A Comparative Study of the Viscosity Coefficients of Some Nematic Liquid Crystalss

Abstract The viscosity coefficients η 1 , η 2 , η 3 and η 12 of 4‐n‐pentyl‐4'‐cyanobiphenyl, 4‐n‐octyl‐4'‐cyanobiphenyl, 4‐methoxy‐4'‐n‐butylazoxybenzene and an aromatic ester mixture were determined in the nematic phase region. Except for 4‐n‐octyl‐4'‐cyanobiphenyl, the viscosity coefficients of the liquid crystals show the following properties: 1) Three of the viscosity coefficients are related by an equation of the form Σ A i η i = 0, in which the corresponding constants A i are very similar for the different liquid crystals. 2) The equation η i =a ij η j + b ij is valid with the exception of the neighbourhood of the clearing point. 3) The reduced viscosity coefficients 3η i /(η 1 + η 2 + η 3 ) of the different liquid crystals exhibit a very similar dependence on the reduced temperature and a nearly linear dependence on the degree of order. The viscosity coefficients η 2 and η 12 of 4‐n‐octyl‐4' ‐cyanobiphenyl show a divergence at the transition to the smectic phase, whereas η 1 and η 3 show the normal temperature dependence.