90 research outputs found
Dynamics of colloidal particles with capillary interactions
We investigate the dynamics of colloids at a fluid interface driven by
attractive capillary interactions. At submillimeter length scales, the
capillary attraction is formally analogous to two-dimensional gravity. In
particular it is a non-integrable interaction and it can be actually relevant
for collective phenomena in spite of its weakness at the level of the pair
potential. We introduce a mean-field model for the dynamical evolution of the
particle number density at the interface. For generic values of the physical
parameters the homogeneous distribution is found to be unstable against
large-scale clustering driven by the capillary attraction. We also show that
for the instability to be observable, the appropriate values for the relevant
parameters (colloid radius, surface charge, external electric field, etc.) are
experimentally well accessible. Our analysis contributes to current studies of
the structure and dynamics of systems governed by long-ranged interactions and
points towards their experimental realizations via colloidal suspensions.Comment: Matches version accepted for publication. New refs. added, misprints
corrected in figs.6,8,9,1
Onset of anomalous diffusion in colloids confined to quasi-monolayers
It has been recently shown that a colloidal monolayer, e.g., formed at a
fluid interface or by means of a suitable confining potential, exhibits
anomalous collective diffusion. This is a consequence of the hydrodynamic
interactions mediated by the three-dimensional (3D) ambient fluid when the
particles are confined to reside on a two-dimensional (2D) manifold. We study
theoretically and with numerical simulations the crossover from normal to
anomalous diffusion as the particles are, in real systems, confined by a 3D
external potential and thus have the possibility to fluctuate out of the 2D
manifold, thus forming actually a quasi-monolayer.Comment: 13 pages, 9 figure
Current Conservation in the Covariant Quark-Diquark Model of the Nucleon
The description of baryons as fully relativistic bound states of quark and
glue reduces to an effective Bethe-Salpeter equation with quark-exchange
interaction when irreducible 3-quark interactions are neglected and separable
2-quark (diquark) correlations are assumed. This covariant quark-diquark model
of baryons is studied with the inclusion of the quark substructure of the
diquark correlations. In order to maintain electromagnetic current conservation
it is then necessary to go beyond the impulse approximation. A conserved
current is obtained by including the coupling of the photon to the exchanged
quark and direct ``seagull'' couplings to the diquark structure. Adopting a
simple dynamical model of constituent quarks and exploring various
parametrisations of scalar diquark correlations, the nucleon Bethe-Salpeter
equation is solved and the proton and neutron electromagnetic form factors are
calculated numerically. The resulting magnetic moments are still about 50% too
small, the improvements necessary to remedy this are discussed. The results
obtained in this framework provide an excellent description of the electric
form factors (and charge radii) of the proton, up to a photon momentum transfer
of 3.5GeV^2, and the neutron.Comment: 30 Pages, LaTeX2e, revised version, minor modifiactions to the text,
some phrases eliminated, some remarks adde
Strong Effect of Weak Charging in Suspensions of Anisotropic Colloids
Suspensions of hard colloidal particles frequently serve as model systems in
studies on fundamental aspects of phase transitions. But often colloidal
particles that are considered as ``hard'' are in fact weakly charged. If the
colloids are spherical, weak charging has a only a weak effect on the
structural properties of the suspension, which can be easily corrected for.
However, this does not hold for anisotropic particles.
We introduce a model for the interaction potential between charged ellipsoids
of revolution (spheroids) based on the Derjaguin approximation of
Debye--H\"uckel Theory and present a computer simulation study on aspects of
the system's structural properties and phase behaviour. In line with previous
experimental observations, we find that even a weak surface charge has a strong
impact on the correlation functions. A likewise strong impact is seen on the
phase behaviour, in particular, we find stable cubatic order in suspensions of
oblate ellipsoids
Electrostatic interactions in critical solvents
The subtle interplay between critical phenomena and electrostatics is
investigated by considering the effective force acting on two parallel walls
confining a near-critical binary liquid mixture with added salt. The
ion-solvent coupling can turn a non-critical repulsive electrostatic force into
an attractive one upon approaching the critical point. However, the effective
force is eventually dominated by the critical Casimir effect, the universal
properties of which are not altered by the presence of salt. This observation
allows a consistent interpretation of recent experimental data.Comment: Submitte
Theory of capillary-induced interactions beyond the superposition approximation
Within a general theoretical framework we study the effective,
deformation-induced interaction between two colloidal particles trapped at a
fluid interface in the regime of small deformations. In many studies, this
interaction has been computed with the ansatz that the actual interface
configuration for the pair is given by the linear superposition of the
interface deformations around the single particles. Here we assess the validity
of this approach and compute the leading term of the effective interaction for
large interparticle separation beyond this so-called superposition
approximation. As an application, we consider the experimentally relevant case
of interface deformations owing to the electrostatic field emanating from
charged colloidal particles. In mechanical isolation, i.e., if the net force
acting on the total system consisting of the particles plus the interface
vanishes, the superposition approximation is actually invalid. The effective
capillary interaction is governed by contributions beyond this approximation
and turns out to be attractive. For sufficiently small surface charges on the
colloids, such that linearization is strictly valid, and at asymptotically
large separations, the effective interaction does not overcome the direct
electrostatic repulsion between the colloidal particles.Comment: Minor typos correcte
Lattice Fundamental Measure Theory beyond 0D Cavities: Dimers on Square Lattices
Using classical density functional theory, we study the behavior of dimers,
i.e. hard rods of length , on a two-dimensional cubic lattice. For
deriving a free energy functional, we employ Levy's prescription which is based
on the minimization of a microscopic free energy with respect to the many-body
probability under the constraint of a fixed density profile. Using that, we
recover the functional originally found by Lafuente and Cuesta and derive an
extension. With this extension, the free energy functional is exact on cavities
that can hold at most two particles simultaneously. The new functional improves
the prediction of the free energy in bulk as well as in highly confined
systems, especially for high packing fractions, in comparison to that of
Lafuente and Cuesta.Comment: 26 pages, 7 figures, Paper is submitted to J.Stat.Phy
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