94 research outputs found
Work fluctuations for a Brownian particle between two thermostats
We explicitly determine the large deviation function of the energy flow of a
Brownian particle coupled to two heat baths at different temperatures. This toy
model, initially introduced by Derrida and Brunet [B. Derrida and E. Brunet, in
"Einstein aujourd'hui", EDP Sciences, 2005], allows not only to sort out the
influence of initial conditions on large deviation functions but also to
pinpoint various restrictions bearing upon the range of validity of the
Fluctuation Relation.Comment: 9 pages, 5 figure
Energetics of active fluctuations in living cells
The nonequilibrium activity taking place in a living cell can be monitored
with a tracer embedded in the medium. While microrheology experiments based on
optical manipulation of such probes have become increasingly standard, we put
forward a number of experiments with alternative protocols that, we claim, will
provide new insight into the energetics of active fluctuations. These are based
on either performing thermodynamic--like cycles in control-parameter space, or
on determining response to external perturbations of the confining trap beyond
simple translation. We illustrate our proposals on an active itinerant Brownian
oscillator modeling the dynamics of a probe embedded in a living medium
Fluctuations of internal energy flow in a vibrated granular gas
The non-equilibrium fluctuations of power flux in a fluidized granular media
have been recently measured in an experiment [Phys. Rev. Lett. 92, 164301,
2004], which was announced to be a verification of the Fluctuation Relation
(FR) by Gallavotti and Cohen. An effective temperature was also identified and
proposed to be a useful probe for such non equilibrium systems. We explain
these results in terms of a two temperature Poisson process. Within this model,
supported by independent Molecular Dynamics simulations, power flux
fluctuations do not satisfy the FR and the nature of the effective temperature
is clarified. In the pursue of a hypothetical global quantity fulfilling the
FR, this points to the need of considering other candidates than the power
flux.Comment: accepted for publication on Physical Review Lette
Dynamics of a tracer granular particle as a non-equilibrium Markov process
The dynamics of a tracer particle in a stationary driven granular gas is
investigated. We show how to transform the linear Boltzmann equation describing
the dynamics of the tracer into a master equation for a continuous Markov
process. The transition rates depend upon the stationary velocity distribution
of the gas. When the gas has a Gaussian velocity probability distribution
function (pdf), the stationary velocity pdf of the tracer is Gaussian with a
lower temperature and satisfies detailed balance for any value of the
restitution coefficient . As soon as the velocity pdf of the gas
departs from the Gaussian form, detailed balance is violated. This
non-equilibrium state can be characterized in terms of a Lebowitz-Spohn action
functional defined over trajectories of time duration . We
discuss the properties of this functional and of a similar functional
which differs from the first for a term which is non-extensive
in time. On the one hand we show that in numerical experiments, i.e. at finite
times , the two functionals have different fluctuations and
always satisfies an Evans-Searles-like symmetry. On the other hand we cannot
observe the verification of the Lebowitz-Spohn-Gallavotti-Cohen (LS-GC)
relation, which is expected for at very large times . We give
an argument for the possible failure of the LS-GC relation in this situation.
We also suggest practical recipes for measuring and
in experiments.Comment: 16 pages, 3 figures, submitted for publicatio
Exact solution of a model DNA-inversion genetic switch with orientational control
DNA inversion is an important mechanism by which bacteria and bacteriophage
switch reversibly between phenotypic states. In such switches, the orientation
of a short DNA element is flipped by a site-specific recombinase enzyme. We
propose a simple model for a DNA inversion switch in which recombinase
production is dependent on the switch state (orientational control). Our model
is inspired by the fim switch in Escherichia coli. We present an exact
analytical solution of the chemical master equation for the model switch, as
well as stochastic simulations. Orientational control causes the switch to
deviate from Poissonian behaviour: the distribution of times in the on state
shows a peak and successive flip times are correlated.Comment: Revised version, accepted for publicatio
It is easy to see, but it is better to foresee: a case report on the favourable alliance between CardioMEMS and levosimendan
Abstract
Background
In the past years, different devices have been investigated to help in identifying early decompensation events in patients with heart failure (HF) and reduced ejection fraction (EF), reducing hospital admissions. In this report, we present the first patient experience with levosimendan infusion led by CardioMEMS.
Case summary
A 68-year-old man with HF and reduced EF with more than 20 hospitalizations for exacerbation of HF was enrolled in our HF Clinic from October 2017. Echocardiogram showed a dilated left ventricle with severely reduced EF (29%) and increased pulmonary artery systolic pressure (40 mmHg). From October 2017 to May 2019, the patient went through numerous hospitalizations, despite optimal medical therapy; subsequently, was adopted a strategy of levosimendan infusions guided by CardioMEMS. Levosimendan infusions improved haemodynamic and pressure profiles. The patient was monitored daily by CardioMEMS, and from June to December 2019, he had only two hospitalizations scheduled for levosimendan infusion and none for HF exacerbation.
Discussion
Our case supports the combination of CardioMEMS and levosimendan for the optimal management of patients with advanced HF. These results further strengthen the development of a randomized clinical trial to demonstrate the clinical usefulness of this device in combination with the levosimendan infusion programme in advanced HF patients
Switching and growth for microbial populations in catastrophic responsive environments
Phase variation, or stochastic switching between alternative states of gene
expression, is common among microbes, and may be important in coping with
changing environments. We use a theoretical model to assess whether such
switching is a good strategy for growth in environments with occasional
catastrophic events. We find that switching can be advantageous, but only when
the environment is responsive to the microbial population. In our model,
microbes switch randomly between two phenotypic states, with different growth
rates. The environment undergoes sudden "catastrophes", the probability of
which depends on the composition of the population. We derive a simple
analytical result for the population growth rate. For a responsive environment,
two alternative strategies emerge. In the "no switching" strategy, the
population maximises its instantaneous growth rate, regardless of catastrophes.
In the "switching" strategy, the microbial switching rate is tuned to minimise
the environmental response. Which of these strategies is most favourable
depends on the parameters of the model. Previous studies have shown that
microbial switching can be favourable when the environment changes in an
unresponsive fashion between several states. Here, we demonstrate an
alternative role for phase variation in allowing microbes to maximise their
growth in catastrophic responsive environments.Comment: 9 pages, 10 figures; replaced with revised versio
Active mechanics reveal molecular-scale force kinetics in living oocytes
Active diffusion of intracellular components is emerging as an important
process in cell biology. This process is mediated by complex assemblies of
molecular motors and cytoskeletal filaments that drive force generation in the
cytoplasm and facilitate enhanced motion. The kinetics of molecular motors have
been precisely characterized in-vitro by single molecule approaches, however,
their in-vivo behavior remains elusive. Here, we study the active diffusion of
vesicles in mouse oocytes, where this process plays a key role in nuclear
positioning during development, and combine an experimental and theoretical
framework to extract molecular-scale force kinetics (force, power-stroke, and
velocity) of the in-vivo active process. Assuming a single dominant process, we
find that the nonequilibrium activity induces rapid kicks of duration 300 s resulting in an average force of 0.4 pN on vesicles
in in-vivo oocytes, remarkably similar to the kinetics of in-vitro myosin-V.
Our results reveal that measuring in-vivo active fluctuations allows extraction
of the molecular-scale activity in agreement with single-molecule studies and
demonstrates a mesoscopic framework to access force kinetics.Comment: 20 pages, 4 figures, see ancillary files for Supplementary Materials,
* equally contributing author
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