3,130 research outputs found
Deposition of general ellipsoidal particles
We present a systematic overview of granular deposits composed of ellipsoidal
particles with different particle shapes and size polydispersities. We study
the density and anisotropy of such deposits as functions of size polydispersity
and two shape parameters that fully describe the shape of a general ellipsoid.
Our results show that, while shape influences significantly the macroscopic
properties of the deposits, polydispersity plays apparently a secondary role.
The density attains a maximum for a particular family of non-symmetrical
ellipsoids, larger than the density observed for prolate or oblate ellipsoids.
As for anisotropy measures, the contact forces show are increasingly preferred
along the vertical direction as the shape of the particles deviates for a
sphere. The deposits are constructed by means of an efficient molecular
dynamics method, where the contact forces are efficiently and accurately
computed. The main results are discussed in the light of applications for
porous media models and sedimentation processes.Comment: 7 pages, 8 figure
Molecular dynamics simulations of the evaporation of particle-laden droplets
We use molecular dynamics simulations to study the evaporation of
particle-laden droplets on a heated surface. The droplets are composed of a
Lennard-Jones fluid containing rigid particles which are spherical sections of
an atomic lattice, and heating is controlled through the temperature of an
atomistic substrate. We observe that sufficiently large (but still nano-sized)
particle-laden drops exhibit contact line pinning, measure the outward fluid
flow field which advects particle to the drop rim, and find that the structure
of the resulting aggregate varies with inter-particle interactions. In
addition, the profile of the evaporative fluid flux is measured with and
without particles present, and is also found to be in qualitative agreement
with earlier theory. The compatibility of simple nanoscale calculations and
micron-scale experiments indicates that molecular simulation may be used to
predict aggregate structure in evaporative growth processes
Free Thermal Convection Driven by Nonlocal Effects
We report and explain a convective phenomenon observed in molecular dynamics
simulations that cannot be classified either as a hydrodynamics instability nor
as a macroscopically forced convection. Two complementary arguments show that
the velocity field by a thermalizing wall is proportional to the ratio between
the heat flux and the pressure. This prediction is quantitatively corroborated
by our simulations.Comment: RevTex, figures is eps, submited for publicatio
Stratified horizontal flow in vertically vibrated granular layers
A layer of granular material on a vertically vibrating sawtooth-shaped base
exhibits horizontal flow whose speed and direction depend on the parameters
specifying the system in a complex manner. Discrete-particle simulations reveal
that the induced flow rate varies with height within the granular layer and
oppositely directed flows can occur at different levels. The behavior of the
overall flow is readily understood once this novel feature is taken into
account.Comment: 4 pages, 6 figures, submitte
Velocity fluctuations and hydrodynamic diffusion in sedimentation
We study non-equilibrium velocity fluctuations in a model for the
sedimentation of non-Brownian particles experiencing long-range hydrodynamic
interactions. The complex behavior of these fluctuations, the outcome of the
collective dynamics of the particles, exhibits many of the features observed in
sedimentation experiments. In addition, our model predicts a final relaxation
to an anisotropic (hydrodynamic) diffusive state that could be observed in
experiments performed over longer time ranges.Comment: 7 pages, 5 EPS figures, EPL styl
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
Using Available Volume to Predict Fluid Diffusivity in Random Media
We propose a simple equation for predicting self-diffusivity of fluids
embedded in random matrices of identical, but dynamically frozen, particles
(i.e., quenched-annealed systems). The only nontrivial input is the volume
available to mobile particles, which also can be predicted for two common
matrix types that reflect equilibrium and non-equilibrium fluid structures. The
proposed equation can account for the large differences in mobility exhibited
by quenched-annealed systems with indistinguishable static pair correlations,
illustrating the key role that available volume plays in transport.Comment: to appear in Physical Review E (12 pages, 4 figures
Star-graph expansions for bond-diluted Potts models
We derive high-temperature series expansions for the free energy and the
susceptibility of random-bond -state Potts models on hypercubic lattices
using a star-graph expansion technique. This method enables the exact
calculation of quenched disorder averages for arbitrary uncorrelated coupling
distributions. Moreover, we can keep the disorder strength as well as the
dimension as symbolic parameters. By applying several series analysis
techniques to the new series expansions, one can scan large regions of the
parameter space for any value of . For the bond-diluted 4-state
Potts model in three dimensions, which exhibits a rather strong first-order
phase transition in the undiluted case, we present results for the transition
temperature and the effective critical exponent as a function of
as obtained from the analysis of susceptibility series up to order 18. A
comparison with recent Monte Carlo data (Chatelain {\em et al.}, Phys. Rev.
E64, 036120(2001)) shows signals for the softening to a second-order transition
at finite disorder strength.Comment: 8 pages, 6 figure
Molecular dynamics in arbitrary geometries : parallel evaluation of pair forces
A new algorithm for calculating intermolecular pair forces in molecular dynamics (MD) simulations on a distributed parallel computer is presented. The arbitrary interacting cells algorithm (AICA) is designed to operate on geometrical domains defined by an unstructured, arbitrary polyhedral mesh that has been spatially decomposed into irregular portions for parallelisation. It is intended for nano scale fluid mechanics simulation by MD in complex geometries, and to provide the MD component of a hybrid MD/continuum simulation. The spatial relationship of the cells of the mesh is calculated at the start of the simulation and only the molecules contained in cells that have part of their surface closer than the cut-off radius of the intermolecular pair potential are required to interact. AICA has been implemented in the open source C++ code OpenFOAM, and its accuracy has been indirectly verified against a published MD code. The same system simulated in serial and in parallel on 12 and 32 processors gives the same results. Performance tests show that there is an optimal number of cells in a mesh for maximum speed of calculating intermolecular forces, and that having a large number of empty cells in the mesh does not add a significant computational overhead
The influence of bond-rigidity and cluster diffusion on the self-diffusion of hard spheres with square-well interaction
Hard spheres interacting through a square-well potential were simulated using
two different methods: Brownian Cluster Dynamics (BCD) and Event Driven
Brownian Dynamics (EDBD). The structure of the equilibrium states obtained by
both methods were compared and found to be almost the identical. Self diffusion
coefficients () were determined as a function of the interaction strength.
The same values were found using BCD or EDBD. Contrary the EDBD, BCD allows one
to study the effect of bond rigidity and hydrodynamic interaction within the
clusters. When the bonds are flexible the effect of attraction on is
relatively weak compared to systems with rigid bonds. increases first with
increasing attraction strength, and then decreases for stronger interaction.
Introducing intra-cluster hydrodynamic interaction weakly increases for a
given interaction strength. Introducing bond rigidity causes a strong decrease
of which no longer shows a maximum as function of the attraction strength
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