typical of self-avoiding walk statistics. The matching between
excluded volume and ideal chains at small q, and the excluded volume power law
behavior at large q are observed for orthogonal to the main
elongation axis but not yet for along the elongation direction
itself, as a result of interferences with finite extensibility effects. Our
simulations support the existence of anisotropic shear blobs for polymers in
good solvent under shear flow for provided chains are sufficiently
long.Comment: 36 pages, 11 figures, submitted to J. Chem. Phy
Bulk viscosity of the Lennard-Jones system at the triple point by dynamical Non Equilibrium Molecular Dynamics
Non-equilibrium Molecular Dynamics (NEMD) calculations of the bulk viscosity
of the triple point Lennard-Jones fluid are performed with the aim of
investigating the origin of the observed disagreement between Green-Kubo
estimates and previous NEMD data. We show that a careful application of the
Doll's perturbation field, the dynamical NEMD method, the instantaneous form of
the perturbation and the "subtraction technique" provides a NEMD estimate of
the bulk viscosity at zero field in full agreement with the value obtained by
the Green-Kubo formula. As previously reported for the shear viscosity, we find
that the bulk viscosity exhibits a large linear regime with the field intensity
which confirms the Lennard-Jones fluid as a genuine Newtonian fluid even at
triple point.Comment: 27 pages, 11 figure
Coarse-graining polymer solutions: a critical appraisal of single- and multi-site models
We critically discuss and review the general ideas behind single- and
multi-site coarse-grained (CG) models as applied to macromolecular solutions in
the dilute and semi-dilute regime. We first consider single-site models with
zero-density and density-dependent pair potentials. We highlight advantages and
limitations of each option in reproducing the thermodynamic behavior and the
large-scale structure of the underlying reference model. As a case study we
consider solutions of linear homopolymers in a solvent of variable quality.
Secondly, we extend the discussion to multi-component systems presenting, as a
test case, results for mixtures of colloids and polymers. Specifically, we
found the CG model with zero-density potentials to be unable to predict
fluid-fluid demixing in a reasonable range of densities for mixtures of
colloids and polymers of equal size. For larger colloids, the polymer volume
fractions at which phase separation occurs are largely overestimated. CG models
with density-dependent potentials are somewhat less accurate than models with
zero-density potentials in reproducing the thermodynamics of the system and,
although they presents a phase separation, they significantly underestimate the
polymer volume fractions along the binodal. Finally, we discuss a general
multi-site strategy, which is thermodynamically consistent and fully
transferable with the number of sites, and that allows us to overcome most of
the limitations discussed for single-site models.Comment: 23 pages, 9 figures, 4 table
Integral-equation analysis of single-site coarse-grained models for polymer-colloid mixtures
We discuss the reliability of integral-equation methods based on several
commonly used closure relations in determining the phase diagram of
coarse-grained models of soft-matter systems characterized by mutually
interacting soft and hard-core particles. Specifically, we consider a set of
potentials appropriate to describe a system of hard-sphere colloids and linear
homopolymers in good solvent, and investigate the behavior when the soft
particles are smaller than the colloids, which is the regime of validity of the
coarse-grained models. Using computer-simulation results as a benchmark, we
find that the hypernetted-chain approximation provides accurate estimates of
thermodynamics and structure in the colloid-gas phase in which the density of
colloids is small. On the other hand, all closures considered appear to be
unable to describe the behavior of the mixture in the colloid-liquid phase, as
they cease to converge at polymer densities significantly smaller than those at
the binodal. As a consequence, integral equations appear to be unable to
predict a quantitatively correct phase diagram.Comment: 16 pages, 11 figures, 3 table
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