An experimental investigation of electromechanical coupling in cholesteric liquid crystals

Madhusudana, N. V. ; Pratibha, R. (1989) An experimental investigation of electromechanical coupling in cholesteric liquid crystals Liquid Crystals, 5 (6). pp. 1827-1840. ISSN 0267-8292

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Related URL: http://dx.doi.org/10.1080/02678298908045691

Abstract

Cholesteric liquid crystals which have a helical arrangement of oriented chiral molecules are expected to show novel cross couplings between fluxes and forces. The most convincing demonstration of these couplings is through a dynamical effect on the structure. Though Lehmann found a rotation of the structure under a temperature gradient in 1900, there has been no subsequent experiment confirming the same. We argue that it is very difficult to obtain a sufficiently weak anchoring of the director at the solid-cholesteric interface which is a necessary condition for the occurrence of Lehmann rotation. In order to achieve a practically zero anchoring energy at the surface, we have devised a simple technique of floating essentially flat cholesteric drops in the isotropic phase. Using this configuration we study the electromechanical coupling which produces a rotation of the structure under the action of a DC electric field. Using measurements on samples with different values of the pitch the relevant electromechanical coefficient of the materials investigated is found to satisfy the relation vE = −0.6 × 10−12(q/m−1)J m−2, where q is the wavevector of the helix, whose sign is positive (negative) for a right (left) handed structure, confirming that vE is hydrodynamic in origin.

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