Liquid Crystal and Optics Group

Several authors [1-5] have studied the effect of a constant shear rate (steady state condition) on the planar structure of a cholesteric liquid crystal (CLC). In these cases the shearing is obtained by constant rotation of a rheogoniometer. The main conclusions are: 1) the CLC layers can slip freely, and a uniform helical axis tilt is produced at low shear rate. 2) a broken structure is induced in which the helix keeps a uniform orientation only inside small swarms. 3) the onset of hydrodynamical instabilities is ob­ served. These conclusions have been criticized by some authors [6] because the rheogoniometer in­ duces also a radial velocity gradient and because in the hydrodynamics backflow was ignored. In our opinion the physical problem of the shearing of a CLC is not yet clarified because the nature of the induced instabilities is not well deFined: structural and hydrodynamical instabilities can both be present and the cited experiments could favor one or the other. In this paper a planar CLC is submitted to a static or slow linear shear deformation. This device simplifies the geometry and the discussion of the results. We performed observations with an optical micro­ scope and looked at the reflection spectra. Our conclusions are: 1) the layers slip, but not freely, 2) the helical axis tilts, 3) structural periodic in­ stabilities take place before the onset of permanent deformations.