131 research outputs found
Stability of a d-Dimensional Thin-Shell Wormhole Surrounded by Quintessence
We study the stability of different higher dimensional thin–shell wormholes (HDTSW) in general relativity with a cosmological constant. We show that a d-dimensional thin–shell wormhole surrounded by quintessence can have three different throat geometries: spherical, planar and hyperbolic. Unlike the spherical geometry, the planar and hyperbolic geometries allow different topologies that can be interpreted as higher-dimensional domain walls or branes connecting two universes. To construct these geometries, we use the cut-and-paste procedure by joining two identical vacuum space-time solutions. Properties such as the null energy condition and geodesics are also studied. A linear stability analysis around the static solutions is carried out, taking into account a more general HDTSW geometry than previous works, so it is possible to recover other well-known stability HDTSW conditions
Phase separation in star polymer-colloid mixtures
We examine the demixing transition in star polymer-colloid mixtures for star
arm numbers f=2,6,16,32 and different star-colloid size ratios. Theoretically,
we solve the thermodynamically self-consistent Rogers-Young integral equations
for binary mixtures using three effective pair potentials obtained from direct
molecular computer simulations. The numerical results show a spinodal
instability. The demixing binodals are approximately calculated, and found to
be consistent with experimental observations.Comment: 4 pages, 4 figures, submitted to PR
Entropy-induced separation of star polymers in porous media
We present a quantitative picture of the separation of star polymers in a
solution where part of the volume is influenced by a porous medium. To this
end, we study the impact of long-range-correlated quenched disorder on the
entropy and scaling properties of -arm star polymers in a good solvent. We
assume that the disorder is correlated on the polymer length scale with a
power-law decay of the pair correlation function . Applying
the field-theoretical renormalization group approach we show in a double
expansion in and that there is a range of
correlation strengths for which the disorder changes the scaling
behavior of star polymers. In a second approach we calculate for fixed space
dimension and different values of the correlation parameter the
corresponding scaling exponents that govern entropic effects. We
find that , the deviation of from its mean field value
is amplified by the disorder once we increase beyond a threshold. The
consequences for a solution of diluted chain and star polymers of equal
molecular weight inside a porous medium are: star polymers exert a higher
osmotic pressure than chain polymers and in general higher branched star
polymers are expelled more strongly from the correlated porous medium.
Surprisingly, polymer chains will prefer a stronger correlated medium to a less
or uncorrelated medium of the same density while the opposite is the case for
star polymers.Comment: 14 pages, 7 figure
Scaling of Star Polymers with one to 80 Arms
We present large statistics simulations of 3-dimensional star polymers with
up to arms, and with up to 4000 monomers per arm for small values of
. They were done for the Domb-Joyce model on the simple cubic lattice. This
is a model with soft core exclusion which allows multiple occupancy of sites
but punishes each same-site pair of monomers with a Boltzmann factor . We
use this to allow all arms to be attached at the central site, and we use the
`magic' value to minimize corrections to scaling. The simulations are
made with a very efficient chain growth algorithm with resampling, PERM,
modified to allow simultaneous growth of all arms. This allows us to measure
not only the swelling (as observed from the center-to-end distances), but also
the partition sum. The latter gives very precise estimates of the critical
exponents . For completeness we made also extensive simulations of
linear (unbranched) polymers which give the best estimates for the exponent
.Comment: 7 pages, 7 figure
Polydisperse star polymer solutions
We analyze the effect of polydispersity in the arm number on the effective
interactions, structural correlations and the phase behavior of star polymers
in a good solvent. The effective interaction potential between two star
polymers with different arm numbers is derived using scaling theory. The
resulting expression is tested against monomer-resolved molecular dynamics
simulations. We find that the theoretical pair potential is in agreement with
the simulation data in a much wider polydispersity range than other proposed
potentials. We then use this pair potential as an input in a many-body theory
to investigate polydispersity effects on the structural correlations and the
phase diagram of dense star polymer solutions. In particular we find that a
polydispersity of 10%, which is typical in experimental samples, does not
significantly alter previous findings for the phase diagram of monodisperse
solutions.Comment: 14 pages, 7 figure
An integral equation approach to effective interactions between polymers in solution
We use the thread model for linear chains of interacting monomers, and the
``polymer reference interaction site model'' (PRISM) formalism to determine the
monomer-monomer pair correlation function for dilute and
semi-dilute polymer solutions, over a range of temperatures from very high
(where the chains behave as self-avoiding walks) to below the
temperature, where phase separation sets in. An inversion procedure, based on
the HNC integral equation, is used to extract the effective pair potential
between ``average'' monomers on different chains. An accurate relation between
, [the pair correlation function between the polymer
centers of mass (c.m.)], and the intramolecular form factors is then used to
determine , and subsequently extract the effective c.m.-c.m. pair
potential by a similar inversion procedure. depends on
temperature and polymer concentration, and the predicted variations are in
reasonable agreement with recent simulation data, except at very high
temperatures, and below the temperature.Comment: 13 pages, 13 figures, revtex ; revised versio
Effective interactions between star polymers and colloidal particles
Using monomer-resolved Molecular Dynamics simulations and theoretical
arguments based on the radial dependence of the osmotic pressure in the
interior of a star, we systematically investigate the effective interactions
between hard, colloidal particles and star polymers in a good solvent. The
relevant parameters are the size ratio q between the stars and the colloids, as
well as the number of polymeric arms f (functionality) attached to the common
center of the star. By covering a wide range of q's ranging from zero (star
against a flat wall) up to about 0.75, we establish analytical forms for the
star-colloid interaction which are in excellent agreement with simulation
results. A modified expression for the star-star interaction for low
functionalities, f < 10 is also introduced.Comment: 37 pages, 14 figures, preprint-versio
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