Nematic liquid crystal viscosity: Inadequacies of microscopic theories

Recently Janik er al. reported measurements of the Miesowicz viscosity coef ficients of the nematic liquid crystal and confronted them with the results of the available theoretical models. They found that none of them can be s uccessfully applied for the purpose of the Miesowicz-type experiment. In th is paper I present an explanation why the microscopic theories seem inadequ ate. In particular, I analyze the applicability and conclusions of the Osip ov-Terentjev model, whose final predictions are highlighted to be the same as those of the Kuzuu-Doi theory. It has been shown that the microscopic th eories can perfectly explain the behavior of the Miesowicz coefficients dif ference eta(3) - eta(2) but are inadequate to study their ratio eta(3) /eta (2), which is contributed by the Leslie coefficient alpha(4). A disagreemen t between experimental data and the theoretical results for alpha(4) is att ributed to the fact that the isotropic contribution to alpha(4) in the nema tic phase is beyond the scope of the theories that are based on the anisotr opic orientational distributions.