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# Direct measurement of the thermomechanical Lehmann coefficient in a compensated cholesteric liquid crystal

Abstract : The thermomechanical Lehmann coefficient \nu is directly measured as a function of temperature in a compensated cholesteric liquid crystal. The method consists of observing the continuous rotation of the director in samples treated for planar sliding anchoring when a temperature gradient is applied perpendicularly to the director. The main result is that there is no relationship between the Lehmann coefficient and the equilibrium twist $q$. In particular, we confirm that $\nu$ does not vanish at the compensation temperature at which $q=0$, in agreement with previous static measurements of Éber and Jánossy (Mol. Cryst. Liq. Cryst. {\bf 72}, (1982) 233) and of ourselves (EuroPhysics Lett. {\bf 80}, (2007) 26001). In addition, the sign of the Lehmann coefficient is determined by observing between crossed polarizers the sense of rotation of the extinction branches of the disclination lines.
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https://hal-ens-lyon.archives-ouvertes.fr/ensl-00351719
Contributor : Patrick Oswald Connect in order to contact the contributor
Submitted on : Saturday, January 10, 2009 - 5:42:01 PM
Last modification on : Friday, February 12, 2021 - 10:58:06 AM
Long-term archiving on: : Tuesday, June 8, 2010 - 5:23:58 PM

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OswDequidtEPL08.pdf
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### Citation

Patrick Oswald, Alain Dequidt. Direct measurement of the thermomechanical Lehmann coefficient in a compensated cholesteric liquid crystal. EPL - Europhysics Letters, European Physical Society/EDP Sciences/Società Italiana di Fisica/IOP Publishing, 2008, 83 (1), pp.16005. ⟨10.1209/0295-5075/83/16005⟩. ⟨ensl-00351719⟩

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