Determination of the rotational viscosity coefficient γ1of nematic liquid crystals

Abstract

An apparatus for the measurement of the rotational viscosity coefficient gamma ₁ of nematic liquid crystals is described. The commonly used method with a rotating magnetic field is improved by using a rotating sample and a fixed magnetic field, which allows larger field strengths and sample diameters. This technique and a precise determination of the torque on the sample lead to relative errors of measurement of the order of 0.3%. Wall effects, which can cause serious errors, are discussed. Their influence on the gamma ₁ values obtained is small if magnetic fields of sufficient strength are used. Kinematic and dynamic rotational viscosity coefficients for the nematic liquid crystals MBBA and 4-n-octyloxyphenyl 4-n-pentyloxybenzoate are presented.