On the creation of director disorder in nematic liquid crystals G.R. Luckhurst School of Chemistry and Southampton Liquid Crystal Institute, University of Southampton, Southampton SO17 1BJ, United Kingdom Available online 8 November 2005 Abstract 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. D 2005 Elsevier B.V. All rights reserved. Keywords: Nematic liquid crystal; Director disorder; Colloid; Gel; Hydrodynamics; Ultrasound; ESR spectroscopy 1. Introduction The defining characteristic of a liquid crystal is the long range orientational order of its constituent molecules. As a conse- quence properties such as the diamagnetic susceptibility and the refractive index are anisotropic and for many liquid crystals these properties are cylindrically symmetric about a unique axis known as the director [1]. At a molecular level the director can be thought of as the direction about which the unique molecular axis tends to align. Another consequence of the long range order is that the director can be aligned by relatively weak perturba- tions or fields from its random distribution in the bulk sample. These perturbations include magnetic and electric fields as well as surface fields [1]. This ability to align the director, especially in nematic phases, is vital for the use of liquid crystals in display devices where the director distribution is changed between aligned states on applying or removing an electric field [2]. Although the creation of the aligned state is usually of primary interest there are occasions when the destruction of the director alignment can also be of value. Indeed in one of the earliest display devices information was written onto the display by destroying the uniform alignment produced by the cell surfaces on application of an electric field [2]. The resulting random director alignment combined with the optical birefrin- gence produces a scattering state. The study of the effects that destroy the uniform state of director alignment can also contribute to our understanding of the factors controlling this alignment and the competition between them. There are also experiments in which it is important to be able to study a liquid crystal in which the director is not uniformly aligned. For example, NMR spectroscopy is an important technique with which to determine the phase symmetry, in particular, for nematics. In these experiments the intrinsic magnetic field of the spectrometer will align the director and a second perturbation needs to be applied to destroy it. In this way it is possible to determine two components of a tensorial magnetic interaction and hence the phase symmetry [3]. Here we describe four methods which can be used to produce a random director distribution. The extent to which the director is disordered has been determined using ESR spectroscopy and the foundations for this methodology are described in the following Section. One way by which a random director distribution can be created is to use surface alignment. The basis for this is given in Section 3 and then in Sections 4 and 5 we describe how colloidal 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.117 E-mail address: gl@soton.ac.uk. Thin Solid Films 509 (2006) 36 – 48 www.elsevier.com/locate/tsf