5,646 research outputs found
A Wigner-Seitz model of charged lamellar colloidal dispersions
A concentrated suspension of lamellar colloidal particles (e. g. clay) is
modelled by considering a single, uniformly charged, finite platelet confined
with co- and counterions to a Wigner-Seitz (WS) cell. The system is treated
within Poisson-Boltzmann theory, with appropriate boundary conditions on the
surface of the WS cell, supposed to account for the confinement effect of
neighbouring platelets. Expressions are obtained for the free energy, osmotic
and disjoining pressures and the capacitance in terms of the local
electrostatic potential and the co- and counterion density profiles. Explicit
solutions of the linearized Poisson-Boltzmann (LPB) equation are obtained for
circular and square platelets placed at the centre of a cylindrical or
parallelepipedic cell. The resulting free energy is found to go through a
minimum as a function of the aspect ratio of the cell, for any given volume
(determined by the macroscopic concentration of platelets), platelet surface
charge and salt concentration. The optimum aspect ratio is found to be nearly
independent of the two latter physical parameters. The osmotic and disjoining
pressures are found to coincide at the free energy minimum, while the total
quadrupole moment of the electric double-layer formed by the platelet and the
surrounding co- and counterions vanishes simultaneously. The osmotic
equation-of-state is calculated for a variety of physical conditions. The limit
of vanishing platelet concentration is considered in some detail, and the force
acting between two coaxial platelets is calculated in that limit as a function
of their separation.Comment: 21 pages, RevTeX, 12 figures available on request to
[email protected], submitted to Physical Review
The plasma-insulator transition of spin-polarized Hydrogen
A mixed classical-quantum density functional theory is used to calculate pair
correlations and the free energy of a spin-polarized Hydrogen plasma. A
transition to an atomic insulator phase is estimated to occur around r_s=2.5 at
T=10^4K, and a pressure . Spin polarization is imposed to
prevent the formation of H_2 molecules.Comment: 10 pages, 4 figure
Onsager model for a variable dielectric permittivity near an interface
Using a generalisation of an Onsager type approach, we are able to predict a
dielectric permittivity profile of an inhomogeneous dipolar fluid in the
presence of a dielectric interface. The reaction and cavity fields are
calculated semi-analytically using bispherical coordinates. An asymptotic
expression for the local permittivity is derived as a function of distance from
the interface.Comment: 20 pages, 4 figures, submitted to Molecular Physic
Clustering, conductor-insulator transition and phase separation of an ultrasoft model of electrolytes
We investigate the clustering and phase separation of a model of ultrasoft,
oppositely charged macroions by a combination of Monte Carlo and Molecular
Dynamics simulations. Static and dynamic diagnostics, including the dielectric
permittivity and the electric conductivity of the model, show that ion pairing
induces a sharp conductor-insulator transition at low temperatures and
densities, which impacts the separation into dilute and concentrated phases
below a critical temperature. Preliminary evidence is presented for a possible
tricritical nature of the phase diagram of the model.Comment: 5 pages, 5 figure
Reversible gelation and dynamical arrest of dipolar colloids
We use molecular dynamics simulations of a simple model to show that
dispersions of slightly elongated colloidal particles with long-range dipolar
interactions, like ferrofluids, can form a physical (reversible) gel at low
volume fractions. On cooling, the particles first self-assemble into a
transient percolating network of cross-linked chains, which, at much lower
temperatures, then undergoes a kinetic transition to a dynamically arrested
state with broken ergodicity. This transition from a transient to a frozen gel
is characterised by dynamical signatures reminiscent of jamming in much denser
dispersions.Comment: 6 pages, 7 figure
Multi-scale coarse-graining of diblock copolymer self-assembly: from monomers to ordered micelles
Starting from a microscopic lattice model, we investigate clustering,
micellization and micelle ordering in semi-dilute solutions of AB diblock
copolymers in a selective solvent. To bridge the gap in length scales, from
monomers to ordered micellar structures, we implement a two-step coarse
graining strategy, whereby the AB copolymers are mapped onto ``ultrasoft''
dumbells with monomer-averaged effective interactions between the centres of
mass of the blocks. Monte Carlo simulations of this coarse-grained model yield
clear-cut evidence for self-assembly into micelles with a mean aggregation
number n of roughly 100 beyond a critical concentration. At a slightly higher
concentration the micelles spontaneously undergo a disorder-order transition to
a cubic phase. We determine the effective potential between these micelles from
first principles.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett
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