Rotational viscosities in ferroelectric smectic liquid crystals

The usefulness of liquid crystals in many applications relies on the possibility to induce structural changes by an externally applied agent, e.g. an electric field. The dynamical behaviour of the related switching processes may often be described by introducing a suitable viscosity term in the equation of motion. Viscosity effects in nematics are handled consistently within the Leslie-Ericksen model, while for the smectic C* phase a simplified nematic-like description is often used. In the present work, the viscosity notation in the nematic and smectic C phases is discussed with special emphasis on rotational viscosities. A scaling rule between the nematic rotational viscosity and the smectic C* Goldstone mode viscosity can be given. The experimental techniques for measuring rotational viscosities in ferroelectric liquid crystals are reviewed.