DISORDERED-SURFACE-LAYER TRANSITION IN NEMATIC LIQUID-CRYSTALS

A disordered nematic surface layer, with negative orientational order or biaxial order, is shown to result from interaction with substrates which favor random planar alignment. We propose a simple interaction Hamiltonian which yields predictions in excellent agreement with measured pretransitional effects for 4-n-pentyl-4'-cyanobiphenyl on clean glass and unrubbed n-methylaminopropyltrimethoxysilane substrates. When the dimensionless interaction strength is larger than a critical value, a pretransitional biaxial boundary layer is predicted for a temperature range which is bounded above by surface biaxial-uniaxial and biaxial-biaxial transitions, and below by a complete wetting transition of the boundary layer.