EFFECTS OF SHEAR ON THE SMECTIC A-PHASE OF THERMOTROPIC LIQUID-CRYSTALS

The theological behaviour of the smectic A phase of the thermotropic l iquid crystal 4-cyano-4'-octylbiphenyl (8CB) is examined. X-ray scatte ring studies under shear flow were performed to probe changes of struc tures. We found that in a certain range of temperatures two states of orientation of lamellae exist. These two steady states of orientation are separated by a first order dynamic transition that becomes continu ous at T-c (a temperature different from that of the smectic/nematic t ransition). At low shear rates, the smectic A phase is non-Newtonian: its viscosity eta varies as (T-c - T)(1/2) . gamma(-1/2) (where gamma is the shear rate and T the temperature). In this regime, the structur e of the system is compatible with multilamellar cylinders oriented al ong the flow direction. At high shear rates, the system becomes Newton ian, its layers are then oriented perpendicular to the shearing plates (as already noticed by Safinya et al. [1]).