Liquid Crystal Displays: Fundamental Physics and Technology

15

The Twisted Nematic

The liquid crystal display prototypes developed at RCA interested Wolfgang Helfrich, a German researcher in Heilmeier’s Group, who was engaged in applying the Ericksen–Leslie dynamic continuum theory to the operation of the display prototypes. While investigating whether a shear stress torque was necessary to produce the Williams domain pattern, he asked himself a cogent question: Is it the twisted stream of molecules in the convection cell that changes the polarization of the incident light that subsequently modulates the intensity? And would it be easier to modulate the light intensity if there were a natural or induced twist in the liquid crystal?

Continuing this train of thought, he decided to apply a thin film compound on the glass substrate and used a cotton swab to trace parallel troughs in the film; because of the long cylindrical shape and soft, flexible ends of the nematic liquid crystal molecules, the rubbed compound troughs would cause the long axes of the molecules to adhere to the glass surface, thereby anchoring them at a predetermined angle. Now if the angle of the rubbing on the top and bottom glass substrates were different, the stream of molecules, anchored as they were at those angles at the top and bottom substrates, would undergo a twist in the middle layers. At left in Figure 15.1 is a micrograph of the anchoring compound as applied, the middle micrograph shows the parallel troughs wrought by “rubbing” the compound, and the right schematic ...