159 research outputs found
Isotropic-Nematic Transition in Liquid-Crystalline Elastomers
In liquid-crystalline elastomers, the nematic order parameter and the induced
strain vary smoothly across the isotropic-nematic transition, without the
expected first-order discontinuity. To investigate this smooth variation, we
measure the strain as a function of temperature over a range of applied stress,
for elastomers crosslinked in the nematic and isotropic phases, and analyze the
results using a variation on Landau theory. This analysis shows that the smooth
variation arises from quenched disorder in the elastomer, combined with the
effects of applied stress and internal stress.Comment: 4 pages, including 4 postscript figures, uses REVTeX
Magnetic moment of welded HTS samples: dependence on the current flowing through the welds
We present a method to calculate the magnetic moments of the high-temperature
superconducting (HTS) samples which consist of a few welded HTS parts. The
approach is generalized for the samples of various geometrical shapes and an
arbitrary number of welds. The obtained relations between the sample moment and
the density of critical current, which flows through the welds, allow to use
the magnetization loops for a quantitative characterization of the weld quality
in a wide range of temperatures and/or magnetic fields.Comment: RevTeX4, 4 pages, 2 figures. Submitted to Supercond. Sci. Techno
Imaging local diffusion in microstructures using NV-based pulsed field gradient NMR
Understanding diffusion in microstructures plays a crucial role in many
scientific fields, including neuroscience, cancer or energy research. While
magnetic resonance (MR) methods are the gold standard for diffusion
measurements, spatial encoding in MR imaging has limitations. Here, we
introduce nitrogen-vacancy (NV) center based nuclear magnetic resonance (NMR)
spectroscopy as a powerful tool to probe diffusion with an optical readouts. We
have developed an experimental scheme combining pulsed gradient spin echo
(PGSE) with optically detected NV-NMR spectroscopy, which allows for the local
quantification of molecular diffusion and flow within microscopic sample
volumes. We demonstrate correlated optical imaging with spatially resolved PGSE
NV-NMR experiments probing anisotropic water diffusion within a model
microstructure. Our optically detected PGSE NV-NMR technique opens up prospects
for extending the current capabilities of investigating diffusion processes
with the future potential of probing single cells, tissue microstructures, or
ion mobility in thin film materials for battery applications.Comment: 37 pages, 5 figures, 2 table
FingerTac - A Wearable Tactile Thimble for Mobile Haptic Augmented Reality Applications
ingerTac is a novel concept for a wearable augmented haptic thimble. It makes use of the limited spatial discrimination capabilities of vibrotactile stimuli at the skin and generates tactile feedback perceived at the bottom center of a fingertip by applying simultaneous vibrations at both sides of the finger. Since the bottom of the finger is thus kept free of obstruction, the device is well promising for augmented haptic applications, where real world interactions need to be enriched or amalgamated with virtual tactile feedback. To minimize its lateral dimension, the vibration actuators are placed on top of the device, and mechanical links transmit the vibrations to the skin. Two evaluation studies with N=10 participants investigate (i) the loss of vibration intensity through these mechanical links, and (ii) the effect of lateral displacement between stimulus and induced vibration. The results of both studies support the introduced concept of the FingerTac
Expanded sampling of New Zealand glass sponges (Porifera: Hexactinellida) provides new insights into biodiversity, chemodiversity, and phylogeny of the class
Glass sponges (Hexactinellida) constitute important parts of ecosystems on the deep-sea floor worldwide. However, they are still an understudied group in terms of their diversity and systematics. Here, we report on new specimens collected during RV Sonne expedition SO254 to the New Zealand region, which has recently emerged as a biodiversity hotspot for hexactinellids. Examination of the material revealed several species new to science or so far unknown from this area. While formal taxonomic descriptions of a fraction of these were published earlier, we here briefly report on the morphology of the remaining new species and use the collection to greatly expand the molecular phylogeny of the group as established with ribosomal DNA and cytochrome oxidase subunit I markers. In addition, we provide a chemical fingerprinting analysis on a subset of the specimens to investigate if the metabolome of glass sponges contains phylogenetic signal that could be used to supplement morphological and DNA-based approaches
Soft and non-soft structural transitions in disordered nematic networks
Properties of disordered nematic elastomers and gels are theoretically
investigated with emphasis on the roles of non-local elastic interactions and
crosslinking conditions. Networks originally crosslinked in the isotropic phase
lose their long-range orientational order by the action of quenched random
stresses, which we incorporate into the affine-deformation model of nematic
rubber elasticity. We present a detailed picture of mechanical quasi-Goldstone
modes, which accounts for an almost completely soft polydomain-monodomain (P-M)
transition under strain as well as a ``four-leaf clover'' pattern in
depolarized light scattering intensity. Dynamical relaxation of the domain
structure is studied using a simple model. The peak wavenumber of the structure
factor obeys a power-law-type slow kinetics and goes to zero in true mechanical
equilibrium. The effect of quenched disorder on director fluctuation in the
monodomain state is analyzed. The random frozen contribution to the fluctuation
amplitude dominates the thermal one, at long wavelengths and near the P-M
transition threshold. We also study networks obtained by crosslinking
polydomain nematic polymer melts. The memory of initial director configuration
acts as correlated and strong quenched disorder, which renders the P-M
transition non-soft. The spatial distribution of the elastic free energy is
strongly dehomogenized by external strain, in contrast to the case of
isotropically crosslinked networks.Comment: 19 pages, 15 EPS figure
On a linear programming approach to the discrete Willmore boundary value problem and generalizations
We consider the problem of finding (possibly non connected) discrete surfaces
spanning a finite set of discrete boundary curves in the three-dimensional
space and minimizing (globally) a discrete energy involving mean curvature.
Although we consider a fairly general class of energies, our main focus is on
the Willmore energy, i.e. the total squared mean curvature Our purpose is to
address the delicate task of approximating global minimizers of the energy
under boundary constraints.
The main contribution of this work is to translate the nonlinear boundary
value problem into an integer linear program, using a natural formulation
involving pairs of elementary triangles chosen in a pre-specified dictionary
and allowing self-intersection.
Our work focuses essentially on the connection between the integer linear
program and its relaxation. We prove that: - One cannot guarantee the total
unimodularity of the constraint matrix, which is a sufficient condition for the
global solution of the relaxed linear program to be always integral, and
therefore to be a solution of the integer program as well; - Furthermore, there
are actually experimental evidences that, in some cases, solving the relaxed
problem yields a fractional solution. Due to the very specific structure of the
constraint matrix here, we strongly believe that it should be possible in the
future to design ad-hoc integer solvers that yield high-definition
approximations to solutions of several boundary value problems involving mean
curvature, in particular the Willmore boundary value problem
Trueness of CAD/CAM digitization with a desktop scanner – an in vitro study
Desktop scanners are devices for digitization of conventional impressions or gypsum casts by indirect Computer-Aided Design/Computer-Assisted Manufacturing (CAD/CAM) in dentistry. The purpose of this in vitro study was: 1, to investigate whether virtual models produced by the extraoral scanner have the same trueness as sectioned casts; and 2, to assess if digitization with an extraoral scanner influences the surface information
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