8,905 research outputs found
Stochastic Hard-Sphere Dynamics for Hydrodynamics of Non-Ideal Fluids
A novel stochastic fluid model is proposed with non-ideal structure factor
consistent with compressibility, and adjustable transport coefficients. This
Stochastic Hard Sphere Dynamics (SHSD) algorithm is a modification of the
Direct Simulation Monte Carlo (DSMC) algorithm and has several computational
advantages over event-driven hard-sphere molecular dynamics. Surprisingly, SHSD
results in an equation of state and pair correlation function identical to that
of a deterministic Hamiltonian system of penetrable spheres interacting with
linear core pair potentials. The fluctuating hydrodynamic behavior of the SHSD
fluid is verified for the Brownian motion of a nano-particle suspended in a
compressible solvent.Comment: This work performed under the auspices of the U.S. Department of
Energy by Lawrence Livermore National Laboratory under Contract
DE-AC52-07NA27344 (LLNL-JRNL-401745). To appear in Phys. Rev. Lett. 200
A Thermodynamically-Consistent Non-Ideal Stochastic Hard-Sphere Fluid
A grid-free variant of the Direct Simulation Monte Carlo (DSMC) method is
proposed, named the Isotropic DSMC (I-DSMC) method, that is suitable for
simulating dense fluid flows at molecular scales. The I-DSMC algorithm
eliminates all grid artifacts from the traditional DSMC algorithm; it is
Galilean invariant and microscopically isotropic. The stochastic collision
rules in I-DSMC are modified to yield a non-ideal structure factor that gives
consistent compressibility, as first proposed in [Phys. Rev. Lett. 101:075902
(2008)]. The resulting Stochastic Hard Sphere Dynamics (SHSD) fluid is
empirically shown to be thermodynamically identical to a deterministic
Hamiltonian system of penetrable spheres interacting with a linear core pair
potential, well-described by the hypernetted chain (HNC) approximation. We
apply a stochastic Enskog kinetic theory for the SHSD fluid to obtain estimates
for the transport coefficients that are in excellent agreement with particle
simulations over a wide range of densities and collision rates. The fluctuating
hydrodynamic behavior of the SHSD fluid is verified by comparing its dynamic
structure factor against theory based on the Landau-Lifshitz Navier-Stokes
equations. We also study the Brownian motion of a nano-particle suspended in an
SHSD fluid and find a long-time power-law tail in its velocity autocorrelation
function consistent with hydrodynamic theory and molecular dynamics
calculations.Comment: 30 pages, revision adding some clarifications and a new figure. See
also arXiv:0803.035
Electrokinetic Lattice Boltzmann solver coupled to Molecular Dynamics: application to polymer translocation
We develop a theoretical and computational approach to deal with systems that
involve a disparate range of spatio-temporal scales, such as those comprised of
colloidal particles or polymers moving in a fluidic molecular environment. Our
approach is based on a multiscale modeling that combines the slow dynamics of
the large particles with the fast dynamics of the solvent into a unique
framework. The former is numerically solved via Molecular Dynamics and the
latter via a multi-component Lattice Boltzmann. The two techniques are coupled
together to allow for a seamless exchange of information between the
descriptions. Being based on a kinetic multi-component description of the fluid
species, the scheme is flexible in modeling charge flow within complex
geometries and ranging from large to vanishing salt concentration. The details
of the scheme are presented and the method is applied to the problem of
translocation of a charged polymer through a nanopores. In the end, we discuss
the advantages and complexities of the approach
Fluctuating hydrodynamics of multi-species, non-reactive mixtures
In this paper we discuss the formulation of the fuctuating Navier-Stokes
(FNS) equations for multi-species, non-reactive fluids. In particular, we
establish a form suitable for numerical solution of the resulting stochastic
partial differential equations. An accurate and efficient numerical scheme,
based on our previous methods for single species and binary mixtures, is
presented and tested at equilibrium as well as for a variety of non-equilibrium
problems. These include the study of giant nonequilibrium concentration
fluctuations in a ternary mixture in the presence of a diffusion barrier, the
triggering of a Rayleigh-Taylor instability by diffusion in a four-species
mixture, as well as reverse diffusion in a ternary mixture. Good agreement with
theory and experiment demonstrates that the formulation is robust and can serve
as a useful tool in the study of thermal fluctuations for multi-species fluids.
The extension to include chemical reactions will be treated in a sequel paper
Influence of Disorder Strength on Phase Field Models of Interfacial Growth
We study the influence of disorder strength on the interface roughening
process in a phase-field model with locally conserved dynamics. We consider two
cases where the mobility coefficient multiplying the locally conserved current
is either constant throughout the system (the two-sided model) or becomes zero
in the phase into which the interface advances (one-sided model). In the limit
of weak disorder, both models are completely equivalent and can reproduce the
physical process of a fluid diffusively invading a porous media, where
super-rough scaling of the interface fluctuations occurs. On the other hand,
increasing disorder causes the scaling properties to change to intrinsic
anomalous scaling. In the limit of strong disorder this behavior prevails for
the one-sided model, whereas for the two-sided case, nucleation of domains in
front of the invading front are observed.Comment: Accepted for publication in PR
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