Forces in nematic liquid crystals constrained to the nanometer scale under hybrid anchoring conditions

Using a surface forces apparatus we have studied two thermotropic nematic liquid crystals (5CB and ME10.5) subjected to hybrid (homeotropic/planar) anchoring conditions. A film of nematic material is constrained between two curved smooth surfaces separated by less than 2500 angstrom. The intersurface force is nonmonotonic with the separation, being repulsive for thicknesses larger than similar or equal to 100 angstrom and strongly adhesive at a shorter scale. While the repulsion can be qualitatively explained by an elastic model of director deformation, including anchoring deviation at the boundaries, the attraction cannot be explained either by elasticity or by dispersive forces. The expected confinement-induced anchoring transition has not been observed for a thickness as small as 200 angstrom.