329 research outputs found
Chaos in Pseudo-Newtonian Black Holes with Halos
The Newtonian as well as the special relativistic dynamics are used to study
the stability of orbits of a test particle moving around a black hole plus a
dipolar halo. The black hole is modeled by either the usual monopole potential
or the Paczynki-Wiita pseudo-Newtonian potential. The full general relativistic
similar case is also considered. The Poincare section method and the Lyapunov
characteristic exponents show that the orbits for the pseudo-Newtonian
potential models are more unstable than the corresponding general relativistic
geodesics.Comment: RevTEX, 7 eps figures, to appear in Astron Astrophy
Very low shot noise in carbon nanotubes
We have performed noise measurements on suspended ropes of single wall carbon
nanotubes (SWNT) between 1 and 300 K for different values of dc current through
the ropes. We find that the shot noise is suppressed by more than a factor 100
compared to the full shot noise 2eI. We have also measured an individual SWNT
and found a level of noise which is smaller than the minimum expected. Another
finding is the very low level of 1/f noise, which is significantly lower than
previous observations. We propose two possible interpretations for this strong
shot noise reduction: i) Transport within a rope takes place through a few
nearly ballistic tubes within a rope and possibly involves non integer
effective charges. ii) A substantial fraction of the tubes conduct with a
strong reduction of effective charge (by more than a factor 50).Comment: Submitted to Eur. Phys. J. B (Jan. 2002) Higher resolution pictures
are posted on http://www.lps.u-psud.fr/Collectif/gr_07/publications.htm
Vortex: A new family of one-way hash functions based on AES rounds and carry-less multiplication
Abstract. We present Vortex a new family of one way hash functions that can produce message digests of 256 bits. The main idea behind the design of these hash functions is that we use well known algorithms that can support very fast diffusion in a small number of steps. We also balance the cryptographic strength that comes from iterating block cipher rounds with SBox substitution and diffusion (like Whirlpool) against the need to have a lightweight implementation with as small number of rounds as possible. We use only 3 AES rounds as opposed to 10 since our goal is not to protect a secret symmetric key but to support perfect mixing of the bits of the input into the hash value. Three AES rounds are followed by our variant of Galois Field multiplication. This achieves cross-mixing between 128-bit sets. We present a set of qualitative arguments why we believe Vortex supports collision resistance and first pre-image resistance
Encrypt-to-self:Securely outsourcing storage
We put forward a symmetric encryption primitive tailored towards a specific application: outsourced storage. The setting assumes a memory-bounded computing device that inflates the amount of volatile or permanent memory available to it by letting other (untrusted) devices hold encryptions of information that they return on request. For instance, web servers typically hold for each of the client connections they manage a multitude of data, ranging from user preferences to technical information like database credentials. If the amount of data per session is considerable, busy servers sooner or later run out of memory. One admissible solution to this is to let the server encrypt the session data to itself and to let the client store the ciphertext, with the agreement that the client reproduce the ciphertext in each subsequent request (e.g., via a cookie) so that the session data can be recovered when required. In this article we develop the cryptographic mechanism that should be used to achieve confidential and authentic data storage in the encrypt-to-self setting, i.e., where encryptor and decryptor coincide and constitute the only entity holding keys. We argue that standard authenticated encryption represents only a suboptimal solution for preserving confidentiality, as much as message authentication codes are suboptimal for preserving authenticity. The crucial observation is that such schemes instantaneously give up on all security promises the moment the key is compromised. In contrast, data protected with our new primitive remains fully integrity protected and unmalleable. In the course of this paper we develop a formal model for encrypt-to-self systems, show that it solves the outsourced storage problem, propose surprisingly efficient provably secure constructions, and report on our implementations
Discrete cilia modelling with singularity distributions
We discuss in detail techniques for modelling flows due to finite and infinite arrays of beating cilia. An efficient technique, based on concepts from previous ‘singularity models’ is described, that is accurate in both near and far-fields. Cilia are modelled as curved slender ellipsoidal bodies by distributing Stokeslet and potential source dipole singularities along their centrelines, leading to an integral equation that can be solved using a simple and efficient discretisation. The computed velocity on the cilium surface is found to compare favourably with the boundary condition. We then present results for two topics of current interest in biology. 1) We present the first theoretical results showing the mechanism by which rotating embryonic nodal cilia produce a leftward flow by a ‘posterior tilt,’ and track particle motion in an array of three simulated nodal cilia. We find that, contrary to recent suggestions, there is no continuous layer of negative fluid transport close to the ciliated boundary. The mean leftward particle transport is found to be just over 1 μm/s, within experimentally measured ranges. We also discuss the accuracy of models that represent the action of cilia by steady rotlet arrays, in particular, confirming the importance of image systems in the boundary in establishing the far-field fluid transport. Future modelling may lead to understanding of the mechanisms by which morphogen gradients or mechanosensing cilia convert a directional flow to asymmetric gene expression. 2) We develop a more complex and detailed model of flow patterns in the periciliary layer of the airway surface liquid. Our results confirm that shear flow of the mucous layer drives a significant volume of periciliary liquid in the direction of mucus transport even during the recovery stroke of the cilia. Finally, we discuss the advantages and disadvantages of the singularity technique and outline future theoretical and experimental developments required to apply this technique to various other biological problems, particularly in the reproductive system
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
Phase diagram of aggregation of oppositely charged colloids in salty water
Aggregation of two oppositely charged colloids in salty water is studied. We
focus on the role of Coulomb interaction in strongly asymmetric systems in
which the charge and size of one colloid is much larger than the other one. In
the solution, each large colloid (macroion) attracts certain number of
oppositely charged small colloids (-ion) to form a complex. If the
concentration ratio of the two colloids is such that complexes are not strongly
charged, they condense in a macroscopic aggregate. As a result, the phase
diagram in a plane of concentrations of two colloids consists of an aggregation
domain sandwiched between two domains of stable solutions of complexes. The
aggregation domain has a central part of total aggregation and two wings
corresponding to partial aggregation. A quantitative theory of the phase
diagram in the presence of monovalent salt is developed. It is shown that as
the Debye-H\"{u}ckel screening radius decreases, the aggregation domain
grows, but the relative size of the partial aggregation domains becomes much
smaller. As an important application of the theory, we consider solutions of
long double-helix DNA with strongly charged positive spheres (artificial
chromatin). We also consider implications of our theory for in vitro
experiments with the natural chromatin. Finally, the effect of different shapes
of macroions on the phase diagram is discussed.Comment: 10 pages, 9 figures. The text is rewritten, but results are not
change
Anomalous Behavior near T_c and Synchronization of Andreev Reflection in Two-Dimensional Arrays of SNS Junctions
We have investigated low-temperature transport properties of two-dimensional
arrays of superconductor--normal-metal--superconductor (SNS) junctions. It has
been found that in two-dimensional arrays of SNS junctions (i) a change in the
energy spectrum within an interval of the order of the Thouless energy is
observed even when the thermal broadening far exceeds the Thouless energy for a
single SNS junction; (ii) the manifestation of the subharmonic energy gap
structure (SGS) with high harmonic numbers is possible even if the energy
relaxation length is smaller than that required for the realization of a
multiple Andreev reflection in a single SNS junction. These results point to
the synchronization of a great number of SNS junctions. A mechanism of the SGS
origin in two-dimensional arrays of SNS junctions, involving the processes of
conventional and crossed Andreev reflection, is proposed.Comment: 5 pages, 5 figure
Generic flow profiles induced by a beating cilium
We describe a multipole expansion for the low Reynolds number fluid flows
generated by a localized source embedded in a plane with a no-slip boundary
condition. It contains 3 independent terms that fall quadratically with the
distance and 6 terms that fall with the third power. Within this framework we
discuss the flows induced by a beating cilium described in different ways: a
small particle circling on an elliptical trajectory, a thin rod and a general
ciliary beating pattern. We identify the flow modes present based on the
symmetry properties of the ciliary beat.Comment: 12 pages, 6 figures, to appear in EPJ
Fluid transport at low Reynolds number with magnetically actuated artificial cilia
By numerical modeling we investigate fluid transport in low-Reynolds-number
flow achieved with a special elastic filament or artifical cilium attached to a
planar surface. The filament is made of superparamagnetic particles linked
together by DNA double strands. An external magnetic field induces dipolar
interactions between the beads of the filament which provides a convenient way
of actuating the cilium in a well-controlled manner. The filament has recently
been used to successfully construct the first artificial micro-swimmer [R.
Dreyfus at al., Nature 437, 862 (2005)]. In our numerical study we introduce a
measure, which we call pumping performance, to quantify the fluid transport
induced by the magnetically actuated cilium and identify an optimum stroke
pattern of the filament. It consists of a slow transport stroke and a fast
recovery stroke. Our detailed parameter study also reveals that for
sufficiently large magnetic fields the artificial cilium is mainly governed by
the Mason number that compares frictional to magnetic forces. Initial studies
on multi-cilia systems show that the pumping performance is very sensitive to
the imposed phase lag between neighboring cilia, i.e., to the details of the
initiated metachronal wave.Comment: 12 pages, 10 figures. To appear in EPJE, available online at
http://dx.doi.org/10.1140/epje/i2008-10388-
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