147 research outputs found
Effective behavior of nematic elastomer membranes
We derive the effective energy density of thin membranes of liquid crystal
elastomers as the Gamma-limit of a widely used bulk model. These membranes can
display fine-scale features both due to wrinkling that one expects in thin
elastic membranes and due to oscillations in the nematic director that one
expects in liquid crystal elastomers. We provide an explicit characterization
of the effective energy density of membranes and the effective state of stress
as a function of the planar deformation gradient. We also provide a
characterization of the fine-scale features. We show the existence of four
regimes: one where wrinkling and microstructure reduces the effective membrane
energy and stress to zero, a second where wrinkling leads to uniaxial tension,
a third where nematic oscillations lead to equi-biaxial tension and a fourth
with no fine scale features and biaxial tension. Importantly, we find a region
where one has shear strain but no shear stress and all the fine-scale features
are in-plane with no wrinkling
Programming complex shapes in thin nematic elastomer and glass sheets
Nematic elastomers and glasses are solids that display spontaneous distortion
under external stimuli. Recent advances in the synthesis of sheets with
controlled heterogeneities have enabled their actuation into non-trivial shapes
with unprecedented energy density. Thus, these have emerged as powerful
candidates for soft actuators. To further this potential, we introduce the key
metric constraint which governs shape changing actuation in these sheets. We
then highlight the richness of shapes amenable to this constraint through two
broad classes of examples which we term nonisometric origami and lifted
surfaces. Finally, we comment on the derivation of the metric constraint, which
arises from energy minimization in the interplay of stretching, bending and
heterogeneity in these sheets
Chandra observation of the Galactic supernova remnant CTB 109 (G109.1-1.0)
Context: We study the X-ray emission of the Galactic supernova remnant (SNR)
CTB 109 (G109.1-1.0), which is well-known for its enigmatic half-shell
morphology both in radio and in X-rays and is associated with the anomalous
X-ray pulsar (AXP) 1E2259+586. Aims: We want to understand the origin of the
X-ray bright feature inside the SNR called the Lobe and the details of the
interaction of the SNR shock wave with the ambient interstellar medium (ISM).
Methods: The Lobe and the northeastern part of the SNR were observed with
Chandra ACIS-I. We analysed the spectrum of the X-ray emission by dividing the
entire observed emission into small regions. The X-ray emission is best
reproduced with one-component or two-component non-equilibrium ionisation
models depending on the position. In the two-component model one emission
component represents the shocked ISM and the other the shocked ejecta. Results:
We detect enhanced element abundances, in particular for Si and Fe, in and
around the Lobe. There is one particular region next to the Lobe with a high Si
abundance of 3.3 (2.6 - 4.0) times the solar value. This is the first,
unequivocal detection of ejecta in CTB 109. Conclusions: The new Chandra data
confirm that the Lobe was created by the interaction of the SNR shock and the
supernova ejecta with dense and inhomogeneous medium in the environment of SNR
CTB 109. The newly calculated age of the SNR is t ~ 1.4 x 10^4 yr.Comment: Accepted for publication in A&A. 9 pages, 10 figure
Patterning nonisometric origami in nematic elastomer sheets
Nematic elastomers dramatically change their shape in response to diverse
stimuli including light and heat. In this paper, we provide a systematic
framework for the design of complex three dimensional shapes through the
actuation of heterogeneously patterned nematic elastomer sheets. These sheets
are composed of \textit{nonisometric origami} building blocks which, when
appropriately linked together, can actuate into a diverse array of three
dimensional faceted shapes. We demonstrate both theoretically and
experimentally that: 1) the nonisometric origami building blocks actuate in the
predicted manner, 2) the integration of multiple building blocks leads to
complex multi-stable, yet predictable, shapes, 3) we can bias the actuation
experimentally to obtain a desired complex shape amongst the multi-stable
shapes. We then show that this experimentally realized functionality enables a
rich possible design landscape for actuation using nematic elastomers. We
highlight this landscape through theoretical examples, which utilize large
arrays of these building blocks to realize a desired three dimensional origami
shape. In combination, these results amount to an engineering design principle,
which we hope will provide a template for the application of nematic elastomers
to emerging technologies
Actuation of thin nematic elastomer sheets with controlled heterogeneity
Nematic elastomers and glasses deform spontaneously when subjected to temperature changes. This property can be exploited in the design of heterogeneously patterned thin sheets that deform into a non-trivial shape when heated or cooled. In this paper, we start from a variational formulation for the entropic elastic energy of liquid crystal elastomers and we derive an effective two-dimensional metric constraint, which links the deformation and the heterogeneous director field. Our main results show that satisfying the metric constraint is both necessary and sufficient for the deformation to be an approximate minimizer of the energy. We include several examples which show that the class of deformations satisfying the metric constraint is quite rich
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