269 research outputs found
The effect of direct interactions on Brownian diffusion
The effect of direct interactions between suspended particles on their diffusion coefficient is investigated starting from the generalized Einstein relation. It is shown that an attractive potential added to the hard core repulsion leads to a decrease of the diffusion coëfficiënt, whereas a repulsive term has the opposite effect. Simple examples of attractive and repulsive potentials are considered in some detail. Using these results the possibility to obtain information on the interaction potential between suspended particles from their diffusion coefficient is discussed
Scaling of dynamics with the range of interaction in short-range attractive colloids
We numerically study the dependence of the dynamics on the range of
interaction for the short-range square well potential. We find that,
for small , dynamics scale exactly in the same way as thermodynamics,
both for Newtonian and Brownian microscopic dynamics. For interaction ranges
from a few percent down to the Baxter limit, the relative location of the
attractive glass line and the liquid-gas line does not depend on . This
proves that in this class of potentials, disordered arrested states (gels) can
be generated only as a result of a kinetically arrested phase separation.Comment: 4 pages, 4 figure
Gelation as arrested phase separation in short-ranged attractive colloid-polymer mixtures
We present further evidence that gelation is an arrested phase separation in
attractive colloid-polymer mixtures, based on a method combining confocal
microscopy experiments with numerical simulations recently established in {\bf
Nature 453, 499 (2008)}. Our results are independent of the form of the
interparticle attractive potential, and therefore should apply broadly to any
attractive particle system with short-ranged, isotropic attractions. We also
give additional characterization of the gel states in terms of their structure,
inhomogeneous character and local density.Comment: 6 figures, to be published in J. Phys. Condens. Matter, special issue
for EPS Liquids Conference 200
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
Phase behaviour of charged colloidal sphere dispersions with added polymer chains
We study the stability of mixtures of highly screened repulsive charged
spheres and non-adsorbing ideal polymer chains in a common solvent using free
volume theory. The effective interaction between charged colloids in an aqueous
salt solution is described by a screened-Coulomb pair potential, which
supplements the pure hard-sphere interaction. The ideal polymer chains are
treated as spheres that are excluded from the colloids by a hard-core
interaction, whereas the interaction between two ideal chains is set to zero.
In addition, we investigate the phase behaviour of charged colloid-polymer
mixtures in computer simulations, using the two-body (Asakura-Oosawa pair
potential) approximation to the effective one-component Hamiltonian of the
charged colloids. Both our results obtained from simulations and from free
volume theory show similar trends. We find that the screened-Coulomb repulsion
counteracts the effect of the effective polymer-mediated attraction. For
mixtures of small polymers and relatively large charged colloidal spheres, the
fluid-crystal transition shifts to significantly larger polymer concentrations
with increasing range of the screened-Coulomb repulsion. For relatively large
polymers, the effect of the screened-Coulomb repulsion is weaker. The resulting
fluid-fluid binodal is only slightly shifted towards larger polymer
concentrations upon increasing the range of the screened-Coulomb repulsion. In
conclusion, our results show that the miscibility of dispersions containing
charged colloids and neutral non-adsorbing polymers increases, upon increasing
the range of the screened-Coulomb repulsion, or upon lowering the salt
concentration, especially when the polymers are small compared to the colloids.Comment: 25 pages,13 figures, accepted for publication on J.Phys.:Condens.
Matte
Phase behaviour of additive binary mixtures in the limit of infinite asymmetry
We provide an exact mapping between the density functional of a binary
mixture and that of the effective one-component fluid in the limit of infinite
asymmetry. The fluid of parallel hard cubes is thus mapped onto that of
parallel adhesive hard cubes. Its phase behaviour reveals that demixing of a
very asymmetric mixture can only occur between a solvent-rich fluid and a
permeated large particle solid or between two large particle solids with
different packing fractions. Comparing with hard spheres mixtures we conclude
that the phase behaviour of very asymmetric hard-particle mixtures can be
determined from that of the large component interacting via an adhesive-like
potential.Comment: Full rewriting of the paper (also new title). 4 pages, LaTeX, uses
revtex, multicol, epsfig, and amstex style files, to appear in Phys. Rev. E
(Rapid Comm.
Elongation and fluctuations of semi-flexible polymers in a nematic solvent
We directly visualize single polymers with persistence lengths ranging from
to 16 m, dissolved in the nematic phase of rod-like {\it fd}
virus. Polymers with sufficiently large persistence length undergo a coil-rod
transition at the isotropic-nematic transition of the background solvent. We
quantitatively analyze the transverse fluctuations of semi-flexible polymers
and show that at long wavelengths they are driven by the fluctuating nematic
background. We extract both the Odijk deflection length and the elastic
constant of the background nematic phase from the data.Comment: 4 pages, 4 figures, submitted to PR
Influence of polymer excluded volume on the phase behavior of colloid-polymer mixtures
We determine the depletion-induced phase-behavior of hard sphere colloids and
interacting polymers by large-scale Monte Carlo simulations using very accurate
coarse-graining techniques. A comparison with standard Asakura-Oosawa model
theories and simulations shows that including excluded volume interactions
between polymers leads to qualitative differences in the phase diagrams. These
effects become increasingly important for larger relative polymer size. Our
simulations results agree quantitatively with recent experiments.Comment: 5 pages, 4 figures submitted to Physical Review Letter
Crystalline Assemblies and Densest Packings of a Family of Truncated Tetrahedra and the Role of Directional Entropic Forces
Polyhedra and their arrangements have intrigued humankind since the ancient
Greeks and are today important motifs in condensed matter, with application to
many classes of liquids and solids. Yet, little is known about the
thermodynamically stable phases of polyhedrally-shaped building blocks, such as
faceted nanoparticles and colloids. Although hard particles are known to
organize due to entropy alone, and some unusual phases are reported in the
literature, the role of entropic forces in connection with polyhedral shape is
not well understood. Here, we study thermodynamic self-assembly of a family of
truncated tetrahedra and report several atomic crystal isostructures, including
diamond, {\beta}-tin, and high- pressure lithium, as the polyhedron shape
varies from tetrahedral to octahedral. We compare our findings with the densest
packings of the truncated tetrahedron family obtained by numerical compression
and report a new space filling polyhedron, which has been overlooked in
previous searches. Interestingly, the self-assembled structures differ from the
densest packings. We show that the self-assembled crystal structures can be
understood as a tendency for polyhedra to maximize face-to-face alignment,
which can be generalized as directional entropic forces.Comment: Article + supplementary information. 23 pages, 10 figures, 2 table
- …