489 research outputs found
Tests of mode coupling theory in a simple model for two-component miscible polymer blends
We present molecular dynamics simulations on the structural relaxation of a
simple bead-spring model for polymer blends. The introduction of a different
monomer size induces a large time scale separation for the dynamics of the two
components. Simulation results for a large set of observables probing density
correlations, Rouse modes, and orientations of bond and chain end-to-end
vectors, are analyzed within the framework of the Mode Coupling Theory (MCT).
An unusually large value of the exponent parameter is obtained. This feature
suggests the possibility of an underlying higher-order MCT scenario for dynamic
arrest.Comment: Revised version. Additional figures and citation
Lattice Boltzmann versus Molecular Dynamics simulation of nano-hydrodynamic flows
A fluid flow in a simple dense liquid, passing an obstacle in a
two-dimensional thin film geometry, is simulated by Molecular Dynamics (MD)
computer simulation and compared to results of Lattice Boltzmann (LB)
simulations. By the appropriate mapping of length and time units from LB to MD,
the velocity field as obtained from MD is quantitatively reproduced by LB. The
implications of this finding for prospective LB-MD multiscale applications are
discussed.Comment: 4 pages, 4 figure
Non-exponential kinetic behavior of confined water
We present the results of molecular dynamics simulations of SPC/E water
confined in a realistic model of a silica pore. The single-particle dynamics
have been studied at ambient temperature for different hydration levels. The
confinement near the hydrophilic surface makes the dynamic behaviour of the
liquid strongly dependent on the hydration level. Upon decrease of the number
of water molecules in the pore we observe the onset of a slow dynamics due to
the ``cage effect''. The conventional picture of a stochastic single-particle
diffusion process thus looses its validity
Molecular Dynamics Computer Simulation of the Dynamics of Supercooled Silica
We present the results of a large scale computer simulation of supercooled
silica. We find that at high temperatures the diffusion constants show a
non-Arrhenius temperature dependence whereas at low temperature this dependence
is also compatible with an Arrhenius law. We demonstrate that at low
temperatures the intermediate scattering function shows a two-step relaxation
behavior and that it obeys the time temperature superposition principle. We
also discuss the wave-vector dependence of the nonergodicity parameter and the
time and temperature dependence of the non-Gaussian parameter.Comment: 5 pages, Latex, 6 postscript figure
Wall-liquid and wall-crystal interfacial free energies via thermodynamic integration: A molecular dynamics simulation study
A method is proposed to compute the interfacial free energy of a
Lennard-Jones system in contact with a structured wall by molecular dynamics
simulation. Both the bulk liquid and bulk face-centered-cubic crystal phase
along the (111) orientation are considered. Our approach is based on a
thermodynamic integration scheme where first the bulk Lennard-Jones system is
reversibly transformed to a state where it interacts with a structureless flat
wall. In a second step, the flat structureless wall is reversibly transformed
into an atomistic wall with crystalline structure. The dependence of the
interfacial free energy on various parameters such as the wall potential, the
density and orientation of the wall is investigated. The conditions are
indicated under which a Lennard-Jones crystal partially wets a flat wall.Comment: 15 pages, 11 figure
Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems
We present a hybrid method for the simulation of colloidal systems, that
combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB
method is used as a model for the solvent in order to take into account the
hydrodynamic mass and momentum transport through the solvent. The colloidal
particles are propagated via MD and they are coupled to the LB fluid by viscous
forces. With respect to the LB fluid, the colloids are represented by uniformly
distributed points on a sphere. Each such point (with a velocity V(r) at any
off-lattice position r is interacting with the neighboring eight LB nodes by a
frictional force F=\xi_0(V(r)-u(r)) with \xi_0 being a friction force and u(r)
being the velocity of the fluid at the position r. Thermal fluctuations are
introduced in the framework of fluctuating hydrodynamics. This coupling scheme
has been proposed recently for polymer systems by Ahlrichs and D"unweg [J.
Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single
colloidal particle in a LB fluid, namely the effective Stokes friction and long
time tails in the autocorrelation functions for the translational and
rotational velocity. Moreover, a charged colloidal system is considered
consisting of a macroion, counterions and coions that are coupled to a LB
fluid. We study the behavior of the ions in a constant electric field. In
particular, an estimate of the effective charge of the macroion is yielded from
the number of counterions that move with the macroion in the direction of the
electric field.Comment: 37 pages, 12 figure
The Influence of Chemical Short Range Order on Atomic Diffusion in Al-Ni Melts
We use inelastic neutron scattering and molecular dynamics (MD) simulation to
investigate the chemical short range order (CSRO), visible through prepeaks in
the structure factors, and its relation to self diffusion in Al-Ni melts. As a
function of composition at 1795K Ni self diffusion coefficients from experiment
and simulation exhibit a non-linear dependence with a pronounced increase on
the Al-rich side. This comes along with a change in CSRO with increasing Al
content that is related to a more dense packing of the atoms in Ni-rich Al-Ni
systems.Comment: 11 pages, 4 figure
Nanoflows through disordered media: a joint Lattice Boltzmann and Molecular Dynamics investigation
We investigate nanoflows through dilute disordered media by means of joint
lattice Boltzmann (LB) and molecular dynamics (MD) simulations -- when the size
of the obstacles is comparable to the size of the flowing particles -- for
randomly located spheres and for a correlated particle-gel. In both cases at
sufficiently low solid fraction, , LB and MD provide similar values
of the permeability. However, for , MD shows that molecular size
effects lead to a decrease of the permeability, as compared to the
Navier-Stokes predictions. For gels, the simulations highlights a surplus of
permeability, which can be accommodated within a rescaling of the effective
radius of the gel monomers.Comment: 4 pages, 4 figure
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