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Abstract

This thesis presents an experimental study of focal conic domains in smectic liquid crystal films on solid substrates. Focal conic domains are complex structures in smectic phases and are likely to play an important role in the future development of novel self-assembling soft matter systems. In smectic films on solid substrates, the generation of focal conic domains can be enforced by antagonistic molecular anchoring conditions at the film interfaces, i. e., random planar at the substrate interface and homeotropic at the air interface. The variation of the anchoring conditions at the substrate interface and the resulting influence on the morphology, properties, and spatial arrangement of focal conic domains were the main objects of the present study.The relation between the diameter of the focal conic domains, the thickness of the smectic film, and the anchoring strength of the substrate has been studied. The strength of the planar anchoring on the solid substrate was systematically varied by coating the substrates with special alkoxysilane compounds. For each anchoring strength value, the relation between the size of the focal conic domains and the film thickness was determined. Analyzing the experimental results by a theoretical model enabled the quantitative determination of the strength of the planar anchoring of the smectic phase on the substrate.The curved arrangement of the smectic layers in focal conic domains leads to a depression in the film/air interface above each domain. The depth of these depressions enables to estimate the magnitude of the elastic modulus controlling the dilation or compression of the smectic layers. The depression depth was measured in di erent smectic phases and at di erent phase transitions. In some cases, e. g., at second-order smectic-A nematic transitions, a pronounced temperature dependence of the depression depth was observed, indicating a softening of the smectic layers.A method to prepare anchoring patterns on the substrates is described. The substrates were structured in a way that areas with homeotropic anchoring conditions alternate with areas possessing random planar anchoring conditions. When smectic films were prepared on such substrates, focal conic domains formed only on the planar anchoring areas, i. e., the anchoring pattern was translated into a presence-and-absence pattern of focal conic domains. The lateral dimensions of the planar anchoring areas determined an upper limit for the diameter of the focal conic domains. Thus, almost arbitrary two-dimensional arrangements of focal conic domains can be achieved by controlling the size and position of individual domains.Smectic liquid crystal films have been also studied on crystalline substrates with uniaxial planar anchoring conditions. On such substrates, all liquid crystal molecules are aligned parallel to the surface and point into one direction. In thin smectic films on these substrates, linear domains with a hemicylinder-like shape were observed. The relation between the film thickness and the distance and depth of the hemicylinder structures was studied. In thicker films, the linear domains are decorated with a complex corrugated pattern, which becomes more pronounced on further increasing the thickness, and finally ends with the formation of circular focal conic domains.