Prof. Ophelia K. C. Tsui
Department of Physics, Boston University
Liquid Crystal Alignment Control by Nano Surface Patterns
When subject to an inhomogeneous surface anchoring field, elasticity of nematic liquid crystal (LC) ensures that the LC local director does not vary faster than the extrapolation length, le (= LC elastic constant/strength of the surface anchoring field). Typical values of le are about 1 micron.
Consider inhomogeneous alignment patterns comprising square domains with local preferred alignment directions alternating between x and y as shown in Fig. 1. At very large pattern period, P, the LC alignment on the pattern is expected to copy the pattern inhomogeneity. The exception occurs within le from the interface between two adjacent domains where the LC alignment must undergo a gradual transition from one orientation to the other (as seen in the LC configurations illustrated for the case labelled P > P* - the transition period - in Fig. 1b). At very small P ≤ le , the interfacial regions would occupy a substantial portion of the domain area so the LC configuration is highly homogeneous and lies in the 45 deg azimuthal direction (as seen in the case labelled P ≤ P* in Fig. 1b). Such homogenization of the LC alignment reduces the elastic strain energy of the system but at the cost of a higher LC azimuthal surface energy (since the LC orientation thereafter is misaligned from the local preferred direction everywhere). If the rise in surface energy is large enough, the LC system may lower its total energy further by undergoing an orientation transition into an alignment state with uniform finite pre-tilt. By adjusting the surface anchoring energy, which is easily achievable by varying the groove depth of the surface pattern, we demonstrated that the pre-tilt angle at the orientation transition could be controlled.
1. "Liquid Crystal Orientation Transition on Microtextured Substrates", Baoshe Zhang, Fuk Kay Lee, Ophelia K.C. Tsui, Ping Sheng, Phys. Rev. Lett. 91, 215501-4 (2003).
2. "First-Order Liquid Crystal Orientation Transition on Inhomogeneous Substrates", Ophelia K. C. Tsui, Fuk Kay Lee, Baoshe Zhang, Ping Sheng, Phys. Rev. E 69, 021704 (2004).
3. "Substrate conditioning for continuous liquid crystal pre-tilt control", Fuk Kay Lee, Baoshe Zhang, Ping Sheng, Hoi Sing Kwok, Ophelia K. C. Tsui., Appl. Phys. Lett., 85, 5556-8 (2004).
4. "Continuous Liquid Crystal Pretilt Control Through Textured Substrates", Fuk Kay Lee, Baoshe Zhang, Ping Sheng, Hoi Sing Kwok, Ophelia K. C. Tsui, Appl. Phys. Lett. 85(23), 5556-8 (2004).
5. "Liquid Crystal Pretilt Control by Inhomogeneous Surfaces", Jones T. K. Wan, Ophelia K. C. Tsui, Hoi-Sing Kwok, Ping Sheng, Phys. Rev. E 72, 021711-1-021711-4 (2005).
6. "Liquid Crystal Alignment Layer and Methods of Making Thereof", H. S. Kwok, Fion S. Y. Yeung, F. Xie, O. K. C. Tsui and P. Sheng, US Patent Application no. 20050260426.
7. "Liquid Crystal Pretilt Angle Control Using Nano-textured Surfaces", by Fion Sze-Yan Yeung, Feng-Chao Xie, Jones Tsz-Kai Wan, Fuk Kay Lee, Ophelia K. C. Tsui, Ping Sheng and Hoi-Sing Kwok, J. Appl. Phys. 99, 124506 (2006).
8. "Substrate Patterning for Liquid Crystal Alignment by Optical Interference", Xuemin Lu, Fuk Kay Lee, Ping Sheng, Hoi Sing Kwok, Vladimir Chigrinov, and Ophelia K. C. Tsui, Appl. Phys. Lett. 88, 243508 (2006).
9. "Variable Liquid Crystal Pretilt Angles by Nanostructured Surfaces", F. S. Y. Yeung, J. Y. Ho, Y. W. Li, F. C. Xie, O. K. C. Tsui, P. Sheng, H. S. Kwok, Appl. Phys. Lett. 88, 051910-1-3 (2006).
Last revised on 28 May 2007.