New Method for Measuring the Twist Elastic Constant $\frac{K_{22}}{{\chi }_a}$ and the Shear Viscosity $\frac{{\gamma }_1}{{\chi }_a}$ for Nematics

By monitoring the change in the interference figure for an oriented nematic (optic axis perpendicular to the incident light cone) as twist deformation is induced by a magnetic field (Freedericksz transition), one is led to a simple and direct method for measuring $\frac{K_{22}}{{\chi }_a}$ and $\frac{{\gamma }_1}{{\chi }_a}$. An analytic expression is obtained for the effect of twist on the interference figure and is verified experimentally.