Zenithal anchoring angle and anchoring strength on oxidized silane plates

Silane coated surfaces are chemically modified by using the post-discharge of an oxygen and nitrogen cold plasma, to produce alignment layers. These o xidized surfaces yield uniform nematic cells, provided that they are previo usly rubbed unidirectionally. Depending on the oxygen concentration arrivin g to the sample surface, the zenithal anchoring orientation may be tuned fr om planar to homeotropic, and tilted in-between. We measure the zenithal an choring strength in the planar region, for 4-n-pentyl-4'-cyanobiphenyl (5CB ), as a function of the oxygen concentration and of temperature. The anchor ing strength is found roughly independent of the oxygen concentration, with an extrapolation length similar to 100 nm for Delta T=1.5K.