Surface-polarization effect in the alignment of nematic liquid crystals

The change in the orientation of nematic liquid crystals due to the increase of the sample thickness is experimentally analysed. The data show that the phenomenon is a critical one, typical of a second order phase transition. The change in the surface polarization induced by a selective ion-adsorption on the two boundary solid substrates is proposed as the cause of the observed facts and discussed. The considered model is correct for angles of tilt small with respect to the initial homeotropic alignment, and the theoretical curves fit both the interferometric and the voltage-threshold data. A few comments on the obtained results are reported