Alignment of nematic liquid crystals on mixed Langmuir-Blodgett mono-layers

Mono-layers of stearic and behenic acids and mixtures of them in different proportions, deposited with the Langmuir-Blodgett technique, were used to study the alignment and the alignment dynamics in nematic liquid crystal cells. A relaxation process from a splay-bend flow induced metastable orientation to the homeotropic one occurs. The lifetime of the metastable state was found to depend on the mono-layer composition. The transition between the homeotropic and the conical anchoring was found to be irreversible in the case of the mixed aligning mono-layers: on cooling from the isotropic phase a quasi-planar nematic state (schlieren texture) appears. It is stable in a range of a few degrees below the clearing point and, on decreasing the temperature, relaxes to the homeotropic state in form of expanding domains.Comment: 9 pages, LaTeX2e article, 8 figures, 11 EPS files, submitted to Thin Solid Film

Germination and predation of Acacia karroo seeds on acid mine drainage polluted soils

A research report submitted to the Faculty of Science, in partial fulfilment of the requirements for the degree of Master of Science, University of the Witwatersrand, Johannesburg, March 2016.The study aims to assess the impacts of Acid Mine Drainage (AMD) polluted soils on Acacia karroo seed germination and viability, seed dry mass and predation, in comparison with trees from the same provenance growing on non-polluted soils. The study was undertaken within the Vaal River Operations mining rights area. This area is bisected by the Vaal River which separates the polluted area from the non-polluted area. Contamination of soils on the northern section of the Vaal River is a result of mining operations, historical tailings spillage as well as an existing pollution plume which has resulted in AMD polluted soils. The rehabilitation of disturbed land is often hindered due to low seedling establishment. The success of germination is one of the most important first steps for seedling establishment and growth and hence towards establishing a self-sustaining vegetation cover over disturbed areas. Dry seed mass was slightly higher from trees in non-polluted (0.051±0.009g) compared to the polluted areas (0.046±0.009g), however no significant difference was found. Seeds collected from the non-polluted area had highest proportion of seeds in the seed mass class 0.0455-0.0904g, compared to the seeds from the polluted areas which were highest in the smaller seed mass class 0.0155-0.454g. At the tree level, the Coefficient of Variation (CV) for dry seed mass was higher for seeds collected from the polluted area (20.5%) compared to the non-polluted area (17.9%), however, no significant difference was found. However, percentage seed predation was significantly lower in the polluted (35±15.76%) relative to the non-polluted areas (48±14.69%). Percentage seed germination was significantly higher in the non-polluted (92±9.35%) compared to the polluted areas (81±20.42%), with a significantly more rapid germination rate of 4.2±0.19 days compared to 4.7±0.45 days, respectively. In conclusion, despite their lower dry seed mass, seeds collected from AMD polluted soils still had high percentage germination, while exhibiting a lower percentage of seed predation compared to those growing on unpolluted soils. Due to A. karroo’s apparent tolerance to the poor conditions on the AMD polluted soils and its regeneration capabilities, it is likely to be a good species for rehabilitation of AMD polluted sites. Further studies should aim to determine seedling performance from those seeds collected from polluted areas in terms of seedling establishment, rates of growth and survival over time when established in AMD polluted soils as well as non-polluted soils, to determine their likely success

Influence of the flow on the anchoring of nematic liquid crystals on a Langmuir-Blodgett monolayer studied by optical second-harmonic generation

The influence of capillary flow on the alignment of the nematic liquid crystal MBBA on fatty acid Langmuir-Blodgett monolayers was studied by optical second-harmonic generation. The surface dipole sensitivity of the technique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the nematic director in the bulk. itself is affected by the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a slight preference for the direction of the previous flow.Comment: LaTeX2e article, 11 figures, 17 EPS files, submitte

Telephone-cord instabilities in thin smectic capillaries

Telephone-cord patterns have been recently observed in smectic liquid crystal capillaries. In this paper we analyse the effects that may induce them. As long as the capillary keeps its linear shape, we show that a nonzero chiral cholesteric pitch favors the SmA*-SmC* transition. However, neither the cholesteric pitch nor the presence of an intrinsic bending stress are able to give rise to a curved capillary shape. The key ingredient for the telephone-cord instability is spontaneous polarization. The free energy minimizer of a spontaneously polarized SmA* is attained on a planar capillary, characterized by a nonzero curvature. More interestingly, in the SmC* phase the combined effect of the molecular tilt and the spontaneous polarization pushes towards a helicoidal capillary shape, with nonzero curvature and torsion.Comment: Submitte

Phase-matched second-harmonic generation in a ferroelectric liquid crystal waveguide

True phase-matched second-harmonic generation in a waveguide of crosslinkable ferroelectric liquid crystals is demonstrated. These materials allow the formation of macroscopically polar structures whose order can be frozen by photopolymerization. Homeotropic alignment was chosen which offers decisive advantages compared to other geometries. All parameters contributing to the conversion efficiency are maximized by deliberately controlling the supramolecular arrangement. The system has the potential to achieve practical level of performances as a frequency doubler for low power laser diodes.Comment: 4 pages, LaTeX2e article, 3 figures, 4 EPS files, submitted to Physical Review Letter

High-contrast imaging of 180{\deg} ferroelectric domains by optical microscopy using ferroelectric liquid crystals

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an electric field gives rise to an immediate and lasting optical response when an appropriately aligned FLC is observed between crossed polarizers, with one field direction yielding a dark image, and the opposite direction yielding a bright image. Here this peculiar electro-optic response is used to image, with high optical contrast, 180{\deg} ferroelectric domains in a crystalline substrate of magnesium-doped lithium niobate. The lithium niobate substrate contains a few domains with upwards electric polarization surrounded by regions with downward electric polarization. In contrast to a reference non-chiral liquid crystal that is unable to show ferroelectric behavior due to its high symmetry, the FLC, which is used as a thin film confined between the lithium niobate substrate and an inert aligning substrate, reveals ferroelectric domains as well as their boundaries, with strong black and white contrast. The results show that FLCs can be used for non-destructive read-out of domains in underlying ferroelectrics, with potential applications in e.g. photonic devices and non-volatile ferroelectric memories.Comment: 12 pages, 3 figure