Twist viscosity of mixtures of low molar mass nematics

Abstract

Measurements of the twist viscosity, γ₁(DLS) and twist elastic coefficient, K₂₂(DLS) by electric-field-dependent dynamic light scattering (EFDLS) are reported for low molar mass nematics (LMMNs) 4′-heptyl-4-cyanobiphenyl (7CB) and 4′-octyl-4-cyanobiphenyl (8CB), and their binary mixtures at several temperatures in the nematic state. The results are compared with values (γ₁(Rheol)=α₃–α₂) computed from rheological measurements of the Leslie viscosities α₂ and α₃. For the binary mixtures, at each temperature, the measured twist viscosity γ₁(DLS) and corresponding twist elastic constant K₂₂(DLS) show approximately a linear additive dependence on concentration. The calculated twist viscosity, γ₁(Rheol), agrees with γ₁(DLS) for the pure components, but is significantly smaller for the binary mixtures. Our observations appear to be consistent with a recent report of a discrepancy between values of the tumbling parameter λ, determined using a small-strain oscillatory optical technique, vs those measured by a rheological method. These results suggest that, in the rheological measurements at large strains, the rate of director rotation for mixtures may be affected by a flow-induced change in structure, e.g., shear-induced biaxiality.