TWISTING TRANSITION IN A CAPILLARY FILLED WITH CHIRAL SMECTIC-C LIQUID-CRYSTAL

We consider a cylindrical capillary filled with the chiral smectic C p hase of a liquid crystal. The smectic layers are stacked along the cap illary axis, and at the capillary wall the director is assumed to be p arallel to the wall. Non-chiral materials in these conditions exhibit a disclination line which coincides with the capillary axis. In chiral smectic C material the tendency for macroscopic twist competes with t he boundary conditions at the wall, and the disclination line can shif t away from the capillary axis and form a helix. We calculate an appro ximate phase diagram for the system, which shows how the phase (helica l or non-helical) depends on material parameters (elastic constants an d chirality) and capillary radius. Both continuous and discontinuous p hase transitions occur: the continuous transition should be observable in ordinary liquid crystals, while the discontinuous transition shoul d be observable in main chain polymer liquid crystals. Observation of the discontinuous transition would provide information about a poorly known elastic constant.