Homeotropic alignments

Freely suspended films provide a perfect homeotropic alignment of smectic LCs since the layers always orient parallel to the LC/air interface.33 The director structure in such films can then be determined by analyzing the optical properties of plane-polarized light reflected from the surface of the films at a slightly oblique angle.34 The technique gains additional power when electrodes are added to the setup, allowing observation of the behavior of the films in the presence of an electric field parallel to the plane of the film.35... 

Note 2 When the alignment of the director in a homeotropic alignment deviates from the perpendieular, the alignment is said to be a pre-tilted homeotropic alignment the pre-tilt angle is the deviation from 90 . 

Moreover, segregation of nanoparticles at liquid crystal/substrate interfaces is also an effective way to promote or alter the alignment of thin nematic liquid crystal films. Depending on their concentration, size, and nature, several types of nanoparticles have been shown to induce homeotropic alignment as well as defects and remarkable defect patterns. 

Fig. 7 Examples of polyhedral oligomeric silsesquioxane (POSS) nanoparticles used to induce homeotropic alignment in nematic liquid crystals...

Recently, Chen and co-workers also presented a combination of photo- and nanoparticle-induced alignment, where the homeotropic alignment induced by POSS nanoparticles can be changed into planar alignment by adsorption of a photo-excited azo-dye [346-348]. 

To solve these equations we need suitable boundary conditions. In the following we will assume that the boundaries have no orienting effect on the director (the homeotropic alignment of the director is only due to the layering and the coupling between the layer normal p and the director h). Any variation of the layer displacement must vanish at the boundaries ... 

A typical experimental apparatus for studying photorefractivity in liquid crystals is illustrated in Fig. 2. Two coherent laser beams from an Ar+ laser are crossed in the sample, with a total of 5 mW of p-polarized output at 514 nm. The beams are unfocused and have a 1/e diameter of 2.5 mm. The liquid crystal composite is sandwiched between two ITO coated glass slides that are coated with octadecyl-silyl groups to induce the liquid crystal director to align perpendicular to the face of the glass slides, that is, homeotropic alignment [43], The cell thickness is determined by a Teflon spacer that is 12 to 100 p,m thick. A small electric field... 

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

Figure 25.9 Polarizing optical micrographs of the equimolar mixture of 14 and 16 in the smectic A liquid crystalline state (a) homeotropically aligned monodomain is formed on a glass substrate when the sample is cooled slowly from the isotropic state at the rate of 3° C min Inset is a conoscopic image (b) unaligned polydomains are formed on a glass substrate when the sample is cooled from the isotropic state at the rate of 20°C min ...

Xiang Y., Lin Z. Y., Li T., Shi J., Huang J. W. and Ji L. N., A homeotropical Alignment Nematic Liquid Crystal Doped with FeTPPCl will Dramatically Decrease the Critical Magnetic Field for the Bend Reorientation in a Static Magnetic Field. Mol. Cryst. Liq. Cryst. 339 (2000) pp. 139-143. 

Figure 4 shows data we obtained at 319.2 K. A sample was prepared homeotropically-aligned by cooling from the melt in a saturating low... 

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