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N A Clark

Publications and source records attributed to N A Clark.

At least 19 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.

Journal Article↗
PubMed2006

Thermoregulatory observations in soccer match play: professional and recreational level applications using an intestinal pill system to measure core temperature.

BACKGROUND: Technological limitations associated with oesophageal and rectal temperature probes have previously limited match play observations of body temperature. OBJECTIVE: To investigate the application of an intestinal pill system to measure core temperature during dynamic, field based, soccer matches. METHODS: Core temperature (T(c)) was assessed using the intestinal pill system (CorTemp 2000) and changes in plasma volume were calculated using the values for haemoglobin and packed cell volume obtained before and after recreational (n = 8) and the professional soccer (n = 7) matches. RESULTS: In the recreational match, there were significant post-match reductions in body mass (-1.6%, p<0.05) and plasma volume (-7.2%, p<0.01). Significant increases were observed in T(c) from rest to half time (p<0.01) and from half time to full time (p<0.05). In the professional match, body mass decreased by 1.9% (p<0.05) and plasma volume by 11.6% (p<0.01). T(c) increased from rest to half time (p<0.01) but was unchanged from half time to full time. Significant associations were observed between maximum oxygen consumption and match play heart rates in the second half of each match but these were not related to changes in plasma volume or T(c). CONCLUSIONS: Intestinal temperature proved a useful method of recording core temperature during soccer match play and assisted in the measurement of alterations in thermoregulatory variables in response to both professional and recreational level soccer matches; however, technological limitations still restrict the wider application of this method, especially at a competitive level.

Adolescent↗
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.

Journal Article↗
PubMed2005

Layer-scale optical chirality of liquid-crystalline phases.

We present a model for the optical chirality of layered liquid-crystalline phases. The model demonstrates that uniform stacking of chiral layers can lead to significant collective optical rotation, even in the absence of a superlayer helix. We predict the optical rotation of the B2 phases of bent-core liquid crystals, which can have optical rotation as large as 1000 times the molecular optical activity.

Journal Article↗
PubMed2005

Pretransitional behavior of a water in liquid crystal microemulsion close to the demixing transition: evidence for intermicellar attraction mediated by paranematic fluctuations.

We present a study of a water-in-oil microemulsion in which surfactant coated water nanodroplets are dispersed in the isotropic phase of the thermotropic liquid-crystal penthyl-cyanobiphenyl (5CB). As the temperature is lowered below the isotropic to nematic phase transition of pure 5CB, the system displays a demixing transition leading to a coexistence of a droplet-rich isotropic phase with a droplet-poor nematic. The transition is anticipated, in the high T side, by increasing pretransitional fluctuations in 5CB molecular orientation and in the nanodroplet concentration. The observed phase behavior supports the notion that the nanosized droplets, while large enough for their statistical behavior to be probed via light scattering, are also small enough to act as impurities, disturbing the local orientational ordering of the liquid crystal and thus experiencing pretransitional attractive interaction mediated by paranematic fluctuations. The pretransitional behavior, together with the topology of the phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model which describes the nanodroplets simply as holes in the liquid crystal.

Biphenyl Compounds↗
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.

Journal Article↗
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.

Journal Article↗
PubMed2003

Fluctuation mediated interaction and phase separation of nanoparticles in a liquid crystal solvent.

Water-in-oil microemulsions of nanodroplets in the isotropic phase of a thermotropic liquid crystal exhibit, with decreasing temperature and in anticipation of a demixing transition, enhanced correlation in fluctuations of both molecular orientation and droplet concentration. Mean field modeling of this pretransition behavior, on the basis of a lattice in which the nanodroplets are introduced as holes, shows that the observed interdroplet attractive interaction is produced by the disordering effect of the droplets on the liquid crystal and mediated solely by paranematic fluctuations.

Journal Article↗
PubMed2003

Second-harmonic generation from rubbed ferroelectric liquid crystal mesogenic monolayer surfaces.

We have designed self-assembled monolayers of molecules containing a ferroelectric liquid crystal mesogen attached to a glass surface through an alkane chain. After mechanical rubbing these layers induce a single domain in a cell containing a high-polarization achiral liquid crystal in the smectic-C phase. We have used optical second-harmonic generation to demonstrate that this behavior is explained by rubbing-induced in-plane anisotropy of the angular distribution function that describes the ensemble of mesogenic units. The surface order parameter is 0.094, a substantial fraction of what has been observed for rubbed polymeric alignment layers.

Journal Article↗
PubMed2002

Liquid-crystal-solid interface structure at the antiferroelectric-ferroelectric phase transition.

Total internal reflection is used to probe the molecular organization at the surface of a tilted chiral smectic liquid crystal at temperatures in the vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are interpreted using an exact analytical solution of a real model for ferroelectric order at the surface. In the mixture T3, ferroelectric surface order is expelled with the bulk ferroelectric-antiferroelectric transition. The conditions for ferroelectric order at the surface of an antiferroelectric bulk are presented.

Journal Article↗
PubMed2001

Universality and scaling in the disordering of a smectic liquid crystal.

We present experimental and theoretical studies of the effects of quenched disorder on one-dimensional crystal ordering in three dimensions. This fragile smectic liquid crystal layering, the material with the simplest positional order, is also the most easily deformed periodic structure and is, therefore, profoundly affected by disorder, introduced here by confinement in silica aerogel. Theory and experiment combine to characterize this system to an extraordinary degree, their close accord producing a coherent picture: crystal ordering is lost, giving way to extended short-range correlations that exhibit universal structure and scaling, anomalous layer elasticity, and glassy dynamics.

Journal Article↗
PubMed2001

Evidence from infrared dichroism, x-ray diffraction, and atomistic computer simulation for a "zigzag" molecular shape in tilted smectic liquid crystal phases.

Infrared dichroism (IR) and atomistic computer simulation are employed to probe molecular shape in smectic liquid crystal phases where the optic axis is tilted relative to the layer normal. Polar plots of absorption profiles due to core (phenyl, C-C) and tail (alkyl or methylene, CH2) vibrations in the tilted synclinic (smectic-C) phase of a variety of materials show the phenyl (core) IR absorbance symmetry axes to be consistently tilted at larger angle from the layer normal than the alkyl or methylene (tail). This suggests that, on average, the tails are less tilted than the cores. Furthermore, we find that optic axis tilt angle is close to the core tilt angle measured by IR dichroism, as expected, since liquid crystal birefringence arises primarily from the cores. These results are in accord with the "zigzag" model of Bartolino, Doucet, and Durand. However, we find that only a small fraction of the tail, the part nearest the core, is tilted, and only this part contributes significantly to layer contraction upon molecular tilt.

Journal Article↗
PubMed2001

Microscopic structure and dynamics of a partial bilayer smectic liquid crystal.

Cyanobiphenyls (nCB's) represent a useful and intensively studied class of mesogens. Many of the peculiar properties of nCB's (e.g., the occurrence of the partial bilayer smectic-A(d) phase) are thought to be a manifestation of short-range antiparallel association of neighboring molecules, resulting from strong dipole-dipole interactions between cyano groups. To test and extend existing models of microscopic ordering in nCB's, we carry out large-scale atomistic simulation studies of the microscopic structure and dynamics of the Sm-A(d) phase of 4-octyl-4'-cyanobiphenyl (8CB). We compute a variety of thermodynamic, structural, and dynamical properties for this material, and make a detailed comparison of our results with experimental measurements in order to validate our molecular model. Semiquantitative agreement with experiment is found: the smectic layer spacing and mass density are well reproduced, translational diffusion constants are similar to experiment, but the orientational ordering of alkyl chains is overestimated. This simulation provides a detailed picture of molecular conformation, smectic layer structure, and intermolecular correlations in Sm-A(d) 8CB, and demonstrates that pronounced short-range antiparallel association of molecules arising from dipole-dipole interactions plays a dominant role in determining the molecular-scale structure of 8CB.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
PubMed2001

Alignment of liquid crystals with patterned isotropic surfaces.

The molecules of a nematic liquid crystal exposed to an isotropic surface adopt a mean tilt relative to the normal but have no in-plane alignment-that is, they are free to have any azimuthal orientation in the surface plane. Pursuing the theoretical suggestion by Meyer that, in spite of this azimuthal degeneracy, spatially inhomogeneous isotropic surfaces combine with liquid crystal elastic anisotropy to produce alignment, we show that a boundary line between two isotropic regions that differ in mean tilt does indeed align the liquid crystal. The boundaries on a patterned surface of distinct isotropic regions thus act as a system of lines that the molecular orientation locally follows. This enables the development of liquid crystal alignment surfaces based on printing or lithographic patterning.

Journal Article↗