2,769 research outputs found
A new model for simulating colloidal dynamics
We present a new hybrid lattice-Boltzmann and Langevin molecular dynamics
scheme for simulating the dynamics of suspensions of spherical colloidal
particles. The solvent is modeled on the level of the lattice-Boltzmann method
while the molecular dynamics is done for the solute. The coupling between the
two is implemented through a frictional force acting both on the solvent and on
the solute, which depends on the relative velocity. A spherical colloidal
particle is represented by interaction sites at its surface. We demonstrate
that this scheme quantitatively reproduces the translational and rotational
diffusion of a neutral spherical particle in a liquid and show preliminary
results for a charged spherical particle. We argue that this method is
especially advantageous in the case of charged colloids.Comment: For a movie click on the link below Fig
Electrophoretic mobility of a charged colloidal particle: A computer simulation study
We study the mobility of a charged colloidal particle in a constant
homogeneous electric field by means of computer simulations. The simulation
method combines a lattice Boltzmann scheme for the fluid with standard Langevin
dynamics for the colloidal particle, which is built up from a net of bonded
particles forming the surface of the colloid. The coupling between the two
subsystems is introduced via friction forces. In addition explicit counterions,
also coupled to the fluid, are present. We observe a non-monotonous dependence
of the electrophoretic mobility on the bare colloidal charge. At low surface
charge density we observe a linear increase of the mobility with bare charge,
whereas at higher charges, where more than half of the ions are co-moving with
the colloid, the mobility decreases with increasing bare charge.Comment: 15 pages, 8 figure
Statistical Mechanics of the Fluctuating Lattice Boltzmann Equation
We propose a new formulation of the fluctuating lattice Boltzmann equation
that is consistent with both equilibrium statististical mechanics and
fluctuating hydrodynamics. The formalism is based on a generalized lattice-gas
model, with each velocity direction occupied by many particles. We show that
the most probable state of this model corresponds to the usual equilibrium
distribution of the lattice Boltzmann equation. Thermal fluctuations about this
equilibrium are controlled by the mean number of particles at a lattice site.
Stochastic collision rules are described by a Monte Carlo process satisfying
detailed balance. This allows for a straightforward derivation of discrete
Langevin equations for the fluctuating modes. It is shown that all
non-conserved modes should be thermalized, as first pointed out by Adhikari et
al.; any other choice violates the condition of detailed balance. A
Chapman-Enskog analysis is used to derive the equations of fluctuating
hydrodynamics on large length and time scales; the level of fluctuations is
shown to be thermodynamically consistent with the equation of state of an
isothermal, ideal gas. We believe this formalism will be useful in developing
new algorithms for thermal and multiphase flows.Comment: Submitted to Physical Review E-11 pages Corrected Author(s) field on
submittal for
Sedimentation and Flow Through Porous Media: Simulating Dynamically Coupled Discrete and Continuum Phases
We describe a method to address efficiently problems of two-phase flow in the
regime of low particle Reynolds number and negligible Brownian motion. One of
the phases is an incompressible continuous fluid and the other a discrete
particulate phase which we simulate by following the motion of single
particles. Interactions between the phases are taken into account using locally
defined drag forces. We apply our method to the problem of flow through random
media at high porosity where we find good agreement to theoretical expectations
for the functional dependence of the pressure drop on the solid volume
fraction. We undertake further validations on systems undergoing gravity
induced sedimentation.Comment: 22 pages REVTEX, figures separately in uudecoded, compressed
postscript format - alternatively e-mail '[email protected]' for
hardcopies
Fluctuating lattice Boltzmann
The lattice Boltzmann algorithm efficiently simulates the Navier Stokes
equation of isothermal fluid flow, but ignores thermal fluctuations of the
fluid, important in mesoscopic flows. We show how to adapt the algorithm to
include noise, satisfying a fluctuation-dissipation theorem (FDT) directly at
lattice level: this gives correct fluctuations for mass and momentum densities,
and for stresses, at all wavevectors . Unlike previous work, which recovers
FDT only as , our algorithm offers full statistical mechanical
consistency in mesoscale simulations of, e.g., fluctuating colloidal
hydrodynamics.Comment: 7 pages, 3 figures, to appear in Europhysics Letter
Star formation in Perseus
We present a complete survey of current star formation in the Perseus
molecular cloud, made at 850 and 450 micron with SCUBA at the JCMT. Covering 3
deg^2, this submillimetre continuum survey for protostellar activity is second
in size only to that of rho Ophiuchus (Johnstone et al. 2004). Complete above
0.4 msun (5 sigma detection in a 14'' beam), we detect a total of 91 protostars
and prestellar cores. Of these, 80% lie in clusters, representative of star
formation across the Galaxy. Two of the groups of cores are associated with the
young stellar clusters IC348 and NGC1333, and are consistent with a steady or
reduced star formation rate in the last 0.5 Myr, but not an increasing one. In
Perseus, 40--60% of cores are in small clusters (< 50 msun) and isolated
objects, much more than the 10% suggested from infrared studies. Complementing
the dust continuum, we present a C^18O map of the whole cloud at 1' resolution.
The gas and dust show filamentary structure of the dense gas on large and small
scales, with the high column density filaments breaking up into clusters of
cores. The filament mass per unit length is 5--11 msun per 0.1 pc. Given these
filament masses, there is no requirement for substantial large scale flows
along or onto the filaments in order to gather sufficient material for star
formation. We find that the probability of finding a submillimetre core is a
strongly increasing function of column density, as measured by C^18O integrated
intensity, prob(core) proportional to I^3.0. This power law relation holds down
to low column density, suggesting that there is no A_v threshold for star
formation in Perseus, unless all the low-A_v submm cores can be demonstrated to
be older protostars which have begun to lose their natal molecular cloud.Comment: 15 pages, 5 figures, bibtex file scubasf_astroph.bbl, included tex
files SSA-clusters-sorted-tidy.te
Fluctuating hydrodynamic modelling of fluids at the nanoscale
A good representation of mesoscopic fluids is required to combine with
molecular simulations at larger length and time scales (De Fabritiis {\it et.
al}, Phys. Rev. Lett. 97, 134501 (2006)). However, accurate computational
models of the hydrodynamics of nanoscale molecular assemblies are lacking, at
least in part because of the stochastic character of the underlying fluctuating
hydrodynamic equations. Here we derive a finite volume discretization of the
compressible isothermal fluctuating hydrodynamic equations over a regular grid
in the Eulerian reference system. We apply it to fluids such as argon at
arbitrary densities and water under ambient conditions. To that end, molecular
dynamics simulations are used to derive the required fluid properties. The
equilibrium state of the model is shown to be thermodynamically consistent and
correctly reproduces linear hydrodynamics including relaxation of sound and
shear modes. We also consider non-equilibrium states involving diffusion and
convection in cavities with no-slip boundary conditions
CO abundances in a protostellar cloud: freeze-out and desorption in the envelope and outflow of L483
CO isotopes are able to probe the different components in protostellar
clouds. These components, core, envelope and outflow have distinct physical
conditions and sometimes more than one component contributes to the observed
line profile. In this study we determine how CO isotope abundances are altered
by the physical conditions in the different components. We use a 3D molecular
line transport code to simulate the emission of four CO isotopomers, 12CO
J=2-1, 13CO J=2-1, C18O J=2-1 and C17O J=2-1 from the Class 0/1 object L483,
which contains a cold quiescent core, an infalling envelope and a clear
outflow. Our models replicate JCMT (James Clerk Maxwell Telescope) line
observations with the inclusion of freeze-out, a density profile and infall.
Our model profiles of 12CO and 13CO have a large linewidth due to a high
velocity jet. These profiles replicate the process of more abundant material
being susceptible to a jet. C18O and C17O do not display such a large linewidth
as they trace denser quiescent material deep in the cloud.Comment: 9 figures, 13 pages, 2 table
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