Alignment change by moisture adsorption in nematic liquid crystals

The abrupt change of the director orientation in nematic liquid crystals, for certain critical values of the cell thickness is described, and explained in terms of a change in surface polarization by moisture adsorption through the free external surface. The measured values of the director mean square angle with respect to the initial homeotropic direction are fitted by a stretched exponential function of the cell thickness with β=1/2. The behaviour of the three other parameters, i.e., the asymptotic deviation, the mean scaling length, and the critical thickness typical of a second order phase transition, is discussed as well.