1,188 research outputs found
Limited Range Fractality of Randomly Adsorbed Rods
Multiple resolution analysis of two dimensional structures composed of
randomly adsorbed penetrable rods, for densities below the percolation
threshold, has been carried out using box-counting functions. It is found that
at relevant resolutions, for box-sizes, , between cutoffs given by the
average rod length and the average inter-rod distance $r_1$, these
systems exhibit apparent fractal behavior. It is shown that unlike the case of
randomly distributed isotropic objects, the upper cutoff $r_1$ is not only a
function of the coverage but also depends on the excluded volume, averaged over
the orientational distribution. Moreover, the apparent fractal dimension also
depends on the orientational distributions of the rods and decreases as it
becomes more anisotropic. For box sizes smaller than the box counting
function is determined by the internal structure of the rods, whether simple or
itself fractal. Two examples are considered - one of regular rods of one
dimensional structure and rods which are trimmed into a Cantor set structure
which are fractals themselves. The models examined are relevant to adsorption
of linear molecules and fibers, liquid crystals, stress induced fractures and
edge imperfections in metal catalysts. We thus obtain a distinction between two
ranges of length scales: where the internal structure of the
adsorbed objects is probed, and where their distribution is
probed, both of which may exhibit fractal behavior. This distinction is
relevant to the large class of systems which exhibit aggregation of a finite
density of fractal-like clusters, which includes surface growth in molecular
beam epitaxy and diffusion-limited-cluster-cluster-aggregation models.Comment: 10 pages, 7 figures. More info available at
http://www.fh.huji.ac.il/~dani/ or
http://www.fiz.huji.ac.il/staff/acc/faculty/biham or
http://chem.ch.huji.ac.il/employee/avnir/iavnir.htm . Accepted for
publication in J. Chem. Phy
Decay Process for Three - Species Reaction - Diffusion System
We propose the deterministic rate equation of three-species in the reaction -
diffusion system. For this case, our purpose is to carry out the decay process
in our three-species reaction-diffusion model of the form . The
particle density and the global reaction rate are also shown analytically and
numerically on a two-dimensional square lattice with the periodic boundary
conditions. Especially, the crossover of the global reaction rate is discussed
in both early-time and long-time regimes.Comment: 6 pages, 3 figures, Late
On Universality in Human Correspondence Activity
Identifying and modeling patterns of human activity has important
ramifications in applications ranging from predicting disease spread to
optimizing resource allocation. Because of its relevance and availability,
written correspondence provides a powerful proxy for studying human activity.
One school of thought is that human correspondence is driven by responses to
received correspondence, a view that requires distinct response mechanism to
explain e-mail and letter correspondence observations. Here, we demonstrate
that, like e-mail correspondence, the letter correspondence patterns of 16
writers, performers, politicians, and scientists are well-described by the
circadian cycle, task repetition and changing communication needs. We confirm
the universality of these mechanisms by properly rescaling letter and e-mail
correspondence statistics to reveal their underlying similarity.Comment: 17 pages, 3 figures, 1 tabl
Exactly Solvable Model of Monomer-Monomer Reactions on a Two-Dimensional Random Catalytic Substrate
We present an \textit{exactly solvable} model of a monomer-monomer reaction on a 2D inhomogeneous, catalytic substrate and study the
equilibrium properties of the two-species adsorbate. The substrate contains
randomly placed catalytic bonds of mean density which connect neighboring
adsorption sites. The interacting and (monomer) species undergo
continuous exchanges with corresponding adjacent gaseous reservoirs. A reaction
takes place instantaneously if and particles
occupy adsorption sites connected by a catalytic bond. We find that for the
case of \textit{annealed} disorder in the placement of the catalytic bonds the
reaction model under study can be mapped onto the general spin (GS1)
model. Here we concentrate on a particular case in which the model reduces to
an exactly solvable Blume-Emery-Griffiths (BEG) model (T. Horiguchi, Phys.
Lett. A {\bf 113}, 425 (1986); F.Y. Wu, Phys. Lett. A, {\bf 116}, 245 (1986))
and derive an exact expression for the disorder-averaged equilibrium pressure
of the two-species adsorbate. We show that at equal partial vapor pressures of
the and species this system exhibits a second-order phase transition
which reflects a spontaneous symmetry breaking with large fluctuations and
progressive coverage of the entire substrate by either one of the species.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Exactly solvable model of A + A \to 0 reactions on a heterogeneous catalytic chain
We present an exact solution describing equilibrium properties of the
catalytically-activated A + A \to 0 reaction taking place on a one-dimensional
lattice, where some of the sites possess special "catalytic" properties. The A
particles undergo continuous exchanges with the vapor phase; two neighboring
adsorbed As react when at least one of them resides on a catalytic site (CS).
We consider three situations for the CS distribution: regular, annealed random
and quenched random. For all three CS distribution types, we derive exact
results for the disorder-averaged pressure and present exact asymptotic
expressions for the particles' mean density. The model studied here furnishes
another example of a 1D Ising-type system with random multi-site interactions
which admits an exact solution.Comment: 7 pages, 3 Figures, appearing in Europhysics Letter
Possible Experimental Test of Continuous Medium Model for Fractal Media
We use the fractional integrals to describe fractal media. We consider the
fractal media as special ("fractional") continuous media. We discuss the
possible experimental testing of the continuous medium model for fractal media
that is suggested in Phys. Lett. A. 336 (2005) 167-174. This test is connected
with measure of period of the Maxwell pendulum with fractal medium cylinder.Comment: 9 page
Binary Reactive Adsorbate on a Random Catalytic Substrate
We study the equilibrium properties of a model for a binary mixture of
catalytically-reactive monomers adsorbed on a two-dimensional substrate
decorated by randomly placed catalytic bonds. The interacting and
monomer species undergo continuous exchanges with particle reservoirs and react
() as soon as a pair of unlike particles appears on sites
connected by a catalytic bond.
For the case of annealed disorder in the placement of the catalytic bonds
this model can be mapped onto a classical spin model with spin values , with effective couplings dependent on the temperature and on the mean
density of catalytic bonds. This allows us to exploit the mean-field theory
developed for the latter to determine the phase diagram as a function of in
the (symmetric) case in which the chemical potentials of the particle
reservoirs, as well as the and interactions are equal.Comment: 12 pages, 4 figure
- …