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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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〉
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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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