573 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
Phase Transitions in Lyotropic Nematic Gels
In this paper, we discuss the equilibrium phases and collapse transitions of
a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel
consists of a cross-linked polymer network with rod-like molecules embedded in
it. Upon decreasing the quality of the solvent, we find that a lyotropic
nematic gel undergoes a discontinuous volume change accompanied by an
isotropic-nematic transition. We also present phase diagrams that these systems
may exhibit. In particular, we show that coexistence of two isotropic phases,
of two nematic phases, or of an isotropic and a nematic phase can occur.Comment: 13 pages Revtex, 10 figures, submitted to EPJ
Connecting the vulcanization transition to percolation
The vulcanization transition is addressed via a minimal
replica-field-theoretic model. The appropriate long-wave-length behavior of the
two- and three-point vertex functions is considered diagrammatically, to all
orders in perturbation theory, and identified with the corresponding quantities
in the Houghton-Reeve-Wallace field-theoretic approach to the percolation
critical phenomenon. Hence, it is shown that percolation theory correctly
captures the critical phenomenology of the vulcanization transition associated
with the liquid and critical states.Comment: 9 pages, 5 figure
Coupled dynamics of RNA folding and nanopore translocation
The translocation of structured RNA or DNA molecules through narrow pores
necessitates the opening of all base pairs. Here, we study the interplay
between the dynamics of translocation and base-pairing theoretically, using
kinetic Monte Carlo simulations and analytical methods. We find that the
transient formation of basepairs that do not occur in the ground state can
significantly speed up translocation.Comment: 4 pages, 3 figures, to appear in Physical Review Letter
Effect of Salt Concentration on the Electrophoretic Speed of a Polyelectrolyte through a Nanopore
In a previous paper [S. Ghosal, Phys. Rev. E 74, 041901 (2006)] a
hydrodynamic model for determining the electrophoretic speed of a
polyelectrolyte through an axially symmetric slowly varying nanopore was
presented in the limit of a vanishingly small Debye length. Here the case of a
finite Debye layer thickness is considered while restricting the pore geometry
to that of a cylinder of length much larger than the diameter. Further, the
possibility of a uniform surface charge on the walls of the nanopore is taken
into account. It is thereby shown that the calculated transit times are
consistent with recent measurements in silicon nanopores.Comment: 4 pages, 2 figure
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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