Switching hydrodynamics in multi - domain , twisted nematic , liquid crystal devices

– We study the switching dynamics in two-domain and four-domain twisted nematic liquid crystal devices. The equilibrium configuration of these devices involves the coexistence of regions characterised by different handedness of the inherent director twist. At the boundaries between these regions there are typically disclinations lines. The dynamics of the disclination lines controls the properties, and in particular the switching speed, of the devices. We describe their motion using a numerical solution of the Beris-Edwards equations of liquid crystal hydrodynamics. Hence we are able to explain why a conventional two-domain device switches off slowly and to propose a device design which circumvents this problem. We also explain the patterns of disclination creation and annihilation that lead to switching in the four-domain twisted nematic device. Introduction. – Twisted nematics (TN) are commonly employed in the construction of flat panel liquid crystal displays [1]. In a TN device, the equilibrium configuration is one in which the director field of the liquid crystal twists across the cell, normally because of conflicting homogeneous anchoring at the boundaries. Though traditional, single domain, TN devices can be built cheaply and easily, it is well known that they do not have ideal viewing angle properties [2]. To circumvent this, a number of possible solutions have been suggested. One partially successful avenue has been the design and construction of multi-domain, TN devices, in which regions of right-handed and left-handed twist alternate in the cell. This director structure is imposed by using suitably patterned boundaries [3–9]. While technological advances will likely make their production easier, there are fundamental problems associated with multi-domain TN devices that need to be investigated and understood theoretically. Most notably, because disclinations necessarily appear at the boundary between domains with different handedness, it is natural to expect that the disclination dynamics will play a major role in determining the switching properties of the devices. Understanding the disclination motion is vital in suggesting better device designs. Therefore in this letter we investigate how disclinations are created, move and are destroyed as two-and four-domain TN devices are switched. To do this we use a lattice Boltzmann algorithm to