Effect of material properties on reverse flow in nematic liquid crystal devices with homeotropic alignment

Abstract

Reverse flow is undesirable in liquid crystal devices with vertical alignment. The influence of the material properties on the onset of backflow is investigated for commercially available negative dielectric liquid crystals. It is shown that the threshold voltage V BF for the occurrence of backflow is an important material characteristic. This threshold is relevant for applications and a large value is desired in devices to avoid backflow while keeping a wide applicable voltage range. Accurate finite element simulation of the liquid crystal hydrodynamics allows extraction of V BF and the unknown Miesowicz coefficients ij . The resulting values are tabulated at 20.0°C. © 2009 American Institute of Physics. ͓doi:10.1063/1.3242018͔The occurrence of reverse nematic flow 1 in liquid crystal devices with homeotropic alignment 2,3 has important consequences for practical applications. For instance, vertically aligned liquid crystal displays ͑VA-LCDs͒ have become very popular for mobile and television applications because of their excellent contrast ratio and wide viewing angle. However, their dynamic behavior 4 is affected by the possible occurrence of reverse flow, also referred to as backflow. When a voltage step with amplitude exceeding a certain threshold V BF is applied, the initial rapid variation causes reverse flow of the negative dielectric liquid crystal which leads to a complex reorientation mechanism ͑including a twist deformation͒ and a bounced transient transmission profile, 5 increasing the switching times. Therefore, the limitations due to backflow must be taken into account as a special case to design improved driving schemes 6 to achieve fast and reliable switching in VA-LCDs. In this paper, the influence of the material properties on the onset of backflow is investigated for a number of commercially available liquid crystal materials with negative dielectric anisotropy. The viscous flow properties of liquid crystals are described by the four Miesowicz 7 coefficients ij . The three principal coefficients express the viscosity of the liquid crystal for a liquid crystal director parallel to the direction of flow ͑ 11 ͒, parallel to the direction of the velocity gradient ͑ 22 ͒, and perpendicular to both previous directions ͑ 33 ͒, respectively. The fourth coefficient 12 is related to the shear viscosity. The ij coefficients are, in general, not known because of the notorious difficulty in measuring their values accurately. 8 It is shown that the threshold voltage V BF for the occurrence of backflow is a material characteristic comparable to the Freedericksz threshold voltage 9 for elastic deformation. The backflow threshold is related to the viscous flow properties of liquid crystals and is relevant for practical devices because backflow is undesired and should be avoided. Furthermore, it is possible to estimate the unknown Miesowicz viscosity coefficients ij based on measurement and simulation of V BF . The ij coefficients are extracted using this approach and tabulated together with the V BF values at...

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