Electrorheological study of the nematic LC 4-n-hepthyl-4'-cyanobiphenyl: experimental and theoretical treatment

TL;DR

This study investigates the electrorheological effects in nematic liquid crystal 4-n-hepthyl-4'-cyanobiphenyl through combined experimental flow measurements and theoretical analysis based on Leslie-Ericksen continuum theory, revealing rheological behaviors and material parameters.

Contribution

It provides a comprehensive experimental and theoretical analysis of ER effects in K21 liquid crystal, interpreting rheological behavior via continuum theory and extracting key material parameters.

Findings

Identification of shear-thinning behavior at intermediate shear rates.

Construction of a master flow curve showing Newtonian plateaus.

Extraction of viscoelastic and dielectric anisotropy parameters.

Abstract

The experimental and theoretical study of the electrorheological (ER) effect observed in the nematic phase of the 4-n-hepthyl-4'-cyanobiphenyl (K21) is the aim of this work. The K21 liquid crystal (LC) appears to be a model system where all the observed rheologial behaviours can be interpreted by the Leslie-Ericksen (L-E) continuum theory for low molecular weight liquid crystals. We present the flow curves of our sample for different temperatures and under the influence of an external electric field, ranging from 0 to 3kV/mm, applied perpendicular to the flow direction. We also present the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. A master flow curve was built, dividing the shear rate by the square of the electric field and multiplying by the square of a reference electric field value, for each temperature, where two Newtonian plateaus appear at low and high shear rate values, connected by a shear-thinning region at intermediate shear rate values. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory. In this description the director alignment angle is a function of the electric field and the flow field - boundary conditions are neglected. In this way it was possible to extract some viscoelastic parameters, as well as the dielectric anisotropy.