26 research outputs found
A Mesoscopic Approach to the ``Negative'' Viscosity Effect in Ferrofluids
We present a mesoscopic approach to analyze the dynamics of a single magnetic
dipole under the influence of an oscillating magnetic field, based on the
formulation of a Fokker-Planck equation. The dissipated power and the viscosity
of a suspension of such magnetic dipoles are calculated from non-equilibrium
thermodynamics of magnetized systems. By means of this method we have found a
non-monotonous behaviour of the viscosity as a function of the frequency of the
field which has been referred to as the ``negative'' viscosity effect.
Moreover, we have shown that the viscosity depends on the vorticity field thus
exhibiting non-Newtonian behaviour. Our analysis is complemented with numerical
simulations which reproduce the behaviour of the viscosity we have found and
extend the scope of our analytical approach to higher values of the magnetic
field.Comment: 9 pages, 2 eps figures, simulations have been adde
Quantum stochastic resonance in electron shelving
Published versio
On the origin of plankton patchiness
Plankton is the productive base of aquatic ecosystems and plays a major role
in the global control of atmospheric carbon dioxide. Nevertheless, after
intensive study, the factors that drive its spatial distribution are still far
from being clear. The models proposed so far show very limited agreement with
actual data as many of their results are not consistent with field
observations. Here we show that fluctuations and turbulent diffusion in
standard prey-predator models are able to accurately and consistently explain
plankton field observations at mesoscales (1-100 km). This includes not only
the spatial pattern but also its temporal evolution. We explicitly elucidate
the interplay between physical and biological factors, suggesting that the form
in which small scale biotic fluctuations are transferred to larger scales may
constitute one of the key elements in determining the spatial distribution of
plankton in the sea.Comment: 16 pages, 3 figure
Coherence Resonance and Noise-Induced Synchronization in Globally Coupled Hodgkin-Huxley Neurons
The coherence resonance (CR) of globally coupled Hodgkin-Huxley neurons is
studied. When the neurons are set in the subthreshold regime near the firing
threshold, the additive noise induces limit cycles. The coherence of the system
is optimized by the noise. A bell-shaped curve is found for the peak height of
power spectra of the spike train, being significantly different from a
monotonic behavior for the single neuron. The coupling of the network can
enhance CR in two different ways. In particular, when the coupling is strong
enough, the synchronization of the system is induced and optimized by the
noise. This synchronization leads to a high and wide plateau in the local
measure of coherence curve. The local-noise-induced limit cycle can evolve to a
refined spatiotemporal order through the dynamical optimization among the
autonomous oscillation of an individual neuron, the coupling of the network,
and the local noise.Comment: five pages, five figure
Gene Regulation in the Pi Calculus: Simulating Cooperativity at the Lambda Switch
Part of the Lecture Notes in Computer Science book series (LNCS, volume 4230).Also part of the Lecture Notes in Bioinformatics book sub series (volume 4230).International audienceWe propose to model the dynamics of gene regulatory networks as concurrent processes in the stochastic pi calculus. As a first case study, we show how to express the control of transcription initiation at the lambda switch, a prototypical example where cooperative enhancement is crucial. This requires concurrent programming techniques that are new to systems biology, and necessitates stochastic parameters that we derive from the literature. We test all components of our model by exhaustive stochastic simulations. A comparison with previous results reported in the literature, experimental and simulation based, confirms the appropriateness of our modeling approach
Do Femtonewton Forces Affect Genetic Function? A Review
Protein-Mediated DNA looping is intricately related to gene expression. Therefore any mechanical constraint that disrupts loop formation can play a significant role in gene regulation. Polymer physics models predict that less than a piconewton of force may be sufficient to prevent the formation of DNA loops. Thus, it appears that tension can act as a molecular switch that controls the much larger forces associated with the processive motion of RNA polymerase. Since RNAP can exert forces over 20 pN before it stalls, a âsubstrate tension switchâ could offer a force advantage of two orders of magnitude. Evidence for such a mechanism is seen in recent in vitro micromanipulation experiments. In this article we provide new perspective on existing theory and experimental data on DNA looping in vitro and in vivo . We elaborate on the connection between tension and a variety of other intracellular mechanical constraints including sequence specific curvature and supercoiling. In the process, we emphasize that the richness and versatility of DNA mechanics opens up a whole new paradigm of gene regulation to explore.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41816/1/10867_2005_Article_9002.pd
Charged-particle nuclear modification factors in PbPb and pPb collisions at â=sNN=5.02 TeV
The spectra of charged particles produced within the pseudorapidity window
|η| < 1 at â
sNN = 5.02 TeV are measured using 404 ”b
â1 of PbPb and 27.4 pbâ1 of pp data
collected by the CMS detector at the LHC in 2015. The spectra are presented over the
transverse momentum ranges spanning 0.5 < pT < 400 GeV in pp and 0.7 < pT < 400 GeV
in PbPb collisions. The corresponding nuclear modification factor, RAA, is measured in
bins of collision centrality. The RAA in the 5% most central collisions shows a maximal
suppression by a factor of 7â8 in the pT region of 6â9 GeV. This dip is followed by an increase,
which continues up to the highest pT measured, and approaches unity in the vicinity
of pT = 200 GeV. The RAA is compared to theoretical predictions and earlier experimental
results at lower collision energies. The newly measured pp spectrum is combined with the
pPb spectrum previously published by the CMS collaboration to construct the pPb nuclear
modification factor, RpA, up to 120 GeV. For pT > 20 GeV, RpA exhibits weak momentum
dependence and shows a moderate enhancement above unity