32,049 research outputs found
Pattern Formation from Defect Chaos --- A Theory of Chevrons
For over 25 years it is known that the roll structure of electroconvection
(EC) in the dielectric regime in planarly aligned nematic liquid crystals has,
after a transition to defect chaos, the tendency to form chevron structures. We
show, with the help of a coarse-grained model, that this effect can generally
be expected for systems with spontaneously broken isotropy, that is lifted by a
small external perturbation. The linearized model as well as a nonlinear
extension are compared to simulations of a system of coupled amplitude
equations which generate chevrons out of defect chaos. The mechanism of chevron
formation is similar to the development of Turing patterns in reaction
diffusion systems.Comment: 17 pages, Latex, 11 PS-figures, submitted to Physica
Transmission wavefront shearing interferometry for photoelastic materials
A general analysis and experimental validation of transmission wavefront shearing interferometry for photoelastic materials are presented. These interferometers applied to optically isotropic materials produce a single interference pattern related to one phase term, but when applied to photoelastic materials, they produce the sum of two different interference patterns with phase terms that are the sum and difference, respectively, of two stress-related phase terms. The two stress-related phase terms may be separated using phase shifting and polarization optics. These concepts are experimentally demonstrated using coherent gradient sensing in full field for a compressed polycarbonate plate with a V-shaped notch with good agreement with theoretical data. The analysis may be applied to any wavefront shearing interferometer by modifying parameters describing the wavefront shearing distance
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