Studies on high strength defects in nematic liquid crystals?

We recently reported the discovery of stable disclinations of strength ±3/2 and ±2 in mixtures of several nematogenic compounds with a non-mesomorphic compound having plate-like molecules. We describe here further detailed observations on these defects, particularly on pairs of high strength disclinations

Elasticity and orientational order in some cyanobiphenyls: Part IV. Reanalysis of the data

We present revised absolute values of the elastic constants and order parameters of several cyanobiphenyls. The calculations make use of our earlier measurements of Freedericksz threshold fields and refractive indices, and corrected values of normalizing constants, particularly in respect of the anisotropy of magnetic susceptibility. The new values agree well with some recent measurements

On the occurrence of point and ring defects in the nematic-nematic coexistence range of a binary mixture of rod-like and disc-like mesogens

We report some observations on the director configurations in the coexistence range of two nematic phases, NR and ND in a binary mixture of rod-like and disc- like nematogens. The director alignment is continuous across the boundary of the NR and ND phases. Point defects of strength + 1 in NR droplets in an otherwise continuous ND medium are found to be annulled by singular ring disclinations of strength - ½ Further, the NR drops which orient homeotropically at the glass plates have a point defect of strength + 1 only if their size is less than about 7 μm. Larger drops have ring disclinations of strength + ½ instead of + 1 point defects. This changeover can be understood on the basis of simple energy considerations

Experimental determination of the elastic constant k<SUB>13</SUB> of a nematic liquid crystal

The elastic constant k13 is the coefficient of a divergence term in the free energy density and involves the second derivative of the director field. Thus its influence is felt only at the surface of the sample. We have analysed the optical path difference of a hybrid aligned nematic liquid crystal PCH7 taken in a cell which has a weak homeotropic alignment on one glass plate and a strong homogeneous alignment on the other. We use the approximation that solutions in the class of Euler-lagrange equations are valid up to the boundaries and extract both k13 and the anchoring energy at the homeotropically aligned surface. In view of the recent argument by Oldano and Barbero that the k13 term by itself may lead to discontinuities in the director configuration near the surfaces our measurements may be taken to yield an 'effective' value which may have contributions from other terms influencing the director configuration at the surface

Component orientational order parameter in binary mixtures

We have used the IR dichroism technique to measure the component order parameters of two different cyanobiphenyl compounds in two binary mixtures. The measurements clearly show that smectic-like short range order considerably enhances the orientational order in the nematic phase. Further, the temperature variation of the orientational order is also increased by virtue of that of the smectic-like short range order

Study of role of Magnesium sulfate in prevention of eclampsia

Background: Hypertensive disorders of pregnancy have a clinical spectrum ranging from non-severe to severe preeclampsia and then potentially to eclampsia. Magnesium sulfate is drug of choice for women with eclampsia and now adapted for prophylaxis of seizures in women with preeclampsia.Methods: It is randomized clinical trial, including 876 patients with preeclampsia fulfilling inclusion criteria. Patients with severe preeclampsia received anticonvulsant prophylaxis with magnesium sulfate. Patients with non-severe preeclampsia were randomized in two groups study group and control group. Patients in study group had received anticonvulsant prophylaxis with magnesium sulfate. The data obtained was analyzed with respect to maternal and perinatal outcome, severity of disease progression and adverse reactions of magnesium sulphate in each group.Results: In this study statistically, significant difference was seen with respect to severity of disease progression and incidence of eclampsia among study and control group.Conclusions: Administration of Magnesium sulfate in patients with non-severe preeclampsia does not affect the progress of labour or perinatal outcome, on contrary it improves maternal outcome by reducing incidence of eclampsia and progression of disease, without significant side effect of drug.

Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rodlike and bent-core molecules

We report on two anomalous trends in the temperature dependences of the splay (K11) and bend (K33) elastic constants in the nematic (N) phase of mixtures of compounds with rodlike (R) and bent-core (BC) molecules: As the sample is cooled from the isotropic to N transition point, (i) K33 increases, attains a maximum value and then decreases, and (ii) close to the N to smectic A (SmA) transition point, K11 decreases sharply. At higher temperatures the bow axes of BC molecules are aligned along the director n^, strongly favoring a bend distortion of n^ as the orientational order parameter is increased. Close to the N-SmA transition point the smecticlike short-range order builds up, and the arrow axes of BC molecules are aligned along n^, facilitating a splay distortion of n^. A simple model calculation brings out the anomalous trend in K33

Electromechanical effect in cholesteric liquid crystals with fixed boundary conditions

We describe a number of experiments to demonstrate the electromechanical effect in cholesteric liquid crystals. The Lehmann rotation phenomenon of drops with stress free boundary conditions is the most convincing effect produced by this coupling. In samples with fixed boundary conditions, the cross coupling produces a distortion of the profile of the azimuthal angles of the director. We have used a conoscopic technique to demonstrate this distortion in thick samples containing negative dielectric anisotropy materials. A strong linear electrooptic effect of electromechanical origin is produced in materials with a weak positive dielectric anisotropy. We provide an evidence that a twisted nematic cell without any chiral molecules also exhibits the electromechanical effect, due to the macroscopic chirality of the medium in such a cell

Two-dimensionally periodic phases in mixtures of compounds made of rodlike and bent-core molecules

We report two distinct types of two-dimensionally periodic phases in different concentration regions of a binary system composed of the bent-core (BC) compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4'-n-dodecylbiphenyl 4'-carboxylate (BC12) and the rodlike (R) compound 4-n-octyloxy 4'-cyanobiphenyl (8OCB). BC12 exhibits only the lamellar B2 phase. 8OCB which has a highly polar cyano end group exhibits the smectic-Ad phase with a lamellar structure of partial bilayers made of antiparallel pairs of the polar molecules. The molecular arrangements in the two-dimensionally periodic phases have been deduced based on optical microscopy, polarized infrared and x-ray diffraction studies. The arrangement in the two-dimensionally periodic phase in lower concentration ranges of BC12 (24 to 35 mol%) is dictated by the partial bilayer structure formed by the rods. The BC molecules arrange themselves with their arrow axes parallel to the long axes of the R molecules and the structure conforms to the c2mm space group symmetry. In higher concentration ranges of BC12 (35 to 60 mol%) the structure is dominated by the BC molecules, to form the familiar B1 phase with the p2mg space group symmetry. The R molecules lie at the interfaces of these domains stabilizing the structure