382 research outputs found
USHER: an algorithm for particle insertion in dense fluids
The insertion of solvent particles in molecular dynamics simulations of
complex fluids is required in many situations involving open systems, but this
challenging task has been scarcely explored in the literature. We propose a
simple and fast algorithm (USHER) that inserts the new solvent particles at
locations where the potential energy has the desired prespecified value. For
instance, this value may be set equal to the system's excess energy per
particle, in such way that the inserted particles are energetically
indistinguishable from the other particles present. During the search for the
insertion site, the USHER algorithm uses a steepest descent iterator with a
displacement whose magnitude is adapted to the local features of the energy
landscape. The only adjustable parameter in the algorithm is the maximum
displacement and we show that its optimal value can be extracted from an
analysis of the structure of the potential energy landscape. We present
insertion tests in periodic and non-periodic systems filled with a
Lennard-Jones fluid whose density ranges from moderate values to high values.Comment: 10 pages (Latex), 8 figures (postscript); J. Chem. Phys. (in press)
200
Emergence of rheological properties in lattice Boltzmann simulations of gyroid mesophases
We use a lattice Boltzmann (LB) kinetic scheme for modelling amphiphilic
fluids that correctly predicts rheological effects in flow. No macroscopic
parameters are included in the model. Instead, three-dimensional hydrodynamic
and rheological effects are emergent from the underlying particulate
conservation laws and interactions. We report evidence of shear thinning and
viscoelastic flow for a self-assembled gyroid mesophase. This purely kinetic
approach is of general importance for the modelling and simulation of complex
fluid flows in situations when rheological properties cannot be predicted {\em
a priori}.Comment: 7 pages, 5 figure
Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids
During the last two years the RealityGrid project has allowed us to be one of
the few scientific groups involved in the development of computational grids.
Since smoothly working production grids are not yet available, we have been
able to substantially influence the direction of software development and grid
deployment within the project. In this paper we review our results from large
scale three-dimensional lattice Boltzmann simulations performed over the last
two years. We describe how the proactive use of computational steering and
advanced job migration and visualization techniques enabled us to do our
scientific work more efficiently. The projects reported on in this paper are
studies of complex fluid flows under shear or in porous media, as well as
large-scale parameter searches, and studies of the self-organisation of liquid
cubic mesophases.
Movies are available at
http://www.ica1.uni-stuttgart.de/~jens/pub/05/05-PhilTransReview.htmlComment: 18 pages, 9 figures, 4 movies available, accepted for publication in
Phil. Trans. R. Soc. London Series
Multiscale modelling of liquids with molecular specificity
The separation between molecular and mesoscopic length and time scales poses
a severe limit to molecular simulations of mesoscale phenomena. We describe a
hybrid multiscale computational technique which address this problem by keeping
the full molecular nature of the system where it is of interest and
coarse-graining it elsewhere. This is made possible by coupling molecular
dynamics with a mesoscopic description of realistic liquids based on Landau's
fluctuating hydrodynamics. We show that our scheme correctly couples
hydrodynamics and that fluctuations, at both the molecular and continuum
levels, are thermodynamically consistent. Hybrid simulations of sound waves in
bulk water and reflected by a lipid monolayer are presented as illustrations of
the scheme
Renormalisation-theoretic analysis of non-equilibrium phase transitions II: The effect of perturbations on rate coefficients in the Becker-Doring equations
We study in detail the application of renormalisation theory to models of
cluster aggregation and fragmentation of relevance to nucleation and growth
processes. In particular, we investigate the Becker-Doring (BD) equations,
originally formulated to describe and analyse non-equilibrium phase
transitions, but more recently generalised to describe a wide range of
physicochemical problems. We consider here rate coefficients which depend on
the cluster size in a power-law fashion, but now perturbed by small amplitude
random noise. Power-law rate coefficients arise naturally in the theory of
surface-controlled nucleation and growth processes. The noisy perturbations on
these rates reflect the effect of microscopic variations in such mean-field
coefficients, thermal fluctuations and/or experimental uncertainties. In the
present paper we generalise our earlier work that identified the nine classes
into which all dynamical behaviour must fall by investigating how random
perturbations of the rate coefficients influence the steady-state and kinetic
behaviour of the coarse-grained, renormalised system. We are hence able to
confirm the existence of a set of up to nine universality classes for such BD
systems.Comment: 30 pages, to appear in J Phys A Math Ge
Lattice Boltzmann simulations of contact line motion in a liquid-gas system
We use a lattice Boltzmann algorithm for liquid-gas coexistence to
investigate the steady state interface profile of a droplet held between two
shearing walls. The algorithm solves the hydrodynamic equations of motion for
the system. Partial wetting at the walls is implemented to agree with Cahn
theory. This allows us to investigate the processes which lead to the motion of
the three-phase contact line. We confirm that the profiles are a function of
the capillary number and a finite size analysis shows the emergence of a
dynamic contact angle, which can be defined in a region where the interfacial
curvature tends to zero.Comment: 13 pages, 5 figures, to appear in Phil. Trans. Roy. Soc. A
(Proceedings of the Xth International Conference on Discrete Simulation of
Fluid Dynamics.
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