285 research outputs found
Dynamics of smectic elastomers
We study the low-frequency, long-wavelength dynamics of liquid crystal
elastomers, crosslinked in the smectic- phase, in their smectic-, biaxial
smectic and smectic- phases. Two different yet related formulations are
employed. One formulation describes the pure hydrodynamics and does not
explicitly involve the Frank director, which relaxes to its local equilibrium
value in a non-hydrodynamic time. The other formulation explicitly treats the
director and applies beyond the hydrodynamic limit. We compare the
low-frequency, long-wavelength dynamics of smectic- elastomers to that of
nematics and show that the two are closely related. For the biaxial smectic and
the smectic- phases, we calculate sound velocities and the mode structure in
certain symmetry directions. For the smectic- elastomers, in addition, we
discuss in some detail their possible behavior in rheology experiments.Comment: 19 pages, 3 figure
Linear hydrodynamics and viscoelasticity of nematic elastomers
We develop a continuum theory of linear viscoelastic response in oriented
monodomain nematic elastomers. The expression for dissipation function is
analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we
propose the relations between the anisotropic rubber moduli and new viscous
coefficients. A new dimensionless number is introduced, which describes the
relative magnitude of viscous and rubber-elastic torques. In an elastic medium
with an independently mobile internal degree of freedom, the nematic director
with its own relaxation dynamics, the model shows a dramatic decrease in the
dynamic modulus in certain deformation geometries. The degree to which the
storage modulus does not altogether drop to zero is shown to be both dependent
on frequency and to be proportional to the semi-softness, the non-ideality of a
nematic network. We consider the most interesting geometry for the
implementation of the theory, calculating the dynamic response to an imposed
simple shear and making predictions for effective moduli and (exceptionally
high) loss factors.Comment: Latex 2e or PDFlatex (4 EPS or JPG figures) - to appear in
Euro.Phys.J.
Supersoft elasticity in polydomain nematic elastomers
We consider the equilibrium stress-strain behavior of polydomain liquid crystal elastomers (PLCEs). We show that there is a fundamental difference between PLCEs cross-linked in the high temperature isotropic and low temperature aligned states. PLCEs cross-linked in the isotropic state then cooled to an aligned state will exhibit extremely soft elasticity (confirmed by recent experiments) and ordered director patterns characteristic of textured deformations. PLCEs cross-linked in the aligned state will be mechanically much harder and characterized by disclination textures
Dynamics, dynamic soft elasticity and rheology of smectic-C elastomers
We present a theory for the low-frequency, long-wavelength dynamics of soft
smectic-C elastomers with locked-in smectic layers. Our theory, which goes
beyond pure hydrodynamics, predicts a dynamic soft elasticity of these
elastomers and allows us to calculate the storage and loss moduli relevant for
rheology experiments as well as the mode structure.Comment: 4 pages, 2 figure
Elasticity of Polydomain Liquid Crystal Elastomers
We model polydomain liquid-crystal elastomers by extending the neo-classical
soft and semi-soft free energies used successfully to describe monodomain
samples. We show that there is a significant difference between polydomains
cross-linked in homogeneous high symmetry states then cooled to low symmetry
polydomain states and those cross-linked directly in the low symmetry
polydomain state. For example, elastomers cross-linked in the isotropic state
then cooled to a nematic polydomain will, in the ideal limit, be perfectly
soft, and with the introduction of non-ideality, will deform at very low stress
until they are macroscopically aligned. The director patterns observed in them
will be disordered, characteristic of combinations of random deformations, and
not disclination patterns. We expect these samples to exhibit elasticity
significantly softer than monodomain samples. Polydomains cross-linked in the
nematic polydomain state will be mechanically harder and contain characteristic
schlieren director patterns. The models we use for polydomain elastomers are
spatially heterogeneous, so rather than solving them exactly we elucidate this
behavior by bounding the energies using Taylor-like (compatible test strain
fields) and Sachs (constant stress) limits extended to non-linear elasticity.
Good agreement is found with experiments that reveal the supersoft response of
some polydomains. We also analyze smectic polydomain elastomers and propose
that polydomain SmC* elastomers cross-linked in the SmA monodomain state are
promising candidates for low field electrical actuation.Comment: 13 pages, 11 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
Photomechanical coupling in photoactive nematic elastomers
Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration in 2001, it has received increasing interest with many recent studies of periodic and buckling behavior. However, theoretical models developed have focused on describing specific deformation modes (e.g., beam bending and uniaxial contraction) in the absence of mechanical loads, with only limited attention to the interplay between mechanical stress and light-induced deformation. This paper explores photomechanical coupling in a photoactive nematic elastomer under both light illumination and mechanical stress. We begin with a continuum framework built on the free energy developed by Corbett and Warner (Phys. Rev. Lett. 2006). Mechanical stress leads to nematic alignment parallel to a uniaxial tensile stress. In the absence of mechanical stress, in the photo-stationary state where the system reaches equilibrium, the nematic director tends to align perpendicular to the polarization of a linearly polarized light. However, sufficient illumination can destroy nematic order through a first-order nematic-isotropic phase transition which is accompanied by a snap through deformation. Combined illumination and mechanical stress can lead to an exchange of stability accompanied by stripe domains. Finally, the stress-intensity phase diagram shows a critical point that may be of interest for energy conversion
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