Liquid crystal dynamic flow control by bidirectional alignment surface

Abstract

We investigate the behavior of liquid crystal dynamic flow in a cell with a bidirectional alignment ͑BDA͒ surface. Numerical simulations show that with a BDA surface having a pitch comparable to the cell gap d, the liquid crystal dynamic flow direction can be controlled by the driving voltage. Such an effect can be applied to bistable twisted nematic displays without the need for anchoring breaking. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3080198͔There are numerous studies of liquid crystal ͑LC͒ dynamic flow on homogenous alignment surfaces. Van Doorn et al. 1 stated that dynamic flow imparts a backward torque on the LC molecules near the center of the device. Berreman et al.2 observed this phenomenon on twisted nematic devices. Bos and Koehler 3 reported that faster LC display response can be achieved by removing this backflow effect. They suggested that for the bend deformation, the backward torque can be eliminated. For the case of bistable twisted nematic ͑BTN͒ devices, dynamic backflow is essential to the switching mechanism. Dozov et al. 4 proposed a switching mechanism based on asymmetric anchoring alignment ͑strong polar anchoring on the master plate and weak polar anchoring on the slave plate͒. When the electric field is turned off rapidly, the master plate director rotates and creates a dynamic flow on the slave plate. Therefore, the cell relaxes rapidly to the transient bend state and then to the -twisted state. If the applied electric field decreases slowly, the backflow effect is weak and the device will relax adiabatically to the homogenous state.It is obvious that the dynamic flow of the LC is very important to determine the dynamic performance of the LC device. In this paper, we propose a new method to control LC flow dynamics using a bidirectional alignment ͑BDA͒ layer. In the following, the mechanism of such idea will first be explained. Then, we demonstrate the idea by applying this technology to the BTN device. A new BTN display, called multidomain BTN ͑MD-BTN͒, which switches between the no-twist and -twisted states, has successfully been fabricated.The BDA surface is a heterogeneous alignment surface with a periodic pattern. Figure 1 shows the detailed structure of the BDA. It has a periodicity of L and is composed of two types of domains D 1 and D 2 . D 1 and D 2 have different pretilt angles 1 and 2 , respectively, on the same plane. The LC molecules are tilted in opposite directions so that 1 + 2 is always larger than 90°. It is intuitively obvious that the LC molecules are either tilted at 1 or 2 , respectively, on the BDA surface, and then relax to a uniform tilt angle in a distance L H , which is determined by the pitch L. The LC deformation inside the bulk system can be calculated by minimizing the total energy consisting of the bulk elastic energy and surface anchoring energywhere n denotes the LC director, W i is the finite polar surface anchoring energy of the domains, and F͑n͒ is the FrankOseen elastic energy given bywhere K 11 , K 22 , and K 33 are the splay, tw...

Arbitrary pretilt and azimuth angles generation by stacked photoalignment layers

“…In conclusion, we have demonstrated a practical method to generate multiple and arbitrary pretilt and azimuth angles for LC devices. Such effect is particularly useful for LC switchable focal lens, wave‐guide devices, fast‐response displays, bistable displays, multi‐domains displays, and LC dynamic flow control studies . The processing window of such method has been maximized.…”

“…Such effect is particularly useful for LC switchable focal lens, 30 wave-guide devices, 31 fast-response displays, 32 bistable displays, 33 multi-domains displays, 34 and LC dynamic flow control studies. 35 The processing window of such method has been maximized. As the self-organized domains are very uniform, highly reproducible results can be obtained even for large-area manufacturing.…”

Thermally excited vortical flow in a microsized nematic cell

“…It should be noted here that these BALC layers were used in the BTN devices. 7 In our calculations the value of the ratio d/L is chosen equal to 0.1. Such choice of the width/length ratio allows us avoid the effect of the bounding lateral walls on the director reorientation inside the BALC cell.…”

“…[3][4][5][6] Recently, a new method to control LC flow dynamics using a bidirectionally aligned liquid crystal (BALC) film has been proposed. 7 It was shown for the case of bistable twisted nematic (BTN) devices, that dynamic flow is essential to the switching mechanism.…”

Formation of vortex flows in thin nematic cells

“…These modes are characterized by the formation of flows in LC cells, which have an effect on the relax ation time of the director field. To reduce the relax ation time of the director field, the double surface ori entation of the director field on one of the bounding surfaces of the LC cell [4] was proposed. This was achieved by that, on one half, e.g., on the lower bounding surface, the director field was oriented at an angle of 45° to this surface; on the other half, it was oriented at an angle of -45° to the same surface.…”