THEORY OF LINEAR VISCOELASTICITY OF CHIRAL LIQUID-CRYSTALS

The governing equations of monodomain isothermal cholesteric liquid cr ystals subjected to small amplitude oscillatory rectilinear shear have been derived for three representative helix orientations. The imposit ion of oscillatory flow excites splay-bend-twist deformations when the helix is aligned along the flow direction, splay-bend deformations wh en the helix is along the vorticity gradient, and twist deformations w hen aligned along the velocity axis. The different nature of the excit ed elastic modes as well as the anisotropic viscosities are reflected in the anisotropy of the linear viscoelastic material functions for sm all amplitude rectilinear oscillatory shear. When the helix is aligned along the flow direction, cholesteric viscoelasticity is strongest, a nd exists in a relatively narrow band of intermediate frequencies. Whe n the helix is aligned along the vorticity direction cholesteric visco elasticity is significant in a relatively broad range of intermediate frequencies. Finally, when the helix is aligned along the velocity gra dient direction, cholesteric viscoelasticity is relatively insignifica nt and only exists in a narrow band of frequencies. The cholesteric pi tch controls the location of viscoelastic region on the frequency spec trum, but only when the helix is not oriented along the vorticity axis .