The emergence of memory in liquid crystals

Current liquid crystal displays exploit the anisotropy, flexibility, and elasticity of the long-ranged molecular ordering of the nematic phase. Because of the intrinsic fluidity and the dielectric uniaxiality, the orientation of a nematic liquid crystal (NLC) within a pixel is readily controlled by an electric field E-1. In turn, the direction of molecular alignment affects the polarization state of transmitted light, thus determining the on and the off state of each pixel. Upon removal of the field, the weak elasticity of nematics drives the pixel back to its off-state, which is a stable, unique, and well-defined state determined by the cell surface alignment. In Fig. 1a and 1b two of the most common schemes used in commercial displays are shown: twisted nematics(2) (TN) and in-plane switching(3,4) (IPS), respectively. An alternative concept for display technology is the use of bistable or multistable materials, in which the electric field is needed to switch the system between two or more states that remain stable without the need for a continuous supply of energy. Several solutions for nematic-based bistable devices have been investigated over the last 30 years, without offering, so far, a practical alternative to the dominant technology used in conventional flat displays. However, the e-book concept and similar applications requiring a refreshable printed paper-like display without an embedded illumination source, is stimulating interest in new multistable display concepts.