352 research outputs found
Coarsening dynamics of ternary amphiphilic fluids and the self-assembly of the gyroid and sponge mesophases: lattice-Boltzmann simulations
By means of a three-dimensional amphiphilic lattice-Boltzmann model with
short-range interactions for the description of ternary amphiphilic fluids, we
study how the phase separation kinetics of a symmetric binary immiscible fluid
is altered by the presence of the amphiphilic species. We find that a gradual
increase in amphiphile concentration slows down domain growth, initially from
algebraic, to logarithmic temporal dependence, and, at higher concentrations,
from logarithmic to stretched-exponential form. In growth-arrested
stretched-exponential regimes, at late times we observe the self-assembly of
sponge mesophases and gyroid liquid crystalline cubic mesophases, hence
confirming that (a) amphiphile-amphiphile interactions need not be long-ranged
in order for periodically modulated structures to arise in a dynamics of
competing interactions, and (b) a chemically-specific model of the amphiphile
is not required for the self-assembly of cubic mesophases, contradicting claims
in the literature. We also observe a structural order-disorder transition
between sponge and gyroid phases driven by amphiphile concentration alone or,
independently, by the amphiphile-amphiphile and the amphiphile-binary fluid
coupling parameters. For the growth-arrested mesophases, we also observe
temporal oscillations in the structure function at all length scales; most of
the wavenumbers show slow decay, and long-term stationarity or growth for the
others. We ascribe this behaviour to a combination of complex amphiphile
dynamics leading to Marangoni flows.Comment: 16 pages, 13 figures. Accepted for publication in Phys. Rev. E.
(Replaced for the latest version, in press.) Higher-quality figures can be
sent upon reques
A Particulate Basis for an Immiscible Lattice-Gas Model
We show that a phenomenological hydrodynamic lattice-gas model of two-phase
flow, developed by Rothman and Keller in 1988 and used extensively for
numerical simulations since then, can be derived from an underlying model of
particle interactions. From this result, we elucidate the nature of the
hydrodynamic limit of the Rothman-Keller model.Comment: 11 pages. Accepted for publication in Computer Physics Communication
A three-dimensional lattice gas model for amphiphilic fluid dynamics
We describe a three-dimensional hydrodynamic lattice-gas model of amphiphilic
fluids. This model of the non-equilibrium properties of oil-water-surfactant
systems, which is a non-trivial extension of an earlier two-dimensional
realisation due to Boghosian, Coveney and Emerton [Boghosian, Coveney, and
Emerton 1996, Proc. Roy. Soc. A 452, 1221-1250], can be studied effectively
only when it is implemented using high-performance computing and visualisation
techniques. We describe essential aspects of the model's theoretical basis and
computer implementation, and report on the phenomenological properties of the
model which confirm that it correctly captures binary oil-water and
surfactant-water behaviour, as well as the complex phase behaviour of ternary
amphiphilic fluids.Comment: 34 pages, 13 figures, high resolution figures available on reques
Large-scale grid-enabled lattice-Boltzmann simulations of complex fluid flow in porous media and under shear
Well designed lattice-Boltzmann codes exploit the essentially embarrassingly
parallel features of the algorithm and so can be run with considerable
efficiency on modern supercomputers. Such scalable codes permit us to simulate
the behaviour of increasingly large quantities of complex condensed matter
systems. In the present paper, we present some preliminary results on the large
scale three-dimensional lattice-Boltzmann simulation of binary immiscible fluid
flows through a porous medium derived from digitised x-ray microtomographic
data of Bentheimer sandstone, and from the study of the same fluids under
shear. Simulations on such scales can benefit considerably from the use of
computational steering and we describe our implementation of steering within
the lattice-Boltzmann code, called LB3D, making use of the RealityGrid steering
library. Our large scale simulations benefit from the new concept of capability
computing, designed to prioritise the execution of big jobs on major
supercomputing resources. The advent of persistent computational grids promises
to provide an optimal environment in which to deploy these mesoscale simulation
methods, which can exploit the distributed nature of compute, visualisation and
storage resources to reach scientific results rapidly; we discuss our work on
the grid-enablement of lattice-Boltzmann methods in this context.Comment: 17 pages, 6 figures, accepted for publication in
Phil.Trans.R.Soc.Lond.
Comparison of Molecular Dynamics with Hybrid Continuum-Molecular Dynamics for a Single Tethered Polymer in a Solvent
We compare a newly developed hybrid simulation method which combines
classical molecular dynamics (MD) and computational fluid dynamics (CFD) to a
simulation consisting only of molecular dynamics. The hybrid code is composed
of three regions: a classical MD region, a continuum domain where the dynamical
equations are solved by standard CFD methods, and an overlap domain where
transport information from the other two domains is exchanged. The exchange of
information in the overlap region ensures that momentum, energy and mass are
conserved. The validity of the hybrid code is demonstrated by studying a single
polymer tethered to a hard wall immersed in explicit solvent and undergoing
shear flow. In classical molecular dynamics simulation a great deal of
computational time is devoted to simulating solvent molecules, although the
solvent itself is of no direct interest. By contrast, the hybrid code simulates
the polymer and surrounding solvent explicitly, whereas the solvent farther
away from the polymer is modeled using a continuum description. In the hybrid
simulations the MD domain is an open system whose number of particles is
controlled to filter the perturbative density waves produced by the polymer
motion.We compare conformational properties of the polymer in both simulations
for various shear rates. In all cases polymer properties compare extremely well
between the two simulation scenarios, thereby demonstrating that this hybrid
method is a useful way to model a system with polymers and under nonzero flow
conditions. There is also good agreement between the MD and hybrid schemes and
experimental data on tethered DNA in flow. The computational cost of the hybrid
protocol can be reduced to less than 6% of the cost of updating the MD forces,
confirming the practical value of the method.Comment: 13 pages, 8 figure
Structural transitions and arrest of domain growth in sheared binary immiscible fluids and microemulsions
We investigate spinodal decomposition and structuring effects in binary
immiscible and ternary amphiphilic fluid mixtures under shear by means of three
dimensional lattice Boltzmann simulations. We show that the growth of
individual fluid domains can be arrested by adding surfactant to the system,
thus forming a bicontinous microemulsion. We demonstrate that the maximum
domain size and the time of arrest depend linearly on the concentration of
amphiphile molecules. In addition, we find that for a well defined threshold
value of amphiphile concentration, the maximum domain size and time of complete
arrest do not change. For systems under constant and oscillatory shear we
analyze domain growth rates in directions parallel and perpendicular to the
applied shear. We find a structural transition from a sponge to a lamellar
phase by applying a constant shear and the occurrence of tubular structures
under oscillatory shear. The size of the resulting lamellae and tubes depends
strongly on the amphiphile concentration, shear rate and shear frequency.Comment: 12 pages, 11 figure
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