An AC Electrooptic Technique for Measuring the Flexoelectric Coefficient (e1 + e3) and Anchoring Energies of Nematics

Abstract

We report an AC technique to measure both the sign and magnitude of (e 1 + e 3 ) using electrooptic measurements on a hybrid aligned nematic cell. A detailed numerical analysis of the optical signal allows us to obtain the temperature dependences of (e 1 + e 3 ), the rotational viscosity coefficient γ 1 as well as the anchoring strengths at the two surfaces. Our measurements on CCH-7 show the following interesting results. a) The extrapolation length L 1 at the homeotropic surface shows a small decrease as the temperature is increased to 66 °C beyond which it shows a sharp increase which we interpret as arising from a softening of the chains of the aligning ODSE layer. b) The slope of the temperature dependence of the extrapolation length L u at the homogeneous surface changes sign around 72 °C. c) (e 1 + e 3 )/S is a constant upto 72 °C and decreases rapidly beyond this temperature. The last two results are attributed to an increase in the conformational freedom between the rings of CCH-7 molecules beyond that temperature.