Improvement of the crystal rotation method to measure the flexoelectric difference (e1-e3) in achiral materials using AC electric fields

摘要

We show that the crystal rotation method for measuring the difference between splay and bend flexoelectric coefficients (e(1)-e(3)) [Outram and Elston, Liq. Cryst. 39, 149 (2012)] can be improved by using AC instead of DC fields. The field frequency is small (-0.05 Hz), yet sufficient to get rid of the difficulties connected with the migration of charge carriers. The method is sensitive enough so as to use fields with amplitudes as small as 1 V/mm. This has the advantage of avoiding the consideration of the interaction due to the dielectric anisotropy, and greatly simplifies the analysis of data. The results are extracted from a simple fitting scheme, which involves two parameters. In particular (e(1)-e(3)) is obtained from a scale factor in a one -parameter fit. Measurements of the temperature dependence of (e(1)-e(3)) are presented for two classical nematic materials.