The Zwetkoff Experiment

The Zwetkoff experiment is described in Section 5.4. This remarkable experiment, involving a magnetic field which rotates relative to a liquid crystal sample, allows the direct measurement of the rotational viscosity 71 in nematics. It also allows a straightforward measurement of the magnetic anisotropy Xa if it is positive, via the observation of a critical angular frequency c c-... 

Figure 5.4 The coordinates and notation introduced to describe the alignment of the director n and magnetic field H in the Zwetkoff experiment. The nematic liquid crystal is contained in a cylinder of radius R and height L as shown. The magnetic field rotates with angular velocity u aroimd the 2-axis, with the phase angles for n and H being given by 0 and respectively.

Boundary conditions have been neglected in the above exposition. For a short discussion on the possible effects of boundary conditions on the Zwetkoff experiment the reader should consult the review by Leslie [168] and the references cited therein. Brief comments on anchoring conditions and the behaviour of possible defects can be fotmd in de Gennes and Frost [110, pp.223-225], together with a description of another similar experiment involving controlled boundary conditions for a sample in... 

In this Zwetkoff experiment, the inward unit normal to the curved surface So of the cylinder is... 

To derive expressions for the dynamic contribution we must make some assumptions as to the relevant variables to include in these terms. It seems reasonable given the experiments of Zwetkoff [4] and Miesowicz [S] to assume that at any material point T and L are functions of n, Vv and w, evaluated at that point at that instant. However, since the gradient of the angular velocity is not included in our list of kinematic variables, it immediately follows from EQN (18) that... 

The experiments of Miesowicz [201] and Zwetkoff [288] suggest that Uj has a linear dependence upon N and A, and accepting such a linear dependence we can "write... 

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