694 research outputs found
Entropic transport - A test bed for the Fick-Jacobs approximation
Biased diffusive transport of Brownian particles through irregularly shaped,
narrow confining quasi-one-dimensional structures is investigated. The
complexity of the higher dimensional diffusive dynamics is reduced by means of
the so-called Fick-Jacobs approximation, yielding an effective one-dimensional
stochastic dynamics. Accordingly, the elimination of transverse, equilibrated
degrees of freedom stemming from geometrical confinements and/or bottlenecks
cause entropic potential barriers which the particles have to overcome when
moving forward noisily. The applicability and the validity of the reduced
kinetic description is tested by comparing the approximation with Brownian
dynamics simulations in full configuration space. This non-equilibrium
transport in such quasi-one-dimensional irregular structures implies for
moderate-to-strong bias a characteristic violation of the Sutherland-Einstein
fluctuation-dissipation relation.Comment: 15 pages, 6 figures ; Phil. Trans. R. Soc. A (2009), in pres
Diffusion of multiple species with excluded-volume effects
Stochastic models of diffusion with excluded-volume effects are used to model
many biological and physical systems at a discrete level. The average
properties of the population may be described by a continuum model based on
partial differential equations. In this paper we consider multiple interacting
subpopulations/species and study how the inter-species competition emerges at
the population level. Each individual is described as a finite-size hard core
interacting particle undergoing Brownian motion. The link between the discrete
stochastic equations of motion and the continuum model is considered
systematically using the method of matched asymptotic expansions. The system
for two species leads to a nonlinear cross-diffusion system for each
subpopulation, which captures the enhancement of the effective diffusion rate
due to excluded-volume interactions between particles of the same species, and
the diminishment due to particles of the other species. This model can explain
two alternative notions of the diffusion coefficient that are often confounded,
namely collective diffusion and self-diffusion. Simulations of the discrete
system show good agreement with the analytic results
Biased random walks on complex networks: the role of local navigation rules
We study the biased random walk process in random uncorrelated networks with
arbitrary degree distributions. In our model, the bias is defined by the
preferential transition probability, which, in recent years, has been commonly
used to study efficiency of different routing protocols in communication
networks. We derive exact expressions for the stationary occupation
probability, and for the mean transit time between two nodes. The effect of the
cyclic search on transit times is also explored. Results presented in this
paper give the basis for theoretical treatment of the transport-related
problems on complex networks, including quantitative estimation of the critical
value of the packet generation rate.Comment: 5 pages (Phys. Rev style), 3 Figure
Deterministic Brownian motion generated from differential delay equations
This paper addresses the question of how Brownian-like motion can arise from
the solution of a deterministic differential delay equation. To study this we
analytically study the bifurcation properties of an apparently simple
differential delay equation and then numerically investigate the probabilistic
properties of chaotic solutions of the same equation. Our results show that
solutions of the deterministic equation with randomly selected initial
conditions display a Gaussian-like density for long time, but the densities are
supported on an interval of finite measure. Using these chaotic solutions as
velocities, we are able to produce Brownian-like motions, which show
statistical properties akin to those of a classical Brownian motion over both
short and long time scales. Several conjectures are formulated for the
probabilistic properties of the solution of the differential delay equation.
Numerical studies suggest that these conjectures could be "universal" for
similar types of "chaotic" dynamics, but we have been unable to prove this.Comment: 15 pages, 13 figure
Equilibrium properties of a Josephson junction ladder with screening effects
In this paper we calculate the ground state phase diagram of a Josephson
Junction ladder when screening field effects are taken into account. We study
the ground state configuration as a function of the external field, the
penetration depth and the anisotropy of the ladder, using different
approximations to the calculation of the induced fields. A series of tongues,
characterized by the vortex density , is obtained. The vortex density
of the ground state, as a function of the external field, is a Devil's
staircase, with a plateau for every rational value of . The width of
each of these steps depends strongly on the approximation made when calculating
the inductance effect: if the self-inductance matrix is considered, the
phase tends to occupy all the diagram as the penetration depth
decreases. If, instead, the whole inductance matrix is considered, the width of
any step tends to a non-zero value in the limit of very low penetration depth.
We have also analyzed the stability of some simple metastable phases: screening
fields are shown to enlarge their stability range.Comment: 16 pp, RevTex. Figures available upon request at
[email protected] To be published in Physical Review B (01-Dec-96
Generalized Fokker-Planck equation, Brownian motion, and ergodicity
Microscopic theory of Brownian motion of a particle of mass in a bath of
molecules of mass is considered beyond lowest order in the mass ratio
. The corresponding Langevin equation contains nonlinear corrections to
the dissipative force, and the generalized Fokker-Planck equation involves
derivatives of order higher than two. These equations are derived from first
principles with coefficients expressed in terms of correlation functions of
microscopic force on the particle. The coefficients are evaluated explicitly
for a generalized Rayleigh model with a finite time of molecule-particle
collisions. In the limit of a low-density bath, we recover the results obtained
previously for a model with instantaneous binary collisions. In general case,
the equations contain additional corrections, quadratic in bath density,
originating from a finite collision time. These corrections survive to order
and are found to make the stationary distribution non-Maxwellian.
Some relevant numerical simulations are also presented
Discrete breathers in nonlinear lattices: Experimental detection in a Josephson array
We present an experimental study of discrete breathers in an underdamped
Josephson-junction array. Breathers exist under a range of dc current biases
and temperatures, and are detected by measuring dc voltages. We find the
maximum allowable bias current for the breather is proportional to the array
depinning current while the minimum current seems to be related to a junction
retrapping mechanism. We have observed that this latter instability leads to
the formation of multi-site breather states in the array. We have also studied
the domain of existence of the breather at different values of the array
parameters by varying the temperature.Comment: 5 pages, 5 figures, submitted to Physical Revie
Brownian motion of a charged particle driven internally by correlated noise
We give an exact solution to the generalized Langevin equation of motion of a
charged Brownian particle in a uniform magnetic field that is driven internally
by an exponentially-correlated stochastic force. A strong dissipation regime is
described in which the ensemble-averaged fluctuations of the velocity exhibit
transient oscillations that arise from memory effects. Also, we calculate
generalized diffusion coefficients describing the transport of these particles
and briefly discuss how they are affected by the magnetic field strength and
correlation time. Our asymptotic results are extended to the general case of
internal driving by correlated Gaussian stochastic forces with finite
autocorrelation times.Comment: 10 pages, 4 figures with subfigures, RevTeX, v2: revise
Characterising epithelial tissues using persistent entropy
In this paper, we apply persistent entropy, a novel topological statistic,
for characterization of images of epithelial tissues. We have found out that
persistent entropy is able to summarize topological and geometric information
encoded by \alpha-complexes and persistent homology. After using some
statistical tests, we can guarantee the existence of significant differences in
the studied tissues.Comment: 12 pages, 7 figures, 4 table
Studying Flow Close to an Interface by Total Internal Reflection Fluorescence Cross Correlation Spectroscopy: Quantitative Data Analysis
Total Internal Reflection Fluorescence Cross Correlation Spectroscopy
(TIR-FCCS) has recently (S. Yordanov et al., Optics Express 17, 21149 (2009))
been established as an experimental method to probe hydrodynamic flows near
surfaces, on length scales of tens of nanometers. Its main advantage is that
fluorescence only occurs for tracer particles close to the surface, thus
resulting in high sensitivity. However, the measured correlation functions only
provide rather indirect information about the flow parameters of interest, such
as the shear rate and the slip length. In the present paper, we show how to
combine detailed and fairly realistic theoretical modeling of the phenomena by
Brownian Dynamics simulations with accurate measurements of the correlation
functions, in order to establish a quantitative method to retrieve the flow
properties from the experiments. Firstly, Brownian Dynamics is used to sample
highly accurate correlation functions for a fixed set of model parameters.
Secondly, these parameters are varied systematically by means of an
importance-sampling Monte Carlo procedure in order to fit the experiments. This
provides the optimum parameter values together with their statistical error
bars. The approach is well suited for massively parallel computers, which
allows us to do the data analysis within moderate computing times. The method
is applied to flow near a hydrophilic surface, where the slip length is
observed to be smaller than 10nm, and, within the limitations of the
experiments and the model, indistinguishable from zero.Comment: 18 pages, 12 figure
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