SHEAR OF MOLECULARLY CONFINED LIQUID-CRYSTALS .1. ORIENTATION AND TRANSITIONS UNDER CONFINEMENT

Using a surface force balance, we have examined the orientation of a n ematic liquid crystal (6CB) confined between mica surfaces. By simulta neous measuring of the force and the refractive index profile, we were able to distinguish between planar, planar twisted, and homeotropic o rientation of the nematic. When the mica was exposed to air for short periods (< 1 h) prior to introducing the 6CB, either planar or twisted planar orientations resulted, depending on the relative orientation o f the mica axes. For longer exposure periods (greater than or equal to 3-4 h) prior to introducing the nematic, a homeotropic orientation wa s observed, presumably because the mica surfaces became sufficiently h ydrophobized. Slow transitions (over a period of days) from a planar t wisted to a planar orientation and from a homeotropic to a planar orie ntation could be observed in certain conditions. We propose that these are associated with anchoring transitions on the mica surfaces, resul ting from adsorption of water molecules penetrating by diffusion from the ambient atmosphere.