Friction Drag on a Particle Moving in a Nematic Liquid Crystal

The flow of a liquid crystal around a particle does not only depend on its shape and the viscosity coefficients but also on the direction of the molecules. We studied the resulting drag force on a sphere moving in a nematic liquid crystal (MBBA) in a low Reynold's number approach for a fixed director field (low Ericksen number regime) using the computational artificial compressibility method. Taking the necessary disclination loop around the sphere into account, the value of the drag force anisotropy (F_\perp/F_\parallel=1.50) for an exactly computed field is in good agreement with experiments (~1.5) done by conductivity diffusion measurements. We also present data for weak anchoring of the molecules on the particle surface and of trial fields, which show to be sufficiently good for most applications. Furthermore, the behaviour of the friction close to the transition point nematic isotropic and for a rod-like and a disc-like liquid crystal will be given.Comment: 23 pages RevTeX, including 3 PS figures, 1 PS table and 1 PS-LaTeX figure; Accepted for publication in Phys. Rev.

Esters of (S)-1,2-propanediol and (R,R)-2,3-butanediol — Chiral Compounds Inducing Cholesteric Phases with a Helix Inversion ·

Mesogenic chiral esters of optically active (S)-1,2-propanediol and (R,R)-2,3-butanediol were synthesized. The compounds, added to a nematic phase induce cholesteric phases exhibiting a helix inversion with temperature variation. This effect is independent of the molecular structure of the nematic solvent. The inversion temperature varies only slightly with concentration but can be influenced by the mesogenic substituent

Smectic A1 — Smectic Ad Transition in Binary Mixtures of Compounds with Strongly Polar Terminal Groups

On the basis of high pressure studies we present here the evidence for the existence of a smectic A, — smectic Aj transition at high pressure in a binary liquid crystal mixture consisting of terminally polar compounds. A feature of this observation is that both the constituent compounds are three phenyl ring systems wherein the bridging dipoles are disposed additive with respect to the polar end group

Re-entrant Nematic Phase Established for Several Homologous Biaromatic Liquid Crystals by Investigation of Binary Mixed Systems

Several homologous series of biaromatic liquid crystals with structure type CnH2n+1O • C6H4 • M•C6H4-CN are investigated. By studying the phase transition temperatures in binary mixed systems a "re-entrant" nematic phase N(re) is established for all compounds exhibiting smectic A phases. The virtual transition temperatures N(re) - SA are found to decrease with increasing chain length of the alkyloxy group. The influence of the middle group M on the re-entrant behaviour can be attributed mainly to its dipole moment. The temperature range of the smectic A phase decreases when the middle group dipole moment is additive to that of the nitrile group. To summarize it can be said that the "re-entrant" behaviour of biaromatic liquid crystals is found to be very similar to that of triaromatic systems reported recently

Optical behaviour of cholesteric blue phases in electric fields

The optical behaviour of cholesteric mixtures of negative dielectric anisotrony under electric fields is reported. A mixture of S 311~ (31.35 %) + N 5 was employed. AC voltages (f = 1000 Hz) betweeen 0 and 150 volts were applied. Cells 23 micron thick, with internal SnO2 electrodes, were used

Density, dielectric and X-ray studies of Smectic A-Smectic A transitions

We report here the results of detailed density, dielectric and X-ray studies on three systems exhibiting different types of Smectic A-Smectic A transitions. It is found that although the layer spacing shows marked changes at the transitions, the corresponding density changes are extremely small. In every case studied, the dielectric anisotropy shows a pronounced decrease on going over to the lower temperature smectic A phase. This decrease can be correlated with the structural changes

An experimental study of the smectic A-smectic C transitions in monolayer, partially bilayer and bilayer systems

The smectic A-smectic C (A-C) transition in several compounds has been shown to be mean-field-like (MF) with a substantial tricritical crossover effect. However, a simple MF behaviour, i.e., with negligible tricritical influence, had not been reported till recently. The heat capacity measurements of the MIT group in two compounds exhibiting the bilayer A2-C2 transition revealed such a behaviour. It is of interest to investigate whether a simple MF behaviour can be observed for partially bilayer AdminusCd and monolayer A1-C1 transitions also. We report the first systematic order parameter measurements in the vicinity of the A2-C2, Ad-Cd and A1-C1 transitions. The data analysed in terms of power law and extended mean field expressions provide clear evidence of a simple MF behaviour in all three cases. The significance of this result is discussed

Electromechanically active pair dynamics in a Gd-doped ceria single crystal

Oxygen-defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cation and oxygen vacancy (V-O) pairs (i.e. Ce-V-O), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-V-O pairs in a grain boundary-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV cm(-1). The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at the Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on density of states and orbitals at the valence band edge. Our research indicates that electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise by one order of magnitude, i.e., from 5 x 10(-4) at room temperature to 5 x 10(-3) at -193 degrees C, due to an increase in the population of the electrically active pairs. At a constant V-O concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not