Extremely small twist elastic constants in lyotropic nematic liquid crystals

Abstract

Recent measurements of the elastic constants in lyotropic chromonic liquid crystals (LCLCs) have revealed an anomalously small twist elastic constant compared to the splay and bend constants. Interestingly, measurements of the elastic constants in the micellar lyotropic liquid crystals (LLCs) that are formed by surfactants, by far the most ubiquitous and studied class of LLCs, are extremely rare and report only the ratios of elastic constants and do not include the twist elastic constant. By means of light scattering, this study presents absolute values of the elastic constants and their corresponding viscosities for the nematic phase of a standard LLC composed of disk-shaped micelles. Very different elastic moduli are found. While the splay elastic constant is in the typical range of 1.5 pN as is true in general for thermotropic nematics, the twist elastic constant is found to be one order of magnitude smaller (0.30 pN) and almost two orders of magnitude smaller than the bend elastic constant (21 pN). These results demonstrate that a small twist elastic constant is not restricted to the special case of LCLCs, but is true for LLCs in general. The reason for this extremely small twist elastic constant very likely originates with the flexibility of the assemblies that are the building blocks of both micellar and chromonic lyotropic liquid crystals.

Lyotropic nematic liquid crystals: interplay between a small twist elastic constant and chirality effects under confined geometries

“…With that, the corresponding viscosities of the splay, twist and bend deformations η splay , η twist and η bend are calculated. A value of (1.65 ± 0.09) kgm −1 s −1 is found for η splay , (5.28 ± 0.32) kgm −1 s −1 for η twist and (87 ± 6) kgm −1 s −1 for η bend [29]. Note that the measurements were performed at room temperature.…”

“…[29]. K 11 = (1.54 ± 0.08) pN and almost two orders of magnitude smaller than the bend elastic constant K 33 = (20.8 ± 1.3) pN [29]. With that, the corresponding viscosities of the splay, twist and bend deformations η splay , η twist and η bend are calculated.…”

“…The table is reprinted with permission from Ref. [29]. K 11 = (1.54 ± 0.08) pN and almost two orders of magnitude smaller than the bend elastic constant K 33 = (20.8 ± 1.3) pN [29].…”

“…With this, it is not only shown that a small K 22 is independent of the shape of the building block, but also that the composition of the building block, and therefore the intermolecular forces being responsible for the aggregation behaviour seem to have little effect on this phenomenon. For all LLCs for which a one order of magnitude smaller K 22 was measured (compared to K 11 and K 33 ), either chromonic, polymeric or micellar [13,14,29,45,47], reflection symmetry-breaking director configurations under confinement are reported [5,24,54]. In other words, confinement induced chiral structures, like the TER or the TP configuration under capillary confinement, point to a small K 22 .…”

“…Furthermore, the addition of intrinsic chirality to this system is studied [28]. Last but not least, depolarised dynamic light scattering was performed to measure the viscoelastic properties of this micellar N D phase [29]. The content of this paper is based on Ref.…”