Wall defects

Note 3 Schlieren textures observed in nematic samples with planar alignment show defect centers with two or four emerging brushes. Schlieren textures in nematic samples with tilted alignments show centers with four brushes centers with two brushes are caused by defect walls. 

Fig. 9 (a) Sketch of the structure of the hexagonal columnar phase of DNA, showing parallel molecules hexagonally packed in the plane perpendicular to their axis, a and 4 are the lattice parameters, (b) COL developable domains observed in polarized microscopy, w indicates defect walls between differently oriented domains, while 7t stands for point defect around which DNA molecules continuously bend (size bar is 10 pm). Adapted with permission from [27]... 

Let us first consider the case where the preferred orientation of the polar director is perpendicular to the tilt plane (K > 0). The spatial variation of the layer normal and the nematic and polar directors is shown in Fig. 12. We see that regions of favorable splay (called blocks or layer fragments in Sect. 2) are intersected by regions of unfavorable splay (defects, walls). In the region of favorable splay the smectic layer is flat. In the defects regions the tilt angle decreases to reduce energy... 

Do DD, Nicholson D, and Do HD. Heat of adsorption and density distribution in slit pores with defective walls GCMC simulation studies and comparison with experimental data. Appl. Surf. Sci., 2007 253(13 SPEC. ISS.) 5580-5586. 

The domain wall displacement is not triggered by any field. There exists a threshold or critical field, H , below which the wall is not displaced. This critical field depends on sample defects walls are affected by any... 

This phase consists of uniform SmA blocks separated by defect walls [22], At each wall, the normal to smectic layers in one blocks mms through a small angle with... 

TGBC C2 X T(2) or blocks of the smectic A (or C ) phases with defect walls (boundaries) between them... 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

The characteristic field, at which the Bragg band disappears, is considerably higher than the critical field ( = 7 V/pm) calculated from the thermodynamic approach, see Eq. (12.29). However, the periodic structure with very thin defect walls separating area of opposite director orientation (

helical structures are also observed at field strengths > in experiments with short voltage pulses when the defects have not enough time to form. 

Optical measurements provide valuable information about alignment kinetics, and electron microscopy of aligned samples provides clues about the alignment process. Microtomed slices were taken fi om PS-PMMA block copolymer materials that were aligned in an electric field (far from the edges of the samples) [65]. Alignment was verified by SAXS. Some slice planes were parallel to the electrodes (parallel slices) and others perpendicular to the electrodes (perpendicular slices), as shown in Fig. 27. The slices were stained with ruthenium tetroxide and viewed with a transmission electron microscope. Several classes of defect structures were observed +1/2 disclination lines and defect walls were most prevalent, and... 

Disclination lines must either form complete loops or end at the edge of the sample or, more likely, at a wall defect (Fig. 35b). For the defect wall to translate, the total length of lines will change. For this reason also, the wall energy loses translation invariance. 

These two simple experimental systems show the presence in liquid crystals of two types of defect lines and point singularities. Liquid crystals contain a large variety of lines with well-defined geometries or topologies. There are also lines that have a continuous core (for example, in the capillary tube) the axial zone corresponds to a maximum of splay and is generally considered to be a defect line, although no discontinuities apart from the singular points are present. This situation is also encountered in the third type of defect - walls. 

The introduction of the disclinations into a liquid crystal structure usually leads to an increase of the free energy of the system. Therefore, defects are not stable, and one generally is able to decrease the number of disclinations by annealing (see Figure 6.2b), or other ways. There are, however, a few cases where disclinations are energetically favorable, and the ordered array of disclinations or defect walls are necessary conditions for the existence of the phase. The existence of these defect phases reflects the fine balance between competing factors. 

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