Interplay between Confinement, Twist Elasticity, and Intrinsic Chirality in Micellar Lyotropic Nematic Liquid Crystals

Abstract

Recent studies have shown that lyotropic nematic liquid crystals (LLCs) are exceptional in their viscoelastic behavior. In particular, LLCs display a remarkable softness to twist deformations, which may lead to chiral director configurations under achiral confinement despite the absence of intrinsic chirality. The twisted escaped radial (TER) and the twisted polar (TP) are the two representative reflection symmetry breaking director configurations in the case of cylindrical confinement with homeotropic anchoring. We demonstrate how such reflection symmetry breaking of micellar LLCs under cylindrical confinement is affected by intrinsic chirality, introduced by the addition of a chiral dopant. Similarities and differences between the effects of intrinsic chirality on the defect-free TER configuration, and on the TP configuration incorporating two half-unit twist disclination lines, are discussed. In the TP case, topological constraints facilitate stable heterochiral systems even in the presence of a small amount of chiral dopant, with unusual regions of rapidly reversing handedness between homochiral domains. At moderate dopant concentrations, the TP structure becomes homochiral. At high dopant concentrations, for which the induced cholesteric pitch is much smaller than the diameter of the capillary, the cholesteric fingerprint structure develops.

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

“…To understand what really happens, it would be necessary to observe the capillaries with a microscope while the magnetic field is applied. Furthermore, such investigations might also provide new information about the fundamental question of the nature of the s = +1 defect core [28]. The TP configuration also occurs in the LCLC Sunset Yellow under capillary confinement, although in this system the TER configuration seems to be the predominant configuration, and of lowest energy, since the TP configuration only forms spontaneously at arbitrary positions like a watersprinkler [5].…”

“…Connecting two half-unit twist disclinations in a capillary confinement, a director field profile suggested as in Figure 6(d-f) emerges, with an additional transverse twist along the capillary diameter between the two twist disclinations. In the double-helix TP configuration, the twist deformations around the two disclination lines have the same sense of chirality, so each pair of disclinations are right handed-right handed (RR) or left handed-left handed (LL) [28]. Since the system under study is achiral, RR and LL must occur with the same probability.…”

“…Adding intrinsic chirality via (R)-mandelic acid as chiral dopant to the micellar N D phase leads to various new structures with which the interplay between confinement, twist elasticity and intrinsic chirality can be studied [28]. In the TP case, the topological constraints of the twist disclinations facilitate stable heterochiral systems even in the presence of a small amount of chiral dopant, with unusual regions of rapidly reversing handedness between homochiral domains.…”

“…Although there has been a certain concentration of chiral dopant at which the TP structure turns from heterochiral to homochiral, there can be no sharp threshold for chiral induction identified, because the TP configuration is topologically constrained. In this sense, the escaped radial configuration is much more sensitive than the TP configuration to chiral doping, because neither topological constraints nor an energy barrier in going from the achiral ER to the chiral TER structure exists for the micellar N D phase [28].…”

“…In the following a detailed study of this lyotropic micellar N D phase confined to cylindrical capillaries with homeotropic boundary conditions is given, analysing the occurring structures by polarised optical microscopy and proposing corresponding director fields of the configurations [24]. 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].…”