Biomedical subjects

S Sprunt

Publications and source records attributed to S Sprunt.

18 recordsLinked to original sources

PubMed2006

Dynamic light scattering study of biaxial ordering in a thermotropic liquid crystal.

Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals.

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PubMed2006

Giant flexoelectricity of bent-core nematic liquid crystals.

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.

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PubMed2006

Critical behavior at the isotropic-to-nematic phase transition in a bent-core liquid crystal.

Magnetic birefringence and dynamic light scattering measurements of orientational order parameter fluctuations at the isotropic-nematic phase transition of a bent-core liquid crystal reveal a pretransitional temperature dependence consistent with the standard Landau-deGennes mean field theory. However, as follows: the transition in the bent-core compound is more weakly first order (TNI-T* approximately 0.4 degrees C), the leading Landau coefficient is approximately 30 times lower, the viscosity associated with nematic order fluctuations is approximately 10 times higher, and the density change is approximately 10 times lower, than typically observed in calamitic (rod-shaped) liquid crystals. One consistent explanation for these anomalies is an optically isotropic phase composed of microscopic complexes or "clusters" of bent-core molecules.

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PubMed2005

Optically isotropic liquid-crystal phase of bent-core molecules with polar nanostructure.

We found that the optically isotropic (I(M)) mesophase observed recently below the nematic phase of the bent-core liquid crystal 4-chlororesorcinol bis[4-(4-n-dodecyloxybenzoyloxy)benzoate] shows ferroelectric-type switching. Polarizing microscopic, electric current, dielectric, and dynamic light scattering studies lead us to propose that the I(M) phase is composed of interconnected orthoconic racemic smectic (Sm-Ca P(F)) nanodomains with random layer orientations. Near the nematic phase, where the polarization can be saturated by electric fields, the system responds in a fashion analogous to the granular structure of a magnetic spin glass--in particular, we observed that the relaxation back to the nonpoled structure follows a similar, inverse logarithmic rule.

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PubMed2005

One nanosecond photon correlation spectroscopy on smectic liquid crystal films.

We have extended photon correlation spectroscopy down to a one-nanosecond time scale, and applied it to a study of layer undulations in freestanding smectic-A films of two cyanobiphenyl liquid crystals. Temporal correlations in the intensity of scattered light reveal an interesting combination of under- and overdamped modes. The underdamped mode is accurately described by a recently calculated correlation function of the smectic layer displacement, although its frequency and damping rate exhibit stronger dispersion at large optical wave vectors than expected from current dynamical models for smectic films.

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PubMed2003

Optical studies of the nematic phase of an oxazole-derived bent-core liquid crystal.

Various optical and dynamic light scattering studies have been conducted on the nematic phase of an oxazole-derived bent-core liquid crystal. At optical length scales and in the absence of applied fields, homogeneously aligned samples of this material, which has an oxazole heterocyclic ring in the central core, are found to behave more like a conventional straight-core nematic than a previously investigated ester-based class of bent-core molecules that have a benzene ring as the core linkage between the two arms of the bow-shaped molecule. In particular, the nematic refractive indices of the oxazole compound combine in the standard way [i.e., square root of ((2no2+ne2)/3)] to match the isotropic value throughout the nematic range, and the observed director fluctuation modes have relaxation rates comparable to those of the usual thermotropics. However, polarized light scattering data reveal evidence of weak biaxial fluctuations, and indications of electric-field-induced biaxiality are observed in the refractive index measurements.

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PubMed2003

Light scattering study of a twist grain boundary liquid crystal.

Dynamic light scattering is used to probe the fluctuation modes of a liquid crystal exhibiting twist grain boundary (TGB) structure. At the chiral nematic to proposed "chiral line liquid" phase transition, anomalous temperature dependence in the fluctuation spectrum and an instability in the helicoidal director structure signify developing TGB order. At lower temperatures, the behavior of the smectic layer-director fluctuations ("soft" mode) indicates that the previously identified commensurate TGB(A) phase may in fact be a TGB(C) phase with an unusually small tilt angle.

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PubMed2002

Dynamics of the nematic phase of a bent-core liquid crystal.

The dielectric fluctuations in the uniaxial nematic phase of a bent-core liquid crystal have been studied by dynamic light scattering. Polarization selection of the scattering cross-section reveals one mode due to ordinary director fluctuations and, in the lower part of the nematic phase, a second mode attributable to fluctuations of the biaxial order parameter. The director fluctuations are approximately 100 times slower than observed in typical nematics based on straight-core molecules, suggesting that the bent core nematogenic units may be microscopic smectic domains ("cybotactic" clusters).

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PubMed2001

Anomalous softening of order parameter fluctuations at the smectic-A to -C* transition in a siloxane-substituted chiral liquid crystal.

Unconventional softening of order-parameter fluctuations is observed at the smectic-A to smectic-C* phase transition in a chiral liquid crystal possessing multiple siloxane substituents on its hydrocarbon chains. Together with an optical "stripe" texture detected above the transition, the atypical dynamics can be explained by the pretransitional development within the smectic layers of a modulated state of the order parameter. Conventional soft-mode behavior is restored when the degree of chain substitution is reduced.

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PubMed2001

Structural properties of the ferrielectric phases of a chiral liquid crystal revealed by dynamic light scattering.

Dynamic light scattering is used to study the antiferro- and ferrielectric phases in a bulk sample of the chiral liquid crystal R-10OTBBB1M7. Comparison of the detected fluctuation modes in the two ferriphases to calculated selection rules for their coupling to light are consistent with the distorted clock structures recently reported by an optical ellipsometry study, but indicate that the distortion has a polar component in both ferriphases. We also find a surprisingly large amplitude and dispersion associated with scattering from fluctuations in the tilt angle magnitude. In terms of the discrete interlayer interaction models currently being used to account for the ferriphases, this finding suggests that the interlayer interaction coefficients are of comparable magnitude to the intralayer coefficients that stabilize the tilt angle.

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PubMed1995