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Journal Articles EPL - Europhysics Letters Year : 2008

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

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Patrick Oswald
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Alain Dequidt

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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Dates and versions

ensl-00351719 , version 1 (10-01-2009)

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Patrick Oswald, Alain Dequidt. Direct measurement of the thermomechanical Lehmann coefficient in a compensated cholesteric liquid crystal. EPL - Europhysics Letters, 2008, 83 (1), pp.16005. ⟨10.1209/0295-5075/83/16005⟩. ⟨ensl-00351719⟩
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