B4 and dark conglomerate phases of bent-core liquid crystals
In 1997, a year after the rediscovery of banana liquid crystals,1,2 the Boulder group showed that achiral, bent-core liquid crystal molecules form polar, chiral smectic phases when the molecules are tilted in the layers, the first example of spontaneous chiral resolution in a fluid.3 The B4 phase was also observed around this time, and the dark conglomerate a few years later, but their underlying structures, though the subject of much speculation, were not understood. Ten years later, optical microscopy, x-ray scattering, AFM and freeze-fracture transmission electron microscopy have resulted in the first complete picture of these mysterious phases. In both phases, the layers have significant saddle splay curvature.The B4 phase is a unique variant of the twist grain boundary phase, being made of bundles of twisted, rope-like smectic ribbons (helical nanofilaments) of finite lateral extent, which pack together to produce macroscopic left- and right-handed domains that rotate plane-polarized light and can typically be resolved in the microscope.4 The dark conglomerate (DC) phase is an optically isotropic, three-dimensional assembly of curved smectic layers that form a sponge-like structure filled with smectic layers that minimizes the global free energy. The dark conglomerate phase exhibits little birefringence but is optically active and exhibits large homochiral domains in cells.5
B4 and DC FFTEM textures

Figure. The B4 banana phase (top) is a nanoporous assembly of helically twisted smectic layers. The sponge-like organization of the layers in the dark conglomerate phase (bottom) is analogous to the plumber's nightmare structure of lytotropic liquid crystals.


[1] T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, Journal of Materials Chemistry 6, 1231 (1996).
[2] T. Akutagawa, Y. Matsunaga, K. Yasuhara, Liquid Crystals 17, 659 (1994).
[3] D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E. Körblova, and D. M. Walba, Science 278 , 1924-1927 (1997).
[4] L. E. Hough, H. T. Jung, D. Krüerke, M. S. Heberling, M. Nakata, C. D. Jones, D. Chen, D. R. Link, J. Zasadzinski, G. Heppke, J. P. Rabe, W. Stocker, E. Körblova, D. M. Walba, M. A. Glaser, N. A. Clark, Science 325, 456 (2009).
[5] L. E. Hough, M. Spannuth, M. Nakata, D. A. Coleman, C. D. Jones, G. Dantlgraber, C. Tschierske, J. Watanabe, E. Körblova, D. M. Walba, J. E. Maclennan, M. A. Glaser, N. A. Clark, Science 325, 452 (2009). The issue includes a Perspectives article (D. Amabilino, Science 325, 402 (2009)).

This research was the subject of Loren Hough's doctoral thesis.