Lehmann effect in a compensated cholesteric liquid crystal: Experimental evidence with fixed and gliding boundary conditions
Laboratoire de Physique, École Normale Supérieure de Lyon, CNRS, Université de Lyon, 46 Allée d'Italie, 69364, Lyon, France
2 Department III, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland
Accepted: 6 February 2008
Published online: 3 April 2008
In a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of Éber and Jánossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). These results prove that one must take into account the chirality of the molecules and the absence of inversion symmetry at the macroscopic scale when deriving the constitutive equations of the phase at the compensation temperature. In this paper, we present the details of our experimental work and a new experiment performed in a sample treated for planar gliding anchoring. The latter experiment, coupled with a numerical simulation, supports the existence of a thermomechanical coupling in a compensated cholesteric.
PACS: 61.30.-v Liquid crystals – / 05.70.Ln Nonequilibrium and irreversible thermodynamics – / 65.40.De Thermal expansion; thermomechanical effects –
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag, 2008