474 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
Glass Rheology: From mode-coupling theory to a dynamical yield criterion
The mode coupling theory (MCT) of glasses, while offering an incomplete
description of glass transition physics, represents the only established route
to first-principles prediction of rheological behavior in nonergodic materials
such as colloidal glasses. However, the constitutive equations derivable from
MCT are somewhat intractable, hindering their practical use and also their
interpretation. Here, we present a schematic (single-mode) MCT model which
incorporates the tensorial structure of the full theory. Using it, we calculate
the dynamic yield surface for a large class of flows
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
Controlling colloidal sedimentation using time dependent shear
Employing a recently developed dynamical density functional theory we study
the response of a colloidal sediment above a wall to shear, demonstrating the
time dependent changes of the density distribution and its center-of-mass after
switching shear either on or off and under oscillatory shear. Following the
onset of steady shear we identify two dynamical mechanisms, distinguished by
their timescales. Shortly after the onset, a transient enhancement of the
packing structure at the wall reflects the self-organization into lanes. On a
much longer timescale these effects are transmitted to the bulk, leading to
migration away from the wall and an increase in the center-of-mass. Under
oscillatory shear flow the center-of-mass enters a stationary state,
reminiscent of a driven damped oscillator.Comment: 6 pages, 4 figure
Dense colloidal suspensions under time-dependent shear
We consider the nonlinear rheology of dense colloidal suspensions under a
time-dependent simple shear flow. Starting from the Smoluchowski equation for
interacting Brownian particles advected by shearing (ignoring fluctuations in
fluid velocity) we develop a formalism which enables the calculation of
time-dependent, far-from-equilibrium averages. Taking shear-stress as an
example we derive exactly a generalized Green-Kubo relation, and an equation of
motion for the transient density correlator, involving a three-time memory
function. Mode coupling approximations give a closed constitutive equation
yielding the time-dependent stress for arbitrary shear rate history. We solve
this equation numerically for the special case of a hard sphere glass subject
to step-strain.Comment: 4 page
Residual Stresses in Glasses
The history dependence of the glasses formed from flow-melted steady states
by a sudden cessation of the shear rate is studied in colloidal
suspensions, by molecular dynamics simulations, and mode-coupling theory. In an
ideal glass, stresses relax only partially, leaving behind a finite persistent
residual stress. For intermediate times, relaxation curves scale as a function
of , even though no flow is present. The macroscopic stress
evolution is connected to a length scale of residual liquefaction displayed by
microscopic mean-squared displacements. The theory describes this history
dependence of glasses sharing the same thermodynamic state variables, but
differing static properties.Comment: submitted to Physical Revie
Capillary Condensation and Interface Structure of a Model Colloid-Polymer Mixture in a Porous Medium
We consider the Asakura-Oosawa model of hard sphere colloids and ideal
polymers in contact with a porous matrix modeled by immobilized configurations
of hard spheres. For this ternary mixture a fundamental measure density
functional theory is employed, where the matrix particles are quenched and the
colloids and polymers are annealed, i.e. allowed to equilibrate. We study
capillary condensation of the mixture in a tiny sample of matrix as well as
demixing and the fluid-fluid interface inside a bulk matrix. Density profiles
normal to the interface and surface tensions are calculated and compared to the
case without matrix. Two kinds of matrices are considered: (i) colloid-sized
matrix particles at low packing fractions and (ii) large matrix particles at
high packing fractions. These two cases show fundamentally different behavior
and should both be experimentally realizable. Furthermore, we argue that
capillary condensation of a colloidal suspension could be experimentally
accessible. We find that in case (ii), even at high packing fractions, the main
effect of the matrix is to exclude volume and, to high accuracy, the results
can be mapped onto those of the same system without matrix via a simple
rescaling.Comment: 12 pages, 9 figures, submitted to PR
A First-Principles Constitutive Equation for Suspension Rheology
Using mode-coupling theory, we derive a constitutive equation for the
nonlinear rheology of dense colloidal suspensions under arbitrary
time-dependent homogeneous flow. Generalizing previous results for simple
shear, this allows the full tensorial structure of the theory to be identified.
Macroscopic deformation measures, such as the Cauchy-Green tensors, thereby
emerge. So does a direct relation between the stress and the distorted
microstructure, illuminating the interplay of slow structural relaxation and
arbitrary imposed flow. We present flow curves for steady planar and uniaxial
elongation and compare these to simple shear. The resulting non-linear Trouton
ratios point to a tensorially nontrivial dynamic yield condition for colloidal
glasses.Comment: accepted to Phys.Rev.Let
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
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