Titles and Abstracts
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Collective molecular reorientation in azobencene Langmuir
monolayers: droplet textures and dynamics of defects
by Enrique Abad
Universitat de Barcelona, Spain
We study the morphology of orientational textures in
Langmuir monolayers of an azobencene derivative displaying trans/cis
isomerization. For proper values of the temperature and the surface
pressure, binary monolayers exhibit phase separation, i.e. trans-rich
droplets develop inside a pool of an isotropic cis medium. Sufficiently
large droplets display complex molecular orientation characterized by a
bend structure in the vicinity of the droplet center and a splay
structure near the droplet boundary. The splay orientation of molecules
near the boundary (pointing either outwards or inwards) turns out to be rather
sensitive to changes in surface pressure. One of the most striking
transition mechanisms between splay-in and splay-out textures involves
the formation of a transient periferic defect (boojum) which moves along
the droplet boundary. We have been able to reproduce this reorientational
phenomenon in the framework of a continuum model based on a free energy
functional including elastic bulk interactions, trans/cis boundary
interactions and the effect of thermal fluctuations.
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Nanohydrodynamics
by Berni J. Alder
Lawrence Livermore National Laboratory, USA
Nanohydrodynamic simulations(molecular dynamics for up to 100 million
particles)exhibit Rayleigh-Taylor instability,initiated by
thermal fluctuations
at the interface,leading to the chaotic regime in the long-time
evolution of
the mixing process.The early time behavior agrees with linear stability
analysis of the Navier-Stokes equations,and the late-time behavior agrees
quantitatively with experimental observations. This algorithm has now been
speeded up by a factor of 100 using the stochastic collision dynamics method
(DSMC) extended to high density (CBA), allowing new insights into the
turbulent mixing process.
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Heat transfer in confined geometry *
by Jean-Louis Barrat,
Université Claude Bernard Lyon I, France
Abstract
I will discuss two problems relating to heat transfer in confined geometry
-the problem of thermal contact resistance (or Kapitsa resistance) at an
interface between a solid and a liquid, which has been determined using
direct NEMD approaches and Green Kubo formulae
-the problem of extrapolating the thermal conductivity determined e.g.
for unidimensional nanometric objects (nanowires) to large sizes,
using a combination of NEMD and classical transport theory approaches
References:
Finite size effects in determination of thermal conductivities
P. Chantrenne and J-L. Barrat, to appear in Journal of Heat Transfer,
http://arxiv.org/abs/cond-mat/0306053
Kapitza resistance at the liquid solid interface
J-L. Barrat and F. Chiaruttini
Molecular Physics, 101, 1605 (2003)
http://arxiv.org/abs/cond-mat/0209607
* Presentation Cancelled.
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Molecular nanosystems at surfaces: Dynamics of self-Assembly
by Johannes V. Barth
Ecole Polytechnique Fédérale de Lausanne, Switzerland
Abstract
Temperature-controlled scanning tunneling microscopy has been
employed to obtain nanoscale insight into the dynamics and
organization of molecular nanosystems at well-defined metal
substrates. We investigated individual functional molecular
species and used ensembles for the assembly of supramolecular
nanosystems directed by H-bonding or metal-ligand interactions.
In particular, carboxylic acids and transition metal centers
have been employed. The careful choice of controllable parameters
(coverage, temperature, substrate chemistry and symmetry, molecular
functionality and stereochemistry, metal center characteristics)
allows to follow the evolution and to steer the organization of
supramolecular architectures. Examples given include single-molecule
level observations of mobility and assembly mechanisms, construction
of low-dimensional hydrogen-bonded architectures, synthesis of
metallosupramolecular compounds and networks.
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Influence of thermal noise and of an electric field
on Liesegang pattern formation
by Iona Bena
University of Geneva, Switzerland
Abstract
Liesegang pattern formation is described in the frame
of a spinodal decomposition scenario.
The influence of (i) thermal noise and
(ii) of an externally applied electric field are studied.
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Kinetics of electrosorption
by Fabienne Berthier
Université Paris-Sud, France
Abstract
Ag / Cu (001) electrodeposition is studied in the framework
of an atomistic kinetic model handled by Monte Carlo
simulations. When the kinetics is controlled by a nucleation
and growth process, the Kolmogorov-Avrami formalism is able
to reproduce it. The number of clusters may remain small
in the simulation box, leading to a dependence of the MC
results with the size of the box. The numerical resolution
of the classical nucleation theory (CNT) approach which
gives a kinetic equation, not for each site, but for the
cluster size distribution N(n, t) defined as the number
of clusters with n atoms at time t, then provides the
expected behavior of an infinite box. We detail first
how it is possible to describe the kinetics issued from
Monte Carlo simulations (which take into account the
atomics interaction) with this nanoscopique approach.
We then relate the physical quantities involved in the
Avrami's equation to the microscopic parameters of the
kinetic model. However, to obtain a quantitative
agreement with Monte Carlo simulations, we show that
it is necessary to take into account the morphology
of the clusters, in particular the concentration of
kinks. This improves the calculation of the edge
free energy and the description of the frequencies
of attachment and detachment of monomers. This leads
to a very good agreement with MC simulations for the
kinetics of electrodeposition and for the physical
quantities that control it, i.e., the steady-state
nucleation rate, the asymptotic growth rate and the
incubation time.
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Mesoscale models for crystalline interfaces:
Phase transitions and interface motion
by W. Craig Carter
Department of Materials Science and Engineering, MIT, USA
Abstract
A diffuse interface model for crystalline
interfaces has been established and will be
described. The model permits both kinetic
analysis of the motion of grain boundaries and the
thermodynamics of grain boundary phase transitions
at the meso-scale. In addition to motion by
curvature, the model predicts that grain rotations
can also contribute to microstuctural evolution.
When the grain boundary undergoes a phase
transition, the rotation kinetics also undergo a
transition. Simulations of grain growth, grain
rotation and grain boundary phase transitions will
be described.
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Propagating waves in 1d lattice reactive system
by Yannick DeDecker
Université Libre de Bruxelles, Belgium
Abstract
We investigate the behavior of discrete systems on a 1-dimensional lattice
composed of localized units interacting with each other through nonlocal,
nonlinear reactive dynamics. In the presence of second order and third order
steps coupling two or three neighboring sites respectively we observe, for
appropriate initial conditions, the propagation of waves which subsist in the
absence of mass transfer by diffusion. For the case of the
third order (bistable) model, a counterintuitive effect is also observed,
whereby the homogeneously less stable state invades the more stable
one under certain conditions.
In the limit of a continuous space and uncorrelated asdorbates
the dynamics of these networks is described by a generic
evolution equation, from which
some analytical predictions can be extracted and
compared with simulations.
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Mesoscopic model for separation of granular material
by Michel Droz
University of Geneva, Switzerland
Abstract
If one puts some granular material, like sand for example, in a box which is
divided into two (or more) equal compartments by a wall which has a narrow
horizontal slit at a certain height and shake the box, one observes that under
certain conditions (e.g., frequency and amplitude of shaking) one compartment of
the box is preferentially filled with sand.
A mesoscopic model describing this spontaneous symmetry breaking will
be introduced. Both dynamical and stationary properties of the model
will be analyzed and its predictions compared with experimental data.
Relations with the so-called zero-range stochastic processes will be
discussed.
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From atomic scale reactant ordering to mesoscale reaction front propagation:
CO-oxidation on Pd(100)
by Jim Evans
Ames Laboratory and Dept of Mathematics, Iowa State University, USA
Abstract
We utilize a heterogeneous coupled lattice-gas (HCLG) approach to
connecting-the-length-scales from a realistic atomistic description
of surface reactions to mesoscale pattern formation [1,2,3].
This method involves parallel simulations of a realistic atomistic
model describing behavior at macroscopic points distributed across
a spatial pattern and appropriately coupled to describe mesoscale
mass transport via surface diffusion. This approach is applied to
provide a realistic description of reaction front structure in a
model for CO oxidation on Pd(100) [4]. Our atomistic model for
the reaction incorporates the observed complex ordering of
locally equilibrated highly-mobile CO adlayers and non-equilibrium
relatively immobile O adlayers. Adspecies interactions are chosen
to match experimental data on ordering, desorption kinetics, and
the heat of adsorption. It is also necessary to incorporate an
accurate description of the adsorption kinetics for oxygen in
order to properly describe the non-equilibrium ordering which
will occur during reaction. Finally, a precise treatment of the
chemical diffusion for interacting CO adlayers in an environment
of coadsorbed O is also needed to appropriately describe mesoscale
mass transport, which determines the structure and propagation of
reaction fronts.
References:
[1] M. Tammaro, M. Sabella, J.W. Evans, J. Chem. Phys. 103 (1995) 10277.
[2] J.W. Evans, D.-J. Liu, M. Tammaro, Chaos 12 (2002) 131.
[3] D.-J. Liu, J.W. Evans, Multiscale Modeling. Sim., submitted (2004)
[4] D.-J. Liu, J.W. Evans, Phys. Rev. B, submitted (2004).
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Nonlinear oscillations and the fluctuation theorem
in nonequilibrium reactions
by Pierre Gaspard
Université Libre de Bruxelles, Belgium
Abstract
Far-from-equilibrium, reacting systems may present oscillatory phenomena.
Such chemical or biochemical clocks exist down to the nanoscale where
molecular fluctuations become important and affect the regularity
of the oscillations. We show that such noisy nonequilibrium systems
are well described by the Onsager-Machlup-Freidlin-Wentzell theory of
stochastic processes. This Hamilton-Jacobi theory allows us to obtain the correlation time
of the noisy oscillations, i.e., the inverse of the
damping rate of the oscillations of the time autocorrelation functions.
Using this result, an estimation is obtained for the minimum number of molecules
required for the oscillations to remain correlated in time.
This estimation puts a fundamental lower limit on the size of chemical clocks.
For typical oscillators, the minimum number of molecules is estimated
between ten and one hundred, which essentially corresponds to nanosystems.
Besides, a fluctuation theorem is derived for stochastic nonequilibrium reactions.
The theorem characterizes the properties of the nonequilibrium fluctuations
around the mean entropy production and the breaking of detailed balance out of thermodynamic
equilibrium. The Onsager and higher-order reciprocity relations,
as well as the Yamamoto-Zwanzig formulas for the reaction constants,
are derived from the fluctuation theorem.
References:
P. Gaspard, "Trace Formula for Noisy Flow", J. Stat. Phys. 106 (2002) 57.
D. Gonze, J. Halloy, and P. Gaspard, "Biochemical clocks and molecular noise:
Theoretical study of robustness factors", J. Chem. Phys. 116 (2002) 10997.
P. Gaspard, "The correlation time of mesoscopic chemical clocks", J. Chem. Phys. 117 (2002) 8905.
P. Gaspard, "Fluctuation theorem for nonequilibrium reactions", J. Chem. Phys. 120 (2004) 8898.
D. Andrieux and P. Gaspard, "Fluctuation theorem and Onsager reciprocity relations", preprint (2004).
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Oscillatory CO oxidation at atmospheric pressure studied with STM
by Bas L. M. Hendriksen
Leiden University, The Netherlands
Abstract
We used a scanning tunneling microscope, which integrated
in a micro-flow reactor, to study the catalytic oxidation of
carbon monoxide on platinum [1] and palladium [2] single-crystal
surfaces at atmospheric pressures and elevated temperatures.
By switching from CO-rich to O2-rich gas flows and vice versa
we reversibly oxidized and reduced the surface, as observed
in STM movies. The formation of the surface oxide had a
dramatic (and surprising) positive effect on the CO2 production,
which was measured online by means of mass spectrometry.
There was significant hysteresis in these reduction-oxidation
cycles resulting in a CO-pressure window where both, the metallic,
low reactive surface and the oxidic surface, with the higher
reactivity, were stable. This bistability also led to
self-sustained oscillations in the CO2 production [2].
Our observations are in full disagreement with existing
models for oscillatory CO oxidation.
References:
[1] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 046101
[2] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci. 552 (2004) 229
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Adsorption and diffusion on surface defects
by Pablo Jensen
UMR 5586 CNRS -
Université Claude Bernard Lyon-1, France
Abstract
Studying the deposition of gold on graphite is interesting from an
experimental point of view, because this is a model system (graphitic
surfaces are planar, gold is not reactive...). We have tried to
understand the interaction of gold atoms and clusters
and the graphite surface, be it ideal or containing defects as steps
or vacancies. This has been carried out by two kinds of interatomic
potentials : semi-empirical (Lennard-Jones) or ab initio. I'll try to
interpret the differences between these two methods...
References:
[1] P Jensen, X Blase (2005) Phys Rev
[2] P Jensen, P Ordejon X Blase (2005) Surf Sci
[3] P Jensen (1999) Rev Mod Phys 71 1695
[4] P. Jensen, L. Lewis and A. Clement (2004) Physica E 21 71
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Growth of molybdenum on a titanium-dioxide substrate
by Peter Krüger
UMR 5613 CNRS-Université de Bourgogne, France
Abstract
Chemi-sorption and clustering of Mo atoms deposited on a Ti02(110)
substrate is studied within density functional theory using a plane
wave pseudo-potential method. It is found that Mo grows in a bcc
structure and the orientational relationship is Mo(001)//Ti(110)
in agreement with experiment. At low coverage ( < 0.25 ML ) the first
nearest neighbor shell of the Mo adatom contains three oxydgen and
one Ti atom. Energies for absorption and cluster formation are
calculated and the energy barriers for these processes are estimated.
The results are used as an input for kinetic Monte Carlo simulations
that model the dynamics and temperature dependance of the various
surface reactions.
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Diffusion on solid surfaces
by Katja Lindenberg
University of California, San Diego, USA
Abstract
We present a numerical and partially analytical study of classical
particles obeying a
Langevin equation that describes diffusion on a
surface modeled by a two dimensional potential. The potential
may be either periodic or random. Depending on the potential and the
damping, we observe superdiffusion, large-step diffusion, diffusion,
and subdiffusion. Superdiffusive behavior is associated with
low damping and is in most cases transient, albeit often long.
Subdiffusive behavior is associated with highly damped particles
in random potentials. In some cases subdiffusive behavior persists
over our entire simulation and may be characterized as metastable.
In any case, we stress that this rich variety of behaviors emerges
naturally from an ordinary Langevin equation for a system described
by ordinary canonical Maxwell-Boltzmann statistics.
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Front propagation and fractality in the Lattice Lotka Volterra
and the Lattice Limit Cycle models
by Astero Provata
National Center for Scientific Research "Demokritos", Greece
Abstract
We study the formation of fronts in low dimensional
interacting systems. As toy models we use the Lattice
Lotka Volterra (LLV) model (which at the Mean Field presents
conservative dynamics and contains only biparticle
interactions) and the Lattice Limit Cycle (LLC) model
(which at the Mean Field level presents dissipative
dynamics and contains one quadriparticle interaction
step). When these models are restricted on low
dimensional lattices fractal clustering and color
front propagation are observed. We study the fractal
dimension of the clusters and the roughening of the
propagating front interfaces. We show that both fractal
dimension and roughening exponents undergo an abrupt
transition in the case of the LLC model when the kinetic
parameters pass near the Hopf bifurcation point, while
no transition is observed in the case of the LLV
model.
References:
A. Shabunin, F. Baras and A. Provata, Phys. Rev. E,
vol. 66, no 036219 (2002).
G. A. Tsekouras and A. Provata, Phys. Rev. E, vol. 65,
no 016204, (2002).
A. Provata and G. A. Tsekouras, Phys. Rev. E, vol. 67,
no 056602, (2003).
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Multi-scale modelling of microstructure formation in metal alloys
by Nicolas Provatas
McMaster University, Canada
Abstract
The selection of microstructure length scales in metal alloys
is critical to their mechanical properties. Therefore understanding
dendritic solidification and grain growth in solid-solid
transformations is essential to be able to optimize metal
alloy performance, an issue of significant industrial importance.
A great deal of research has been done in predictive modeling
of microstructure formation. One of the greatest limitations
in this area has been the computational challenge of describing
a fundamentally multi-scale, non-equilibrium and free-surface
problem. Many approaches in the past have often relied on
semi-empirical formulations that typically rely on inputs
that require some a-priori knowledge of the microstructure
itself. A relatively recent innovation in materials science
that has overcome this problem is the phase-field method.
This approach combines the ability to effortlessly tack
free surface dynamics while incorporating the meso-scale
physics involved in microstructure formation. This can
include bulk and interface thermodynamics, surface tension,
heat and mass transport and dislocation density dynamics.
Combined with novel multi-scale adaptive-mesh,
finite element algorithms, the phase-field method has
gained increased use and popularity as a method of choice
for simulating microstructure evolution at experimentally
relevant length and times scales.
In this talk we report new phase-field simulation results
on microstructure evolution in two alloy systems.
The first involves dendritic finger selection in directional
solidification of rapidly cooled binary alloys of Pivalic
Acid and Succinonitrile, systems referred to as organic
analogues of metals. Our simulations show that dendritic
primary finger spacing is strongly dependent on pulling
velocity, cooling gradients and alloy composition. We find,
however, that when our data is appropriately scaled in terms
of length and time scales of the problem, the dendritic
spacing is described by one universal scaling function that
spans the regime from the onset to solidification cells up
to the emergence of side-branched dendrites. These results
are shown to be in excellent agreement with experiments of
directional solidification in similar systems. Our findings
suggest that dendritic spacing in commercially cast alloys
can similarly be predicted using principles of universal
scaling. The second system we discuss examines the role of
dislocations on spinodal decomposition in Al-Zn alloys. We
show that dislocations can both retard and enhance spinodal
coarsening, depending on their mobility. These findings are
used to resolve several experimental observations that paint
a conflicting picture of spinodal coarsening in alloys. We
construct a crossover scaling function that unambiguously
describes the time dependence of spinodal coarsening in
the presence of dislocations.
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Discrete combustion waves in heterogeneous media:
experimental results and modelling.
by A.S.Rogachev, A.E.Grigoryan, N.A.Kochetov
Russian Academy of Sciences, Russia
Abstract
Many exothermic heterogeneous reactions can propagate throughout
the reaction medium in a wave-like mode, which is usually called
gasless combustion (e.g., reactions M+C, M+B, M+Si, M+Al, where M
is Ti, Zr, Hf, Nb, Ta or other transition metal; Ni+Al, Ti+Ni,
thermit reactions, etc.). These processes are widely used for
production of new materials. Gasless combustion may take place
in heterogeneous mixture of fine reactants (from nanoscale up
to hundreds microns). Early experimental study of the process
demonstrated steady-state macroscopic behavior of gasless
reaction wave, which gave ground to quasi-homogeneous mathematical
models of gasless combustion assumed "thermal homogeneity" of
reactive media, which allows to apply quasi-homogeneous laws
for heat evolution and heat transfer for the combustion of
heterogeneous mixtures. However, development of experimental
methods, especially high-speed microscopic video recording,
has shown that assumption of "thermal homogeneity" can not be
accepted for the most of gasless combustible mixtures. New
experimental results prove that movement of the reaction front
occurs by means of stochastic scintillations (appearance and
disappearance of hot spots) at microscopic level, with living
time of each scintillation about 1 ms or less. The combustion
wave has a discrete structure rather than continuous one. Thus,
new experimental and theoretical approaches are required in
order to understand mechanisms of gasless combustion.
New experimental results presented in this work shed some light
on the following aspects of the problem:
- dynamics of heterogeneous reaction and heat evolution in a single
reaction cell (mesoscale level);
- heat exchange between reaction cells (existing of continuous
solid metal networks in the reaction medium, features of inter-particle
contacts, role of nano-scale gaps in thermal conductivity of the
contact and effective thermal diffusivity of the reaction mixture);
- formation of stochastic thermal heterogeneous patterns in the
combustion wave (microscopic level) and their correlation with
microstructure of the reaction medium;
- influence of the meso- and microscale processes on the macroscopic
behavior of the combustion wave.
Influences of reactants sizes as well as reaction mixture
density are studied in order to determine boundary between
micro-heterogeneous (scintillating) and quasi-homogeneous
combustion regimes. Mathematical model are developed on the
base of obtained experimental results and on the assumption
that reaction medium consists of elemental reaction cells,
and heat dissipation within each cell occurs much faster than
heat exchange between neighborhoods cells. Dependences of
combustion velocity on the initial temperature, rate of heat
release, kinetics of chemical reaction are determined by
computer simulation ("virtual experiments"), they are
compared with experimental data and quasi-homogeneous theories.
The work is supported by the Russian Foundation of Basic Research
(grant 02-03-33186 and 04-03-32654).
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Dissipative particle dynamics simulations of polymer melts
at different coarse-grainning levels
by X. Guerrault, B. Rousseau and J. Farago
Université Paris-Sud, France
Abstract
In the past decade, a lot of effort has been invested
in polymer simulations, mostly due to the fact that they
are at one and the same time omnipresent objects in the
industry and impossible to handle from all points of view
with a single method. Simulation of polymers at an atomic
scale is complex because of the large length scale and the
wide range of time scales at stake. Therefore, several
descriptions called "coarse-grained" models have been
developed, all aiming at reducing both the number of particles
taken into account and the computational time, while
attempting to preserve at least the qualitative agreement
with the microscopic reality. We present here the derivation
of coarse-grained forcefields for two types of polymers,
polyethylene (PE) and cis-polybutadiene (cis-PB), using the
concept of potential of mean force. Forcefields were obtained
for several coarse-graining levels, i.e. different numbers
of monomers per mesoscopic unit called ``bead'', and then
used in Dissipative Particle Dynamics (DPD) simulations.
Structural and dynamical properties for polymer melts
(end-to-end distance, self-diffusion coefficients,
relaxation time of the end-to-end vector) from DPD
simulations are presented and discussed as a function
of the coarse-graining level. DPD simulation data are
compared with microscopic and experimental data, when
available.
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A discrete model for wound healing *
by Leonard M. Sander
University of Michigan, USA
Abstract
When a wound heals, surrounding cells fill the wounded area by enhanced
motility (i.e. diffusion) and enhanced proliferation. 'Contact
inhibition' (hard-core exclusion) plays an important role. Previous
theoretical treatments of this process employ a reaction-diffusion
diffusion equation for the cell density equivalent to the
Fisher-Kolmogorov (FK) equation. However, much recent work has shown that
the FK equation does not represent systems where discreteness plays a
large role. We have formulated a very simple discrete model for wound
healing which takes proliferation, diffusion, and contact inhibition into
account. In some limits it reduces to the FK equation, but not in the
biologically relevant regime. We have numerical results for front
propogation, front width, and kinetic roughening of the front in this
model.
* Presentation Cancelled.
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Formation and evolution of spatial clusters in active media of
chemical nature
by Alexey Shabunin
Saratov State University, Russia
Abstract
We considered two approaches to the modeling of lattice compatible
chemical reactions, namely the Mean-Field (MF)
equations and the Kinetic Monte-Carlo (KMC)
simulations. The proposed model is a four-species
cyclic reactive scheme taking place on the two-dimensional
catalytic support.
The MF approach to this reactive scheme demonstrates
chaotic or quasi-periodic conservative motion the character of which
depends on the initial concentrations and the parameters. The
characteristics of the KMC oscillations depend only on the parameters
of the system. They do not depend on the initial values and in this
sense they are closer to the behavior of dissipative systems. The
character of oscillations remains chaotic at all parameters values. In
the limit of large lattices the oscillations vanish, while the systems
always oscillate locally.
When this model is realised by KMC simulations on a 2-d square lattice
clusters of homologous particles are formed with fractal boundaries.
The different clusters interact with each other through
their boundaries
and these interaction leads to the observed local
oscillations. The form of the cluster size distribution differ
for lattices of small and large sizes. Lattices of small sizes are
characterized by pure power law size
distribution, while for the large lattices the distribution
follows a combined law (the product of power law and exponential decay).
This combined law evidences the co-existence of two independent
processes: cluster formation at the small length scales
and mixing and interaction between clusters at the large length scales.