13,746 research outputs found
Phase transitions of nematic rubbers
Single crystal nematic elastomers undergo a transition from a strongly
ordered phase N to an "isotropic" phase I. We show that: (a) samples produced
under tension by the Finkelmann procedure are intrinsically anisotropic and
should show a small (temperature dependent) birefringence in the high
temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal
limit but for the N->I transition via heating there is no soft mode at the
standard spinodal temperature. (c) the N->I transition is reminiscent of a
martensitic transformation: nucleation of the I phase should occur in the form
of platelets, making a well defined angle with the director.Comment: 7 pages, 3 figures (To appear in Europhys. Lett.
Comparing the performance of geometrically similar airplanes
This note has been prepared for the National Advisory Committee for Aeronautics. It deals with the model rules relating to aeronautical problems, and shows how the characteristics of one airplane can be determined from those of another airplane of different weight or size, and of similar type. If certain rules for the ratios of the dimensions, the weights and the horsepower are followed, a small low-powered airplane can be used for obtaining information as to performance, stability, controllability and maneuverability of a larger prototype, and contrariwise
Extended N=2 Superconformal Structure of Gravity and W-Gravity Coupled to Matter
We show that almost all string theories, including the bosonic string, the
superstring and -string theories, possess a twisted N=2 superconformal
symmetry. This enables us to establish a connection between topological gravity
and the field theoretical description of matter coupled to gravity. We also
show how the \brs operators of the -string can be obtained by hamiltonian
reduction of . The tachyonic and ground ring states of -strings
are described in the light of the N=2 superconformal structure, and the ground
ring generators for the non-critical -string are explicitly constructed.
The relationship to models and quantum integrable systems is also briefly
described.Comment: 48 page
Imprinted Networks as Chiral Pumps
We investigate the interaction between a chirally imprinted network and a
solvent of chiral molecules. We find, a liquid crystalline polymer network is
preferentially swollen by one component of a racemic solvent. This ability to
separate is linked to the chiral order parameter of the network, and can be
reversibly controlled via temperature or a mechanical deformation. It is
maximal near the point at which the network loses its imprinted structure. One
possible practical application of this effect would be a mechanical device for
sorting mixed chiral molecules.Comment: 4 pages, 5 figure
Photonic gaps in cholesteric elastomers under deformation
Cholesteric liquid crystal elastomers have interesting and potentially very
useful photonic properties. In an ideal monodomain configuration of these
materials, one finds a Bragg-reflection of light in a narrow wavelength range
and a particular circular polarization. This is due to the periodic structure
of the material along one dimension. In many practical cases, the cholesteric
rubber possesses a sufficient degree of quenched disorder, which makes the
selective reflection broadband. We investigate experimentally the problem of
how the transmittance of light is affected by mechanical deformation of the
elastomer, and the relation to changes in liquid crystalline structure. We
explore a series of samples which have been synthesized with photonic stop-gaps
across the visible range. This allows us to compare results with detailed
theoretical predictions regarding the evolution of stop-gaps in cholesteric
elastomers
Smectic-C tilt under shear in Smectic-A elastomers
Stenull and Lubensky [Phys. Rev. E {\bf 76}, 011706 (2007)] have argued that
shear strain and tilt of the director relative to the layer normal are coupled
in smectic elastomers and that the imposition of one necessarily leads to the
development of the other. This means, in particular, that a Smectic-A elastomer
subjected to a simple shear will develop Smectic-C-like tilt of the director.
Recently, Kramer and Finkelmann [arXiv:0708.2024, Phys. Rev. E {\bf 78}, 021704
(2008)] performed shear experiments on Smectic-A elastomers using two different
shear geometries. One of the experiments, which implements simple shear,
produces clear evidence for the development of Smectic-C-like tilt. Here, we
generalize a model for smectic elastomers introduced by Adams and Warner [Phys.
Rev. E {\bf 71}, 021708 (2005)] and use it to study the magnitude of
Smectic-C-like tilt under shear for the two geometries investigated by Kramer
and Finkelmann. Using reasonable estimates of model parameters, we estimate the
tilt angle for both geometries, and we compare our estimates to the
experimental results. The other shear geometry is problematic since it
introduces additional in-plane compressions in a sheet-like sample, thus
inducing instabilities that we discuss.Comment: 8 pages, 5 figure
Iteration Method to Derive Exact Rotation Curves from Position-Velocity Diagrams of Spiral Galaxies
We present an iteration method to derive exact rotation curves (RC) of spiral
galaxies from observed position-velocity diagrams (PVD), which comprises the
following procedure. An initial rotation curve, RC0, is adopted from an
observed PV diagram (PV0), obtained by any simple method such as the
peak-intensity method. Using this rotation curve and an observed radial
distribution of intensity (emissivity), we construct a simulated PV diagram
(PV1). The difference between a rotation curve obtained from this PV1 and the
original RC (e.g., difference between peak-intensity velocities) is used to
correct the initial RC to obtain a corrected rotation curve, RC1. This RC1 is
used to calculated another PVD (PV2) using the observed intensity distribution,
and to obtain the second iterated RC (RC2). This iteration is repeated until
PV converges to PV0, so that the differences between PV and PV0 becomes
minimum. Finally RC is adopted as the most reliable rotation curve. We apply
this method to some observed PVDs of nearby galaxies, and show that the
iteration successfully converges to give reliable rotation curves. We show that
the method is powerful to detect central massive objects.Comment: To appear in ApJ.Letters, 5 pages Latex with 4 figure
Radio and near-infrared observations of the steep spectrum Galactic plane radio source WKB 0314+57.8
Radio and near-infared observations towards the steep spectrum Galactic plane
radio source WKB 0314+57.8 are presented, in order to clarify the nature of
this source. The radio observations include archival and survey data, together
with new Giant Metrewave Radio Telescope observations at 617 MHz. The
near-infrared observations are in the J and K bands, from the Gemini instrument
on the Shane 3-m telescope. The radio observations show that WKB 0314+57.8 is
extended, with an very steep spectrum (with flux density proportional to
frequency to -2.5 power between 40 MHz and 1.5 GHz). The colour--magnitude
diagram constructed from near-infrared observations of the field suggests the
presence of a z approx 0.08 galaxy cluster behind the Galactic plane, reddened
by about 6 magnitudes of visual extinction. Although the steep spectrum source
has no obvious identification, two other radio sources in the field covered by
the near-infrared observations have tentative identifications with galaxies.
These observations indicate that WKB 0314+57.8 is a relic source in a cluster
of galaxies, not a pulsar.Comment: 6 pages, to appear in MNRAS, typos correcte
Untwisting of a cholesteric elastomer by a mechanical field
A mechanical strain field applied to a monodomain cholesteric elastomer will
unwind the helical director distribution. There is an analogy with the
classical problem of an electric field applied to a cholesteric liquid crystal,
but with important differences. Frank elasticity is of minor importance unless
the gel is very weak. The interplay is between director anchoring to the rubber
elastic matrix and the external mechanical field. Stretching perpendicular to
the helix axis induces the uniform unwound state via the elimination of sharp,
pinned twist walls above a critical strain. Unwinding through conical director
states occurs when the elastomer is stretched along the helical axis.Comment: 4 pages, RevTeX 3 style, 3 EPS figure
Dwarf Nova Oscillations and Quasi-Periodic Oscillations in Cataclysmic Variables - VIII. VW Hyi in outburst observed with the Southern African Large Telescope
We analyse four light curves obtained at high time resolution (~ 0.1 s) with
the 11-m Southern African Large Telescope, at the ends of two normal outbursts
and one superoutburst of the dwarf nova VW Hyi. All of these contain at least
some Dwarf Nova Oscillations (DNOs), which, when at their highest amplitudes,
are seen in unprecedented detail. In addition to the expected DNOs with periods
> 20 s we find a previously unknown modulation at 13.39 s, but none at shorter
periods. The various DNOs and their interaction with the longer period
Quasi-periodic Oscillations are interpreted in terms of the model of
magnetically controlled flow from an accretion disc proposed earlier in this
series of papers. Our observations include rare DNOs very late in outburst; we
find that the fundamental period does not increase beyond ~ 90 s, which is the
same value that the independent ``longer period DNOs'' converge on.Comment: Accepted for publication in MNRAS. 7 pages, 10 figures, 2 table
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