Liquid crystal is a state of a matter in which molecules maintain their orientations and their respected positions just like solid, but they can move anywhere just like liquid. These liquid crystals are affected by electrical voltages. When electrical voltage is applied, molecules in liquid crystal rotate into specific angle depending on magnitude of electrical voltage.

Cholesteric liquid crystals contain nematic mesogenic molecules with chiral center. Basically, cholesteric liquid crystals are liquid crystals in which their rigid parts are pointing at the same direction and their molecular structure in the center are differed from its mirror image. Their molecules are in the shape of helix and are stacked together. The distance that the director (assigned direction of a molecule) takes to rotate in one full turn is called the “pitch”. This is important because in cholesteric nematic phase (the phase where molecules are not in any positional order in liquid crystal), helical structure of the molecules can filter reflected light into a specific wavelength which happens to equal to its pitch from the mesophase. Therefore, to create an image, electrical voltage is applied to the cholesteric liquid crystal so that molecules can rotate and produce a pitch where certain colors can be generated.

Cholesteric liquid crystal works great for e-paper because it is bistable meaning that it requires no power to sustain an image on its display. ChLCD uses reflected light from outside source thus required even less power. However, this technology’s downfall is its low reflective brightness of the display. Major electronics companies such as IBM, Fujitsu, and HP are developing ChLCD technology for e-paper into commercial use.

This is the third post of our "How e-paper works?" series. Other posts are available at:

(via http://electronics.howstuffworks.com/ and http://plc.cwru.edu/ , image credit: Kent Display)