Hexagonal order in crystalline and columnar phases of hard rods

We report a study of colloidal suspensions of highly monodisperse semiflexible chiral rodlike viruses, denoted fd, in the range of high concentrations. Small angle x-ray scattering experiments reveal the existence of two hexagonal phases: the first one is crystalline and the second one is hexatic columnar, as shown by its short-range positional order. The suspension of rodlike viruses is the first experimental system showing the whole phase sequence with increasing particle concentration theoretically predicted for systems of hard rods, ranging from the chiral nematic via the smectic to columnar and crystalline phases.     

NMR relaxation study of molecular dynamics in columnar and smectic phases of a PAMAM liquid-crystalline co-dendrimer

We present the first results obtained by proton (¹H) nuclear magnetic relaxation studies of molecular dynamics in a supermolecular liquid-crystal dendrimer exhibiting columnar rectangular and smectic-A phases. The ¹H spin-lattice relaxation time (T₁) dispersions are interpreted using two relaxation mechanisms associated with collective motions and local molecular reorientations of the dendritic segments in the low- and high-frequency ranges, respectively. The T₁ values show a drop around 2.3 MHz that is attributed to a contribution coming from cross-relaxation between ¹H and nitrogen nuclear spins. In the high-frequency range the motions appear to be of similar nature in both mesophases and are ascribed to reorientations of dendritic segments (belonging to the core and/or to the mesogenic units) characterized by two correlation times. Notable differences in the dynamics between the columnar and layered phases are observed in the low-frequency range. Depending on the mesophase they are discussed in terms of elastic deformations of the columns and layer undulations. In this study we find that the dendritic core influences the dynamics of the mesogenic units both for local and collective motions. These results can be understood in terms of spatial constraints imposed by the dendritic architecture and by the supermolecular arrangement in the mesophases.     

Phase diagram of hard colloidal platelets: a theoretical account

We construct the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons–Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases. The stability of the solid (columnar) state at high packing fraction is included by invoking a simple equation of state based on a Lennard–Jones–Devonshire cell model which has proven to be quantitatively reliable over a large range of packing fractions. By employing an asymptotic analysis based on the Gaussian approximation we are able to show that the nematic–columnar transition is universal and independent of particle shape. The predicted phase diagram is in qualitative agreement with simulation results.     

A generic equation of state for liquid crystalline phases of hard-oblate particles

A generic equation of state (EoS) is developed for the hard cylindrical disc model to describe the isotropic phase of hard cut-sphere particles introducing a correction parameter to incorporate the negative contributions from higher-order virial coefficients. The isotropic–nematic–columnar phase diagram of hard cut-sphere fluids is investigated combining the new EoS with a scaled Onsager free energy for the nematic phase and an extended cell theory for columnar phase. By mapping the virial coefficients of an oblate spherocylinder on to those of the cylindrical disc (which are known algebraically), the new generic EoS is used to describe the isotropic and nematic phases of hard oblate spherocylinder particles. The predictions of the generic EoS are compared with available simulation data.     

Conditions for growth of columnar and filamentary crystals

The computer model of the initial stage of condensation processes is developed. The stochastic processes of adsorption, surface diffusion, and desorption occur on the 70×70 lattice of adsorption vacancies. The model was tested by an example close to water vapor sorption on a conventional crystal surface. Computed results demonstrate the processes of nucleation, growth of crystals, and developed condensation. Computations were carried out for various adsorption energies. Results showed that with decreasing energy of adsorption the character and rate of nucleation and subsequent condensation vary. With decreasing energy of adsorption the probability of nucleation of islets on substrate reduces, the filling of the next atomic layers on islets occurs earlier than the filling of the substrate, the growth rate of islets in height increases as compared to condensation of a continuous film, and the growth rate of area of islets decreases. There is a typical energy of adsorption at which the growth rate of islets in height reaches its maximum. At a further drop of adsorption energy, there is a growth of islets only in height, the growth rate, however, reduces. The phenomena revealed in computations illustrate the mechanism of origin of columnar and filamentary crystals.     

Optical and thermal studies on viscous lamellar smectic phases in binary mixture of DTAC and ortho-phosphoric acid

The binary mixture of two non-mesogenic compounds, namely, dodecyl trimethylammonium chloride (DTAC) and ortho-phosphoric acid (H₃PO₄) exhibits very interesting liquid crystalline smectic phases at large range of concentrations and temperature. The mixture with lower and higher concentrations of DTAC exhibits SmA, SmD, SmB and SmE phases, sequentially when the specimen is cooled from its isotropic phase. Different liquid crystalline phases observed in the mixture were studied using optical microscopic techniques. The temperature variations of optical anisotropy and electrical conductivity have also been discussed. Helfrich potential and elastic moduli have also been estimated in the smectic phase using the Helfrich model.     

Phase field simulation of the columnar dendritic growth and microsegregation in a binary alloy

This paper applies a phase field model for polycrystalline solidification in binary alloys to simulate the formation and growth of the columnar dendritic array under the isothermal and constant cooling conditions. The solidification process and microsegregation in the mushy zone are analysed in detail. It is shown that under the isothermal condition solidification will stop after the formation of the mushy zone, but dendritic coarsening will progress continuously, which results in the decrease of the total interface area. Under the constant cooling condition the mushy zone will solidify and coarsen simultaneously. For the constant cooling solidification, microsegregation predicted by a modified Brody- Flemings model is compared with the simulation results. It is found that the Fourier number which characterizes microsegregation is different for regions with different microstructures. Dendritic coarsening and the larger area of interface should account for the enhanced Fourier number in the region with well developed second dendritic arms.     

Chromonic liquid crystalline phase transition involving a biphasic region of nematic and hexagonal phases

We report the results of our studies on the optical and thermal properties of binary mixture of two compounds viz. abietic acid and poly ethylene glycol (PEG). The mixture shows a very interesting co-existent biphasic region of lyotropic nematic (N + I) and intermediate hexagonal (M + I) phases, sequentially when the specimen is cooled from its isotropic phase at different concentrations of abietic acid in PEG. The aggregated molecular size has been confirmed by X-ray studies. The temperature variation of optical anisotropy, electrical conductivity, ultrasonic velocity, and molar and adiabatic compressibility has also been discussed.     

Effect of far-field flow on a columnar crystal in the convective undercooled melt

The growth behavior of a columnar crystal in the convective undercooled melt affected by the far-field uniform flow is studied and the asymptotic solution for the interface evolution of the columnar crystal is derived by means of the asymptotic expansion method.The results obtained reveal that the far-field flow induces a significant change of the temperature around the columnar crystal and the convective flow caused by the far-field flow accelerates the growth velocity of the interface of the growing columnar crystal in the upstream direction and inhibits its growth velocity in the downstream direction.Our results are similar to the experimental data and numerical simulations.     

Stability and vibrations of catenoid-shaped smectic films

Catenoid-shaped smectic films are spanned between two coaxial circular frames separated by a distance H. It is shown that there exists a critical height H^* such that below it two shapes of the catenoid are possible. The stability of these two shapes is analysed in terms of their vibrations. The spectrum of eigenfrequencies is calculated as a function of the catenoid height. It is shown that the frequency of the fundamental mode is real for the stable shape and imaginary for the other shape. Experimental study of vibrational eigenmodes performed on stable SCE4 films confirms this theoretical prediction. Received: 25 November 1997 / Received in final form: 30 January 1998 / Accepted: 9 March 1998     

On the critical non-additivity driving segregation of asymmetric binary hard sphere fluids

A previously proposed version of thermodynamic perturbation theory, appropriate for singular pair interactions between particles, is applied to binary mixtures of hard spheres with non-additive diameters. The critical non-additivity Δ_C required to drive fluid–fluid phase separation is determined as a function of the ratio ξ ≤ 1 of the diameters of the two species. Δ_C(ξ) is found to decrease with ξ and to go through a minimum for ξ ? 0.015 before increasing sharply as ξ → 0, irrespective of the total packing-fraction η of the mixture. These results are the basis of an estimate of the range of size ratios for which a binary mixture of additive hard spheres exhibit a fluid–fluid miscibility gap. This range is conjectured to be 0.01 ? ξ ? 0.1.     

Features of the smectic e phases in some strongly polar biphenyls and tolanes

Four compounds with the isothiocyanate terminal group (–NCS) in tolane or biphenyl molecules, revealing E phase in a broad temperature range, have been investigated by means of the DSC, polarizing microscope, miscibility and X-ray methods. The investigations have confirmed that internal smectic E phase in those compounds was not identical and it has been characterized by different distortions of the orthoconic cell and interlayer correlation.     

层间作用对HgS/CdS/HgS柱状纳米系统电子能量的影响

确立了柱状纳米系统电子势能随距离的变化关系，以HgS/CdS/HgS柱状纳米系统为例，讨论了层间作用对电子能谱的影响.结果表明：电子能量随势垒宽度和波矢的增大而增大，随势阱宽度的增大而减小；层间互作用会增大电子能量，但不会改变电子能量随势阱和势垒以及波矢的变化趋势. 关键词： 层间作用势 HgS/CdS/HgS柱状纳米系统 电子能量     

Effects of copper nanoparticles on the thermodynamic,electrical and optical properties of a disc-shaped liquid crystalline material showing columnar phase

Thermodynamic, electrical and optical studies have been carried out on a discotic liquid crystal (DLC), namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene (possessing columnar phase) and its copper nanoparticles (0.6 wt%) based composite. The ionic conductivity of DLC–copper nanocomposite has increased by about two orders of magnitude as compared to the pure system. Dielectric permittivity has also increased. The absorption spectra for pure and nanocomposites have been studied by UV–Vis spectrophotometer. The optical study suggests that surface plasmon resonance has been introduced in DLC due to the incorporation of copper nanoparticles. It has been observed that the presence of nanoparticles has decreased the optical band gap to 3.3 eV from 4.2 eV of the pure DLC. Enhanced properties are useful for one-dimensional conduction and photovoltaic applications.     

Observation of undulations of layers in the smectic B and H phases of a liquid crystal

We have observed scattering of light by static undulations, and by the undulation instability of the layers, in the B and H smectic phases. This would indicate that the B and H phases are closer to a liquid crystal than to a “plastic crystal”.     

Evidence of the hexagonal columnar liquid-crystal phase of hard colloidal platelets by high-resolution SAXS

A vast majority of compounds with bent core or banana shaped molecules exhibit the phase sequence B₆-B₁-B₂ as the chain length is increased in a homologous series. The B₆ phase has an intercalated fluid lamellar structure with a layer spacing of half the molecular length. The B₁ phase has a two dimensionally periodic rectangular columnar structure. The B₂ phase has a monolayer fluid lamellar structure with molecules tilted with respect to the layer normal. Neglecting the tilt order of the molecules in the B₂ phase, we have developed a frustrated packing model to describe this phase sequence qualitatively. The model has some analogy with that of the frustrated smectics exhibited by highly polar rod like molecules.     

Flexoelectricity and piezoelectricity: the reason for the rich variety of phases in antiferroelectric smectic liquid crystals

The free energy of antiferroelectric smectic liquid crystals which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric, and flexoelectric interactions to the nearest layers, and dipolar electrostatic interactions to the nearest and to the next-nearest layers, induce indirect tilt interactions with chiral and achiral properties, which extend to the third- and to the fourth-nearest layers. Although the strength of microscopic interactions changes monotonically with decreasing temperature, the effective interlayer interactions change nonmonotonically and give rise to a nonmonotonic change of the modulation period through various phases. Increased chirality changes the phase sequence.     

Prediction of the acoustic behavior of a parallel assembly of hollow cylinders

In this paper, an approach to predict the sound absorption coefficient and sound transmission loss of a parallel assembly of hollow cylinders is presented. This approach is based on image processing and the Parallel Transfer Matrix Method (PTMM) using four Johnson–Champoux–Allard effective fluids. First, effective parameters of each fluid are identified using geometrical considerations and numerical simulations. Then, the approach is validated for a stack of uniform plastic straws, and used to model a natural stack of non-uniform switchgrass straws. Finally, two parametric studies are conducted to evaluate the effects of the geometric parameters of the straws on the acoustic behavior of their stack. It is shown that there are optimal parameters that maximize the acoustic behavior at specific frequencies.     

Dielectric permittivity studies of nematogenic compounds and their binary mixtures showing induced smectic Ad phase

The binary mixture of 4-n-pentyl phenyl 4-n′-hexyloxy benzoate (ME6O.5) and p-cyanophenyl trans-4-pentyl cyclohexane carboxylate (CPPCC) shows the presence of induced Smectic A_d phase. In the present work the phase diagram as well as static dielectric permittivity measurements of different mixtures throughout the composition range, are reported. The observed variation of dielectric anisotropy with molar concentration in the nematic and smectic phases is explained, assuming the formation of both homo- and hetero-dimers in the mixture.