Giant-Block Twist Grain Boundary Smectic Phases

• Published in: Proceedings of the National Academy of Sciences - PNAS 2005-10, Vol.102 (40), p.14191-14196
• Main Authors: Fernsler, J., Hough, L., R.-F. Shao, MacLennan, J. E., Navailles, L., Brunet, M., Madhusudana, N. V., Mondain-Monval, O., Boyer, C., Zasadzinski, J., Rego, J. A., Walba, D. M., Clark, N. A., Lubensky, Tom C.
• Format: Article
• Language: English
• Subjects: Chirality; Cooling; Electric fields; Flux density; Grain boundaries; Liquid crystals; Materials research; Materials science; Physical Sciences; Physics; Screw dislocations; Surface texture; Temperature; Temperature dependence
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0500664102

Study of a diverse set of chiral smectic materials, each of which has twist grain boundary (TGB) phases over a broad temperature range and exhibits grid patterns in the Grandjean textures of the TGB helix, shows that these features arise from a common structure: "giant" smectic blocks of planar layers of thickness$l_{b}>200$nm terminated by GBs that are sharp, mediating large angular jumps in layer orientation between blocks (60° < Δ < 90°), and lubricating the thermal contraction of the smectic layers within the blocks. This phenomenology is well described by basic theoretical models applicable in the limit that the ratio of molecular tilt penetration length-to-layer coherence length is large, and featuring GBs in which smectic ordering is weak, approaching thin, melted (nematic-like) walls. In this limit the energy cost of change of the block size is small, leading to a wide variation of block dimension, depending on preparation conditions. The models also account for the temperature dependence of the TGB helix pitch.