32 research outputs found
Low Reynolds number hydrodynamics of asymmetric, oscillating dumbbell pairs
Active dumbbell suspensions constitute one of the simplest model system for
collective swimming at low Reynolds number. Generalizing recent work, we derive
and analyze stroke-averaged equations of motion that capture the effective
hydrodynamic far-field interaction between two oscillating, asymmetric
dumbbells in three space dimensions. Time-averaged equations of motion, as
those presented in this paper, not only yield a considerable speed-up in
numerical simulations, they may also serve as a starting point when deriving
continuum equations for the macroscopic dynamics of multi-swimmer suspensions.
The specific model discussed here appears to be particularly useful in this
context, since it allows one to investigate how the collective macroscopic
behavior is affected by changes in the microscopic symmetry of individual
swimmers.Comment: 10 pages, to appear in EPJ Special Topic
Effective swimming strategies in low Reynolds number flows
The optimal strategy for a microscopic swimmer to migrate across a linear
shear flow is discussed. The two cases, in which the swimmer is located at
large distance, and in the proximity of a solid wall, are taken into account.
It is shown that migration can be achieved by means of a combination of sailing
through the flow and swimming, where the swimming strokes are induced by the
external flow without need of internal energy sources or external drives. The
structural dynamics required for the swimmer to move in the desired direction
is discussed and two simple models, based respectively on the presence of an
elastic structure, and on an orientation dependent friction, to control the
deformations induced by the external flow, are analyzed. In all cases, the
deformation sequence is a generalization of the tank-treading motion regimes
observed in vesicles in shear flows. Analytic expressions for the migration
velocity as a function of the deformation pattern and amplitude are provided.
The effects of thermal fluctuations on propulsion have been discussed and the
possibility that noise be exploited to overcome the limitations imposed on the
microswimmer by the scallop theorem have been discussed.Comment: 14 pages, 5 figure
Limits on the monopole magnetic field from measurements of the electric dipole moments of atoms, molecules and the neutron
A radial magnetic field can induce a time invariance violating electric
dipole moment (EDM) in quantum systems. The EDMs of the Tl, Cs, Xe and Hg atoms
and the neutron that are produced by such a field are estimated. The
contributions of such a field to the constants, of the T,P-odd
interactions and are also estimated for the TlF, HgF and YbF molecules (where
() is the electron (nuclear) spin and is the molecular
axis). The best limit on the contact monopole field can be obtained from the
measured value of the Tl EDM. The possibility of such a field being produced
from polarization of the vacuum of electrically charged magnetic monopoles
(dyons) by a Coulomb field is discussed, as well as the limit on these dyons.
An alternative mechanism involves chromomagnetic and chromoelectric fields in
QCD.Comment: Uses RevTex, 16 pages, 4 postscript figures. An explanation of why
there is no orbital contribution to the EDM has been added, and the
presentation has been improved in genera
Hydrodynamic Synchronisation of Model Microswimmers
We define a model microswimmer with a variable cycle time, thus allowing the
possibility of phase locking driven by hydrodynamic interactions between
swimmers. We find that, for extensile or contractile swimmers, phase locking
does occur, with the relative phase of the two swimmers being, in general,
close to 0 or pi, depending on their relative position and orientation. We show
that, as expected on grounds of symmetry, self T-dual swimmers, which are
time-reversal covariant, do not phase-lock. We also discuss the phase behaviour
of a line of tethered swimmers, or pumps. These show oscillations in their
relative phases reminiscent of the metachronal waves of cilia.Comment: 17 pages, 8 figure
A circle swimmer at low Reynolds number
Swimming in circles occurs in a variety of situations at low Reynolds number.
Here we propose a simple model for a swimmer that undergoes circular motion,
generalising the model of a linear swimmer proposed by Najafi and Golestanian
(Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres
arranged in a triangular configuration, joined by two links of time-dependent
length. For small strokes, we discuss the motion of the swimmer as a function
of the separation angle between its links. We find that swimmers describe
either clockwise or anticlockwise circular motion depending on the tilting
angle in a non-trivial manner. The symmetry of the swimmer leads to a
quadrupolar decay of the far flow field. We discuss the potential extensions
and experimental realisation of our model.Comment: 9 pages, 9 Figure
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Locais e período de alimentação da cigarrinha vetora de Xylella fastidiosa, Bucephalogonia xanthophis (Berg) (Hemiptera: Cicadellidae), em mudas cítricas
Closed-Form transformation between geodetic and ellipsoidal coordinates
We present formulas for direct closed-form transformation between geodetic coordinates(Φ, λ, h) and ellipsoidal coordinates (β, λ, u) for any oblate ellipsoid of revolution.These will be useful for those dealing with ellipsoidal representations of the Earth's gravityfield or other oblate ellipsoidal figures. The numerical stability of the transformations for nearpolarand near-equatorial regions is also considered