74,349 research outputs found
Chemical Oscillations out of Chemical Noise
The dynamics of one species chemical kinetics is studied. Chemical reactions
are modelled by means of continuous time Markov processes whose probability
distribution obeys a suitable master equation. A large deviation theory is
formally introduced, which allows developing a Hamiltonian dynamical system
able to describe the system dynamics. Using this technique we are able to show
that the intrinsic fluctuations, originated in the discrete character of the
reagents, may sustain oscillations and chaotic trajectories which are
impossible when these fluctuations are disregarded. An important point is that
oscillations and chaos appear in systems whose mean-field dynamics has too low
a dimensionality for showing such a behavior. In this sense these phenomena are
purely induced by noise, which does not limit itself to shifting a bifurcation
threshold. On the other hand, they are large deviations of a short transient
nature which typically only appear after long waiting times. We also discuss
the implications of our results in understanding extinction events in
population dynamics models expressed by means of stoichiometric relations
Destructive Impact of Molecular Noise on Nanoscale Electrochemical Oscillators
We study the loss of coherence of electrochemical oscillations on meso- and
nanosized electrodes with numeric simulations of the electrochemical master
equation for a prototypical electrochemical oscillator, the hydrogen peroxide
reduction on Pt electrodes in the presence of halides. On nanoelectrodes, the
electrode potential changes whenever a stochastic electron-transfer event takes
place. Electrochemical reaction rate coefficients depend exponentially on the
electrode potential and become thus fluctuating quantities as well. Therefore,
also the transition rates between system states become time-dependent which
constitutes a fundamental difference to purely chemical nanoscale oscillators.
Three implications are demonstrated: (a) oscillations and steady states shift
in phase space with decreasing system size, thereby also decreasing
considerably the oscillating parameter regions; (b) the minimal number of
molecules necessary to support correlated oscillations is more than 10 times as
large as for nanoscale chemical oscillators; (c) the relation between
correlation time and variance of the period of the oscillations predicted for
chemical oscillators in the weak noise limit is only fulfilled in a very
restricted parameter range for the electrochemical nano-oscillator.Comment: 18 pages, 9 figure
Effects of external global noise on the catalytic CO oxidation on Pt(110)
Oxidation reaction of CO on a single platinum crystal is a reaction-diffusion
system that may exhibit bistable, excitable, and oscillatory behavior. We
studied the effect of a stochastic signal artificially introduced into the
system through the partial pressure of CO. First, the external signal is
employed as a turbulence suppression tool, and second, it modifies the
boundaries in the bistable transition between the CO and oxygen covered phases.
Experiments using photoemission electron microscopy (PEEM) together with
numerical simulations performed with the Krischer-Eiswirth-Ertl (KEE) model are
presented.Comment: 15 pages, 7 figures, accepted in J. Chem. Phy
Nonlinear Unsteady Motions and NOx Production in Gas Turbine Combustors
Chiefly for improved efficiency, the trend to increasing use of gas turbine engines in stationary
powerplants has been firmly established. The requirement for minimum NOx production has motivated
operation as close as practically possible near the lean flammability limit, to reduce flame temperatures and
consequently reduce formation of nitrogen oxides via the Zeldovich thermal mechanism. However,
experience has shown that under these conditions, stability of the chamber is compromised, often leading
to the presence of sustained oscillations in the combustor. That possibility raises the problem of the
influence of oscillatory motions on the production of nitrogen oxides. Numerically calculating these
influences for a complex geometry gas turbine combustor is too computationally expensive at this ?me.
Nonlinear analytical methods making use of these influences are a promising direction for simplei ways to
design and develop operational gas turbine combustors. However, this analysis needs results on which to
base unsteady models of the interaction between nonlinear oscillations and species production within a gas
turbine combustor. In this paper, two methods are explored briefly as an initial step. The first is based on
a configuration of perfectly stirred and plug flow reactors to approximate the flow in a combustion
chamber. A complete representation of the chemical processes is accommodated, but the geometry is
simplified. The second is a full numerical simulation for a realistic geometry, but at this stage the
chemistry is simplified
Bloch-Redfield equations for modeling light-harvesting complexes
We challenge the misconception that Bloch-Redfield equations are a less
powerful tool than phenomenological Lindblad equations for modeling exciton
transport in photosynthetic complexes. This view predominantly originates from
an indiscriminate use of the secular approximation. We provide a detailed
description of how to model both coherent oscillations and several types of
noise, giving explicit examples. All issues with non-positivity are overcome by
a consistent straightforward physical noise model. Herein also lies the
strength of the Bloch-Redfield approach because it facilitates the analysis of
noise-effects by linking them back to physical parameters of the noise
environment. This includes temporal and spatial correlations and the strength
and type of interaction between the noise and the system of interest. Finally
we analyze a prototypical dimer system as well as a 7-site Fenna-Matthews-Olson
(FMO) complex in regards to spatial correlation length of the noise, noise
strength, temperature and their connection to the transfer time and transfer
Resonant spike propagation in coupled neurons with subthreshold activity
Màster en Biofísica, curs 2006-2007Chemical coupling between neurons is only active when the presynaptic neuron is firing, and thus it does not allow for the propagation of subthreshold activity. Electrical coupling via gap junctions, on the other hand, is also ubiquitous and, due to its diffusive nature, transmits both subthreshold and suprathreshold activity between neurons. We study theoretically the propagation of spikes between two neurons that exhibit subthreshold oscillations, and which are coupled via both chemical synapses and gap junctions. Due to the electrical coupling, the periodic subthreshold activity is synchronized in the two neurons, and affects propagation of spikes in such a way that for certain values of the delay in the synaptic coupling, propagation is not possible. This effect could provide a mechanism for the modulation of information transmission in neuronal networks
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