511 research outputs found
Stability of Texture and Shape of Circular Domains of Langmuir Monolayers
Finite domains of a Langmuir monolayer in a phase with tilted molecules can
be modeled by a simple elastic free energy of an XY order parameter with
isotropic and anisotropic line tension terms. The domains can and often do
contain nontrivial textures, which in turn influence the shape of the domains.
Herein we investigate the properties of a simplified isotropic model with a
single elastic constant. For circular domains a first-order phase transition is
found between two distinct textures: an exterior defect (or ``virtual boojum'')
texture, and an interior defect texture. Starting with a circular domain and
either of these two textures as a ground state, we find shape instabilities
develop that depend on the elastic constants and line tensions in the
simplified model. In both cases a necessary but not sufficient condition for
the onset of shape instabilities is the possibility for a local negative
effective line tension to develop from the anisotropic line tension term.Comment: 14 double column RevTex pages, 8 eps figure
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
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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Interactions between Membrane Inclusions on Fluctuating Membranes
We model membrane proteins as anisotropic objects characterized by
symmetric-traceless tensors and determine the coupling between these
order-parameters and membrane curvature. We consider the interactions between
transmembrane proteins that respect up-down (reflection) symmetry of bilayer
membranes and that have circular or non-circular cross-sectional areas in the
tangent-plane of membranes. Using a field theoretic approach, we find
non-entropic interactions between reflection-symmetry-breaking
transmembrane proteins with circular cross-sectional area and entropic
interactions between transmembrane proteins with circular
cross-section that do not break up-down symmetry in agreement with previous
calculations. We also find anisotropic interactions between
reflection-symmetry-conserving transmembrane proteins with non-circular
cross-section, anisotropic interactions between
reflection-symmetry-breaking transmembrane proteins with non-circular
cross-section, and non-entropic many-particle interactions among
non-transmembrane proteins. For large , these interactions might provide the
dominant force inducing aggregation of the membrane proteins.Comment: REVTEX, 29 pages with 4 postscript figures compressed using uufiles.
Introduction and Discussion sections revised. To appear in J. Phys. France I
(September
Measure Factors, Tension, and Correlations of Fluid Membranes
We study two geometrical factors needed for the correct construction of
statistical ensembles of surfaces. Such ensembles appear in the study of fluid
bilayer membranes, though our results are more generally applicable. The naive
functional measure over height fluctuations must be corrected by these factors
in order to give correct, self-consistent formulas for the free energy and
correlation functions of the height. While one of these corrections -- the
Faddeev-Popov determinant -- has been studied extensively, our derivation
proceeds from very simple geometrical ideas, which we hope removes some of its
mystery. The other factor is similar to the Liouville correction in string
theory. Since our formulas differ from those of previous authors, we include
some explicit calculations of the effective frame tension and two-point
function to show that our version indeed secures coordinate-invariance and
consistency to lowest nontrivial order in a temperature expansion.Comment: 24 pp; plain Te
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