Cooperative chiral order in the B-Z transition in random sequences of DNA

We present a theory for cooperative chiral order in the transition between right-handed B-DNA and left-handed Z-DNA. This theory, based on the random-field Ising model, predicts the characteristic length scale of Z-DNA segments. This length scale depends on whether the DNA is a homopolymer or a random sequence: it is approximately 4000 nucleotides in a homopolymer but only approximately 25 nucleotides in a random sequence. These theoretical results are consistent with experiments on DNA homopolymers and random sequences

Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid Crystals Under an Electric Field

Chiral liquid crystals often exhibit periodic modulations in the molecular director; in particular, thin films of the smectic-C* phase show a chiral striped texture. Here, we investigate whether similar chiral modulations can occur in the induced molecular tilt of the smectic-A phase under an applied electric field. Using both continuum elastic theory and lattice simulations, we find that the state of uniform induced tilt can become unstable when the system approaches the smectic-A--smectic-C* transition, or when a high electric field is applied. Beyond that instability point, the system develops chiral stripes in the tilt, which induce corresponding ripples in the smectic layers. The modulation persists up to an upper critical electric field and then disappears. Furthermore, even in the uniform state, the system shows chiral fluctuations, including both incipient chiral stripes and localized chiral vortices. We compare these predictions with observed chiral modulations and fluctuations in smectic-A liquid crystals.Comment: 11 pages, including 9 postscript figures, uses REVTeX 3.0 and epsf.st

The electroclinic effect and modulated phases in smectic liquid crystals

We explore the possibility that the large electroclinic effect observed in ferroelectric liquid crystals arises from the presence of an ordered array of disclination lines and walls. If the spacing of these defects is in the subvisible range, this modulated phase would be similar macroscopically to a smectic A phase. The application of an electric field distorts the array, producing a large polarization, and hence a large electroclinic effect. We show that with suitable elastic parameters and sufficiently large chirality, the modulated phase is favored over the smectic A and helically twisted smectic C* phases. We propose various experimental tests of this scenario.Comment: 9 pages, 7 figures; new version includes dipolar interactions and bend-twist couplin

Dynamics of orientational ordering in fluid membranes

We study the dynamics of orientational phase ordering in fluid membranes. Through numerical simulation we find an unusually slow coarsening of topological texture, which is limited by subdiffusive propagation of membrane curvature. The growth of the orientational correlation length $\xi$ obeys a power law $\xi \propto t^w$ with $w < 1/4$ in the late stage. We also discuss defect profiles and correlation patterns in terms of long-range interaction mediated by curvature elasticity.Comment: 5 pages, 3 figures (1 in color); Eq.(9) correcte

Two-Component Fluid Membranes Near Repulsive Walls: Linearized Hydrodynamics of Equilibrium and Non-equilibrium States

We study the linearized hydrodynamics of a two-component fluid membrane near a repulsive wall, via a model which incorporates curvature- concentration coupling as well as hydrodynamic interactions. This model is a simplified version of a recently proposed one [J.-B. Manneville et al. Phys. Rev. E, 64, 021908 (2001)] for non-equilibrium force-centres embedded in fluid membranes, such as light-activated bacteriorhodopsin pumps incorporated in phospholipid (EPC) bilayers. The pump/membrane system is modeled as an impermeable, two-component bilayer fluid membrane in the presence of an ambient solvent, in which one component, representing active pumps, is described in terms of force dipoles displaced with respect to the bilayer midpoint. We first discuss the case in which such pumps are rendered inactive, computing the mode structure in the bulk as well as the modification of hydrodynamic properties by the presence of a nearby wall. We then discuss the fluctuations and mode structure in steady state of active two-component membranes near a repulsive wall. We find that proximity to the wall smoothens membrane height fluctuations in the stable regime, resulting in a logarithmic scaling of the roughness even for initially tensionless membranes. This explicitly non-equilibrium result, a consequence of the incorporation of curvature-concentration coupling in our treatment, also indicates that earlier scaling arguments which obtained an increase in the roughness of active membranes near repulsive walls may need to be reevaluated.Comment: 39 page Latex file, 3 encapsulated Postscript figure

Spherical surface models with directors

A triangulated spherical surface model is numerically studied, and it is shown that the model undergoes phase transitions between the smooth phase and the collapsed phase. The model is defined by using a director field, which is assumed to have an interaction with a normal of the surface. The interaction between the directors and the surface maintains the surface shape. The director field is not defined within the two-dimensional differential geometry, and this is in sharp contrast to the conventional surface models, where the surface shape is maintained only by the curvature energies. We also show that the interaction makes the Nambu-Goto model well-defined, where the bond potential is given by the area of triangles; the Nambu-Goto model is well-known as an ill-defined one even when the conventional two-dimensional bending energy is included in the Hamiltonian.Comment: 18 pages, 13 figure

Dynamics of the diluted Ising Antiferromagnet Fe0.46_{0.46}Zn0.54_{0.54}F2_2 in the (H,T)(H,T) plane

URL: http://www-spht.cea.fr/articles/s93/035International audienceWe present a detailed study of the dynamics of the diluted Ising antiferromagnet ${\rm Fe}_{46\%} {\rm Zn}_{54\%} {\rm F}_2,$ a test model for the random field Ising model (RFIM), over a wide range of temperatures and magnetic fields below the equilibrium transition temperature $T_c(H).$ Using a novel high field SQUID magnetometer, we measure the macroscopic magnetization $M$ through a variety of cooling and field cycling protocols. We find a dynamic behavior, at high fields below the transition temperature, considerably richer than expected in both magnetization-relaxation and field-cycling experiments. At very low temperatures the dynamics is frozen due to the Ising excitation gap. Our results are summarized in a dynamical phase diagram

Accordion-like actuators of multiple 3D patterned liquid crystal polymer films

This work describes the fabrication, characterization, and modelling of liquid crystalline polymer network films with a multiple patterned 3D nematic director profile, a stimuli-responsive material that exhibits complex mechanical actuation under change of temperature or pH. These films have a discrete alternating striped or checkerboard director profile in the plane, and a 90-degree twist through the depth of the film. When actuated via heating, the striped films deform into accordion-like folds, while the film patterned with a checkerboard microstructure buckles out-of-plane. Furthermore, striped films are fabricated so that they also deform into an accordion shaped fold, by a change of pH in an aqueous environment. Three-dimensional finite element simulations and elasticity analysis provide insight into the dependence of shape evolution on director microstructure and the sample's aspect ratio