Bistable nematic liquid crystal device with flexoelectric switching

Abstract

Motivated generally by potential applications in the liquid crystal display industry [8,35], and specifically by recent experimental, theoretical and numerical work [6,7,13,14,21,25,30,31], we consider a thin film of nematic liquid crystal (NLC), sandwiched between two parallel plates. Under certain simplifying assumptions, laid out in £2, we find that for monostable surfaces (i.e. only a single preferred director anchoring angle at each surface), two optically-distinct, steady, stable (equal energy) configurations of the director are achievable, that is, a bistable device. Moreover, it is found that the stability of both of these steady states may be destroyed by the application of a sufficiently large electric field, and that switching between the two states is possible, via the flexoelectric effect. Such a phenomenon could be used in NLC display devices, to reduce power consumption drastically. Previous theoretical demonstrations of such (switchable) bistable devices have either relied on having bistable bounding surfaces, that is, surfaces at which there are two preferred director orientations at the surface [7,14]; on having special (nonplanar) surface morphology within the cell that allows for two stable states (the zenithal bistable device (ZBD) [4,21], or, in the case of the Nemoptic BiNem technology [11,19], on flow effects and a very carefully applied electric field to effect the switching.

Substrate-induced gliding in a nematic liquid crystal layer

“…We follow several authors (e.g. Davidson & Mottram [3], Cummings et al [12,13], Kedney & Leslie [14]) in assuming that the system evolves in the direction that minimizes its total free energy (a gradient flow). This approach leads to the following time dependent problem:…”

“…These anchoring angles are used to obtain a new director solution using Eqs. (12) and (13) at the new time step. To ensure accuracy, we use relatively small time step, dt = 10 −3 ; we have verified that such dt leads to results that are accurate to 0.1%.…”

“…[3,[12][13][14] and references therein. In [12], bistability is obtained in a special case where the anchoring angles are π/2 out of phase and the anchoring strengths are the same at both boundaries; switching between the states is obtained through the application of a transient electric field and in particular, two-way switching is possible for weak anchoring only. In [13] (12); here f (a) has three roots, only two of which represent stable solutions, as discussed below.…”

“…Switching between the two stable steady states in an initially bistable system is possible in the absence of gliding [12,13]: with the application of a suitable transient electric field, one can achieve two way switching in the appropriate parameter regimes. Motivated by the relevance of switching in devices that are both flexible and bistable, and by our results in Sec.…”

“…(Gliding should not, however, be an issue for such devices in only short term or intermittent use.) Another situation where gliding could be an issue is in polymer-based devices that use bistable technology [3,[12][13][14][15][16]. In a bistable device, the NLC layer can sustain two stable field-free director configurations that are optically distinct, in the absence of an applied field.…”