674 research outputs found
Transition from a simple yield stress fluid to a thixotropic material
From MRI rheometry we show that a pure emulsion can be turned from a simple
yield stress fluid to a thixotropic material by adding a small fraction of
colloidal particles. The two fluids have the same behavior in the liquid regime
but the loaded emulsion exhibits a critical shear rate below which no steady
flows can be observed. For a stress below the yield stress, the pure emulsion
abruptly stops flowing, whereas the viscosity of the loaded emulsion
continuously increases in time, which leads to an apparent flow stoppage. This
phenomenon can be very well represented by a model assuming a progressive
increase of the number of droplet links via colloidal particles.Comment: Published in Physical Review E.
http://pre.aps.org/abstract/PRE/v76/i5/e05140
Bottlenecks to vibrational energy flow in OCS: Structures and mechanisms
Finding the causes for the nonstatistical vibrational energy relaxation in
the planar carbonyl sulfide (OCS) molecule is a longstanding problem in
chemical physics: Not only is the relaxation incomplete long past the predicted
statistical relaxation time, but it also consists of a sequence of abrupt
transitions between long-lived regions of localized energy modes. We report on
the phase space bottlenecks responsible for this slow and uneven vibrational
energy flow in this Hamiltonian system with three degrees of freedom. They
belong to a particular class of two-dimensional invariant tori which are
organized around elliptic periodic orbits. We relate the trapping and
transition mechanisms with the linear stability of these structures.Comment: 13 pages, 13 figure
Self-learning Kinetic Monte-Carlo method: application to Cu(111)
We present a novel way of performing kinetic Monte Carlo simulations which
does not require an {\it a priori} list of diffusion processes and their
associated energetics and reaction rates.
Rather, at any time during the simulation, energetics for all possible
(single or multi-atom) processes, within a specific interaction range, are
either computed accurately using a saddle point search procedure, or retrieved
from a database in which previously encountered processes are stored. This
self-learning procedure enhances the speed of the simulations along with a
substantial gain in reliability because of the inclusion of many-particle
processes.
Accompanying results from the application of the method to the case of
two-dimensional Cu adatom-cluster diffusion and coalescence on Cu(111) with
detailed statistics of involved atomistic processes and contributing diffusion
coefficients attest to the suitability of the method for the purpose.Comment: 18 pages, 9 figure
Thermonuclear burn-up in deuterated methane
The thermonuclear burn-up of highly compressed deuterated methane CD is
considered in the spherical geometry. The minimal required values of the
burn-up parameter are determined for various
temperatures and densities . It is shown that thermonuclear burn-up
in becomes possible in practice if its initial density exceeds
. Burn-up in CDT methane
requires significantly ( 100 times) lower compressions. The developed
approach can be used in order to compute the critical burn-up parameters in an
arbitrary deuterium containing fuel
Deformation and flow of a two-dimensional foam under continuous shear
We investigate the flow properties of a two-dimensional aqueous foam
submitted to a quasistatic shear in a Couette geometry. A strong localization
of the flow (shear banding) at the edge of the moving wall is evidenced,
characterized by an exponential decay of the average tangential velocity.
Moreover, the analysis of the rapid velocity fluctuations reveals self-similar
dynamical structures consisting of clusters of bubbles rolling as rigid bodies.
To relate the instantaneous (elastic) and time-averaged (plastic) components of
the strain, we develop a stochastic model where irreversible rearrangements are
activated by local stress fluctuations originating from the rubbing of the
wall. This model gives a complete description of our observations and is also
consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure
Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)
The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated
using thermodynamic integration within the transition state theory. The results
are found to be in excellent agreement with the essentially exact values from
molecular-dynamics simulations. The activation energy and related entropy are
shown to be effectively independent of temperature, thus establishing the
validity of the Arrhenius law over a wide range of temperatures. Our study
demonstrates the equivalence of diffusion rates calculated using thermodynamic
integration within the transition state theory and direct molecular-dynamics
simulations.Comment: 4 pages (revtex), two figures (postscript
The Escape Problem for Irreversible Systems
The problem of noise-induced escape from a metastable state arises in
physics, chemistry, biology, systems engineering, and other areas. The problem
is well understood when the underlying dynamics of the system obey detailed
balance. When this assumption fails many of the results of classical
transition-rate theory no longer apply, and no general method exists for
computing the weak-noise asymptotics of fundamental quantities such as the mean
escape time. In this paper we present a general technique for analysing the
weak-noise limit of a wide range of stochastically perturbed continuous-time
nonlinear dynamical systems. We simplify the original problem, which involves
solving a partial differential equation, into one in which only ordinary
differential equations need be solved. This allows us to resolve some old
issues for the case when detailed balance holds. When it does not hold, we show
how the formula for the mean escape time asymptotics depends on the dynamics of
the system along the most probable escape path. We also present new results on
short-time behavior and discuss the possibility of focusing along the escape
path.Comment: 24 pages, APS revtex macros (version 2.1) now available from PBB via
`get oldrevtex.sty
A Scaling Theory of Bifurcations in the Symmetric Weak-Noise Escape Problem
We consider the overdamped limit of two-dimensional double well systems
perturbed by weak noise. In the weak noise limit the most probable
fluctuational path leading from either point attractor to the separatrix (the
most probable escape path, or MPEP) must terminate on the saddle between the
two wells. However, as the parameters of a symmetric double well system are
varied, a unique MPEP may bifurcate into two equally likely MPEP's. At the
bifurcation point in parameter space, the activation kinetics of the system
become non-Arrhenius. In this paper we quantify the non-Arrhenius behavior of a
system at the bifurcation point, by using the Maslov-WKB method to construct an
approximation to the quasistationary probability distribution of the system
that is valid in a boundary layer near the separatrix. The approximation is a
formal asymptotic solution of the Smoluchowski equation. Our analysis relies on
the development of a new scaling theory, which yields `critical exponents'
describing weak-noise behavior near the saddle, at the bifurcation point.Comment: LaTeX, 60 pages, 24 Postscript figures. Uses epsf macros to include
the figures. A file in `uufiles' format containing the figures is separately
available at ftp://platinum.math.arizona.edu/pub/papers-rsm/paperF/figures.uu
and a Postscript version of the whole paper (figures included) is available
at ftp://platinum.math.arizona.edu/pub/papers-rsm/paperF/paperF.p
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