Twisted topological defect lines in achiral nematic liquid crystal confined in glass capillaries

Chirality occupies a remarkable space in biology and soft matter. In liquid crystals, chirality at the molecular scale propagates to the macroscale in non-trivial ways and can emerge even from achiral components. Here we report that common nonchiral thermotropic liquid crystals confined in square capillary tubes with homeotropic anchoring can show a spontaneous twist during the phase transition from smectic-A to nematic. The chiral structure is transient, but it can be stabilized for a few hours by accurate control of the temperature. We hypothesize that this chiral structure is related to the different rate of change of the twist and bend elastic constants at the phase transition. However, also the shape of the confining capillary plays a key role. In fact, this effect is clearly seen in capillaries, with square cross-section: the edges of the capillary, on the one hand, stabilize disclination lines in the nematic phase, and on the other hand they induce the formation of many defects in the smectic phase.