Two shear flow regimes in nematic liquid crystals: Near a charged surface and in the bulk

Laminar and tumbling flow regimes in a high shear flow, as well as transition from a flow alignment regime to a tumbling situation are investigated in the polar liquid crystal p-n-hexyloxybenzylidenep(')-aminobenzonitrile (HBAB) near a charged surface. The relaxation of the alignment angle theta(tau) to its equilibrium theta(bulk) is calculated using the conventional Erickson-Leslie theory for the case of homeotropic alignment of HBAB at a charged indium tin oxide-coated glass plate and for the temperature range in which HBAB exhibits a nematic phase. In accordance with earlier Couette flow experiments, our calculations of the relaxation process for temperatures below 365 K show that the alignment angle is characterized by relaxing to a theta(bulk) of zero value, indicating that the steady laminar flow may break up into a tumbling flow. The nature of the hydrodynamic instability under high shear flow rates is also discussed for HBAB. (C) 2002 American Institute of Physics.