457 research outputs found
Structural precursor to freezing: An integral equation study
Recent simulation studies have drawn attention to the shoulder which forms in
the second peak of the radial distribution function of hard-spheres at
densities close to freezing and which is associated with local crystalline
ordering in the dense fluid. We address this structural precursor to freezing
using an inhomogeneous integral equation theory capable of describing local
packing constraints to a high level of accuracy. The addition of a short-range
attractive interaction leads to a well known broadening of the fluid-solid
coexistence region as a function of attraction strength. The appearence of a
shoulder in our calculated radial distribution functions is found to be
consistent with the broadened coexistence region for a simple model potential,
thus demonstrating that the shoulder is not exclusively a high density packing
effect
Effect of mixing and spatial dimension on the glass transition
We study the influence of composition changes on the glass transition of
binary hard disc and hard sphere mixtures in the framework of mode coupling
theory. We derive a general expression for the slope of a glass transition
line. Applied to the binary mixture in the low concentration limits, this new
method allows a fast prediction of some properties of the glass transition
lines. The glass transition diagram we find for binary hard discs strongly
resembles the random close packing diagram. Compared to 3D from previous
studies, the extension of the glass regime due to mixing is much more
pronounced in 2D where plasticization only sets in at larger size disparities.
For small size disparities we find a stabilization of the glass phase quadratic
in the deviation of the size disparity from unity.Comment: 13 pages, 8 figures, Phys. Rev. E (in print
Density profiles of a colloidal liquid at a wall under shear flow
Using a dynamical density functional theory we analyze the density profile of
a colloidal liquid near a wall under shear flow. Due to the symmetries of the
system considered, the naive application of dynamical density functional theory
does not lead to a shear induced modification of the equilibrium density
profile, which would be expected on physical grounds. By introducing a
physically motivated dynamic mean field correction we incorporate the missing
shear induced interparticle forces into the theory. We find that the shear flow
tends to enhance the oscillations in the density profile of hard-spheres at a
hard-wall and, at sufficiently high shear rates, induces a nonequilibrium
transition to a steady state characterized by planes of particles parallel to
the wall. Under gravity, we find that the center-of-mass of the density
distribution increases with shear rate, i.e., shear increases the potential
energy of the particles
Density functional theory and demixing of binary hard rod-polymer mixtures
A density functional theory for a mixture of hard rods and polymers modeled
as chains built of hard tangent spheres is proposed by combining the functional
due to Yu and Wu for the polymer mixtures [J. Chem. Phys. {\bf 117}, 2368
(2002)] with the Schmidt's functional [Phys. Rev. E {\bf 63}, 50201 (2001)] for
rod-sphere mixtures. As a simple application of the functional, the demixing
transition into polymer-rich and rod-rich phases is examined. When the chain
length increases, the phase boundary broadens and the critical packing fraction
decreases. The shift of the critical point of a demixing transition is most
noticeable for short chains.Comment: 4 pages,2 figures, in press, PR
Effective interactions in active Brownian suspensions
Active colloids exhibit persistent motion, which can lead to motility-induced phase separation (MIPS). However, there currently exists no microscopic theory to account for this phenomenon. We report a first-principles theory, free of fit parameters, for active spherical colloids, which shows explicitly how an effective many-body interaction potential is generated by activity and how this can rationalize MIPS. For a passively repulsive system the theory predicts phase separation and pair correlations in quantitative agreement with simulation. For an attractive system the theory shows that phase separation becomes suppressed by moderate activity, consistent with recent experiments and simulations, and suggests a mechanism for reentrant cluster formation at high activity
Accurate description of bulk and interfacial properties in colloid-polymer mixtures
Large-scale Monte Carlo simulations of a phase-separating colloid-polymer
mixture are performed and compared to recent experiments. The approach is based
on effective interaction potentials in which the central monomers of
self-avoiding polymer chains are used as effective coordinates. By
incorporating polymer nonideality together with soft colloid-polymer repulsion,
the predicted binodal is in excellent agreement with recent experiments. In
addition, the interfacial tension as well as the capillary length are in
quantitative agreement with experimental results obtained at a number of points
in the phase-coexistence region, without the use of any fit parametersComment: 4 pages, 4 figure
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Equilibrium properties of highly asymmetric star-polymer mixtures
We employ effective interaction potentials to study the equilibrium structure
and phase behavior of highly asymmetric mixtures of star polymers. We consider
in particular the influence of the addition of a component with a small number
of arms and a small size on a concentrated solution of large stars with a high
functionality. By employing liquid integral equation theories we examine the
evolution of the correlation functions of the big stars upon addition of the
small ones, finding a loss of structure that can be attributed to a weakening
of the repulsions between the large stars due to the presence of the small
ones. We analyze this phenomenon be means of a generalized depletion mechanism
which is supported by computer simulations. By applying thermodynamic
perturbation theory we draw the phase diagram of the asymmetric mixture,
finding that the addition of small stars melts the crystal formed by the big
ones. A systematic comparison between the two- and effective one-component
descriptions of the mixture that corroborates the reliability of the
generalized depletion picture is also carried out.Comment: 26 pages, 9 figures, submitted to Phys. Rev.
Dynamic Glass Transition in Two Dimensions
The question about the existence of a structural glass transition in two
dimensions is studied using mode coupling theory (MCT). We determine the
explicit d-dependence of the memory functional of mode coupling for
one-component systems. Applied to two dimensions we solve the MCT equations
numerically for monodisperse hard discs. A dynamic glass transition is found at
a critical packing fraction phi_c^{d=2} = 0.697 which is above phi_c^{d=3} =
0.516 by about 35%. phi^d_c scales approximately with phi^d_{\rm rcp} the value
for random close packing, at least for d=2, 3. Quantities characterizing the
local, cooperative 'cage motion' do not differ much for d=2 and d=3, and we
e.g. find the Lindemann criterion for the localization length at the glass
transition. The final relaxation obeys the superposition principle, collapsing
remarkably well onto a Kohlrausch law. The d=2 MCT results are in qualitative
agreement with existing results from MC and MD simulations. The mean squared
displacements measured experimentally for a quasi-two-dimensional binary system
of dipolar hard spheres can be described satisfactorily by MCT for monodisperse
hard discs over four decades in time provided the experimental control
parameter Gamma (which measures the strength of dipolar interactions) and the
packing fraction phi are properly related to each other.Comment: 14 pages, 15 figure
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
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