Lehmann rotatory power: a new concept in cholesteric liquid crystals

Abstract

By doping a nematic phase with a chiral molecule one obtains a cholesteric phase. Each chiral molecule is characterised by its helical twisting power (HTP) which is defined as HTP = q/(2πC) where q is the equilibrium pitch of the cholesteric phase and C the concentration (in wt%) of chiral molecules. In a similar way, we define the Lehmann rotatory power (LRP) as LRP = v/(2πC) where v is the thermomechanical Lehmann coefficient. By making compensated mixtures, we measured the HTP and the LRP of five chiral molecules (R811, S2011, CC, CB15 and CE4) dissolved in an eutectic mixture 8CB/8OCB. We found that, although these quantities were different, their ratio R = LRP/HTP changed little from one molecule to another. This result shows that the Lehmann effect is closely, but not completely, related to the twist of the phase.

Keywords:

• liquid crystals
• cholesteric
• helical twisting power
• thermomechanical Lehmann effect
• compensated mixture

Acknowledgements

A.Z. would like to express his thanks to the Centre National de la Recherche Scientifique for financial support and to the Ecole Normale Supérieure of Lyon for hospitality. The authors are grateful to A. Pretrosyan for lending them his rubbing machine to prepare the planar samples and to A. Dequidt for fruitful discussions.

Notes

1. Note that there was an error in the value of K ₃₂ given in [4].

2. Note that this value is compatible with that we can calculate from the data given in (4). In this work, we found for , which gives in very good agreement with the value given in the present work.