398 research outputs found
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
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
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
Volume terms for charged colloids: a grand-canonical treatment
We present a study of thermodynamic properties of suspensions of charged
colloids on the basis of linear Poisson-Boltzmann theory. We calculate the
effective Hamiltonian of the colloids by integrating out the ionic degrees of
freedom grand-canonically. This procedure not only yields the well-known
pairwise screened-Coulomb interaction between the colloids, but also additional
volume terms which affect the phase behavior and the thermodynamic properties
such as the osmotic pressure. These calculations are greatly facilitated by the
grand-canonical character of our treatment of the ions, and allow for
relatively fast computations compared to earlier studies in the canonical
ensemble. Moreover, the present derivation of the volume terms are relatively
simple, make a direct connection with Donnan equilibrium, yield an explicit
expression for the effective screening constant, and allow for extensions to
include, for instance, nonlinear effects.Comment: 16 pages, 6 figures, published in Phys.Rev.
Critical behavior in colloid-polymer mixtures: theory and simulation
We extensively investigated the critical behavior of mixtures of colloids and
polymers via the two-component Asakura-Oosawa model and its reduction to a
one-component colloidal fluid using accurate theoretical and simulation
techniques. In particular the theoretical approach, hierarchical reference
theory [Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of
long-range fluctuations on phase separation giving exponents which differ
strongly from their mean-field values, and are in good agreement with those of
the three-dimensional Ising model. Computer simulations combined with
finite-size scaling analysis confirm the Ising universality and the accuracy of
the theory, although some discrepancy in the location of the critical point
between one-component and full-mixture description remains. To assess the limit
of the pair-interaction description, we compare one-component and two-component
results.Comment: 15 pages, 10 figures. Submitted to Phys. Rev.
Flow curves of colloidal dispersions close to the glass transition: Asymptotic scaling laws in a schematic model of mode coupling theory
The flow curves, viz. the curves of stationary stress under steady shearing,
are obtained close to the glass transition in dense colloidal dispersions using
asymptotic expansions in a schematic model of mode coupling theory. The shear
thinning of the viscosity in fluid states and the yielding of glassy states is
discussed. At the transition between fluid and shear-molten glass, simple and
generalized Herschel-Bulkley laws are derived with power law exponents that can
be computed for different particle interactions from the equilibrium structure
factor.Comment: 14 pages, 14 figures, 4 tables, Eur. Phys. J. E (submitted
Three-dimensional jamming and flows of soft glassy materials
Various disordered dense systems such as foams, gels, emulsions and colloidal
suspensions, exhibit a jamming transition from a liquid state (they flow) to a
solid state below a yield stress. Their structure, thoroughly studied with
powerful means of 3D characterization, exhibits some analogy with that of
glasses which led to call them soft glassy materials. However, despite its
importance for geophysical and industrial applications, their rheological
behavior, and its microscopic origin, is still poorly known, in particular
because of its nonlinear nature. Here we show from two original experiments
that a simple 3D continuum description of the behaviour of soft glassy
materials can be built. We first show that when a flow is imposed in some
direction there is no yield resistance to a secondary flow: these systems are
always unjammed simultaneously in all directions of space. The 3D jamming
criterion appears to be the plasticity criterion encountered in most solids. We
also find that they behave as simple liquids in the direction orthogonal to
that of the main flow; their viscosity is inversely proportional to the main
flow shear rate, as a signature of shear-induced structural relaxation, in
close similarity with the structural relaxations driven by temperature and
density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm
- …