782 research outputs found
Geometric depolarization in patterns formed by backscattered light
We formulate a framework for the depolarization of linearly polarized
backscattered light based on the concept of geometric phase, {\it i.e} Berry's
phase. The predictions of this theory are applied to the patterns formed by
backscattered light between crossed or parallel polarizers. This theory should
be particularly adapted to the situation in which polarized light is scattered
many times but predominantly in the forward direction. We apply these ideas to
the patterns which we obtained experimentally with backscattered polarized
light from a colloidal suspension.Comment: 3 pages and 3 figure
Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles
Using the framework of stochastic thermodynamics, we present an experimental
study of a doublet of magnetic colloidal particles which is manipulated by a
time-dependent magnetic field. Due to hydrodynamic interactions, each bead
experiences a state-dependent friction, which we characterize using a
hydrodynamic model. In this work, we compare two estimates of the dissipation
in this system: the first one is energy based since it relies on the measured
interaction potential, while the second one is information based since it uses
only the information content of the trajectories. While the latter only offers
a lower bound of the former, we find it to be simple to implement and of
general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5
figure
Effective zero-thickness model for a conductive membrane driven by an electric field
The behavior of a conductive membrane in a static (DC) electric field is
investigated theoretically. An effective zero-thickness model is constructed
based on a Robin-type boundary condition for the electric potential at the
membrane, originally developed for electrochemical systems. Within such a
framework, corrections to the elastic moduli of the membrane are obtained,
which arise from charge accumulation in the Debye layers due to capacitive
effects and electric currents through the membrane and can lead to an
undulation instability of the membrane. The fluid flow surrounding the membrane
is also calculated, which clarifies issues regarding these flows sharing many
similarities with flows produced by induced charge electro-osmosis (ICEO).
Non-equilibrium steady states of the membrane and of the fluid can be
effectively described by this method. It is both simpler, due to the zero
thickness approximation which is widely used in the literature on fluid
membranes, and more general than previous approaches. The predictions of this
model are compared to recent experiments on supported membranes in an electric
field.Comment: 14 pages, 5 figure
Electrostatic and electrokinetic contributions to the elastic moduli of a driven membrane
We discuss the electrostatic contribution to the elastic moduli of a cell or
artificial membrane placed in an electrolyte and driven by a DC electric field.
The field drives ion currents across the membrane, through specific channels,
pumps or natural pores. In steady state, charges accumulate in the Debye layers
close to the membrane, modifying the membrane elastic moduli. We first study a
model of a membrane of zero thickness, later generalizing this treatment to
allow for a finite thickness and finite dielectric constant. Our results
clarify and extend the results presented in [D. Lacoste, M. Cosentino
Lagomarsino, and J. F. Joanny, Europhys. Lett., {\bf 77}, 18006 (2007)], by
providing a physical explanation for a destabilizing term proportional to
\kps^3 in the fluctuation spectrum, which we relate to a nonlinear ()
electro-kinetic effect called induced-charge electro-osmosis (ICEO). Recent
studies of ICEO have focused on electrodes and polarizable particles, where an
applied bulk field is perturbed by capacitive charging of the double layer and
drives flow along the field axis toward surface protrusions; in contrast, we
predict "reverse" ICEO flows around driven membranes, due to curvature-induced
tangential fields within a non-equilibrium double layer, which hydrodynamically
enhance protrusions. We also consider the effect of incorporating the dynamics
of a spatially dependent concentration field for the ion channels.Comment: 22 pages, 10 figures. Under review for EPJ
Filaments in observed and mock galaxy catalogues
Context. The main feature of the spatial large-scale galaxy distribution is
an intricate network of galaxy filaments. Although many attempts have been made
to quantify this network, there is no unique and satisfactory recipe for that
yet. Aims. The present paper compares the filaments in the real data and in the
numerical models, to see if our best models reproduce statistically the
filamentary network of galaxies. Methods. We apply an object point process with
interactions (the Bisous process) to trace and describe the filamentary network
both in the observed samples (the 2dFGRS catalogue) and in the numerical models
that have been prepared to mimic the data.We compare the networks. Results. We
find that the properties of filaments in numerical models (mock samples) have a
large variance. A few mock samples display filaments that resemble the observed
filaments, but usually the model filaments are much shorter and do not form an
extended network. Conclusions. We conclude that although we can build numerical
models that are similar to observations in many respects, they may fail yet to
explain the filamentary structure seen in the data. The Bisous-built filaments
are a good test for such a structure.Comment: 13 pages, accepted for publication in Astronomy and Astrophysic
Large Faraday rotation of resonant light in a cold atomic cloud
We experimentally studied the Faraday rotation of resonant light in an
optically-thick cloud of laser-cooled rubidium atoms. Measurements yield a
large Verdet constant in the range of 200 000 degrees/T/mm and a maximal
polarization rotation of 150 degrees. A complete analysis of the polarization
state of the transmitted light was necessary to account for the role of the
probe laser's spectrum
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