Anchoring surface alignment

We have not considered here alignment techniques, such as SiOx deposition or alignement by monolayer (Langmuir) deposition on surfaces. Details on these techniques of surface alignment can be found elsewhere [5-9]. We also do not discuss recently introduced active (command) surfaces, where the anchoring conditions are controlled by light [55], electrochemistry [56] and electric field [57]. 

Here we consider only homeotropic grating surface alignment. A zenithal bistable device surface can also be realized with planar surface anchoring on the grating surface. ... 

Surfaces act to induce alignment of the liquid crystalline director through the phenomenon of surface anchoring. Surface anchoring, although poorly understood, is critical in the performance of most liquid crystal devices. In the absence of external... 

More recently, however, it has become apparent that the situation can be more complex with the realization that surface anchoring in nematics can be bistable, as found by Jerome, Pieranski and Boix [24] and Monkade, Boix and Durand [25]. Shortly thereafter, Barberi, Boix and Durand [26] showed that one can switch the surface alignment from one anchoring to the other using an electric field. As Nobili and Durand [27] discuss, one must consider rather more complex forms for the surface energy... 

In the limit of weak anchoring, li , U, which means that b a, i.e., A9s b, i.e., external constraints disrupt the surface alignment. 

Two cases of conical anchoring are related to the strong anchoring conditions which have been mentioned in the previous Section. The first occurs when = tt/2 in Fig. 2.4, so that n is parallel to i/, in which case the boundary condition is homeotropic. The second occurs when = 0 and the director n becomes perpendicular to I/, which results in a surface alignment that is degenerately homogeneous. 

An essential requirement for device applications is that the orientation of the molecules at the cell boundaries be controllable. At present there are many techniques used to control liquid crystal alignment which involve either chemical or mechanical means. However the relative importance of these two is uncertain and the molecular origin of liquid crystal anchoring remains unclear. Phenomenological models invoke a surface anchoring energy which depends on the so-called surface director , fij. In the case where there exists cylindrical symmetry about a preferred direction, hp the potential is usually expressed in the form of Rapini and Popoular [48]... 

Stelzer et al. [109] have studied the case of a nematic phase in the vicinity of a smooth solid wall. A distance-dependent potential was applied to favour alignment along the surface normal near the interface that is, a homeotropic anchoring force was applied. The liquid crystal was modelled with the GB(3.0, 5.0, 2, 1) potential and the simulations were run at temperatures and densities corresponding to the nematic phase. Away from the walls the molecules behave just as in the bulk. However, as the wall is approached, oscillations appear in the density profile indicating that a layered structure is induced by the interface, as we can see from the snapshot in Fig. 19. These layers are... 

The alignment of the director at the nematic free surface of real systems is not found to exhibit universal behaviour. Depending on the mesogen, homeotro-pic, tilted and planar anchoring have been observed. Clearly, to study this interface in a simulation a potential which exhibits a nematic phase in co-... 

An alumina matrix may be prepared with high pore density (more than 60 %) and pore diameters ranging from 5 to 250 nm. Ruiz-Hitzky et al. [214] immobilized GOD in nanoporous alumina membranes with regular hexagonal arrays of highly ordered cylindrical pores aligned perpendicularly to the membrane surface. GOD was anchored in the membrane by the highly hydrophilic chitosan biopolymer. Full activity was maintained for at least 50 hours. 

The concept of local perturbations of the director around nanoparticles, often linked to homeotropic anchoring to the nanoparticle surface, is a concept often brought forward in discussions of thermal, optical and electro-optic properties of nanoparticle-doped nematic liquid crystals, which adds a slightly different perspective to the invisibility of smaller particles in aligned nematics. This appears to be of particular relevance for particles coated with either hydrocarbon chains or pro-mesogenic as well as mesogenic units. 

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