Liquid crystal anchoring

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]... 

At present there have been no attempts to explore the molecular electronic basis for liquid crystal anchoring or to calculate Rapini-Popoular coefficients for real systems using first principles methods. However, there have been a number of theoretical treatments which have suggested that the ordering of... 

Figure 5.25 demonstrates the dependences of RMS on the reduced liquid crystal anchoring energy at the substrate Uw = Wod/Kss, It can be seen that with the rise of Uw the curve h(o s) becomes smoother and its maximum shifts toward larger values of ujs- Note that for low anchoring energy, Uw 0.5, the deformation amplitude (0 )max drastically increases, i.e., the assumption of the small director deviation (5.84), (5.85) becomes... 

POLYVINYL ALCOHOL BORIC ACID—A MATRIX WITH HIGH LIQUID CRYSTAL ANCHORING POWER... 

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... 

Fig. 3 Principle anchoring conditions of nematic liquid crystals at nanoparticle surfaces (a) planar anchoring, and (b) vertical (or homeotropic) anchoring...

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. 

Chen et al. reported on a general approach by which the polarization of the emission from semiconductor nanorods can be manipulated by an external bias. In their device, the composite of a nematic liquid crystal mixture (E7, Merck) and nanorods (CdS) filled into an ITO-coated cell with an optimized concentration of one CdS nanorod per 1010 LC molecules was used to achieve the highest polarization ratio of the suspended nanorods [447, 448]. The nematic liquid crystal in this system acts as a solvent and media whose direction of alignment can be tuned by an applied electric field. Hence, the orientation of the CdS nanorods can be fine-tuned by an external bias because of the anchoring force between the liquid crystal... 

The self-organization of both thermotropic and lyotropic liquid crystals make these ordered fluids remarkable media for the dispersion and organization (alignment) of CNTs. This subject has been the focus of a recent excellent review by Scalia [231], theoretical work on anchoring at the liquid crystal/CNT interface by Popa-Nita and Kralj [458], and a number of earlier experimental reports on liquid crystal/CNT composites demonstrating that liquid crystal orientational order can be transferred to dispersed CNTs, which is commonly illustrated using polarized Raman spectroscopy [459 -62]. 

Fig. 14 (a) Chemical structures of the polyphilic dispersion-promoter molecules, (b) Tailor-designed polyphilic molecules promoting CNT dispersion in the nematic host. Pyrene anchoring group (blue), mesogenic CB unit (dark red), flexible hydrocarbon or ethylene oxide spacer (green), and liquid crystal host (light red) [464]. (Reproduced by permission of The Royal Society of Chemistry)... 

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