8,315 research outputs found
Chiral Lyotropic Liquid Crystals: TGB Phases and Helicoidal Structures
The molecules in lyotropic membranes are typically aligned with the surface
normal. When these molecules are chiral, there is a tendency for the molecular
direction to twist. These competing effects can reach a compromise by producing
helicoidal defects in the membranes. Unlike thermotropic smectics, the centers
of these defects are hollow and thus their energy cost comes from the line
energy of an exposed lamellar surface. We describe both the
twist-grain-boundary phase of chiral lamellar phases as well as the isolated
helicoidal defects.Comment: 10 pages, plain TeX, two included figures, revision corrects figures
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Twisted Line Liquids
We propose a model of directed lines where the average direction has the
nature of a cholesteric liquid crystal. This model, for instance, would
describe the liquid of screw dislocations in the twist-grain-boundary (TGB)
phase of liquid crystals. We show that the presence of lines does not alter the
long wavelength elasticity of a cholesteric and, therefore, does not stabilize
Landau-Peierls instability of the cholesteric phase. We discuss other possible
mechanisms for stabilizing the twist-grain-boundary phase.Comment: 10 pages, tex file (macros included), IASSNS-HEP-93/2
Discretizing Gravity in Warped Spacetime
We investigate the discretized version of the compact Randall-Sundrum model.
By studying the mass eigenstates of the lattice theory, we demonstrate that for
warped space, unlike for flat space, the strong coupling scale does not depend
on the IR scale and lattice size. However, strong coupling does prevent us from
taking the continuum limit of the lattice theory. Nonetheless, the lattice
theory works in the manifestly holographic regime and successfully reproduces
the most significant features of the warped theory. It is even in some respects
better than the KK theory, which must be carefully regulated to obtain the
correct physical results. Because it is easier to construct lattice theories
than to find exact solutions to GR, we expect lattice gravity to be a useful
tool for exploring field theory in curved space.Comment: 17 pages, 4 figures; references adde
Thermal storage experience at the MSSTF and plans for the future
The background of thermal storage development at the Midtemperature Solar Systems Test Facility is reviewed. The problems which were encountered are discussed and a course of action for resolving the problems is outlined. Scaling effects of going from laboratory models to full-size applications were determined and applied to thermal storage needs in near-term solar projects
Cosmology and Hierarchy in Stabilized Warped Brane Models
We examine the cosmology and hierarchy of scales in models with branes
immersed in a five-dimensional curved spacetime subject to radion
stabilization. When the radion field is time-independent and the inter-brane
spacing is stabilized, the universe can naturally find itself in the
radiation-dominated epoch. This feature is independent of the form of the
stabilizing potential. We recover the standard Friedmann equations without
assuming a specific form for the bulk energy-momentum tensor. In the models
considered, if the observable brane has positive tension, a solution to the
hierarchy problem requires the presence of a negative tension brane somewhere
in the bulk. We find that the string scale can be as low as the electroweak
scale. In the situation of self-tuning branes where the bulk cosmological
constant is set to zero, the brane tensions have hierarchical values. In the
case of a polynomial stabilizing potential no new hierarchy is created.Comment: Version to appear in PL
Elasticity Theory of a Twisted Stack of Plates
We present an elastic model of B-form DNA as a stack of thin, rigid plates or
base pairs that are not permitted to deform. The symmetry of DNA and the
constraint of plate rigidity limit the number of bulk elastic constants
contributing to a macroscopic elasticity theory of DNA to four. We derive an
effective twist-stretch energy in terms of the macroscopic stretch epsilon
along and relative excess twist sigma about the DNA molecular axis. In addition
to the bulk stretch and twist moduli found previously, we obtain a
twist-stretch modulus with the following remarkable properties: 1) it vanishes
when the radius of the helical curve following the geometric center of each
plate is zero, 2) it vanishes with the elastic constant K_{23} that couples
compression normal to the plates to a shear strain, if the plates are
perpendicular to the molecular axis, and 3) it is nonzero if the plates are
tilted relative to the molecular axis. This implies that a laminated helical
structure carved out of an isotropic elastic medium will not twist in response
to a stretching force, but an isotropic material will twist if it is bent into
the shape of a helix.Comment: 19 pages, plain LaTeX, 1 included eps figur
Chirality in Liquid Crystals: from Microscopic Origins to Macroscopic Structure
Molecular chirality leads to a wonderful variety of equilibrium structures,
from the simple cholesteric phase to the twist-grain-boundary phases, and it is
responsible for interesting and technologically important materials like
ferroelectric liquid crystals. This paper will review some recent advances in
our understanding of the connection between the chiral geometry of individual
molecules and the important phenomenological parameters that determine
macroscopic chiral structure. It will then consider chiral structure in
columnar systems and propose a new equilibrium phase consisting of a regular
lattice of twisted ropes.Comment: 20 pages with 6 epsf figure
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