Shear Flow in the Presence of Electric Fields: A Method for Separate Determination of Elastic and Viscous Constants of Nematics

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Abstract
We have investigated a method for separate determination of elastic and viscous constants for nematics in a rotational shear flow experiment. By inducing a shear flow perpendicular to the initial molecular alignment and simultaneously applying an electric a.c. field at right angles to the flow it is possible to determine not only k 11/k 33, (γ 2-γ 1)/k 33 and γ z,/γ 1, but split the combinations into k 11, k 33, γ 1, γ 2 and n 2 (Miesowicz). Although the flow solutions tend to be instable in the case of negative dielectric anisotropy, the required electric fields can be shown to lie far below the instability thresholds. Thus the method can be quite generally applied to all nematics, of both ϵ > 0 and ϵ < 0. Together with the widely used method for determining k 22 (unwinding) it offers a rapid and accurate access to the most important elastic and viscous constants of nematic liquids.