Biomedical subjects

J E Maclennan

Publications and source records attributed to J E Maclennan.

9 recordsLinked to original sources

PubMed2006

Electric-field-induced chirality flipping in smectic liquid crystals: the role of anisotropic viscosity.

We demonstrate the homogeneous and permanent reversal of the chirality of a condensed phase by an applied electric field. Tilted chiral smectic layers exhibit a coupled polarization density and molecular orientation fields which reorient about the layer normal as couple of fixed handedness in response to small applied electric fields. Experiments on some bent-core smectics show that above a threshold field the induced rotation can occur instead about the molecular long axis and that, as a result, the handedness of the phase can be flipped. The effect is quantitatively described by a nonequilibrium dissipative model of chiral smectic dynamics with anisotropic rotational viscosities.

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PubMed2005

Giant-block twist grain boundary smectic phases.

Study of a diverse set of chiral smectic materials, each of which has twist grain boundary (TGB) phases over a broad temperature range and exhibits grid patterns in the Grandjean textures of the TGB helix, shows that these features arise from a common structure: "giant" smectic blocks of planar layers of thickness l(b) > 200 nm terminated by GBs that are sharp, mediating large angular jumps in layer orientation between blocks (60 degrees < Delta < 90 degrees ), and lubricating the thermal contraction of the smectic layers within the blocks. This phenomenology is well described by basic theoretical models applicable in the limit that the ratio of molecular tilt penetration length-to-layer coherence length is large, and featuring GBs in which smectic ordering is weak, approaching thin, melted (nematic-like) walls. In this limit the energy cost of change of the block size is small, leading to a wide variation of block dimension, depending on preparation conditions. The models also account for the temperature dependence of the TGB helix pitch.

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PubMed2003

Control of molecular orientation in electrostatically stabilized ferroelectric liquid crystals.

The continuously reorientable (XY-like) ferroelectric polarization density of a chiral smectic liquid crystal is shown experimentally to produce nearly complete screening of the applied electric field in an appropriate cell geometry. This screening, combined with the expulsion of polarization charge for large polarization materials, is shown to produce electrostatic control of the orientation of a uniform optic axis or polarization field.

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PubMed2003

Polarization-modulated smectic liquid crystal phases.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

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PubMed2001

Biaxial model of the surface anchoring of bent-core smectic liquid crystals.

In synclinically tilted smectic phases, bent-core liquid crystal molecules aligned with the director in the plane of a cell boundary will, in general, have their molecular (bow) planes parallel to the boundary, normal to it, or at a well-defined intermediate orientation. A model describing the interaction of such bent-core (banana-shaped) molecules with planar surfaces that distinguishes energetically between molecules lying flat on the surface and those oriented edge on is given by a biaxial modification of the uniaxial surface anchoring expression used for chiral smectics of rod-shaped molecules. When combined with a field-induced straightening of the smectic layers, the model provides a mechanism for the transition from an analog to a bistable director response observed electro-optically in the ferroelectric banana-shaped material (R,S)-MHOBOW.

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PubMed2001

Unusual thickness-dependent thermal behavior and anticlinic coupling in chiral smectic free-standing liquid-crystal films.

We observe, in free-standing films of a chiral smectic liquid crystal, a series of discrete transitions in the relative orientation of the tilt of the interior and surface layers. These transitions include a remarkable reentrant synclinic-anticlinic-synclinic ordering sequence of the film surfaces in the presence of an electric field upon cooling. The profiles of the associated heat-capacity anomalies are found to be strongly thickness dependent and exhibit a novel crossover behavior in reduced dimensions. We measure the anticlinic coupling between tilted surface layers in the smectic- A phase.

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PubMed2001

Electro-optic behavior of liquid-crystal-filled silica opal photonic crystals: effect of liquid-crystal alignment.

Photonic crystals made of nematic liquid crystal intercalated into the void space of close-packed silica spheres (synthetic porous opal) exhibit significant electric-field-induced shift of the optical Bragg reflection peak when the liquid crystal has the long molecular axis oriented parallel to the sphere surfaces. No such effect is observed for comparable fields when the long-axis orientation is normal to the sphere surfaces.

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PubMed2001

Influence of ions on the "V-shaped" electro-optic response of ferroelectric liquid crystals.

It has recently been shown that in surface-stabilized ferroelectric liquid crystal cells with high spontaneous polarization, the polarization charge self-interaction leads to a "V-shaped" optical response to an applied voltage. The presence of ionic free charges in the liquid crystal changes the internal electric field, and therefore also the spatial dependence of the polarization and optic axis orientation. We have numerically solved the nonlinear ion diffusion equation in the electric field due to external voltage, spontaneous polarization, and ions, and calculated the electro-optic response to a triangular applied voltage. When the period of the driving voltage is smaller than the zero-field diffusion time of the ions across the cell, and larger than the ion transit time in the applied field, an inverse hysteresis in the electro-optic response is obtained. At these time scales ions also cause characteristic changes in the shape of the electro-optic response.

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PubMed2000

Ring-pattern dynamics in smectic-C* and smectic-C*A freely suspended liquid crystal films.

Ring patterns of concentric 2pi solitons in molecular orientation form in freely suspended chiral smectic-C films in response to an in-plane rotating electric field. We present measurements of the driven dynamics of ring formation under conditions of synchronous winding and of the zero-field relaxation of ring patterns, and propose a simple model which enables their quantitative description in low polarization DOBAMBC. In smectic-C*A TFMHPOBC we observe an odd-even layer number effect, with odd layer number films exhibiting order of magnitude slower relaxation rates than even layer films. We show that this rate difference is due to a much larger spontaneous polarization in odd layer number films.

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