The twist bend nematic: a case of mistaken identity.

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' NTB. We show that since 2011 a second, lower-temperature nematic phase observed in odd methylene-linked cyanobiphenyl dimers discovered by Toriumi and called NX, has been incorrectly identified as NTB. Moreover, as more quantitative data on the NX emerged, Meyer's simple prediction has been distorted to accommodate those findings. In fact, the molecular organisation in the NX conforms to the NPT phase, a polar, twisted arrangement of nonlinear mesogens advanced in 2016. The attributes of the NPT are summarised and differentiated from those of the NTB in an effort to contribute to a better understanding of the NX 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. [ABSTRACT FROM AUTHOR]