Comment on "Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic...

We argue that the experimental determination of the flexoelectric coefficients and the surface polarization of the p-pentyl-cyanobiphenil liquid crystal, recently reported by Mazzulla et al. [Phys. Rev. E 64, 021708 (2001)] is questionable. This conclusion follows from the hypothesis performed by the authors to extract the flexoelectric coefficients and the surface polarization from the reflectivity curves taken under an applied voltage. In fact, they assume that the liquid crystal can be considered as a perfect insulating material and that the ionic charges do not contribute to the electric field profile inside the sample. Using our recent results concerning nematic liquid crystals submitted to an external electric field we show that this hypothesis is far from being correct. In the experimental conditions of Mazzulla et al., due to the low frequency of the applied alternative square pulsed signal, the ions play an important role in the molecular orientation of the liquid crystal induced by the external field.