In October, Jim learned that John Linkhauer had not filed the TN-LCD patent disclosure. Linkhauer promised to submit it soon. But in December, 1970, Jim found out that Linkhauer still hadn’t filed the disclosure. From then on, Murray did Jim’s patent work himself. He filed Jim’s U.S. application on February 9, 1971.
Expert Fields Effect Cells
After spending a few minutes savoring his invention, Jim walked briskly to his office. Halfway down the hallway, he hopped up, tapped his heels together and shouted, “Eureka!”
What was it about the collapsing twist that excited him? The ability of nematic liquid crystals to freely rotate is an important concept to understanding how the twist collapses. The mechanism of collapse was one of Jim’s key conceptual insights into how a twisted nematic device would behave.
In the absence of an electric field, the TN-LCD deformation consists of pure twist. When a field is applied, the first change occurs in the middle of the layer where the director begins to tilt toward normal orientation. This causes a small degree of splay which is projected into the regions above and below the middle and causes bend deformation and a structure similar to the bend-splay balance in Figure b.
As the field increases the center swings though a substantial portion of 90° and reaches a threshold at which the twist cannot be sustained because it is able to release stress by freely rotating around the axis of the director. The twist unwinds and the TN-LCD closes. The twist does not unwind completely unless the voltage is very high. A portion of it remains.
Splay, bend and twist can be thought of as “springs” that attract each molecule to the molecules around it, but are not attached to points on them. The molecules are free to move in three directions, oscillate around their centers and spin. The director(s) that represent the structures formed by splay, bend and twist can also be thought of as having some of the properties of a spring: the structures resist distortion by external forces but removal of these forces cause the structures to “snap back.” Like a spring, the forces of these elastic constants store energy.
Figure c represents free rotation which means that there is no resistance to rotation around the axis of the director. If the structures represented in figures a and b were rotated there would be slight resistance, due to the twist elastic constant. After released, the structures would snap back to their original configuration. However, if the structure in Figure c was rotated, there would be no resistance and the structures would not snap back.