731 research outputs found
Brownian motors
In systems possessing a spatial or dynamical symmetry breaking thermal
Brownian motion combined with unbiased, non-equilibrium noise gives rise to a
channelling of chance that can be used to exercise control over systems at the
micro- and even on the nano-scale. This theme is known as ``Brownian motor''
concept. The constructive role of (the generally overdamped) Brownian motion is
exemplified for a noise-induced transport of particles within various set-ups.
We first present the working principles and characteristics with a
proof-of-principle device, a diffusive temperature Brownian motor. Next, we
consider very recent applications based on the phenomenon of signal mixing. The
latter is particularly simple to implement experimentally in order to optimize
and selectively control a rich variety of directed transport behaviors. The
subtleties and also the potential for Brownian motors operating in the quantum
regime are outlined and some state-of-the-art applications, together with
future roadways, are presented.Comment: 20 pages, 9 figures (slightly changed version
Transverse rectification of disorder-induced fluctuations in a driven system
We study numerically the overdamped motion of particles driven in a two
dimensional ratchet potential. In the proposed design, of the so-called
geometrical-ratchet type, the mean velocity of a single particle in response to
a constant force has a transverse component that can be induced by the presence
of thermal or other unbiased fluctuations. We find that additional quenched
disorder can strongly enhance the transverse drift at low temperatures, in
spite of reducing the transverse mobility. We show that, under general
conditions, the rectified transverse velocity of a driven particle fluid is
equivalent to the response of a one dimensional flashing ratchet working at a
drive-dependent effective temperature, defined through generalized Einstein
relations.Comment: 4.5 pages, 3 fig
Stochastic ratcheting of two dimensional colloids : Directed current and dynamical transitions
We present results of molecular dynamics simulations for two-dimensional
repulsively interacting colloids driven by a one dimensional asymmetric and
commensurate ratchet potential, switching on and off stochastically. This
drives a time-averaged directed current of colloids, exhibiting resonance with
change in ratcheting frequency, where the resonance frequency itself depends
non-monotonically on density. Using scaling arguments, we obtain analytic
results that show good agreement with numerical simulations. With increasing
ratcheting frequency, we find non-equilibrium re-entrant transitions between
solid and modulated liquid phases.Comment: paper and supplementary; published versio
Ratchet Effect and Nonlinear Transport for Particles on Random Substrates with Crossed ac Drives
We show in simulations that overdamped interacting particles in two
dimensions with a randomly disordered substrate can exhibit novel
nonequilibrium transport phenomena including a transverse ratchet effect, where
a combined dc drive and circular ac drive produce a drift velocity in the
direction transverse to the applied dc drive. The random disorder does not
break any global symmetry; however, in two dimensions, symmetry breaking occurs
due to the chirality of the circular drive. In addition to inducing the
transverse ratchet effect, increasing the ac amplitude also strongly affects
the longitudinal velocity response and can produce what we term an overshoot
effect where the longitudinal dc velocity is higher in the presence of the ac
drive than it would be for a dc drive alone. We also find a dynamical
reordering transition upon increasing the ac amplitude. In the absence of a dc
drive, it is possible to obtain a ratchet effect when the combined ac drives
produce particle orbits that break a reflection symmetry. In this case, as the
ac amplitude increases, current reversals can occur. These effects may be
observable for vortices in type II superconductors as well as for colloids
interacting with random substrates.Comment: 11 pages, 16 postscript figure
Lattice effects and current reversal in superconducting ratchets
Competition between the vortex lattice and a lattice of asymmetric artificial
defects is shown to play a crucial role in ratchet experiments in
superconducting films. We present a novel and collective mechanism for current
reversal based on a reconfiguration of the vortex lattice. In contrast to
previous models of vortex current reversal, the mechanism is based on the
global response of the vortex lattice to external forces.Comment: 12 pages, 7 figure
Molecular machines operating on nanoscale: from classical to quantum
The main physical features and operating principles of isothermal
nanomachines in microworld are reviewed, which are common for both classical
and quantum machines. Especial attention is paid to the dual and constructive
role of dissipation and thermal fluctuations, fluctuation-dissipation theorem,
heat losses and free energy transduction, thermodynamic efficiency, and
thermodynamic efficiency at maximum power. Several basic models are considered
and discussed to highlight generic physical features. Our exposition allows to
spot some common fallacies which continue to plague the literature, in
particular, erroneous beliefs that one should minimize friction and lower the
temperature to arrive at a high performance of Brownian machines, and that
thermodynamic efficiency at maximum power cannot exceed one-half. The emerging
topic of anomalous molecular motors operating sub-diffusively but highly
efficiently in viscoelastic environment of living cells is also discussed
Hidden symmetries, instabilities, and current suppression in Brownian ratchets
The operation of Brownian motors is usually described in terms of
out-of-equilibrium and symmetry-breaking settings, with the relevant
spatiotemporal symmetries identified from the analysis of the equations of
motion for the system at hand. When the appropriate conditions are satisfied,
symmetry-related trajectories with opposite current are thought to balance each
other, yielding suppression of transport. The direction of the current can be
precisely controlled around these symmetry points by finely tuning the driving
parameters. Here we demonstrate, by studying a prototypical Brownian ratchet
system, the existence of {\it hidden} symmetries, which escape the
identification by the standard symmetry analysis, and require different
theoretical tools for their revelation. Furthermore, we show that system
instabilities may lead to spontaneous symmetry breaking with unexpected
generation of directed transport.Comment: To appear in Phys. Rev. Let
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