133 research outputs found
Viscosity of Colloidal Suspensions
Simple expressions are given for the Newtonian viscosity as
well as the viscoelastic behavior of the viscosity of
neutral monodisperse hard sphere colloidal suspensions as a function of volume
fraction and frequency over the entire fluid range, i.e., for
volume fractions . These expressions are based on an
approximate theory which considers the viscosity as composed as the sum of two
relevant physical processes: , where is the
infinite frequency (or very short time) viscosity, with the solvent
viscosity, the equilibrium hard sphere radial distribution
function at contact, and the contribution due to the
diffusion of the colloidal particles out of cages formed by their neighbors, on
the P\'{e}clet time scale , the dominant physical process in
concentrated colloidal suspensions. The Newtonian viscosity agrees very well with the extensive experiments of Van
der Werff et al and others. Also, the asymptotic behavior for large is
of the form , in agreement
with these experiments, but the theoretical coefficient differs by a
constant factor from the exact coefficient, computed from the
Green-Kubo formula for . This still enables us to predict
for practical purposes the visco-elastic behavior of monodisperse spherical
colloidal suspensions for all volume fractions by a simple time rescaling.Comment: 51 page
Transport in a highly asymmetric binary fluid mixture
We present molecular dynamics calculations of the thermal conductivity and
viscosities of a model colloidal suspension with colloidal particles roughly
one order of magnitude larger than the suspending liquid molecules. The results
are compared with estimates based on the Enskog transport theory and effective
medium theories (EMT) for thermal and viscous transport. We find, in
particular, that EMT remains well applicable for predicting both the shear
viscosity and thermal conductivity of such suspensions when the colloidal
particles have a ``typical'' mass, i.e. much larger than the liquid molecules.
Very light colloidal particles on the other hand yield higher thermal
conductivities, in disagreement with EMT. We also discuss the consequences of
these results to some proposed mechanisms for thermal conduction in
nanocolloidal suspensions.Comment: 13 pages, 6 figures, to appear in Physical Review E (2007
Mesoscopic two-phase model for describing apparent slip in micro-channel flows
The phenomenon of apparent slip in micro-channel flows is analyzed by means
of a two-phase mesoscopic lattice Boltzmann model including non-ideal
fluid-fluid and fluid-wall interactins. The weakly-inhomogeneous limit of this
model is solved analytically.
The present mesoscopic approach permits to access much larger scales than
molecular dynamics, and comparable with those attained by continuum methods.
However, at variance with the continuum approach, the existence of a gas layer
near the wall does not need to be postulated a priori, but emerges naturally
from the underlying non-ideal mesoscopic dynamics. It is therefore argued that
a mesoscopic Lattice Boltzmann approach with non-ideal fluid-fluid and
fluid-wall interactions might achieve an optimal compromise between physical
realism and computational efficiency for the study of channel micro-flows.Comment: 5 pages, 3 figure
Nonlinear viscoelasticity of metastable complex fluids
Many metastable complex fluids such as colloidal glasses and gels show
distinct nonlinear viscoelasticity with increasing oscillatory-strain
amplitude; the storage modulus decreases monotonically as the strain amplitude
increases whereas the loss modulus has a distinct peak before it decreases at
larger strains. We present a qualitative argument to explain this ubiquitous
behavior and use mode coupling theory (MCT) to confirm it. We compare
theoretical predictions to the measured nonlinear viscoelasticity in a dense
hard sphere colloidal suspensions; reasonable agreement is obtained. The
argument given here can be used to obtain new information about linear
viscoelasticity of metastable complex fluids from nonlinear strain
measurements.Comment: 7 pages, 3 figures, accepted for publication in Europhys. Let
Lattice-Boltzmann and finite-difference simulations for the permeability for three-dimensional porous media
Numerical micropermeametry is performed on three dimensional porous samples
having a linear size of approximately 3 mm and a resolution of 7.5 m. One
of the samples is a microtomographic image of Fontainebleau sandstone. Two of
the samples are stochastic reconstructions with the same porosity, specific
surface area, and two-point correlation function as the Fontainebleau sample.
The fourth sample is a physical model which mimics the processes of
sedimentation, compaction and diagenesis of Fontainebleau sandstone. The
permeabilities of these samples are determined by numerically solving at low
Reynolds numbers the appropriate Stokes equations in the pore spaces of the
samples. The physical diagenesis model appears to reproduce the permeability of
the real sandstone sample quite accurately, while the permeabilities of the
stochastic reconstructions deviate from the latter by at least an order of
magnitude. This finding confirms earlier qualitative predictions based on local
porosity theory. Two numerical algorithms were used in these simulations. One
is based on the lattice-Boltzmann method, and the other on conventional
finite-difference techniques. The accuracy of these two methods is discussed
and compared, also with experiment.Comment: to appear in: Phys.Rev.E (2002), 32 pages, Latex, 1 Figur
Square root singularity in the viscosity of neutral colloidal suspensions at large frequencies
The asymptotic frequency , dependence of the dynamic viscosity of
neutral hard sphere colloidal suspensions is shown to be of the form , where has been determined as a
function of the volume fraction , for all concentrations in the fluid
range, is the solvent viscosity and the P\'{e}clet time. For
a soft potential it is shown that, to leading order steepness, the asymptotic
behavior is the same as that for the hard sphere potential and a condition for
the cross-over behavior to is given. Our result for the hard
sphere potential generalizes a result of Cichocki and Felderhof obtained at low
concentrations and agrees well with the experiments of van der Werff et al, if
the usual Stokes-Einstein diffusion coefficient in the Smoluchowski
operator is consistently replaced by the short-time self diffusion coefficient
for non-dilute colloidal suspensions.Comment: 18 pages LaTeX, 1 postscript figur
Thixotropy in macroscopic suspensions of spheres
An experimental study of the viscosity of a macroscopic suspension, i.e. a
suspension for which Brownian motion can be neglected, under steady shear is
presented. The suspension is prepared with a high packing fraction and is
density-matched in a Newtonian carrier fluid. The viscosity of the suspension
depends on the shear rate and the time of shearing. It is shown for the first
time that a macroscopic suspension shows thixotropic viscosity, i.e.
shear-thinning with a long relaxation time as a unique function of shear. The
relaxation times show a systematic decrease with increasing shear rate. These
relaxation times are larger when decreasing the shear rates, compared to those
observed after increasing the shear. The time scales involved are about 10000
times larger than the viscous time scale and about 1000 times smaller than the
thermodynamic time scale. The structure of the suspension at the outer cylinder
of a viscometer is monitored with a camera, showing the formation of a
hexagonal structure. The temporal decrease of the viscosity under shear
coincides with the formation of this hexagonal pattern
The Physics of the Colloidal Glass Transition
As one increases the concentration of a colloidal suspension, the system
exhibits a dramatic increase in viscosity. Structurally, the system resembles a
liquid, yet motions within the suspension are slow enough that it can be
considered essentially frozen. This kinetic arrest is the colloidal glass
transition. For several decades, colloids have served as a valuable model
system for understanding the glass transition in molecular systems. The spatial
and temporal scales involved allow these systems to be studied by a wide
variety of experimental techniques. The focus of this review is the current
state of understanding of the colloidal glass transition. A brief introduction
is given to important experimental techniques used to study the glass
transition in colloids. We describe features of colloidal systems near and in
glassy states, including tremendous increases in viscosity and relaxation
times, dynamical heterogeneity, and ageing, among others. We also compare and
contrast the glass transition in colloids to that in molecular liquids. Other
glassy systems are briefly discussed, as well as recently developed synthesis
techniques that will keep these systems rich with interesting physics for years
to come.Comment: 56 pages, 18 figures, Revie
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