Applications of Flexoelectricity

Book chapter, 2012

There have been a number of attempts to utilize the flexoelectric effect for electro-optic devices. Most of these have been aimed at new display modes, sometimes inspired by the twisted nematic (TN) cell and often with the aim of attaining bistability. The proposed modes so far do not consider smectic materials (where the flexoelectric phenomena are more complex) but deal with nematics or cholesterics (chiral nematics with a sufficiently short pitch to be negatively uniaxial). While success so far has been modest, two of the proposed devices stand out: the flexoelectro-optic device using cholesterics and the zenithal bistable device (ZBD) using non-chiral nematics. The first is a fast analogue device with several remarkable and unique properties. It is characterized by symmetric switching and a flexoelectric polarization is induced at every switching step. As its physics is relatively transparent and analytically accessible it is discussed here at some length, in particular because it gives valuable clues to both the potential and the problems of this device, which has a good chance to become a viable technology in the years to come. The second device is based on the direct flexoelectric effect, which means that a bulk polarization of a particular sign is essentially inverted in the switching operation. It has just reached the market, after a long time of experiments and modelling, in the form of a passively driven display with memory. Its physics is very complex and it can only be modelled to some extent in two dimensions and there is hardly any analytical theory that satisfactorily describes its global working mechanism. However, the basic ideas are simple and beautifully illustrate the physics of flexoelectricity and, as we will describe, the realization of a viable device represents no small engineering achievement.