Backflow, shear viscosity

The anisotropy x l of the magnetic susceptibility must be determined in a separate experiment. The relaxation time after a rapid rotation by a small angle obeys Fq. (43). For angles above 45°, and especially for angles around 90°, the director relaxation will be strongly inhomogeneous due to backflow [36]. This effect can be utilized to determine shear viscosities besides the rotational viscosity for high viscosity and polymeric materials. 

As the transmission curve of a liquid crystal display during the switching process depends on all the Leslie coefficients due to backflow effects, it is possible to determine the coefficients from the transmission curve. In analogy to light scattering, the coefficients are obtained with different accuracies [38, 39]. The investigation of torsional shear flow in a liquid crystal [31-35] allows the determination of quantities from which some Leslie coefficients can be determined, if one shear viscosity coefficient is known. 

An externally applied torque on the director can only be transmitted to the surfaces of a vessel without shear, if the director rotation is homogeneous throughout the sample as assumed in Sect. 8.1.2 for the rotational viscosity. Otherwise this transmission occurs partially by shear stresses. The resulting shear flow is called backflow. As there is usually a fixed director orientation at the surfaces of the sample container, a director rotation in the bulk of the sample by application of a routing magnetic field leads to an inhomogeneous rotation of the director and to a backflow [42]. 

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