Liquid-Crystal Displays: Fabrication and Measurement of a Twisted Nematic Liquid-Crystal Cell

This article describes a set of experiments designed to explore some of the basic properties of liquid crystals. The relationship between molecular shape, molecular order, and macroscopic properties, such as optical birefringence, phase transitions, and electrically–controlled birefringence, as demonstrated by the electro–optic effect, are investigated. Students are encouraged to consider the importance of surface phenomena on these properties and how molecular order at a surface can be transferred throughout the bulk of such a highly associated fluid. Experiments are performed using either a sandwich cell or a twist cell containing the aligned nematic liquid crystal 4'-pentyl-4-biphenylcarbonitrile. In the first part, students investigate the anisotropic nature of this liquid crystalline material using the wave-retarding properties of an aligned sample confined within a sandwich cell. In the second part, students manufacture a twisted nematic liquid-crystal cell and characterize the important elements required in a working display device based on electro–optical principles. The experiments are suitable as a second-year laboratory and are performed by students in two three-hour sessions. The experiments could be adapted to a three-hour laboratory by omitting either the sandwich, or the twist cell section, without compromising the learning objectives of the lab.