19,646 research outputs found
The formation of the kamacite phase in metallic meteorites
Diffusion analysis of concentrated gradients in metallic meteorites kamacit
Applications of the Analytical Electron Microscope to Materials Science
In the last 20 years, the analytical electron microscope (AEM) as allowed investigators to obtain chemical and structural information from less than 50 nanometer diameter regions in thin samples of materials and to explore problems where reactions occur at boundaries and interfaces or within small particles or phases in bulk samples. Examples of the application of the AEM to materials science problems are presented in this paper and demonstrate the usefulness and the future potential of this instrument
Vortex-type elastic structured media and dynamic shielding
The paper addresses a novel model of metamaterial structure. A system of
spinners has been embedded into a two-dimensional periodic lattice system. The
equations of motion of spinners are used to derive the expression for the
chiral term in the equations describing the dynamics of the lattice. Dispersion
of elastic waves is shown to possess innovative filtering and polarization
properties induced by the vortextype nature of the structured media. The
related homogenised effective behavior is obtained analytically and it has been
implemented to build a shielding cloak around an obstacle. Analytical work is
accompanied by numerical illustrations.Comment: 24 pages, 13 figure
Contraction of broken symmetries via Kac-Moody formalism
I investigate contractions via Kac-Moody formalism. In particular, I show how
the symmetry algebra of the standard 2-D Kepler system, which was identified by
Daboul and Slodowy as an infinite-dimensional Kac-Moody loop algebra, and was
denoted by , gets reduced by the symmetry breaking term,
defined by the Hamiltonian For this I
define two symmetry loop algebras , by
choosing the `basic generators' differently. These
can be mapped isomorphically onto subalgebras of , of
codimension 2 or 3, revealing the reduction of symmetry. Both factor algebras
, relative to the corresponding
energy-dependent ideals , are isomorphic to
and for , respectively, just as for the
pure Kepler case. However, they yield two different non-standard contractions
as , namely to the Heisenberg-Weyl algebra or to an abelian Lie algebra, instead of the Euclidean algebra
for the pure Kepler case. The above example suggests a
general procedure for defining generalized contractions, and also illustrates
the {\em `deformation contraction hysteresis'}, where contraction which involve
two contraction parameters can yield different contracted algebras, if the
limits are carried out in different order.Comment: 21 pages, 1 figur
Galactic Spiral Structure
We describe the structure and composition of six major stellar streams in a
population of 20 574 local stars in the New Hipparcos Reduction with known
radial velocities. We find that, once fast moving stars are excluded, almost
all stars belong to one of these streams. The results of our investigation have
lead us to re-examine the hydrogen maps of the Milky Way, from which we
identify the possibility of a symmetric two-armed spiral with half the
conventionally accepted pitch angle. We describe a model of spiral arm motions
which matches the observed velocities and composition of the six major streams,
as well as the observed velocities of the Hyades and Praesepe clusters at the
extreme of the Hyades stream. We model stellar orbits as perturbed ellipses
aligned at a focus in coordinates rotating at the rate of precession of
apocentre. Stars join a spiral arm just before apocentre, follow the arm for
more than half an orbit, and leave the arm soon after pericentre. Spiral
pattern speed equals the mean rate of precession of apocentre. Spiral arms are
shown to be stable configurations of stellar orbits, up to the formation of a
bar and/or ring. Pitch angle is directly related to the distribution of orbital
eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve
to form bars and rings. We show that orbits of gas clouds are stable only in
bisymmetric spirals. We conclude that spiral galaxies evolve toward grand
design two-armed spirals. We infer from the velocity distributions that the
Milky Way evolved into this form about 9 Gyrs ago.Comment: Published in Proc Roy Soc A. A high resolution version of this file
can be downloaded from http://papers.rqgravity.net/SpiralStructure.pdf. A
simplified account with animations begins at
http://rqgravity.net/SpiralStructur
Mixing by Swimming Algae
In this fluid dynamics video, we demonstrate the microscale mixing
enhancement of passive tracer particles in suspensions of swimming microalgae,
Chlamydomonas reinhardtii. These biflagellated, single-celled eukaryotes (10
micron diameter) swim with a "breaststroke" pulling motion of their flagella at
speeds of about 100 microns/s and exhibit heterogeneous trajectory shapes.
Fluorescent tracer particles (2 micron diameter) allowed us to quantify the
enhanced mixing caused by the swimmers, which is relevant to suspension feeding
and biogenic mixing. Without swimmers present, tracer particles diffuse slowly
due solely to Brownian motion. As the swimmer concentration is increased, the
probability density functions (PDFs) of tracer displacements develop strong
exponential tails, and the Gaussian core broadens. High-speed imaging (500 Hz)
of tracer-swimmer interactions demonstrates the importance of flagellar beating
in creating oscillatory flows that exceed Brownian motion out to about 5 cell
radii from the swimmers. Finally, we also show evidence of possible cooperative
motion and synchronization between swimming algal cells.Comment: 1 page, APS-DFD 2009 Gallery of Fluid Motio
The determination of the apsidal angles and Bertrand's theorem
We derive an expression for the determination of the apsidal angles that
holds good for arbitrary central potentials. Then we discuss under what
conditions the apsidal angles remain independent of the mechanical energy and
angular momentum in the central force problem. As a consequence, an alternative
and non-perturbative proof of Bertrand's theorem is obtained.Comment: Latex file, one figure; submitted for publicatio
- …