179 research outputs found
Surface extrapolation length and director structures in confined nematics
We report the results of Monte Carlo simulations of the Lebwohl--Lasher model
of nematic liquid crystals confined to cylindrical cavities with homeotropic
anchoring. We show that the ratio of the bulk to surface couplings is not in
general equal to the corresponding parameter K/W used in elastic theory (where
K is the Frank elastic constant in the one constant approximation and W is the
surface anchoring strength). By measuring the temperature dependence of K/W
(which is equivalent to the surface extrapolation length) we are able to
reconcile the results of our simulations as well as others with the predictions
of elastic theory. We find that the rate at which we cool the system from the
isotropic to nematic phase plays a crucial role in the development of the final
director structure, because of a large free energy barrier separating different
director structures as well as the temperature dependence of . With a
suitably fast cooling rate we are able to keep the system out of a metastable
planar state and form an escaped radial structure for large enough systems.Comment: 6 pages, 5 figure
Piezoelectricity of Cholesteric Elastomers
We consider theoretically the properties of piezoelectricity in cholesteric
elastomers. We deduce using symmetry considerations the piezoelectric
contributions to the free energy in the context of a coarse-grained description
of the material. In contrast to previous work we find that compressions or
elongations of the material along the pitch axis do not produce a piezoelectric
response, in agreement with fundamental symmetry considerations. Rather only
suitable shear strains or local rotations produce a polarization. We propose
some molecular mechanisms to explain these effects.Comment: 11 pages, 1 Postscript figure; Late
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