436 research outputs found
Work fluctuation theorems for harmonic oscillators
The work fluctuations of an oscillator in contact with a thermostat and
driven out of equilibrium by an external force are studied experimentally and
theoretically within the context of Fluctuation Theorems (FTs). The oscillator
dynamics is modeled by a second order Langevin equation. Both the transient and
stationary state fluctuation theorems hold and the finite time corrections are
very different from those of a first order Langevin equation. The periodic
forcing of the oscillator is also studied; it presents new and unexpected short
time convergences. Analytical expressions are given in all cases
Nonequilibrium fluctuations in a resistor
In small systems where relevant energies are comparable to thermal agitation,
fluctuations are of the order of average values. In systems in thermodynamical
equilibrium, the variance of these fluctuations can be related to the
dissipation constant in the system, exploiting the Fluctuation-Dissipation
Theorem (FDT). In non-equilibrium steady systems, Fluctuations Theorems (FT)
additionally describe symmetry properties of the probability density functions
(PDFs) of the fluctuations of injected and dissipated energies. We
experimentally probe a model system: an electrical dipole driven out of
equilibrium by a small constant current , and show that FT are
experimentally accessible and valid. Furthermore, we stress that FT can be used
to measure the dissipated power in the system by just
studying the PDFs symmetries.Comment: Juillet 200
Fluctuation theorems for harmonic oscillators
We study experimentally the thermal fluctuations of energy input and
dissipation in a harmonic oscillator driven out of equilibrium, and search for
Fluctuation Relations. We study transient evolution from the equilibrium state,
together with non equilibrium steady states. Fluctuations Relations are
obtained experimentally for both the work and the heat, for the stationary and
transient evolutions. A Stationary State Fluctuation Theorem is verified for
the two time prescriptions of the torque. But a Transient Fluctuation Theorem
is satisfied for the work given to the system but not for the heat dissipated
by the system in the case of linear forcing. Experimental observations on the
statistical and dynamical properties of the fluctuation of the angle, we derive
analytical expressions for the probability density function of the work and the
heat. We obtain for the first time an analytic expression of the probability
density function of the heat. Agreement between experiments and our modeling is
excellent
Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media
Taking into account stress fluctuations due to thermal noise, we study
thermally activated irreversible crack growth in disordered media. The
influence of material disorder on sub-critical growth of a single crack in
two-dimensional brittle elastic material is described through the introduction
of a rupture threshold distribution. We derive analytical predictions for crack
growth velocity and material lifetime in agreement with direct numerical
calculations. It is claimed that crack growth process is inhibited by disorder:
velocity decreases and lifetime increases with disorder. More precisely,
lifetime is shown to follow a super-Arrhenius law, with an effective
temperature theta - theta_d, where theta is related to the thermodynamical
temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter
Experimental test of the Gallavotti-Cohen fluctuation theorem in turbulent flows
We test the fluctuation theorem from measurements in turbulent flows. We
study the time fluctuations of the force acting on an obstacle, and we consider
two experimental situations: the case of a von K\'arm\'an swirling flow between
counter-rotating disks (VK) and the case of a wind tunnel jet. We first study
the symmetries implied by the Gallavotti-Cohen fluctuation theorem (FT) on the
probability density distributions of the force fluctuations; we then test the
Sinai scaling. We observe that in both experiments the symmetries implied by
the FT are well verified, whereas the Sinai scaling is established, as
expected, only for long times
A dynamical law for slow crack growth in polycarbonate films
We study experimentally the slow growth of a single crack in polycarbonate
films submitted to uniaxial and constant imposed stress. For this visco-plastic
material, we uncover a dynamical law that describes the dependence of the
instantaneous crack velocity with experimental parameters. The law involves a
Dugdale-Barenblatt static description of crack tip plastic zones associated to
an Eyring's law and an empirical dependence with the crack length that may come
from a residual elastic field
Slow crack growth in polycarbonate films
We study experimentally the slow growth of a single crack in polycarbonate
films submitted to uniaxial and constant imposed stress. The specificity of
fracture in polycarbonate films is the appearance of flame shaped macroscopic
process zones at the tips of the crack. Supported by an experimental study of
the mechanical properties of polycarbonate films, an analysis of the stress
dependence of the mean ratio between the process zone and crack lengths, during
the crack growth, show a quantitative agreement with the Dugdale-Barenblatt
model of the plastic process zone. We find that the fracture growth curves obey
strong scaling properties that lead to a well defined growth master curve
Spinal anesthesia for cesarean delivery in a woman with neuromyelitis optica.
Neuromyelitis optica (NMO), or Devic's disease, is an idiopathic severe demyelinating disease that preferentially affects the optic nerve and spinal cord. Neuraxial anesthesia in women with multiple sclerosis is widely accepted, but reports of the use of neuraxial anesthesia in patients with NMO are scarce. We report the case of a morbidly obese primigravida undergoing a planned cesarean delivery at 32 weeks' gestation due to an acute exacerbation of NMO, managed with spinal anesthesia. Other than increased intraoperative hyperalgesia requiring inhaled nitrous oxide/oxygen, the mother experienced no apparent anesthetic-related complications
On the intermittent energy transfer at viscous scales in turbulent flows
In this letter we present numerical and experimental results on the scaling
properties of velocity turbulent fields in the range of scales where viscous
effects are acting. A generalized version of Extended Self Similarity capable
of describing scaling laws of the velocity structure functions down to the
smallest resolvable scales is introduced. Our findings suggest the absence of
any sharp viscous cutoff in the intermittent transfer of energy.Comment: 10 pages, plain Latex, 6 figures available upon request to
[email protected]
Subcritical crack growth in fibrous materials
We present experiments on the slow growth of a single crack in a fax paper
sheet submitted to a constant force . We find that statistically averaged
crack growth curves can be described by only two parameters : the mean rupture
time and a characteristic growth length . We propose a model
based on a thermally activated rupture process that takes into account the
microstructure of cellulose fibers. The model is able to reproduce the shape of
the growth curve, the dependence of on as well as the effect of
temperature on the rupture time . We find that the length scale at which
rupture occurs in this model is consistently close to the diameter of cellulose
microfibrils
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