Shape model for the molecular interpretation of the flexoelectric effect

A mean-field model for the flexoelectric polarization in nematics is presented, based on a continuous description of director deformations coupled to the molecular degrees of freedom via surface interactions. In such a framework, a consistent picture of the flexoelectric effect is obtained, including both dipolar and quadrupolar contributions, with a realistic account of the molecular characteristics of shape and charge distribution. The method is aimed at establishing a quantitative link between chemical structure and flexoelectric response. It provides numerical estimates of the effect and its temperature dependence and allows the recognition of the relevant molecular features for its emergence. Application to some representative systems, comprising mesogenic molecules and photoisomerizable dopants, is considered; it is shown that simple interpretative schemes can be misleading and a comparison with experimental data is reported.