Measurements of the Rotational Viscosity Coefficient and the Shear-Alignment Angle in Nematic Liquid Crystals

A method is described for the direct measurement of the shear-alignment angle in a nematic liquid crystal. Results of such measurements are reported for the nematic phases of three compounds: PAA ($p$, ${p}^{\ensuremath{'}}$-azoxydianisole), MBBA ($p$-methoxybenzylidene-${p}^{\ensuremath{'}}\ensuremath{-}n$-butylaniline), and HBAB ($p\ensuremath{-}n$-hexyloxybenzylidene-${p}^{\ensuremath{'}}$-aminobenzonitrile). We also report measurements of the rotational viscosity coefficient for the same compounds. In contrast to a recent report by G\"ahwiller, no orientational instability was found in HBAB.