Influence of a surface electric field on the anchoring characteristics of nematic phases at rubbed polyimides

We report measurements of the anchoring energy of 5CB (4-cyano-4′-n-pentyl-biphenyl) at a commercial polyimide (AL1051) with a method using a large applied electric field on thick measurement cells. The experiments reveal a deviation from the expected behavior of the system and cannot be described by the usual Frank elastic bulk energy density plus anchoring surface energy within the thermodynamic limit. We, therefore, discuss the effect of a surface electric field coupled to director distortions by the flexoelectric effect. Numerical calculations show, indeed, that the experiments can be described reasonably by the model. Experiments for thin cells indicate that the Debye screening length does not depend on the cell thickness. We derive an easy way to get an estimate of the Debye screening length from the experiments.