On the creation of director disorder in nematic liquid crystals

One of the striking features of a nematic liquid crystal is the ease with which the random director distribution, characteristic of an unperturbed system, can be converted into a state of uniform alignment with weak magnetic fields. Here we are concerned with how this order can be destroyed not only because new liquid crystal physics may be involved in the process but because there are uses for the disordered state. The torque responsible for creating the disorder, which must compete with the uniform magnetic field, should not only exceed some threshold value but must also be random. These conditions can be achieved using surface and elastic torques produced by a suspension of colloidal particles and by the network of a gelator. The hydrodynamics produced by sample spinning also competes with the magnetic field but in an apparently coherent manner. However, a random element is introduced during the spinning and this produces a random distribution of the director in the plane orthogonal to the spinning axis. Pressure waves seem to have little influence on the director distribution but at the onset of cavitation the director alignment is destroyed, presumably as a result of the implosion of bubbles created in the low-pressure regions. Our studies have benefited from the use of ESR spectroscopy to determine the extent of director disorder and the basis of this powerful technique is described here.