Spontaneous periodic deformations in nonchiral planar-aligned bimesogens with a nematic-nematic transition and a negative elastic constant

Hydrocarbon linked mesogenic dimers are found to exhibit an additional nematic phase below the conventional uniaxial nematic phase as confirmed by x-ray diffraction. The phase produces unusual periodic stripe domains in planar cells. The stripes are found to be parallel to the rubbing direction (in rubbed cells) with a well-defined period equal to double the cell gap. The stripes appear without external electromagnetic field, temperature or thickness gradients, rubbing or hybrid alignment treatments. Simple modeling proposes a negative sign for at least one of the two elastic constants: splay and twist, as a necessary condition for the observed pattern.     

Ferrocene-containing liquid crystals bearing a cholesteryl unit

The synthesis and characterization of new liquid crystalline compounds containing ferrocene azo or imino aromatic and cholesteryl units is reported. The purpose of this research was to combine the properties of metals (colour, electronic density, magnetism and polarizability) with those of chiral nematics (high birefringence and presence of physical colours) and correlate relationships between chemical structures and properties in the condensed state.     

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 10² to 10⁹ Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.     

Influence of absorbed pump profile on the temperature distribution within a diode side-pumped laser rod

In this paper, an analytical model for temperature distribution of the side-pumped laser rod is extracted. This model can be used for side-pumped laser rods whose absorbed pump profile is a Gaussian profile. Then, it is validated by numerical results which exhibit a good agreement with the analytical results. Afterwards, by considering a general expression for super-Gaussian and top-hat profiles, and solving the heat equation, the influence of profile width and super-Gaussian exponent of the profile on temperature distribution are investigated. Consequently, the profile width turns out to have a greater influence on the temperature compared to the type of the profile.     

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.     

Singular-value-like decomposition for complex matrix triples

The classical singular value decomposition for a matrix A∈C^m×n is a canonical form for A that also displays the eigenvalues of the Hermitian matrices AA^∗ and A^∗A. In this paper, we develop a corresponding decomposition for A that provides the Jordan canonical forms for the complex symmetric matrices and . More generally, we consider the matrix triple , where are invertible and either complex symmetric or complex skew-symmetric, and we provide a canonical form under transformations of the form , where X,Y are nonsingular.     

Computer simulation of crystallization kinetics and morphology in fiber-reinforced thermoplastic composites. III. Thermal nucleation

A computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials based on thermally nucleated crystallizable matrices. As demonstrated for athermally nucleated composites, the presence of reinforcing fibers increases the complexity of the system. Fibers are shown to have a dual effect on the spherulitic crystallization process. The influence that fibers have depends on the interplay between the enhancing effects that fibers have on nucleation and the depressing effects that fibers have on spherulitic growth. Fibers that do not provide additional nuclei to the system depress the rate of crystallization relative to an unreinforced polymer, while fibers that add nuclei to the system increase the rate of crystallization. The transcrystalline morphologies that develop in thermally nucleated fiber-reinforced polymers are controlled primarily by the relative numbers of bulk and fiber nuclei. The extent of transcrystalline regions can be suppressed either by increasing the rate of bulk nucleation, or by decreasing the rate of fiber nucleation. Finally, the qualitative appearance of the morphology in the transcrystalline region was found to be indicative of the mode of fiber nucleation. © 1995 John Wiley & Sons, Inc.     

Mechanistic studies on the repair of a novel DNA photolesion: the spore photoproduct

[formula: see text] UV irradiation of spores results in the formation of the spore photoproduct. This novel DNA photolesion is repaired in the germinating spore in a reaction catalyzed by the spore photoproduct lyase. Model studies, using a simple bispyrimidine, suggest that this repair reaction proceeds by hydrogen abstraction from C6 of the spore photoproduct followed by beta-scission of the bond linking the two pyrimidines and back hydrogen atom transfer.     

On the combustion and sooting behavior of standard and hydro-treated jet fuels: An experimental and modeling study on the compositional effects

In a context of growing level of environmental awareness, emission from aviation are the subject of increasing scrutiny. This situation poses important challenges because, due to safety, practical and economic factors, aero-transportation technologies are not likely to undergo rapid paradigm shifts. An area where important innovations are being introduced is fuel technology: fuels from alternative processes, potentially from renewable sources, offer the opportunity of limiting the carbon footprint of transportation, moreover, a better control on fuel quality can contribute to reducing emissions.Hydro-treating of oil based fuels can reduce their sulfur and aromatic content promoting a cleaner combustion. In order to better understand the impact of hydro-treating on emissions of PAHs and soot from jet fuels, new speciation data covering oxidation intermediates and soot precursors were measured in a flow reactor for a standard jet fuel and its hydro-treated counterpart. Using a detailed kinetic mechanism and complex surrogate blends mimicking the composition of the real fuels, the speciation data from the flow reactor were simulated. Additionally, soot formation trends were calculated and compared with previously published data. Using the kinetic model, which is based on mechanistic principles, it was possible to separate the relative contribution of different processes and, for the fuel blends of interest, the role played by specific components in the PAHs and soot formation. The results obtained provide useful information towards more effective fuel formulation strategies and fuel blends modeling.