Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications. Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate. Here we present a technique to measure the bend modulus K33 of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal ChOH cholesteric structure. K33 is typically smaller than 0.6 pN, showing a nonmonotonous temperature dependence with a slight increase near the transition to the twist-bend phase. K33 depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB) and rodlike molecules such as pentylcyanobiphenyl (5CB) show a K33 value that is 5 times smaller than K33 of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, K33 in CB11CB doped with a chiral agent is noticeably smaller than K33 in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals.