ORIENTATION MODE SELECTION MECHANISMS FOR SHEARED NEMATIC LIQUID-CRYSTALLINE MATERIALS

An extensive analysis of the flow orientation modes of sheared liquid crystalline materials is performed using a complete nonlinear nonequil ibrium theory that takes into account short and long range order elast icity and viscous flow effects. It is found that there an two main ori entation modes in shear flow: (i) in-plane modes, where the average mo lecular orientation is in the shear plane (v-del v plane, where v is t he velocity vector); and (ii) out-of-plane modes where the average mol ecular orientation has a nonzero component along the vorticity (del x v) axis. It is found that there are four in-plane orientation modes, a nd five out-of-plane modes, depending on the magnitude of the ratio of short to long range elasticity (R), and the magnitude of the ratio of viscous force to long range elastic force (Ericksen number: Er). The spatial configuration of the orientation field shows a bulk region and two boundary layers, which are smoothly and continuously connected by the action of compatibilization mechanisms. The system has two differ ent compatibilization mechanisms at the boundary between the bulk and surface layer regions: (i) scalar order parameter adjustment, and (ii) director orientation changes. The activations of these two mechanisms are self-selected, and depend on the parametric (R, Er) conditions. A t lower R the system easily adopts the scalar order parameter compatib ilization mechanism, and at higher R and at moderate Er the system ado pts the director compatibilization mechanism. Multistable nonplanar or ientation modes arise in certain parametric regions. Multistability in nonplanar modes arises due to possible choices in the direction of th e director escape from the shear plane (i.e., left or right), and the nucleation time of the out-of-plane orientation. These two degrees of freedom cause the appearance of chirality in the director field. The n onplanar mode selection and its chirality are stochastic, although the equations are deterministic. The complete theory unifies previously u sed classical theories (Doi and Leesie-Ericksen), but its predictions transcend in number and nature the predictions of the classical theori es.