37 research outputs found
A Stability Diagram for Dense Suspensions of Model Colloidal Al2O3-Particles in Shear Flow
In Al2O3 suspensions, depending on the experimental conditions very different
microstructures can be found, comprising fluid like suspensions, a repulsive
structure, and a clustered microstructure. For technical processing in
ceramics, the knowledge of the microstructure is of importance, since it
essentially determines the stability of a workpiece to be produced. To
enlighten this topic, we investigate these suspensions under shear by means of
simulations. We observe cluster formation on two different length scales: the
distance of nearest neighbors and on the length scale of the system size. We
find that the clustering behavior does not depend on the length scale of
observation. If inter-particle interactions are not attractive the particles
form layers in the shear flow. The results are summarized in a stability
diagram.Comment: 15 pages, 10 figures, revised versio
Hydrodynamic interactions and Brownian forces in colloidal suspensions: Coarse-graining over time and length-scales
We describe in detail how to implement a coarse-grained hybrid Molecular
Dynamics and Stochastic Rotation Dynamics simulation technique that captures
the combined effects of Brownian and hydrodynamic forces in colloidal
suspensions. The importance of carefully tuning the simulation parameters to
correctly resolve the multiple time and length-scales of this problem is
emphasized. We systematically analyze how our coarse-graining scheme resolves
dimensionless hydrodynamic numbers such as the Reynolds number, the Schmidt
number, the Mach number, the Knudsen number, and the Peclet number. The many
Brownian and hydrodynamic time-scales can be telescoped together to maximize
computational efficiency while still correctly resolving the physically
relevant physical processes. We also show how to control a number of numerical
artifacts, such as finite size effects and solvent induced attractive depletion
interactions. When all these considerations are properly taken into account,
the measured colloidal velocity auto-correlation functions and related self
diffusion and friction coefficients compare quantitatively with theoretical
calculations. By contrast, these calculations demonstrate that, notwithstanding
its seductive simplicity, the basic Langevin equation does a remarkably poor
job of capturing the decay rate of the velocity auto-correlation function in
the colloidal regime, strongly underestimating it at short times and strongly
overestimating it at long times. Finally, we discuss in detail how to map the
parameters of our method onto physical systems, and from this extract more
general lessons that may be relevant for other coarse-graining schemes such as
Lattice Boltzmann or Dissipative Particle Dynamics.Comment: 31 pages, 14 figure
Frictionless bead packs have macroscopic friction, but no dilatancy
The statement of the title is shown by numerical simulation of homogeneously
sheared packings of frictionless, nearly rigid beads in the quasistatic limit.
Results coincide for steady flows at constant shear rate γ in the
limit of small γ and static approaches, in which packings are equilibrated
under growing deviator stresses. The internal friction angle ϕ, equal to
5.76 0.22 degrees in simple shear, is independent on the average pressure
P in the rigid limit. It is shown to stem from the ability of stable
frictionless contact networks to form stress-induced anisotropic fabrics. No
enduring strain localization is observed. Dissipation at the macroscopic level
results from repeated network rearrangements, like the effective friction
of a frictionless slider on a bumpy surface. Solid fraction Φ remains
equal to the random close packing value ≃ 0.64 in slowly or statically
sheared systems. Fluctuations of stresses and volume are observed to regress in
the large system limit, and we conclude that the same friction law for simple
shear applies in the large psystem limit if normal stress or density is
externally controlled. Defining the inertia number as I = γ m/(aP),
with m the grain mass and a its diameter, both internal friction
coefficient ∗ = tan ϕ and volume 1/Φ increase as
powers of I in the quasistatic limit of vanishing I, in which all mechanical
properties are determined by contact network geometry. The microstructure of
the sheared material is characterized with a suitable parametrization of the
fabric tensor and measurements of connectivity and coordination numbers
associated with contacts and near neighbors.Comment: 19 pages. Additional technical details may be found in v
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
Shear Viscosity of Clay-like Colloids in Computer Simulations and Experiments
Dense suspensions of small strongly interacting particles are complex
systems, which are rarely understood on the microscopic level. We investigate
properties of dense suspensions and sediments of small spherical Al_2O_3
particles in a shear cell by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We study structuring effects and
the dependence of the suspension's viscosity on the shear rate and shear
thinning for systems of varying salt concentration and pH value. To show the
agreement of our results to experimental data, the relation between bulk pH
value and surface charge of spherical colloidal particles is modeled by
Debye-Hueckel theory in conjunction with a 2pK charge regulation model.Comment: 15 pages, 8 figure
Transport Phenomena and Structuring in Shear Flow of Suspensions near Solid Walls
In this paper we apply the lattice-Boltzmann method and an extension to
particle suspensions as introduced by Ladd et al. to study transport phenomena
and structuring effects of particles suspended in a fluid near sheared solid
walls. We find that a particle free region arises near walls, which has a width
depending on the shear rate and the particle concentration. The wall causes the
formation of parallel particle layers at low concentrations, where the number
of particles per layer decreases with increasing distance to the wall.Comment: 14 pages, 14 figure
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
Lattice Boltzmann simulations of soft matter systems
This article concerns numerical simulations of the dynamics of particles
immersed in a continuum solvent. As prototypical systems, we consider colloidal
dispersions of spherical particles and solutions of uncharged polymers. After a
brief explanation of the concept of hydrodynamic interactions, we give a
general overview over the various simulation methods that have been developed
to cope with the resulting computational problems. We then focus on the
approach we have developed, which couples a system of particles to a lattice
Boltzmann model representing the solvent degrees of freedom. The standard D3Q19
lattice Boltzmann model is derived and explained in depth, followed by a
detailed discussion of complementary methods for the coupling of solvent and
solute. Colloidal dispersions are best described in terms of extended particles
with appropriate boundary conditions at the surfaces, while particles with
internal degrees of freedom are easier to simulate as an arrangement of mass
points with frictional coupling to the solvent. In both cases, particular care
has been taken to simulate thermal fluctuations in a consistent way. The
usefulness of this methodology is illustrated by studies from our own research,
where the dynamics of colloidal and polymeric systems has been investigated in
both equilibrium and nonequilibrium situations.Comment: Review article, submitted to Advances in Polymer Science. 16 figures,
76 page
Everything you always wanted to know about SDPD⋆ (⋆but were afraid to ask)
An overview of the smoothed dissipative particle dynamics (SDPD) method is presented in a format that tries to quickly answer questions that often arise among users and newcomers. It is hoped that the status of SDPD is clarified as a mesoscopic particle model and its potentials and limitations are highlighted, as compared with other methods
Shear- and vibration-induced order-disorder transitions in granular media
By molecular dynamics simulations we investigate the order-disorder transitions induced in granular media by an applied drive combining vibrations and shear. As the steady state is attained, the pack is found in disordered configurations for comparatively high intensities of the drive; conversely, ordering and packing fractions exceeding the random close packing are found when vibrations and shear are weak. As forcing amplitudes get smaller, we find diverging time scales in the dynamics, as the system enters a jamming region. Under this perspective, our picture supports the intuition that externally applied forcing has, in driven granular media, a role similar to temperature in thermal systems