Effet Lehmann dans les cristaux liquides cholestériques

Abstract : Cholesteric liquid cristals usually form a macroscopic helicoidal structure due to the presence of chiral molecules. In these particular systems, a linear thermomechanical coupling can be observed, called the Lehmann effect: a temperature gradient is able to generate a torque on the local orientation of the molecules. To explain this effect, Leslie introduced a phenomenological coupling coefficent ν, which can be non-zero only if the phase is chiral.
Moreover, there exist cholesterics called compensated, the helicoidal structure of which spontaneously unwinds at a particular temperature, leading to a non chiral structure. In this thesis, we show experimentally, that ν does not vanish at the unwinding temperature. This implies that the phase is still chiral at that temperature, a point that was up to now contentious. Furthermore, the static and dynamical experiments presented here enable to estimate the order of magnitude of the coefficient ν.
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Alain Dequidt. Effet Lehmann dans les cristaux liquides cholestériques. Dynamique des Fluides [physics.flu-dyn]. Ecole normale supérieure de lyon - ENS LYON, 2008. Français. ⟨tel-00376381⟩

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