Measurements of Hydrodynamic Parameters for Nematic 5CB

Abstract

A nematic liquid crystal, pentyl-cyano-biphenyl (5CB) is studied in two different flow situations: Poiseuille flow and torsional shear flow, in both cases with and without the simultaneous application of an electric field. The Poiseuille flow set-up gives an accurate determination of the Miesowicz viscosity η_c and a less precise estimation of η_b. With η_c known it is possible to separately determine the elastic constants K ₁₁ and K ₃₃ and the viscous coefficients α₂, α₃, and thereby γ₁, γ₂ from the torsional shear flow experiment. Using the data we have also been able to calculate the actual director profile in the liquid crystal as a function of applied shear and electric field. Curves are given of K ₁₁, K ₃₃, K ₁₁/K ₃₃, α₂, α₃, γ₁, γ₂, γ₁/γ₂, η_b and η_c for 5CB as a function of temperature through the whole nematic range from 22.0 °C to 35.1 °C. After the completion of our manuscript we have noticed the recent article by J. Wahl, Z. Natur-forsch. 34a, 818 (1979). Wahl discusses shear flow in the presence of an electric field for MBBA which is a material with negative dielectric anisotropy. This means that the field itself has a similar effect as the shear, instead of counteracting it as in our case. Wahl's analysis is considerably different from ours. In particular he makes the simplification that the electric field has no influence on the velocity gradient, which might be valid in his case of MBBA, where the dielectric anisotropy is about a tenth of that for 5CB.