MAGNETIC-FIELD-INDUCED FREEDERICKSZ TRANSITION AND THE DYNAMIC-RESPONSE OF NEMATIC LIQUID-CRYSTAL FILMS WITH A FREE-SURFACE

Abstract

The Freedericksz transition and the dynamic response of homeotropical liquid-crystal films with a free surface (FS) induced by a perpendicular dc magnetic field have been studied theoretically and experimentally for two nematic substances: 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4'-n-octyl-4-cyanobiphenyl (8CB). For either one of these two materials, it is found that the critical magnetic field of these two nematic liquid-crystal (NLC) films with a free surface is equal to that of the NLC film with hard boundaries (HB) of the same thickness. This thus demonstrates a very strong orientational anchoring for 5CB and 8CB at the free surface. The intrinsic structures of FS samples are homeotropic, just as the HB ones are. For the dynamic response, the turn-off rates of FS samples are found to be about two times faster than those of the HB ones. This is interpreted with the backflow theory and the lack of positional anchoring on the free surface. Finally, a sensitive approach to determine the viscosity coefficients, gamma-1 and eta-c, of the NLC film is suggested.