29 research outputs found
Threshold feedback control for a collective flashing ratchet: threshold dependence
We study the threshold control protocol for a collective flashing ratchet. In
particular, we analyze the dependence of the current on the values of the
thresholds. We have found analytical expressions for the small threshold
dependence both for the few and for the many particle case. For few particles
the current is a decreasing function of the thresholds, thus, the maximum
current is reached for zero thresholds. In contrast, for many particles the
optimal thresholds have a nonzero finite value. We have numerically checked the
relation that allows to obtain the optimal thresholds for an infinite number of
particles from the optimal period of the periodic protocol. These optimal
thresholds for an infinite number of particles give good results for many
particles. In addition, they also give good results for few particles due to
the smooth dependence of the current up to these threshold values.Comment: LaTeX, 10 pages, 7 figures, improved version to appear in Phys. Rev.
Cold atom realizations of Brownian motors
Brownian motors are devices which "rectify" Brownian motion, i.e. they can
generate a current of particles out of unbiased fluctuations. Brownian motors
are important for the understanding of molecular motors, and are also promising
for the realization of new nanolelectronic devices. Among the different systems
that can be used to study Brownian motors, cold atoms in optical lattices are
quite an unusual one: there is no thermal bath and both the potential and the
fluctuations are determined by laser fields. In this article recent
experimental implementations of Brownian motors using cold atoms in optical
lattices are reviewed
Efficiency of Brownian Motors
The efficiency of different types of Brownian motors is calculated
analytically and numerically. We find that motors based on flashing ratchets
present a low efficiency and an unavoidable entropy production. On the other
hand, a certain class of motors based on adiabatically changing potentials,
named reversible ratchets, exhibit a higher efficiency and the entropy
production can be arbitrarily reduced.Comment: LaTeX 209, 6 pages, 7 postscript figures, uses psfi
Online adaptive planning methods for intensity-modulated radiotherapy
Online adaptive radiation therapy aims at adapting a patient's treatment plan to their current anatomy to account for inter-fraction variations before daily treatment delivery. As this process needs to be accomplished while the patient is immobilized on the treatment couch, it requires time-efficient adaptive planning methods to generate a quality daily treatment plan rapidly. The conventional planning methods do not meet the time requirement of online adaptive radiation therapy because they often involve excessive human intervention, significantly prolonging the planning phase. This article reviews the planning strategies employed by current commercial online adaptive radiation therapy systems, research on online adaptive planning, and artificial intelligence's potential application to online adaptive planning.<br/
Overview: Unsolved problems of noise and fluctuations
Noise and fluctuations are at the seat of all physical phenomena. It is well known that, in linear systems, noise plays a destructive role. However, an emerging paradigm for nonlinear systems is that noise can play a constructive role—in some cases information transfer can be optimized at nonzero noise levels. Another use of noise is that its measured characteristics can tell us useful information about the system itself. Problems associated with fluctuations have been studied since 1826 and this Focus Issue brings together a collection of articles that highlight some of the emerging hot unsolved noise problems to point the way for future research
Bulk and Interfacial Shear Thinning of Immiscible Polymers
Nonequilibrium molecular dynamics simulations are used to study the shear
thinning behavior of immiscible symmetric polymer blends. The phase separated
polymers are subjected to a simple shear flow imposed by moving a wall parallel
to the fluid-fluid interface. The viscosity begins to shear thin at much lower
rates in the bulk than at the interface. The entire shear rate dependence of
the interfacial viscosity is consistent with a shorter effective chain length
that also describes the width of the interface. This is independent
of chain length and is a function only of the degree of immiscibility of
the two polymers. Changes in polymer conformation are studied as a function of
position and shear rate.Shear thinning correlates more closely with a decrease
in the component of the radius of gyration along the velocity gradient than
with elongation along the flow. At the interface, this contraction of chains is
independent of and consistent with the bulk behavior for chains of length
. The distribution of conformational changes along chains is also studied.
Central regions begin to stretch at a shear rate that decreases with increasing
, while shear induced changes at the ends of chains are independent of .Comment: 8 pages, 8 figure
Disorder Induced Diffusive Transport In Ratchets
The effects of quenched disorder on the overdamped motion of a driven
particle on a periodic, asymmetric potential is studied. While for the
unperturbed potential the transport is due to a regular drift, the quenched
disorder induces a significant additional chaotic ``diffusive'' motion. The
spatio-temporal evolution of the statistical ensemble is well described by a
Gaussian distribution, implying a chaotic transport in the presence of quenched
disorder.Comment: 10 pages, 4 EPS figures; submitted to Phys. Rev. Letter
Fluctuation - induced forces in critical fluids
The current knowledge about fluctuation - induced long - ranged forces is
summarized. Reference is made in particular to fluids near critical points, for
which some new insight has been obtained recently. Where appropiate, results of
analytic theory are compared with computer simulations and experiments.Comment: Topical review, 24 pages RevTeX, 6 figure