3,038 research outputs found
Equilibrium properties of highly asymmetric star-polymer mixtures
We employ effective interaction potentials to study the equilibrium structure
and phase behavior of highly asymmetric mixtures of star polymers. We consider
in particular the influence of the addition of a component with a small number
of arms and a small size on a concentrated solution of large stars with a high
functionality. By employing liquid integral equation theories we examine the
evolution of the correlation functions of the big stars upon addition of the
small ones, finding a loss of structure that can be attributed to a weakening
of the repulsions between the large stars due to the presence of the small
ones. We analyze this phenomenon be means of a generalized depletion mechanism
which is supported by computer simulations. By applying thermodynamic
perturbation theory we draw the phase diagram of the asymmetric mixture,
finding that the addition of small stars melts the crystal formed by the big
ones. A systematic comparison between the two- and effective one-component
descriptions of the mixture that corroborates the reliability of the
generalized depletion picture is also carried out.Comment: 26 pages, 9 figures, submitted to Phys. Rev.
Reverse-selective diffusion in nanocomposite membranes
The permeability of certain polymer membranes with impenetrable
nanoinclusions increases with the particle volume fraction (Merkel et al.,
Science, 296, 2002). This intriguing observation contradicts even qualitative
expectations based on Maxwell's classical theory of conduction/diffusion in
composites with homogeneous phases. This letter presents a simple theoretical
interpretation based on classical models of diffusion and polymer physics. An
essential feature of the theory is a polymer-segment depletion layer at the
inclusion-polymer interface. The accompanying increase in free volume leads to
a significant increase in the local penetrant diffusivity, which, in turn,
increases the bulk permeability while exhibiting reverse selectivity. This
model captures the observed dependence of the bulk permeability on the
inclusion size and volume fraction, providing a straightforward connection
between membrane microstructure and performance
Stabilization of colloidal suspensions by means of highly-charged nanoparticles
We employ a novel Monte Carlo simulation scheme to elucidate the
stabilization of neutral colloidal microspheres by means of highly-charged
nanoparticles [V. Tohver et al., Proc. Natl. Acad. Sci. U.S.A. 98, 8950
(2001)]. In accordance with the experimental observations, we find that small
nanoparticle concentrations induce an effective repulsion that prevents
gelation caused by the intrinsic van der Waals attraction between colloids.
Higher nanoparticle concentrations induce an attractive potential which is,
however, qualitatively different from the regular depletion attraction. We also
show how colloid-nanoparticle size asymmetry and nanoparticle charge can be
used to manipulate the effective interactions.Comment: Accepted for publication in Physical Review Letters. See also S.
Karanikas and A.A. Louis, cond-mat/0411279. Updated to synchronize with
published versio
Description of the fluctuating colloid-polymer interface
To describe the full spectrum of surface fluctuations of the interface
between phase-separated colloid-polymer mixtures from low scattering vector q
(classical capillary wave theory) to high q (bulk-like fluctuations), one must
take account of the interface's bending rigidity. We find that the bending
rigidity is negative and that on approach to the critical point it vanishes
proportionally to the interfacial tension. Both features are in agreement with
Monte Carlo simulations.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in Phys. Rev.
Let
Colloidal stabilization via nanoparticle haloing
We present a detailed numerical study of effective interactions between
micron-sized silica spheres, induced by highly charged zirconia nanoparticles.
It is demonstrated that the effective interactions are consistent with a
recently discovered mechanism for colloidal stabilization. In accordance with
the experimental observations, small nanoparticle concentrations induce an
effective repulsion that counteracts the intrinsic van der Waals attraction
between the colloids and thus stabilizes the suspension. At higher nanoparticle
concentrations an attractive potential is recovered, resulting in reentrant
gelation. Monte Carlo simulations of this highly size-asymmetric mixture are
made possible by means of a geometric cluster Monte Carlo algorithm. A
comparison is made to results obtained from the Ornstein-Zernike equations with
the hypernetted-chain closure
Accurate description of bulk and interfacial properties in colloid-polymer mixtures
Large-scale Monte Carlo simulations of a phase-separating colloid-polymer
mixture are performed and compared to recent experiments. The approach is based
on effective interaction potentials in which the central monomers of
self-avoiding polymer chains are used as effective coordinates. By
incorporating polymer nonideality together with soft colloid-polymer repulsion,
the predicted binodal is in excellent agreement with recent experiments. In
addition, the interfacial tension as well as the capillary length are in
quantitative agreement with experimental results obtained at a number of points
in the phase-coexistence region, without the use of any fit parametersComment: 4 pages, 4 figure
Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity
Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.Comment: 14 pages, 10 figure
Monte Carlo simulations of the solid-liquid transition in hard spheres and colloid-polymer mixtures
Monte Carlo simulations at constant pressure are performed to study
coexistence and interfacial properties of the liquid-solid transition in hard
spheres and in colloid-polymer mixtures. The latter system is described as a
one-component Asakura-Oosawa (AO) model where the polymer's degrees of freedom
are incorporated via an attractive part in the effective potential for the
colloid-colloid interactions. For the considered AO model, the polymer
reservoir packing fraction is eta_p^r=0.1 and the colloid-polymer size ratio is
q=sigma_p/\sigma=0.15 (with sigma_p and sigma the diameter of polymers and
colloids, respectively). Inhomogeneous solid-liquid systems are prepared by
placing the solid fcc phase in the middle of a rectangular simulation box
creating two interfaces with the adjoined bulk liquid. By analyzing the growth
of the crystalline region at various pressures and for different system sizes,
the coexistence pressure p_co is obtained, yielding p_co=11.576 k_BT/sigma^3
for the hard sphere system and p_co=8.0 k_BT/sigma^3 for the AO model (with k_B
the Boltzmann constant and T the temperature). Several order parameters are
introduced to distinguish between solid and liquid phases and to describe the
interfacial properties. From the capillary-wave broadening of the solid-liquid
interface, the interfacial stiffness is obtained for the (100) crystalline
plane, giving the values gamma=0.49 k_BT/sigma^2 for the hard-sphere system and
gamma=0.95 k_BT/sigma^2 for the AO model.Comment: 11 pages, 13 figure
Third-order thermodynamic perturbation theory for effective potentials that model complex fluids
We have performed Monte Carlo simulations to obtain the thermodynamic properties of fluids with two kinds of hard-core plus attractive-tail or oscillatory potentials. One of them is the square-well potential with small well width. The other is a model potential with oscillatory and decaying tail. Both model potentials are suitable for modeling the effective potential arising in complex fluids and fluid mixtures with extremely-large-size asymmetry, as is the case of the solvent-induced depletion interactions in colloidal dispersions. For the former potential, the compressibility factor, the excess energy, the constant-volume excess heat capacity, and the chemical potential have been obtained. For the second model potential only the first two of these quantities have been obtained. The simulations cover the whole density range for the fluid phase and several temperatures. These simulation data have been used to test the performance of a third-order thermodynamic perturbation theory (TPT) recently developed by one of us [ S. Zhou Phys. Rev. E 74 031119 (2006)] as compared with the well-known second-order TPT based on the macroscopic compressibility approximation due to Barker and Henderson. It is found that the first of these theories provides much better accuracy than the second one for all thermodynamic properties analyzed for the two effective potential models
Surface-mediated attraction between colloids
We investigate the equilibrium properties of a colloidal solution in contact
with a soft interface. As a result of symmetry breaking, surface effects are
generally prevailing in confined colloidal systems. In this Letter, particular
emphasis is given to surface fluctuations and their consequences on the local
(re)organization of the suspension. It is shown that particles experience a
significant effective interaction in the vicinity of the interface. This
potential of mean force is always attractive, with range controlled by the
surface correlation length. We suggest that, under some circumstances,
surface-induced attraction may have a strong influence on the local particle
distribution
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