How does an electric field change the orientation of liquid crystal molecules?
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), an electric field is used to manipulate the orientation of liquid crystal molecules, which in turn alters the material's optical properties.1
When a small voltage is applied, the liquid crystal molecules align their axes either parallel or perpendicular to the direction of the electric field. The specific preferred direction of this alignment is determined by the moleculeās unique chemical structure.1
In common applications like the twisted-nematic cell, the molecules are normally arranged to rotate the polarization of light, allowing it to pass through crossed polarizers. When an electric field is applied, the molecules reorient to align with the field, losing their ability to rotate the light's polarization. This prevents the light from passing through the top polarizer, causing that portion of the display to appear opaque or dark. By selectively switching these fields on and off across a grid, the device creates the light and dark patterns necessary to form images.2