An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4′-cyanobiphenyl

Abstract

An experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4′-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie–Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm⁻¹, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log–log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie–Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.

Keywords:

• electrorheology
• liquid crystals
• 7CB
• continuum theory

Acknowledgement

The authors are grateful to Professor João Paulo Casquilho for many fruitful discussions.