Bistable liquid crystal displays (LCD) are attractive for low-power mobile products and paper-like applications, such as e-dictionaries, e-books and e-newspapers. Once written, they require no power to maintain the displayed image, due to the intrinsic memory of the pixel. The high-level multiplexability of the bistable LCD enables unlimited screen resolution in passive matrices. Several bulk- or surface-controlled bistable devices has been proposed so far, based on smectic, cholesteric or nematic liquid crystals.

One of these devices is the BiNem® bistable nematic display, proposed a few years ago¹ and developed by Nemoptic. The two BiNem bistable textures are almost planar and differ by a p-twist, resulting in excellent optical contrast and viewing angle of the display. Without field, the two textures are blocked by the monostable surface anchoring on the two substrates, giving infinite bistability (helped by a suitable chiral doping). Under strong field, the weaker anchoring on one of the surfaces is broken: the surface director orients along the field, almost perpendicular to the surface easy axis. On field removal, the surface relaxation toward the one or the other bistable texture is dynamically controlled by the driving pulse waveform, favouring the elastic or the hydrodynamic couplings between bulk and surface.

Here we study the physical mechanisms of the BiNem switching. The surface anchoring breaking, continuous or discontinuous depending on the surface pretilt, is treated in a simple analytical approximation. More detailed description of the surface relaxation and the bulk-surface couplings, taking into account the elastic and the viscosity anisotropies, is provided by numerical simulation, especially developed at Nemoptic for the study of the switching in weak-anchoring devices. The numerical description of the physical phenomena involved in the switching enable us to define the “target” anchoring properties needed to optimize the BiNem display and to reach in real devices the theoretically expected performances, e.g. line refreshment times as short as a few tens of microseconds.

Figure 1: QVGA (left) and VGA (right) black&white BiNem® reflective screens.

1.        I. Dozov, M. Nobili, G. Durand, Appl. Phys. Lett. 70, 1179 (1997).

2.        I. Dozov et. al., SID Intl Symp Digest Tech Papers, 32, pp. 224-227 (2001); P. Martinot-Lagarde, I. Dozov, Proceedings of SPIE 5003, 25-34 (2003).