Growth of toric domains in mesophases of oxadiazoles

Abstract

In a previous paper (Phase Transitions, 80(9), 987, 2007) we discussed the smectic and nematic textures of some mesomorphic oxadiazole compounds with terminal Cl-substituent. Optical microscope investigations showed a very interesting behaviour of smectic and nematic phases; the smectic phase has fan-shaped and toric textures and the nematic phase has spherulitic domains, which disappear as the sample is further heated, the texture changing into a smooth one. Here, we investigate four oxadiazole compounds with the same structure but terminal Br-substituent. The behaviour of the smectic and nematic phases is like that observed in the compounds with Cl. The focus of the paper is on the growth of toric domains from the nematic melt and on the role of defects in the domain structure.

Ringed spherulitic and undulated textures in the nematic phase of a mixture of trans-4-hexylcyclohexanecarboxilic and benzoic acids

“…The existence of cybotactic clusters in liquid crystal compounds is receiving an increasing interest. We have recently observed a texture transition in the nematic phase of an oxadiazole compound [19,20]. We proposed the existence of cybotactic clusters to explain the spherulitic nematic phase in of this oxadiazole compounds.…”

“…We have already observed spherulitic patterns in the nematic phase of oxadiazoles. In that case [19,20], on cooling the sample, it was possible to observe the transition of these nematic structures in the toric domains of a smectic phase. In the case of the mixture C6/benzoic acid, the transition to the smectic phase was not observed, because it is below the room-temperature.…”

“…Let us remember that the existence of cybotactic clusters in liquid-crystal compounds is receiving a renewed interest, in particular for oxadiazole compounds [19][20][21]. The reason is that one of these materials displayed a ferroelectric behaviour in its nematic phase [21].…”

An image-processing analysis of skin textures

“…However, it is necessary to check whether the hypothesis of homogeneity is really verified and whether preferred directions are displayed in the image frame (hypothesis of isotropy). We introduced a typical length characterising the texture size, which was very useful for characterisation of smectic and nematic phases [13,14]. Instead of measuring the homogeneity, by evaluating the histogram's entropy of intensity difference versus distance from a point of the image frame (see for instance [15]), or by calculating the spatial organisation by means of `run-length statistics' [16,17], we compute a set of coherence lengths defined in the following way.…”

“…References [11] and [12] have faced the skin topography characterisation by processing the skin profile obtained with a capacitance device, to investigate the ageing of skin. Here, we propose to apply at skin characterisation, the image processing procedure previously used to investigate texture transitions in nematic liquid crystals [13][14]. This processing is suitable for images with smooth, scarcely regular textures, as those observed in the microscopy investigation of certain nematic liquid crystal cells.…”