2,025 research outputs found
The Role of Spin Anisotropy in the Unbinding of Interfaces
We study the ground state of a classical X-Y model with -fold spin
anisotropy in a uniform external field, . An interface is introduced
into the system by a suitable choice of boundary conditions. For large , as
, we prove using an expansion in that the interface unbinds
from the surface through an infinite series of layering transitions. Numerical
work shows that the transitions end in a sequence of critical end points.Comment: 7 pages RevTeX, plus 1 postscript figure available from the authors
OUTP-94-41
Dissipative particle dynamics: the equilibrium for finite time steps
Dissipative particle dynamics (DPD) is a relatively new technique which has
proved successful in the simulation of complex fluids. We caution that for the
equilibrium achieved by the DPD simulation of a simple fluid the temperature
depends strongly on the time step. An analytic expression for the dependence is
obtained and shown to agree well with simulation results.Comment: 5 pages, LaTeX, 1 Postscript figure, submitted to Europhys.Letts.,
Algebraic corrections made to final resul
Lattice Boltzmann Algorithm for three-dimensional liquid crystal hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics in three dimensions. The equations of motion are written in terms
of a tensor order parameter. This allows both the isotropic and the nematic
phases to be considered. Backflow effects and the hydrodynamics of topological
defects are naturally included in the simulations, as are viscoelastic effects
such as shear-thinning and shear-banding. We describe the implementation of
velocity boundary conditions and show that the algorithm can be used to
describe optical bounce in twisted nematic devices and secondary flow in
sheared nematics with an imposed twist.Comment: 12 pages, 3 figure
Transport coefficients of a mesoscopic fluid dynamics model
We investigate the properties of stochastic rotation dynamics
(Malevanets-Kapral method), a mesoscopic model used for simulating fluctuating
hydrodynamics. Analytical results are given for the transport coefficients. We
discuss the most efficient way of measuring the transport properties and obtain
excellent agreement between the theoretical and numerical calculations.Comment: 12 pages, 9 figures, submitted to J. Chem. Phy
Modeling microscopic swimmers at low Reynolds number
We employ three numerical methods to explore the motion of low Reynolds
number swimmers, modeling the hydrodynamic interactions by means of the Oseen
tensor approximation, lattice Boltzmann simulations and multiparticle collision
dynamics. By applying the methods to a three bead linear swimmer, for which
exact results are known, we are able to compare and assess the effectiveness of
the different approaches. We then propose a new class of low Reynolds number
swimmers, generalized three bead swimmers that can change both the length of
their arms and the angle between them. Hence we suggest a design for a
microstructure capable of moving in three dimensions. We discuss multiple bead,
linear microstructures and show that they are highly efficient swimmers. We
then turn to consider the swimming motion of elastic filaments. Using
multiparticle collision dynamics we show that a driven filament behaves in a
qualitatively similar way to the micron-scale swimming device recently
demonstrated by Dreyfus et al.Comment: 12 pages, 10 figure
Space missions to comets
The broad impact of a cometary mission is assessed with particular emphasis on scientific interest in a fly-by mission to Halley's comet and a rendezvous with Tempel 2. Scientific results, speculations, and future plans are discussed
A Coarse Grained Model for DNA and Polymer Packaging: Statics and Dynamics
We present a numerical characterization of the statics and dynamics of the packaging of a semi-flexible polymer inside a sphere. The study is motivated by recent experiments on the packaging of DNA inside viral capsids. It is found that the force required to confine the coarse-grained polymer is in fair agreement with that found in experiments for the packaging of the phi29 bacteriophage genome. Despite its schematic nature, the model is capable of reproducing the most salient dynamical features of packaging experiments such as the presence of pauses during individual packaging processes and the trend of the resisting force as a function of chain packed fraction
Magnetic properties and critical behavior of disordered Fe_{1-x}Ru_x alloys: a Monte Carlo approach
We study the critical behavior of a quenched random-exchange Ising model with
competing interactions on a bcc lattice. This model was introduced in the study
of the magnetic behavior of Fe_{1-x}Ru_x alloys for ruthenium concentrations
x=0%, x=4%, x=6%, and x=8%. Our study is carried out within a Monte Carlo
approach, with the aid of a re-weighting multiple histogram technique. By means
of a finite-size scaling analysis of several thermodynamic quantities, taking
into account up to the leading irrelevant scaling field term, we find estimates
of the critical exponents \alpha, \beta, \gamma, and \nu, and of the critical
temperatures of the model. Our results for x=0% are in excellent agreement with
those for the three-dimensional pure Ising model in the literature. We also
show that our critical exponent estimates for the disordered cases are
consistent with those reported for the transition line between paramagnetic and
ferromagnetic phases of both randomly dilute and Ising models. We
compare the behavior of the magnetization as a function of temperature with
that obtained by Paduani and Branco (2008), qualitatively confirming the
mean-field result. However, the comparison of the critical temperatures
obtained in this work with experimental measurements suggest that the model
(initially obtained in a mean-field approach) needs to be modified
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