Ordered interfaces for dual easy axes in liquid crystals

Using nCB films adsorbed on MoS2 substrates studied by X-ray diffraction, optical microscopy and scanning tunneling microscopy, we demonstrate that ordered interfaces with well-defined orientations of adsorbed dipoles induce planar anchoring locked along the adsorbed dipoles or the alkyl chains, which play the role of easy axes. For two alternating orientations of the adsorbed dipoles or dipoles and alkyl chains, bi-stability of anchoring can be obtained. The results are explained by introducing fourth order terms in the phenomenological anchoring potential, leading to the demonstration of first order anchoring transition in these systems. Using this phenomenological anchoring potential, we show how the nature of anchoring in the presence of dual easy axes (inducing bi-stability or average orientation between the two easy axes) can be related to the microscopical nature of the interface.