The Twist Bend Nematic: A Case of Mistaken Identity

Abstract

We review the physics underlying Meyer's conjecture of how macroscopic-scale twist and bend conspire within the Frank-Oseen elasticity theory of nematics to create a heliconical arrangement of the uniaxial, apolar nematic director, the so-called "twist bend nematic" ${\mathrm{N}}_{\mathrm{TB}}$. We show that since 2011 a second, lower-temperature nematic phase observed in odd methylene-linked cyanobiphenyl dimers discovered by Toriumi and called ${\mathrm{N}}_{\mathrm{X}}$, has been incorrectly identified as ${\mathrm{N}}_{\mathrm{TB}}$. Moreover, as more quantitative data on the ${\mathrm{N}}_{\mathrm{X}}$ emerged, Meyer's simple prediction has been distorted to accommodate those findings. In fact, the molecular organization in the ${\mathrm{N}}_{\mathrm{X}}$ conforms to the ${\mathrm{N}}_{\mathrm{PT}}$ phase, a polar, twisted arrangement of nonlinear mesogens advanced in 2016. The attributes of the ${\mathrm{N}}_{\mathrm{PT}}$ are summarized and differentiated from those of the ${\mathrm{N}}_{\mathrm{TB}}$ in an effort to contribute to a better understanding of the ${\mathrm{N}}_{\mathrm{X}}$ phase and, equally important, to encourage researchers to continue to search for a liquid crystal that exhibits Meyer's pioneering theoretical suggestion, namely that form-chirality can exist in simple nematics composed of achiral molecules.

10 pages, 2 figures

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