Flow-Induced Twist-Compression in a Twisted Nematic Cell

Abstract

An optical convergent-beam guided-wave technique is used to explore in detail the dynamic flow effects in a twisted nematic cell. During switch-on it is found that the dynamic flow compresses the director twist to regions close to the cell walls. For high fields this twist compression takes the cell far beyond the Mauguin limit and it no longer effectively guides the polarization of the light through the cell. This results in a very fast switch to a transient dark state. DOI: 10.1103/PhysRevLett.90.168701 PACS numbers: 83.80.Xz, 47.17.+e, 42.79.Gn, 78.20.-e Nematic liquid crystals form the basis of the majority of flat screen displays. Their switching dynamics, key to their functionality as video displays, is understood in terms of the foundational theory pioneered by Ericksen [1,2] and Leslie [3]. Developed in the 1960s, and simplified by Berreman and van Doorn [4,5], this theory of flow in anisotropic fluids is still very widely used. However, there exist very few detailed experimental confirmations of many of its predictions. This is particularly true as regards to switch-on dynamics where the influence of flow in a relatively fast, field-driven process has barely been explored. Some simple experiments [6,7] have confirmed the theoretically predicted ''backflow'' in a twisted nematic (TN) cell during switch-off by recording an optical ''bounce'' effect in transmission. However, almost all the experiments [8] are based on simple transmission or reflection observations during switching and as such the signals from the liquid crystal cell are an integrated response. Only recently, using a convergentbeam optical guided-wave technique [9,10], have details of the director profile in the liquid crystal cell during switch-off been unambiguously obtained, thereby establishing the validity of the theory. Further there has been no published work which details the change of director profile within a cell during switch-on, a much faster process providing a more rigorous test of the theory.In this present study a convergent-beam system is used to investigate the switch-on dynamics of a twisted nematic cell detailing directly the strong influence of flow on the twist profile, and, in particular, the strong compression of director twist to the cell walls.The liquid crystal cell comprises two low index glass plates (n 1:5170 at 632.8 nm), each coated inside by a thin ( 50 nm) layer of indium tin oxide (ITO), on top of which are surface aligning layers of rubbed polyimide. The plates are assembled together with 6:0 m spacers in between. A twisted director profile is established by setting the angle between the rubbing directions of the upper and lower plates to about 87 . The cell is filled with liquid crystal ZLI-2293 (Merck) in the isotropic phase and then slowly cooled down to room temperature forming a monodomain.The cell is then inserted between the two glass hemispheres, optical contact being achieved with matching fluid. The glass hemispheres and the matching fluid have the same index as the glass substrates of the ...

Delay effect of switch-on in a supertwisted nematic cell

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