90 research outputs found
Intermediate range chemical ordering of cations in simple molten alkali halides
The presence of first sharp diffraction peaks in the partial structure
factors is investigated in computer simulations of molten mixtures of alkali
halides. An intermediate range ordering appears for the Li+ ions only, which is
associated with clustering of this species and is not reflected in the
arrangement of other ions. This ordering is surprising in view of the
simplicity of the interionic interactions in alkali halides. The clustering
reflects an incomplete mixing of the various species on a local length scale,
which can be demonstrated by studying the complementary sub-space of cations in
the corresponding pure alkali halides by means of a void analysis.Comment: 5 pages, 5 figure
Long-Wavelength Anomalies in the Asymptotic Behavior of Mode-Coupling Theory
We discuss the dynamic behavior of a tagged particle close to a classical
localization transition in the framework of the mode-coupling theory of the
glass transition. Asymptotic results are derived for the order parameter as
well as the dynamic correlation functions and the mean-squared displacement
close to the transition. The influence of an infrared cutoff is discussed.Comment: 15 pages, 8 figures, to appear in J Phys Condens Matte
Lattice Boltzmann simulations of a viscoelastic shear-thinning fluid
We present a hybrid lattice Boltzmann algorithm for the simulation of flow
glass-forming fluids, characterized by slow structural relaxation, at the level
of the Navier-Stokes equation. The fluid is described in terms of a nonlinear
integral constitutive equation, relating the stress tensor locally to the
history of flow. As an application, we present results for an integral
nonlinear Maxwell model that combines the effects of (linear) viscoelasticity
and (nonlinear) shear thinning. We discuss the transient dynamics of
velocities, shear stresses, and normal stress differences in planar
pressure-driven channel flow, after switching on (startup) and off (cessation)
of the driving pressure. This transient dynamics depends nontrivially on the
channel width due to an interplay between hydrodynamic momentum diffusion and
slow structural relaxation
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
Channel Flow of a Tensorial Shear-Thinning Maxwell Model: Lattice Boltzmann Simulations
We introduce a nonlinear generalized tensorial Maxwell-type constitutive
equation to describe shear-thinning glass-forming fluids, motivated by a recent
microscopic approach to the nonlinear rheology of colloidal suspensions. The
model captures a nonvanishing dynamical yield stress at the glass transition
and incorporates normal-stress differences. A modified lattice-Boltzmann (LB)
simulation scheme is presented that includes non-Newtonian contributions to the
stress tensor and deals with flow-induced pressure differences. We test this
scheme in pressure-driven 2D Poiseuille flow of the nonlinear generalized
Maxwell fluid. In the steady state, comparison with an analytical solution
shows good agreement. The transient dynamics after startup and cessation of the
pressure gradient are studied; the simulation reproduces a finite stopping time
for the cessation flow of the yield-stress fluid in agreement with previous
analytical estimates
Mobile particles in an immobile environment: Molecular Dynamics simulation of a binary Yukawa mixture
Molecular dynamics computer simulations are used to investigate thedynamics
of a binary mixture of charged (Yukawa) particles with a size-ratio of 1:5. We
find that the system undergoes a phase transition where the large particles
crystallize while the small particles remain in a fluid-like (delocalized)
phase. Upon decreasing temperature below the transition, the small particles
become increasingly localized on intermediate time scales. This is reflected in
the incoherent intermediate scattering functions by the appearance of a plateau
with a growing height. At long times, the small particles show a diffusive
hopping motion. We find that these transport properties are related to
structural correlations and the single-particle potential energy distribution
of the small particles.Comment: 7 pages, 5 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
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