2,945 research outputs found
Vibrational ratchets
Transport in a one-dimensional symmetric device can be activated by the
combination of thermal noise and a bi-harmonic drive. For the study case of an
overdamped Brownian particle diffusing on a periodic one-dimensional substrate,
we distinguish two apparently different bi-harmonic regimes: (i) Harmonic
mixing, where the two drive frequencies are commensurate and of the order of
some intrinsic dynamical relaxation rate. A comparison of new simulation
results with earlier theoretical predictions shows that the analytical
understanding of this frequency mixing mechanism is not satisfactory, yet; (ii)
Vibrational mixing, where one harmonic drive component is characterized by a
high frequency but finite amplitude-to-frequency ratio. Its effect on the
device response to either a static or a low-frequency additional input signal
is accurately reproduced by rescaling each spatial Fourier component of the
substrate potential, separately. Contrary to common wisdom based on the linear
response theory, we show that extremely high-frequency modulations can indeed
influence the response of slowly (or dc) operated devices, with potential
applications in sensor technology and cellular physiology. Finally, the mixing
of two high-frequency beating signal is also investigated both numerically and
analytically.Comment: 8 pages, 9 figure
Nonequilibrium steady state of the kinetic Glauber-Ising model under an alternating magnetic field
When periodically driven by an external magnetic field, a spin system can
enter a phase of steady entrained oscillations with nonequilibrium probability
distribution function. We consider an arbitrary magnetic field switching its
direction with frequency comparable with the spin-flip rate and show that the
resulting nonequilibrium probability distribution can be related to the system
equilibrium distribution in the presence of a constant magnetic field of the
same magnitude. We derive convenient approximate expressions for this exact
relation and discuss their implications.Comment: 6 pages, 4 figure
Recycled Noise Rectification: A Dumb Maxwell's Daemon
The one dimensional motion of a massless Brownian particle on a symmetric
periodic substrate can be rectified by re-injecting its driving noise through a
realistic recycling procedure. If the recycled noise is multiplicatively
coupled to the substrate, the ensuing feed-back system works like a passive
Maxwell's daemon, capable of inducing a net current that depends on both the
delay and the autocorrelation times of the noise signals. Extensive numerical
simulations show that the underlying rectification mechanism is a resonant
nonlinear effect: The observed currents can be optimized for an appropriate
choice of the recycling parameters with immediate application to the design of
nanodevices for particle transport.Comment: 7 pages, 6 figure
Taxis of Artificial Swimmers in a Spatio-Temporally Modulated Activation Medium
Contrary to microbial taxis, where a tactic response to external stimuli is
controlled by complex chemical pathways acting like sensor-actuator loops,
taxis of artificial microswimmers is a purely stochastic effect associated with
a non-uniform activation of the particles' self-propulsion. We study the tactic
response of such swimmers in a spatio-temporally modulated activating medium by
means of both numerical and analytical techniques. In the opposite limits of
very fast and very slow rotational particle dynamics, we obtain analytic
approximations that closely reproduce the numerical description. A swimmer
drifts on average either parallel or anti-parallel to the propagation direction
of the activating pulses, depending on their speed and width. The drift in line
with the pulses is solely determined by the finite persistence length of the
active Brownian motion performed by the swimmer, whereas the drift in the
opposite direction results from the combination of ballistic and diffusive
properties of the swimmer's dynamics.Comment: 19 pages, 6 figures; Entropy (in press
AC Driven Jumps Distribution on a Periodic Substrate
A driven Brownian particle (e.g. an adatom on a surface) diffusing on a
low-viscosity, periodic substrate may execute multiple jumps. In the presence
of an additional periodic drive, the jump lengths and time durations become
statistically modulated according to a syncronyzation mechanism reminiscent of
asymmetric stochastic resonance. Here, too, bistability plays a key role, but
in a dynamical sense, inasmuch as a particle switches between locked and
running states.Comment: 4 pages, 4 figures, RevTeX, to be published in Surface Science
Letter
String ratchets: ac driven asymmetric kinks.
We simulated numerically the time evolution of a one-kink bearing, damped elastic string sitting on noiseless periodic substrates of two types: (I) asymmetric, time independent, (II) symmetric, periodically deformable. An asymmetric kink subjected to an ac drive is shown to drift steadily with finite average speed independent of its initial kinetic conditions. In the overdamped regime the resulting net kink transport can be attributed to the rectification of the Brownian motion of a pointlike particle with oscillating mass. For intermediate to low damping completely different features show up, due to the finite size of the objects being transported; in particular, the kink current hits a maximum for an optimal value of the damping constant, resonates at the kink internal-mode frequency and, finally, reverses sign within a certain range of the drive parameters
Rectification of spatial disorder
We demonstrate that a large ensemble of noiseless globally coupled-pinned
oscillators is capable of rectifying spatial disorder with spontaneous current
activated through a dynamical phase transition mechanism, either of first or
second order, depending on the profile of the pinning potential. In the
presence of an external weak drive, the same collective mechanism can result in
an absolute negative mobility, which, though not immediately related to
symmetry breaking, is most prominent at the phase transition
Enhanced buoyancy of active particles in convective flows
The authors study the influence of activity on particles advected by convection rolls and observe that they float on the surface, even if they are denser than the suspension fluid
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