*3.6.2 Homeotropic alignment*

In homeotropic alignment the LC molecule directors are oriented perpendicular to the electrode surface (**Figure 17(b)**). It can be achieved by treating the surface with a surfactant such as hexadecyltrimethylammonium bromide (HTAB), lecithin and polymers [57]. The polar head of a surfactant chemically attaches to the substrate, and the hydrocarbon tail points out, perpendicular to its surface. At this point maximum intermolecular interaction between LC molecules and surfactant promotes perpendicular anchoring of the nematic LC director. However, surfactant-based homeotropic alignment is not stable against humidity and heat [58, 59].

Apart from these two standard alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc. which are employed in various LCD devices.
