  Using nCB films adsorbed on MoS 2 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 using the introduction of 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 finally show how the nature of anchoring in 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.
Introduction Understanding the interactions between liquid crystal (LC) and a
solid substrate is of clear applied interest, the vast majority of LC displays
relying on control of interfaces. However this concerns also fundamental
problems like wetting phenomena and all phenomena of orientation of soft matter
bulk induced by the presence of an interface. In LCs at interfaces, the
so-called easy axes correspond to the favoured orientations of the LC director
close to the interface. If one easy axis only is defined for one given
interface, the bulk director orients along or close to this axis [1]. It is
well known that, in anchoring phenomena, two major effects compete to impose
the anchoring directions of a liquid crystal, first, the interactions between
molecules and the interface, second, the substrate roughness whose role has
been analyzed by Berreman [2]. The influence of adsorbed molecular functional
groups at the interface is most often dominant with, for example in carbon
substrates, a main influence of unsaturated carbon bonds orientation at the
interface [3]. In common LC displays, there is one unique easy axis, but
modifications of surfaces have allowed for the discovery of promising new
anchoring-related properties. For instance, the first anchoring bi-stability
has been established on rough surfaces, associated with electric
ordo-polarization [4] and the competition between a stabilizing short-range
term and a destabilizing long-range term induced by an external field, can
induce a continuous variation of anchoring orientation [5]. More recently,
surfaces with several easy axes have been studied extensively. It has been
shown that control of a continuous variation of director pretilt, obtained in
several systems [6, 7], is associated with the presence of two different easy
axes, one perpendicular to the substrate (homeotropic) and one planar [7, 8].
Similar models can explain the continuous evolution of anchoring between two
planar orientations observed on some crystalline substrates [9]. However, in
the same time, two easy axes can also lead to anchoring bi-stability [10, 11]
or discontinuous transitions of anchoring [9], which is not compatible with the
model established to interpret observed control of pretilt. In order to be able
to predict if bi-stability or continuous combination of the two easy axes
occurs for one given system, it becomes necessary to understand the microscopic
origin of the easy axes.
