Anchoring and orientational wetting of nematic liquid crystals on self-assembled monolayer substrates: An evanescent wave ellipsometric study

An evanescent wave ellipsometric technique is used to study the orientational wetting of 4^',4- alkylcyanobiphenyls (nCB,n=5--9) at the interface with a series of self-assembled monolayers. Brewster angle measurements are interpreted in terms of a global orientational wetting phase diagram (T,n,h₁), where T denotes temperature, n is the alkyl chain length, and h₁ is the cosine of the contact angle of water with the monolayer. At temperatures below the isotropic-nematic phase transition temperature TIN, an anchoring transition from planar to homeotropic alignment occurs as the surface of the monolayer is made more hydrophobic. Homeotropic anchoring of the nematic director at T<TIN is associated with complete orientational wetting on approaching the isotropic-nematic phase boundary from temperatures above TIN. The anchoring-wetting transition shifts to less hydrophobic surfaces as n increases.