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 (e₁ + e₃) 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 (e₁ + e₃), the rotational viscosity coefficient γ1 as well as the anchoring strengths at the two surfaces. Our measurements on CCH-7 show the following interesting results. a) The extrapolation length L₁ 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) (e₁ + e₃)/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.