A simple method to measure the twist elastic constant of a nematic liquid crystal

Abstract

We demonstrate a simple method for measuring the twist elastic constant (K 22 ) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180°in a homogeneous cell, which is known as 180°super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K 22 and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K 22 values agree well with those reported by using other methods.

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

“…Xiang et al [6] explored the very same dimer CB7CB at the temperature 106 0 C and found that 11 K =5.7 pN and 22 K = 2.6 pN, i.e., 11 22 / 2.2 KK  , as expected; for CB7CB doped with 1wt % of the chiral dopant S811, 33 K was determined to be 0.3 pN. Finally, Parthasarathi et al [18] measured 33 K by the extrapolation approach similar to that of Yun et al [16] but arrived at a very different conclusion that 33 K of pure CB7CB is monotonically increasing as the sample is cooled down. It is only when CB7CB is mixed with a monomer 7OCB in some proportions that 33 K shows a non-monotonous behavior, first increasing and then decreasing as the temperature is lowered from the clearing point ( NI T ).…”

“…The latter result is somewhat surprising, as 11 22 / KK is expected to be larger than 2 in CB7CB, since this material does exhibit the Ntb phase rather than a splay-bend nematic, as demonstrated in the freeze-fracture transmission electron microscopy studies by Borshch et al [4] and by the resonant carbon soft X-ray scattering by Zhu et al [17]. Xiang et al [6] explored the very same dimer CB7CB at the temperature 106 0 C and found that 11 K =5.7 pN and 22 K = 2.6 pN, i.e., 11 22 / 2.2 KK  , as expected; for CB7CB doped with 1wt % of the chiral dopant S811, 33 K was determined to be 0.3 pN. Finally, Parthasarathi et al [18] measured 33 K by the extrapolation approach similar to that of Yun et al [16] but arrived at a very different conclusion that 33 K of pure CB7CB is monotonically increasing as the sample is cooled down.…”

“…Finally, Parthasarathi et al [18] measured 33 K by the extrapolation approach similar to that of Yun et al [16] but arrived at a very different conclusion that 33 K of pure CB7CB is monotonically increasing as the sample is cooled down. It is only when CB7CB is mixed with a monomer 7OCB in some proportions that 33 K shows a non-monotonous behavior, first increasing and then decreasing as the temperature is lowered from the clearing point ( NI T ).…”

“…In particular, Dozov [9] suggested that the twist-bend nematic could appear upon cooling of a uniaxial nematic formed by the molecules of a bent shape. The model [9] suggests that the bent shape can result in spontaneous bend deformation of the director and a change of the sign of the bend elastic constant 33 K from positive to negative as one crosses the N -Ntb transition point ( Ntb T ).…”

“…Balachandran et al [19] used the splay Frederiks transition approach for another typical bimesogen, CB11CB, and found that as the temperature decreases, 33 K becomes smaller than 11 K ; namely, 33 K =6 pN, while 11 K =15.5 pN near Ntb T . Adlem et al [20] explored multicomponent mixtures of dimeric materials by the dynamic light scattering; the latter allows one to extract all three bulk elastic constants of the material in the region close to the phase transition.…”