175 research outputs found
Entropic Interactions in Suspensions of Semi-Flexible Rods: Short-Range Effects of Flexibility
We compute the entropic interactions between two colloidal spheres immersed
in a dilute suspension of semi-flexible rods. Our model treats the
semi-flexible rod as a bent rod at fixed angle, set by the rod contour and
persistence lengths. The entropic forces arising from this additional
rotational degree of freedom are captured quantitatively by the model, and
account for observations at short range in a recent experiment. Global fits to
the interaction potential data suggest the persistence length of fd-virus is
about two to three times smaller than the commonly used value of .Comment: 4 pages, 5 figures, submitted to PRE rapid communication
Diffusive Evolution of Stable and Metastable Phases II: Theory of Non-Equilibrium Behaviour in Colloid-Polymer Mixtures
By analytically solving some simple models of phase-ordering kinetics, we
suggest a mechanism for the onset of non-equilibrium behaviour in
colloid-polymer mixtures. These mixtures can function as models of atomic
systems; their physics therefore impinges on many areas of thermodynamics and
phase-ordering. An exact solution is found for the motion of a single, planar
interface separating a growing phase of uniform high density from a
supersaturated low density phase, whose diffusive depletion drives the
interfacial motion. In addition, an approximate solution is found for the
one-dimensional evolution of two interfaces, separated by a slab of a
metastable phase at intermediate density. The theory predicts a critical
supersaturation of the low-density phase, above which the two interfaces become
unbound and the metastable phase grows ad infinitum. The growth of the stable
phase is suppressed in this regime.Comment: 27 pages, Latex, eps
Glasses in hard spheres with short-range attraction
We report a detailed experimental study of the structure and dynamics of
glassy states in hard spheres with short-range attraction. The system is a
suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear
polymer which induces a depletion attraction between the particles. Observation
of crystallization reveals a re-entrant glass transition. Static light
scattering shows a continuous change in the static structure factors upon
increasing attraction. Dynamic light scattering results, which cover 11 orders
of magnitude in time, are consistent with the existence of two distinct kinds
of glasses, those dominated by inter-particle repulsion and caging, and those
dominated by attraction. Samples close to the `A3 point' predicted by mode
coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure
Effects of polymer polydispersity on the phase behaviour of colloid-polymer mixtures
We study the equilibrium behaviour of a mixture of monodisperse hard sphere
colloids and polydisperse non-adsorbing polymers at their -point, using
the Asakura-Oosawa model treated within the free-volume approximation. Our
focus is the experimentally relevant scenario where the distribution of polymer
chain lengths across the system is fixed. Phase diagrams are calculated using
the moment free energy method, and we show that the mean polymer size at which gas-liquid phase separation first occurs decreases with increasing
polymer polydispersity . Correspondingly, at fixed mean polymer size,
polydispersity favours gas-liquid coexistence but delays the onset of
fluid-solid separation. On the other hand, we find that systems with different
but the same {\em mass-averaged} polymer chain length have nearly
polydispersity-independent phase diagrams. We conclude with a comparison to
previous calculations for a semi-grandcanonical scenario, where the polymer
chemical potentials are imposed, which predicted that fluid-solid coexistence
was over gas-liquid in some areas of the phase diagram. Our results show that
this somewhat counter-intuitive result arose because the actual polymer size
distribution in the system is shifted to smaller sizes relative to the polymer
reservoir distribution.Comment: Changes in v2: sketch in Figure 1 corrected, other figures improved;
added references to experimental work and discussion of mapping from polymer
chain length to effective radiu
Can Polymer Coils be modeled as "Soft Colloids"?
We map dilute or semi-dilute solutions of non-intersecting polymer chains
onto a fluid of ``soft'' particles interacting via a concentration dependent
effective pair potential, by inverting the pair distribution function of the
centers of mass of the initial polymer chains. A similar inversion is used to
derive an effective wall-polymer potential; these potentials are combined to
successfully reproduce the calculated exact depletion interaction induced by
non-intersecting polymers between two walls. The mapping opens up the
possibility of large-scale simulations of polymer solutions in complex
geometries.Comment: 4 pages, 3 figures ReVTeX[epsfig,multicol,amssymb] references update
Phase equilibria and glass transition in colloidal systems with short-ranged attractive interactions. Application to protein crystallization
We have studied a model of a complex fluid consisting of particles
interacting through a hard core and a short range attractive potential of both
Yukawa and square-well form. Using a hybrid method, including a self-consistent
and quite accurate approximation for the liquid integral equation in the case
of the Yukawa fluid, perturbation theory to evaluate the crystal free energies,
and mode-coupling theory of the glass transition, we determine both the
equilibrium phase diagram of the system and the lines of equilibrium between
the supercooled fluid and the glass phases. For these potentials, we study the
phase diagrams for different values of the potential range, the ratio of the
range of the interaction to the diameter of the repulsive core being the main
control parameter. Our arguments are relevant to a variety of systems, from
dense colloidal systems with depletion forces, through particle gels,
nano-particle aggregation, and globular protein crystallization.Comment: 20 pages, 10 figure
The low temperature interface between the gas and solid phases of hard spheres with a short-ranged attraction
At low temperature, spheres with a very short-ranged attraction exist as a
close-packed solid coexisting with an infinitely dilute gas. We find that the
ratio of the interfacial tension between these two phases to the thermal energy
diverges as the range of the attraction goes to zero. The large tensions when
the interparticle attractions are short-ranged may be why globular proteins
only crystallise over a narrow range of conditions.Comment: 6 pages, no figures (v2 has change of notation to agree with that of
Stell
Nonergodicity transitions in colloidal suspensions with attractive interactions
The colloidal gel and glass transitions are investigated using the idealized
mode coupling theory (MCT) for model systems characterized by short-range
attractive interactions. Results are presented for the adhesive hard sphere and
hard core attractive Yukawa systems. According to MCT, the former system shows
a critical glass transition concentration that increases significantly with
introduction of a weak attraction. For the latter attractive Yukawa system, MCT
predicts low temperature nonergodic states that extend to the critical and
subcritical region. Several features of the MCT nonergodicity transition in
this system agree qualitatively with experimental observations on the colloidal
gel transition, suggesting that the gel transition is caused by a low
temperature extension of the glass transition. The range of the attraction is
shown to govern the way the glass transition line traverses the phase diagram
relative to the critical point, analogous to findings for the fluid-solid
freezing transition.Comment: 11 pages, 7 figures; to be published in Phys. Rev. E (1 May 1999
Macromolecular theory of solvation and structure in mixtures of colloids and polymers
The structural and thermodynamic properties of mixtures of colloidal spheres
and non-adsorbing polymer chains are studied within a novel general
two-component macromolecular liquid state approach applicable for all size
asymmetry ratios. The dilute limits, when one of the components is at infinite
dilution but the other concentrated, are presented and compared to field theory
and models which replace polymer coils with spheres. Whereas the derived
analytical results compare well, qualitatively and quantitatively, with
mean-field scaling laws where available, important differences from ``effective
sphere'' approaches are found for large polymer sizes or semi-dilute
concentrations.Comment: 23 pages, 10 figure
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