Visual observation of dispirations in liquid crystals

ArticleinPhysical review. B, Condensed matter 45(14):7684-7689 · May 1992with4 Reads
DOI: 10.1103/PhysRevB.45.7684 · Source: PubMed
In this work, wedge-screw and twist-edge dispirations in liquid crystals were visually observed. In smectic phases, such as the antiferroelectric SmCA and the dimeric SmC2, where the molecules or the mesogenic groups in the adjacent smectic layers tilt in opposite senses, we found the schlieren texture with the strength of m=+/-1/2 which should not be seen in the normal SmC phase. The observation indicates the existence of a wedge-screw dispiration; i.e., a pi-wedge disclination (m=1/2) accompanied by a screw dislocation that is characterized by the Burgers vector b whose magnitude is equal to the layer thickness d. A threadlike defect line with +/-pi-wedge disclinations at the ends observed in freely suspended films manifests a twist-edge dispiration; i.e., a linked structure of a pi-twist disclination and a ||b||=d edge dislocation.
    • "Thus, in a regular SmC phase without any inclusions, we can normally expect no more than two defects per shell interface. However, in the SmC a phase, the sign invariance is re-established through the possibility of dispirations [28], a special type of defect allowed by the anticlinic tilting arrangement that is the hallmark of this smectic-C type phase. In a shell made from this phase, we can thus again expect four defects per shell interface, although the alignment is quasi-homeotropic as induced by standard ionic surfactants. "
    [Show abstract] [Hide abstract] ABSTRACT: Thin liquid crystalline shells surrounding and surrounded by aqueous phases can be conveniently produced using a nested capillary microfluidic system, as was first demonstrated by Fernandez-Nieves et al. in 2007. By choosing particular combinations of stabilizers in the internal and external phases, different types of alignment, uniform or hybrid, can be ensured within the shell. Here, we investigate shells in the nematic and smectic phases under varying boundary conditions, focusing in particular on textural transformations during phase transitions, on the interaction between topological defects in the director field and inclusions in the liquid crystal (LC), and on the possibility to relocate defects within the shell by rotating the shell in the gravitational field. We demonstrate that inclusions in a shell can seed defects that cannot form in a pristine shell, adding a further means of tuning the defect configuration, and that shells in which the internal aqueous phase is not density matched with the LC will gently rearrange the internal structure upon a rotation that changes the influence of gravity. Because the defects can act as anchor points for added linker molecules, allowing self-assembly of adjacent shells, the various arrangements of defects developing in these shells and the possibility of tuning the result by modifying boundary conditions, LC phase, thickness and diameter of the shell or applying external forces make this new LC configuration very attractive.
    Full-text · Article · Apr 2013
  • [Show abstract] [Hide abstract] ABSTRACT: We have closely measured the layer thicknesses of three different series of liquid crystals by X-ray diffraction. The layer thickness in the three series of compounds was found to be correlated with the degree of stabilizing ability for the antiferroelectric structure, though the molecular structures are almost the same. The compounds with the antiferroelectric SmCA* phase were of somewhat lesser layer thickness than that which the stable SmC* phase would be expected to have. This fact supports a previously proposed idea that the molecular dimerization in adjacent layers is important as the origin for the appearance of the SmCA* phase.
    Article · Jan 1993
  • [Show abstract] [Hide abstract] ABSTRACT: The differences in the Langmuir film properties of a liquid crystalline homopolymer, its corresponding mesogenic side group moiety, and two copolymers are discussed. The results from Brewster angle microscopy are compared with those from the standard methods that are commonly used to infer the structure of a monolayer (i.e., isotherms). As expected, the viscosity and stability of the films increased for the homopolymer while the onset mean molecular area (MmA) remained similar to that of the monomer. Although this similarity may lead to the assumption that the structure of the two films is the same, Brewster angle microscopy showed a distinct difference between both compounds. Whereas the monomer exhibits liquid crystalline condensed phases with large domains in which the molecular tilt is uniform, the homopolymer is in a solid amorphous state without any uniformly tilted regions large enough to be observed by the microscope. In contrast to the homopolymer, the copolymer is highly fluid with a distinctly higher MmA. Again, no domains with a uniform tilt were observed, probably due to the increased distance between the mesogens.
    Article · Mar 1993
Show more