268 research outputs found
Dynamical Heterogeneities and Cooperative Motion in Smectic Liquid Crystals
Using simulations of hard rods in smectic-A states, we find non-gaussian
diffusion and heterogeneous dynamics due to the equilibrium periodic smectic
density profiles, which give rise to permanent barriers for layer-to-layer
diffusion. This relaxation behavior is surprisingly similar to that of
non-equilibrium supercooled liquids, although there the particles are trapped
in transient (instead of permanent) cages. Interestingly, we also find
stringlike clusters of up to 10 inter-layer rods exhibiting dynamic
cooperativity in this equilibrium state.Comment: 10 pages, 4 figure
Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?
We study sedimentation equilibrium of low-salt suspensions of binary mixtures
of charged colloids, both by Monte Carlo simulations of an effective
colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture.
We show that the theoretically predicted lifting and layering effect, which
involves the entropy of the screening ions and a spontaneous macroscopic
electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480
(2005)], can also be understood on the basis of an effective colloid-only
system with pairwise screened-Coulomb interactions. We consider, by theory and
by simulation, both repelling like-charged colloids and attracting oppositely
charged colloids, and we find a re-entrant lifting and layering phenomenon when
the charge ratio of the colloids varies from large positive through zero to
large negative values
The sediment of mixtures of charged colloids: segregation and inhomogeneous electric fields
We theoretically study sedimentation-diffusion equilibrium of dilute binary,
ternary, and polydisperse mixtures of colloidal particles with different
buoyant masses and/or charges. We focus on the low-salt regime, where the
entropy of the screening ions drives spontaneous charge separation and the
formation of an inhomogeneous macroscopic electric field. The resulting
electric force lifts the colloids against gravity, yielding highly
nonbarometric and even nonmonotonic colloidal density profiles. The most
profound effect is the phenomenon of segregation into layers of colloids with
equal mass-per-charge, including the possibility that heavy colloidal species
float onto lighter ones
A Case Study of Sedimentation of Charged Colloids: The Primitive Model and the Effective One-Component Approach
Sedimentation-diffusion equilibrium density profiles of suspensions of
charge-stabilized colloids are calculated theoretically and by Monte Carlo
simulation, both for a one-component model of colloidal particles interacting
through pairwise screened-Coulomb repulsions and for a three-component model of
colloids, cations, and anions with unscreened-Coulomb interactions. We focus on
a state point for which experimental measurements are available [C.P. Royall et
al., J. Phys.: Cond. Matt. {\bf 17}, 2315 (2005)]. Despite the apparently
different picture that emerges from the one- and three-component model
(repelling colloids pushing each other to high altitude in the former, versus a
self-generated electric field that pushes the colloids up in the latter), we
find similar colloidal density profiles for both models from theory as well as
simulation, thereby suggesting that these pictures represent different view
points of the same phenomenon. The sedimentation profiles obtained from an
effective one-component model by MC simulations and theory, together with MC
simulations of the multi-component primitive model are consistent among
themselves, but differ quantitatively from the results of a theoretical
multi-component description at the Poisson-Boltzmann level. We find that for
small and moderate colloid charge the Poisson-Boltzmann theory gives profiles
in excellent agreement with the effective one-component theory if a smaller
effective charge is used. We attribute this discrepancy to the poor treatment
of correlations in the Poisson-Boltzmann theory.Comment: 9 pages, 7 figure
Colloid-oil-water-interface interactions in the presence of multiple salts: charge regulation and dynamics
We theoretically and experimentally investigate colloid-oil-water-interface
interactions of charged, sterically stabilized, poly(methyl-methacrylate)
colloidal particles dispersed in a low-polar oil (dielectric constant
) that is in contact with an adjacent water phase. In this model
system, the colloidal particles cannot penetrate the oil-water interface due to
repulsive van der Waals forces with the interface whereas the multiple salts
that are dissolved in the oil are free to partition into the water phase. The
sign and magnitude of the Donnan potential and/or the particle charge is
affected by these salt concentrations such that the effective interaction
potential can be highly tuned. Both the equilibrium effective colloid-interface
interactions and the ion dynamics are explored within a Poisson-Nernst-Planck
theory, and compared to experimental observations.Comment: 13+2 pages, 5+3 figures; V2: small clarifications in the tex
Fluids of platelike particles near a hard wall
Fluids consisting of hard platelike particles near a hard wall are
investigated using density functional theory. The density and orientational
profiles as well as the surface tension and the excess coverage are determined
and compared with those of a fluid of rodlike particles. Even for low densities
slight orientational packing effects are found for the platelet fluid due to
larger intermolecular interactions between platelets as compared with those
between rods. A net depletion of platelets near the wall is exhibited by the
excess coverage, whereas a change of sign of the excess coverage of hard-rod
fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure
A Triangular Tessellation Scheme for the Adsorption Free Energy at the Liquid-Liquid Interface: Towards Non-Convex Patterned Colloids
We introduce a new numerical technique, namely triangular tessellation, to
calculate the free energy associated with the adsorption of a colloidal
particle at a flat interface. The theory and numerical scheme presented here
are sufficiently general to handle non-convex patchy colloids with arbitrary
surface patterns characterized by a wetting angle, e.g., amphiphilicity. We
ignore interfacial deformation due to capillary, electrostatic, or
gravitational forces, but the method can be extended to take such effects into
account. It is verified that the numerical method presented is accurate and
sufficiently stable to be applied to more general situations than presented in
this paper. The merits of the tessellation method prove to outweigh those of
traditionally used semi-analytic approaches, especially when it comes to
generality and applicability.Comment: 21 pages, 11 figures, 0 table
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