3,858 research outputs found
Foundations of Dissipative Particle Dynamics
We derive a mesoscopic modeling and simulation technique that is very close
to the technique known as dissipative particle dynamics. The model is derived
from molecular dynamics by means of a systematic coarse-graining procedure.
Thus the rules governing our new form of dissipative particle dynamics reflect
the underlying molecular dynamics; in particular all the underlying
conservation laws carry over from the microscopic to the mesoscopic
descriptions. Whereas previously the dissipative particles were spheres of
fixed size and mass, now they are defined as cells on a Voronoi lattice with
variable masses and sizes. This Voronoi lattice arises naturally from the
coarse-graining procedure which may be applied iteratively and thus represents
a form of renormalisation-group mapping. It enables us to select any desired
local scale for the mesoscopic description of a given problem. Indeed, the
method may be used to deal with situations in which several different length
scales are simultaneously present. Simulations carried out with the present
scheme show good agreement with theoretical predictions for the equilibrium
behavior.Comment: 18 pages, 7 figure
Dissipative particle dynamics for interacting systems
We introduce a dissipative particle dynamics scheme for the dynamics of
non-ideal fluids. Given a free-energy density that determines the
thermodynamics of the system, we derive consistent conservative forces. The use
of these effective, density dependent forces reduces the local structure as
compared to previously proposed models. This is an important feature in
mesoscopic modeling, since it ensures a realistic length and time scale
separation in coarse-grained models. We consider in detail the behavior of a
van der Waals fluid and a binary mixture with a miscibility gap. We discuss the
physical implications of having a single length scale characterizing the
interaction range, in particular for the interfacial properties.Comment: 25 pages, 12 figure
Dissipative Particle Dynamics with energy conservation
Dissipative particle dynamics (DPD) does not conserve energy and this
precludes its use in the study of thermal processes in complex fluids. We
present here a generalization of DPD that incorporates an internal energy and a
temperature variable for each particle. The dissipation induced by the
dissipative forces between particles is invested in raising the internal energy
of the particles. Thermal conduction occurs by means of (inverse) temperature
differences. The model can be viewed as a simplified solver of the fluctuating
hydrodynamic equations and opens up the possibility of studying thermal
processes in complex fluids with a mesoscopic simulation technique.Comment: 5 page
Dissipative Particle Dynamics with Energy Conservation
The stochastic differential equations for a model of dissipative particle
dynamics with both total energy and total momentum conservation in the
particle-particle interactions are presented. The corresponding Fokker-Planck
equation for the evolution of the probability distribution for the system is
deduced together with the corresponding fluctuation-dissipation theorems
ensuring that the ab initio chosen equilibrium probability distribution for the
relevant variables is a stationary solution. When energy conservation is
included, the system can sustain temperature gradients and heat flow can be
modeled.Comment: 7 pages, submitted to Europhys. Let
New parametrization method for dissipative particle dynamics
We introduce an improved method of parameterizing the Groot-Warren version of Dissipative Particle Dynamics (DPD) by exploiting a correspondence between DPD and
Scatchard-Hildebrand regular solution theory. The new parameterization scheme widens the realm of applicability of DPD by first removing the restriction of equal repulsive
interactions between like beads, and second, by relating all conservative interactions between beads directly to cohesive energy densities.
We establish the correspondence by deriving an expression for the Helmoltz free energy of mixing obtaining a heat of mixing which is exactly the same form as that for a
regular mixture (quadratic in the volume fraction) and an entropy of mixing which reduces to the ideal entropy of mixing for equal molar volumes. We equate the conservative interaction parameters in the DPD force law to the cohesive energy densities of the pure fluids providing an alternative method of calculating the self-interaction
parameters as well as a route to the cross-interaction parameter.
We validate the new parameterization by modelling the binary system: SnI4/SiCl4, which displays liquid-liquid coexistence below an upper critical solution temperature
around 140°C. A series of DPD simulations were conducted at a set of temperatures ranging from 0°C to above the experimental upper critical solution temperature using
conservative parameters based on extrapolated experimental data. These simulations can be regarded as being equivalent to a quench from a high temperature to a lower one at constant volume.
Our simulations recover the expected phase behaviour ranging from solid-liquid coexistence to liquid-liquid co-existence and eventually leading to a homogeneous single
phase system. The results yield a binodal curve in close agreement with one predicted using regular solution theory, but, significantly, in closer agreement with actual solubility
measurements
Phase Diagram for Self-assembly of Amphiphilic Molecule C12E6 by Dissipative Particle Dynamics Simulation
In a previous study, dissipative particle dynamics simulation was used to
qualitatively clarify the phase diagram of the amphiphilic molecule
hexaethylene glycol dodecyl ether (C12E6). In the present study, the
hydrophilicity dependence of the phase structure was clarified qualitatively by
varying the interaction potential between hydrophilic molecules and water
molecules in a dissipative particle dynamics (DPD) simulation using the Jury
model. By varying the coefficient of the interaction potential between
hydrophilic beads and water molecules as x=-20, 0, 10, and 20, at a
dimensionless temperature of T=0.5 and a concentration of amphiphilic molecules
in water of phi=50% the phase structures grew to lamellar (x=-20), hexagonal
(x=0), and micellar (x=10) phases. For x=20, phase separation occurs between
hydrophilic beads and water molecules
Static and Dynamic Properties of Dissipative Particle Dynamics
The algorithm for the DPD fluid, the dynamics of which is conceptually a
combination of molecular dynamics, Brownian dynamics and lattice gas automata,
is designed for simulating rheological properties of complex fluids on
hydrodynamic time scales. This paper calculates the equilibrium and transport
properties (viscosity, self-diffusion) of the thermostated DPD fluid explicitly
in terms of the system parameters. It is demonstrated that temperature
gradients cannot exist, and that there is therefore no heat conductivity.
Starting from the N-particle Fokker-Planck, or Kramers' equation, we prove an
H-theorem for the free energy, obtain hydrodynamic equations, and derive a
non-linear kinetic equation (the Fokker-Planck-Boltzmann equation) for the
single particle distribution function. This kinetic equation is solved by the
Chapman-Enskog method. The analytic results are compared with numerical
simulations.Comment: 22 pages, LaTeX, 3 Postscript figure
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