An experimental investigation of electromechanical coupling in cholesteric liquid crystals

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 v_E = −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 v_E is hydrodynamic in origin.