MAGNETOHYDRODYNAMIC EFFECTS IN NEMATIC LIQUID-CRYSTALS

The behaviour of a nematic liquid crystal when it is spun about an axis orthogonal to a magnetic field is predicted to be controlled by the critical angular velocity, OMEGA(c). For spinning speeds below OMEGA(c) theory shows that the director makes an increasing angle with the field until at OMEGA(c) this angle is 45-degrees. Above OMEGA(c) the director should rotate with an angular velocity slightly less than that of the sample. Observation in both regimes allows OMEGA(c) to be determined; since it depends on the ratio of the diamagnetic anisotropy to the rotational viscosity coefficient of the nematic, this ratio can be measured. However, an experimental investigation by Eastman et al. [1], suggests that the theoretical relationship between OMEGA(c) and this ratio may be in eff or by a factor of about four. We have reanalysed their data in an attempt to check this important claim and have found that there is in fact good agreement between theory and experiment.