496 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
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
From Equilibrium to Steady State: The Transient Dynamics of Colloidal Liquids under Shear
We investigate stresses and particle motion during the start up of flow in a
colloidal dispersion close to arrest into a glassy state. A combination of
molecular dynamics simulation, mode coupling theory and confocal microscopy
experiment is used to investigate the origins of the widely observed stress
overshoot and (previously not reported) super-diffusive motion in the transient
dynamics. A link between the macro-rheological stress versus strain curves and
the microscopic particle motion is established. Negative correlations in the
transient auto-correlation function of the potential stresses are found
responsible for both phenomena, and arise even for homogeneous flows and almost
Gaussian particle displacements.Comment: 24 pages, 14 figures, J. Phys.: Condens. Matter, in pres
Superadiabatic dynamical density functional study of Brownian hard-spheres in time-dependent external potentials
Superadiabatic dynamical density functional theory (superadiabatic-DDFT), a
first-principles approach based on the inhomogeneous two-body correlation
functions, is employed to investigate the response of interacting Brownian
particles to time-dependent external driving. Predictions for the
superadiabatic dynamics of the one-body density are made directly from the
underlying interparticle interactions, without need for either adjustable fit
parameters or simulation input. The external potentials we investigate have
been chosen to probe distinct aspects of structural relaxation in dense,
strongly interacting liquid states. Nonequilibrium density profiles predicted
by the superadiabatic theory are compared with those obtained from both
adiabatic DDFT and event-driven Brownian dynamics simulation. Our findings show
that superadiabatic-DDFT accurately predicts the time-evolution of the one-body
density
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
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
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.
Microscopic theory of solvent mediated long range forces: influence of wetting
We show that a general density functional approach for calculating the force
between two big particles immersed in a solvent of smaller ones can describe
systems that exhibit fluid-fluid phase separation: the theory captures effects
of strong adsorption (wetting) and of critical fluctuations in the solvent. We
illustrate the approach for the Gaussian core model, a simple model of a
polymer mixture in solution and find extremely attractive, long ranged solvent
mediated potentials between the big particles for state points lying close to
the binodal, on the side where the solvent is poor in the species which is
favoured by the big particles.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
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