Rotation in a magnetic field of nematic liquid crystalline samples with a negative anisotropy of their magnetic susceptibility

The behaviour of the mesophase directors when a sample with a negative anisotropy Deltachi in the magnetic susceptibility is rotated at a speed omega in a magnetic field B0 about an axis inclined at an angle beta is investigated by deuterium NMR spectroscopy. There are combinations of beta and omega which lead to the directors aligning along the spinning axis (stable region), and other combinations where the directors are either unaffected by the rotation, or have a timedependent distribution (unstable region). These two regions are separated by a line (the q line) whose position depends on beta and omegac DeltachiB02/2mu0gamma1, where gamma1 is a twist viscosity coefficient. Experiments on 4-n-propyl-4-cyanobicyclohexane (CCH3) are used to demonstrate how the q line, and hence omegac may be determined. Combining this result with a measurement of the anisotropy in the magnetic susceptibility, Deltachi, yields a value for gamma1.