On the flexoelectric effect in nematics

Flexoelectricity is a general and fundamental phenomenon in liquid crystals. It describes the linear coupling between an applied electric field and gradients in the director field. Whereas flexoelectricity has for decades been regarded as only of academic interest, we think it is time to point out its considerable application potential, for instance in the case of the flexoelectro-optic effect, and to urge a revival of interest in the subject. As a result of long-time neglect, published data on flexoelectric coefficients are scarce and inconsistent, even with regard to the sign of the reported effect. In this paper we critically review the possible definitions of flexocoefficients in order to propose an international standard. We point out that the absence of such a standard obstructs the understanding of the physical basis, microscopically as well as macroscopically, of the effect, and leads to the introduction of nonsensical concepts like 'flexoelectric anisotropy'. Based on the only natural convention, we finally propose a simple method for measuring sign and magnitude of the effective flexoelectric coefficient which is the control parameter in electro-optic effects.