How do rubbed alignment layers guide liquid crystal molecules inside a display?
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 a liquid crystal display (LCD), rubbed alignment layers are essential for controlling the orientation of liquid crystal molecules. These layers consist of a thin polymer coating, typically a few tens of nanometers thick, applied to the inner surfaces of the display's substrate plates.1
The guiding process works as follows:
- Surface Interaction: The polymer surface is rubbed with a cloth in a single, specific direction. This mechanical process creates microscopic grooves or an orientational preference that forces the liquid crystal molecules in direct contact with the layer to align approximately parallel to the surface and along the direction of the rubbing.21
- Twisted Configuration: In a twisted nematic (TN) cell, the top and bottom substrate plates are assembled so that their respective rubbing directions are perpendicular to each other.1
- Molecular Alignment: Because the liquid crystal molecules prefer to align with the surface, the molecules at the top plate are oriented in one direction, while those at the bottom are oriented at a 90Β° angle. This forces the entire layer of liquid crystal molecules to adopt a twisted configuration across the gap between the plates.21
This controlled twist is what allows the display to manipulate polarized light, enabling the device to switch between transparent and dark states when an electric field is applied.21