How do nematic liquid crystal molecules arrange themselves in an LCD?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
In nematic liquid crystals, molecules are characterized by their rodlike shape and a lack of positional order, meaning they are disordered in their locations but share a common orientation. The long axes of these molecules align nearly parallel to one another along a common direction, known as the director.12
In the context of a twisted nematic (TN) display, the arrangement is specifically engineered:
- Surface Alignment: The liquid crystal molecules are placed between two substrate plates coated with a polymer layer that has been rubbed in a single direction. This causes the molecules in contact with the plates to align parallel to the surface.3
- Twisted Configuration: The top and bottom plates are assembled so that their respective alignment directions are perpendicular to each other. This forces the liquid crystal molecules to adopt a twisted, helical configuration as they transition from one plate to the other.3
- Voltage Response: When an electric voltage is applied, the twisted state is destroyed, and the molecules reorient themselves to be perpendicular to the substrate plates.31