We have investigated the voltage induced switching in the Zenithal Bistable Display (ZBD) device both theoretically and experimentally. The ZBD device we consider consists of a liquid crystal material sandwiched between a planar upper surface and a periodic grating-like lower substrate (see Figure), both of which induce homeotropic alignment in the liquid crystal molecules close to the substrates. It has previously been shown theoretically, using a continuum model of the director field, that such a device exhibits two stable zero-voltage nematic director configurations. However, the presence of defects in this device means that a more complicated model, which includes a variance in the nematic order parameter, is needed to model the switching dynamics when a voltage is applied. We have used a Landau-de Gennes Q-tensor approach to phenomenologically describe the free energy of the system, which includes anisotropic elasticity, dielectric effects, flexoelectricity and weak surface anchoring. We also fully model the electric potential within the device by solving Maxwell's equations coupled to the liquid crystal equations. Using a Rayleigh dissipation principle to incorporate temporal changes in the Q-tensor we can then model the dynamical behaviour.

Using an averaged Berreman model for the optics of the device we can then compare the transmission in the theoretical model and from experimental results. We find that all key features of the experimental transmission curve are reproduced and can be explained using the theoretical model. We conclude that flexoelectricity is essential for switching between the two stable states and that defect motion close to the grating substrate dominates the dynamic behaviour of the device. Further we are able to predict that tailored voltage pulses will allow extremely fast switching due to a latching process of one of the defects.