675 research outputs found
Slow Dynamics in Ion-Conducting Sodium Silicate Melts: Simulation and Mode-Coupling Theory
A combination of molecular-dynamics (MD) computer simulation and
mode-coupling theory (MCT) is used to elucidate the structure-dynamics relation
in sodium-silicate melts (NSx) of varying sodium concentration. Using only the
partial static structure factors from the MD as an input, MCT reproduces the
large separation in relaxation time scales of the sodium and the silicon/oxygen
components. This confirms the idea of sodium diffusion channels which are
reflected by a prepeak in the static structure factors around 0.95 A^-1, and
shows that it is possible to explain the fast sodium-ion dynamics peculiar to
these mixtures using a microscopic theory.Comment: 4 pages, 4 figure
Diffusion and Interdiffusion in Binary Metallic Melts
We discuss the dependence of self- and interdiffusion coefficients on
temperature and composition for two prototypical binary metallic melts, Al-Ni
and Zr-Ni, in molecular-dynamics (MD) computer simulations and the
mode-coupling theory of the glass transition (MCT). Dynamical processes that
are mainly entropic in origin slow down mass transport (as expressed through
self diffusion) in the mixture as compared to the ideal-mixing contribution.
Interdiffusion of chemical species is a competition of slow kinetic modes with
a strong thermodynamic driving force that is caused by non-entropic
interactions. The combination of both dynamic and thermodynamic effects causes
qualitative differences in the concentration dependence of self-diffusion and
interdiffusion coefficients. At high temperatures, the thermodynamic
enhancement of interdiffusion prevails, while at low temperatures, kinetic
effects dominate the concentration dependence, rationalized within MCT as the
approach to its ideal-glass transition temperature . The Darken equation
relating self- and interdiffusion qualitatively reproduces the
concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the
kinetic contributions to interdiffusion can be slower than the lower bound
suggested by the Darken equation. As temperature is decreased, the agreement
with Darken's equation improves, due to a strong coupling of all kinetic modes
that is a generic feature predicted by MCT.Comment: 16 pages, 12 figure
Amorphous silica between confining walls and under shear: a computer simulation study
Molecular dynamics computer simulations are used to investigate a silica melt
confined between walls at equilibrium and in a steady-state Poisseuille flow.
The walls consist of point particles forming a rigid face-centered cubic
lattice and the interaction of the walls with the melt atoms is modelled such
that the wall particles have only a weak bonding to those in the melt, i.e.
much weaker than the covalent bonding of a Si-O unit. We observe a pronounced
layering of the melt near the walls. This layering, as seen in the total
density profile, has a very irregular character which can be attributed to a
preferred orientational ordering of SiO4 tetrahedra near the wall. On
intermediate length scales, the structure of the melt at the walls can be well
distinguished from that of the bulk by means of the ring size distribution.
Whereas essentially no structural changes occur in the bulk under the influence
of the shear fields considered, strong structural rearrangements in the ring
size distribution are present at the walls as far as there is a slip motion.
For the sheared system, parabolic velocity profiles are found in the bulk
region as expected from hydrodynamics and the values for the shear viscosity as
extracted from those profiles are in good agreement with those obtained in pure
bulk simulations from the appropriate Green-Kubo formula.Comment: 23 pages of Late
The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results
The high-pressure dynamics of a computer-modeled silica melt is studied in
the framework of the mode-coupling theory of the glass transition (MCT) using
static-structure input from molecular-dynamics (MD) computer simulation. The
theory reproduces the experimentally known viscosity minimum (diffusivity
maximum) as a function of density or pressure and explains it in terms of a
corresponding minimum in its critical temperature. This minimum arises from a
gradual change in the equilibrium static structure which shifts from being
dominated by tetrahedral ordering to showing the cageing known from
high-density liquids. The theory is in qualitative agreement with computer
simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in
Journal of Physic
Bose-representation for a strongly coupled nonequilibrim fermionic superfluid in a time-dependent trap
Using the functional integral formulation of a nonequilibrium quantum
many-body theory we develop a regular description of a Fermi system with a
strong attractive interaction in the presence of an external time-dependent
potential. In the strong coupling limit this fermionic system is equivalent to
a noequilibrium dilute Bose gas of diatomic molecules. We also consider a
nonequilibrim strongly coupled Bardeen-Cooper-Schrieffer (BCS) theory and show
that it reduces to the full nonlinear time-dependent Gross-Pitaevski (GP)
equation, which determines an evolution of the condensate wave function.Comment: RevTeX 4, 6 pages, 2 eps figure
Localization dynamics of fluids in random confinement
The dynamics of two-dimensional fluids confined within a random matrix of
obstacles is investigated using both colloidal model experiments and molecular
dynamics simulations. By varying fluid and matrix area fractions in the
experiment, we find delocalized tracer particle dynamics at small matrix area
fractions and localized motion of the tracers at high matrix area fractions. In
the delocalized region, the dynamics is subdiffusive at intermediate times, and
diffusive at long times, while in the localized regime, trapping in finite
pockets of the matrix is observed. These observations are found to agree with
the simulation of an ideal gas confined in a weakly correlated matrix. Our
results show that Lorentz gas systems with soft interactions are exhibiting a
smoothening of the critical dynamics and consequently a rounded
delocalization-to-localization transition.Comment: 5 pages, 3 figure
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
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