985 research outputs found
Three and four current reversals versus temperature in correlation ratchets with a simple sawtooh potential
Transport of Brownian particles on a simple sawtooth potential subjected to
both unbiased thermal and nonequilibrium symmetric three-level Markovian noise
is considered. The new effects of three and four current reversals as a
function of temperature are established in such correlation ratchets. The
parameter space coordinates of the fixed points associated with these current
reversals and the necessary and sufficient conditions for the existence of the
novel current reversals are found.Comment: 4 pages, 5 figures; some changes introduced; accepted for publication
in Physical Review
Separation quality of a geometric ratchet
We consider an experimentally relevant model of a geometric ratchet in which
particles undergo drift and diffusive motion in a two-dimensional periodic
array of obstacles, and which is used for the continuous separation of
particles subject to different forces. The macroscopic drift velocity and
diffusion tensor are calculated by a Monte-Carlo simulation and by a
master-equation approach, using the correponding microscopic quantities and the
shape of the obstacles as input. We define a measure of separation quality and
investigate its dependence on the applied force and the shape of the obstacles
Multiple current reversals in forced inhomogeneous ratchets
Transport properties of overdamped Brownian paricles in a rocked thermal
ratchet with space dependent friction coefficient is studied. By tuning the
parameters, the direction of current exhibit multiple reversals, both as a
function of the thermal noise strength as well as the amplitude of rocking
force. Current reversals also occur under deterministic conditions and exhibits
intriguing structure. All these features arise due to mutual interplay between
potential asymmetry,noise, driving frequency and inhomogeneous friction.Comment: 6 figure
Feynman's ratchet and pawl: an exactly solvable model
We introduce a simple, discrete model of Feynman's ratchet and pawl,
operating between two heat reservoirs. We solve exactly for the steady-state
directed motion and heat flows produced, first in the absence and then in the
presence of an external load. We show that the model can act both as a heat
engine and as a refrigerator. We finally investigate the behavior of the system
near equilibrium, and use our model to confirm general predictions based on
linear response theory.Comment: 19 pages + 10 figures; somewhat tighter presentatio
Irreversible and reversible modes of operation of deterministic ratchets
We discuss a problem of optimization of the energetic efficiency of a simple
rocked ratchet. We concentrate on a low-temperature case in which the
particle's motion in a ratchet potential is deterministic. We show that the
energetic efficiency of a ratchet working adiabatically is bounded from above
by a value depending on the form of ratchet potential. The ratchets with
strongly asymmetric potentials can achieve ideal efficiency of unity without
approaching reversibility. On the other hand we show that for any form of the
ratchet potential a set of time-protocols of the outer force exist under which
the operation is reversible and the ideal value of efficiency is also achieved.
The mode of operation of the ratchet is still quasistatic but not adiabatic.
The high values of efficiency can be preserved even under elevated
temperatures
Influence of the Barrier Shape on Resonant Activation
The escape of a Brownian particle over a dichotomously fluctuating barrier is
investigated for various shapes of the barrier. The problem of resonant
activation is revisited with the attention on the effect of the barrier shape
on optimal value of the mean escape time in the system. The characteristic
features of resonant behavior are analyzed for barriers switching either
between different heights, or "on" and "off" positions. PACS number(s):
05.10-a, 02.50.-r, 82.20.-wj.Comment: 7 pages, 8 figures, RevTex4. Manuscript has been revised and
enhanced. Pictures have been made more clear and some of them have been
cancelled. Additional references have been added. The paper has been
submitted to Phys. Rev.
Transition from Secondary school CLIL to EMI at University: Initial evidence from research in Italy
Through vocabulary tests, interviews with students and recordings of classes/lectures, we measured students' English vocabulary knowledge against the vocabulary in the teachers' talk, the nature of classroom interaction in the two settings and the students' approaches to their learning in general and their in-class listening strategies in particular. In this way we aimed to identify the challenges students faced in English-medium education in the secondary and tertiary educational phases
Efficient Dynamic Importance Sampling of Rare Events in One Dimension
Exploiting stochastic path integral theory, we obtain \emph{by simulation}
substantial gains in efficiency for the computation of reaction rates in
one-dimensional, bistable, overdamped stochastic systems. Using a well-defined
measure of efficiency, we compare implementations of ``Dynamic Importance
Sampling'' (DIMS) methods to unbiased simulation. The best DIMS algorithms are
shown to increase efficiency by factors of approximately 20 for a
barrier height and 300 for , compared to unbiased simulation. The
gains result from close emulation of natural (unbiased), instanton-like
crossing events with artificially decreased waiting times between events that
are corrected for in rate calculations. The artificial crossing events are
generated using the closed-form solution to the most probable crossing event
described by the Onsager-Machlup action. While the best biasing methods require
the second derivative of the potential (resulting from the ``Jacobian'' term in
the action, which is discussed at length), algorithms employing solely the
first derivative do nearly as well. We discuss the importance of
one-dimensional models to larger systems, and suggest extensions to
higher-dimensional systems.Comment: version to be published in Phys. Rev.
Survey of verotoxin-producing Escherichia coli and faecal coliforms in beef carcasses destined for export at slaughterhouses in Brazil.
ABSTRACT This study investigated the presence of generic and verotoxin-producing E. coli as well as enumerated faecal coliforms in 30 beef carcasses in different parts of the slaughter process (after skinning, washing and cooling) at each of three slaughterhouses of the state of Mato Grosso do Sul, Brazil. Among the total number of carcasses examined (n = 90), 39 (43.3%) had generic E. coli. Among the 270 samples analysed, 25 (9.3%) were positive after skinning, 14 (5.2%) were positive after washing and nine (3.3%) were positive after cooling. The majority of isolates of E. coli was collected from samples after skinning, which is considered a critical point of the microbial contamination of carcasses. However, the highest concentration of faecal coliforms was found after the washing step. The cooling step proved to be important to reducing the amount of hygiene-indicator microorganisms. The E. coli isolates had no stx1 or stx2 genes associated with virulence
From the cell membrane to the nucleus: unearthing transport mechanisms for Dynein
Mutations in the motor protein cytoplasmic dynein have been found to cause Charcot-Marie-Tooth disease, spinal muscular atrophy, and severe intellectual disabilities in humans. In mouse models, neurodegeneration is observed. We sought to develop a novel model which could incorporate the effects of mutations on distance travelled and velocity. A mechanical model for the dynein mediated transport of endosomes is derived from first principles and solved numerically. The effects of variations in model parameter values are analysed to find those that have a significant impact on velocity and distance travelled. The model successfully describes the processivity of dynein and matches qualitatively the velocity profiles observed in experiments
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