Transient director patterns upon flow start-up of nematic liquid crystals (an explanation for stress oscillation damping)

In this work we attempt to determine the origin of damped stress oscillatio ns upon flow start-up of a nematic liquid crystalline monodomain. These dam ped stress oscillations were first observed by Gu et al. (1993) in the cone -plate flow cell and have since also been observed by Mather et al. (1997) in the parallel disk cell. Although Mather's work explained the cause of th e stress oscillation damping in the torsional flow cell, the origin of the damping in the cone-plate device remains a mystery. Here we report finding similar damped stress oscillations in the cylindrical Couette cell and comb ined with the optical experiments reported earlier by Cladis and Torza (197 5, 1976) we are able to propose an explanation for the damping in this geom etry. We also report new optical experiments using the cone-plate cell in h opes of determining a cause to the damping in the cone-plate cell.