Shear deformation of homeotropic monodomains: Temperature dependence of stress response for flow‐aligning and tumbling nematics

The rheological behavior of two low molar mass nematogens (LMMN), pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB), is studied. Over a major part of its nematic phase, 8CB shows shear stress oscillations in flow startup and step flow, characteristic of director tumbling, whereas pure 5CB has no oscillatory response but, over its entire nematic range, shows a stress overshoot, typical of flow alignment. For 8CB, the oscillation periodicity is a strong function of temperature, particularly near the Sm A–N transition, and near the N–I transition where the crossover from tumbling to flow‐aligning response occurs. The transient shear response functions of each material are analyzed at different temperatures via Ericksen’s transversely isotropic fluid model to extract the Leslie viscosity coefficients α₁, α₂, and α₃ and the Miesowicz viscosity η_b. Our results are in good agreement with reported literature values.