Surface anchoring energy of an octyl-cyanobiphenyl liquid crystal (8CB) deduced from the shear stress response

Abstract

The surface anchoring energy of the nematic liquid crystal 4-octyl-4ʹ-cyanobiphenyl (8CB) has been deduced from the transient shear stress response, making use of the fact that shearing a tumbling nematic with strong anchoring on the shearing plate generates elastic forces that push the director toward the vorticity axis in the bulk. This feature is manifested in 8CB near the nematic–smectic transition by a long-term decay in the shear viscosity, whose rate depends on the anchoring energy, A_s . The calculations have been performed for the natural boundary conditions for the director associated with the stationary states of the total free energy, including a surface term proportional to A_s . It has been found that A_s ≈ 3.3 × 10⁻⁶ J m⁻² for metallic plates covered with lecithin, in agreement with data obtained by the classical method for glass substrates covered with lecithin.

Keywords:

• surface anchoring energy
• nematic 8CB
• transient shear stress
• rheology

Acknowledgements

This study was partly supported by Fundação para a Ciência e a Tecnologia (Portugal) under the research contract C2007–443–CENIMAT–1 with A. Véron.

Notes

1. The Leslie viscosities satisfy the Parodi relation α₂ + α₃ = α₆ − α₅.

2. For aligning nematics a difference would occur only in a thin layer close to the plate and may be considered as unimportant for bulk properties.

3. By Neumann-like boundary conditions we mean a relation between the normal gradients of the director and the director on the plate.

4. and denotes the part of the shear stress components and that depends only on the molecular field.