Alignment uniform parallel

For low concentrations of polymer network, / external field, the liquid crystal is aligned uniformly parallel to the polymer fibers, as shown by in Figure 11.35(a). 

From these considerations it appears plausible that by rubbing the substrate, in other words producing grooves in the surface, the liquid crystal molecules can be aligned uniformly parallel to the rubbed surface and brought into a stable position. Moreover it can be concluded that only in special cases the orientation of the liquid crystal molecules normal to the grooves (maximum deformation energy of the liquid crystal) is stable [41, 42]. 

We now calculate the threshold of domain formation following the concepts of HelfrichJ We consider a thin planar cell with electrodes on the two surfaces. We assume that the dielectric anisotropy is negative (Ae = 6 — < 0) and that the liquid crystal is aligned uniformly parallel to the surface by suitable surface treatment. The geometry is that of Fig. 1 the director lies in the x-z plane and is parallel to the jc-axis at the surfaces. There is no variation in the y-direction and all variations in the jc-direction are periodic with period A. In this geometry, the electric field alone does not distort the liquid and any instability must be of hydrodynamic nature. 

Hartwick (17) aligned uniformly sized fibers into a densely packed hexagonal array. The interstices between the fibers represented the flow channels. There was no transport between the channels. The performance of the device was low relative to its permeability. This is not unexpected A key property of a packed bed is the radial mass transfer, which evens out flow nonuniformities. Tto is not possible in a device consisting of parallel independent flow paths. In an array of circular parallel channels, the breakthrough time for an unretained sample is inversely proportional to the square of the diameter of the channel. To obtain a plate count of 10,000 plates, it would be necessary that the relative standard deviation of the channel diameter is under 0.5% (see also the footnote in Section 2.1.4). This is clearly a tall order. For retained peaks, similar demands would need to be placed on the uniformity of the stationary phase from channel to channel. 

Because of the dielectric anisotropy property of LCs, the LC molecules can align either parallel or perpendicular to the electric field, theoretically, according to their values of dielectric anisotropy [44]. However, under certain conditions, the uniform director reorientation in an a-c electric field is unfavorable the domain structure corresponding to a minimum free energy is formed. The domain patterns can be classified into two main types orientational domains with pure director rotation without fluid motion and the electrohydrodynamic domains caused by the combined effects of the periodic director reorientation and regular vortices of material moving [44]. This kind of movement of LC materials is called hydrodynamic flow, mainly resulting from the effects of conductivity anisotropy of LC molecules and ionic electric current. 

The experimental evidence presented here indicates that anisotropic Si02 films can be deposited in a beam r.f. plasma system. The films cause parallel alignment for commonly used liquid crystal material and for appropriate beam incidence angles uniform parallel alignment is obtained in a single deposition. 

The nematogen used in the experiments was 5CB containing a small quantity of L-dye D81 (EM Chemicals). The glass sample cell spacing was 10 or 100 jum, (Hellma Cells). Uniform parallel alignment of the nematic was achieved by the rubbed PVA process. The glass sample cell was inserted into a temperature controlled aluminum block. 

When electric fields are applied to liquid crystals, the molecules tend to align—either parallel to the field (for Sa > 0) or perpendicular (for < 0). For the case of nematics, which already have a preferred direction, the director is simply reoriented without breaking the symmetry. However, the helical phase has two nonequivalent directions the twist axis, and the director, which rotates spatially about the twist axis. If > 0, such a helical director is clearly incompatible with a uniform field. For this case, an increasing field first distorts the helix, then stretches out the pitch, and finally causes the well-known cholesteric-nematic transition [1], If <0, the helical director is only compatible with a uniform field if the twist axis and field are parallel. 

In thermoplastic materials, linear macromolecules can align themselves uniformly parallel to one another in microscopic regions and form crystallites, with or without regularly arranged substituents that are not too large. Polymer materials with crystalline regions invariably contain additional amorphous zones where the macromolecules cannot be ordered, which is why they are called semi-crystalline. Besides semi-crystalline materials there are also amorphous thermoplastics [1]. 

Given that nematics tend to align uniformly with the anisotropic axis everywhere parallel, Ericksen [18] argues that this must represent a state of minimum energy, and thus assumes that... 

In the above we have assumed that the lowest energy state is one of uniform parallel alignment of the director however, it is possible to modify these expressions for situations where the lowest energy state might be one of uniform splay, bend or twist. Of these, the last is important because it describes the helical liquid crystal phases that result from chiral molecules. Such states arise if there are terms in the free energy that are linear in the strain, and for a chiral nematic the free energy density becomes ... 

There are two other examples of fluid orientation that are closely related to uniform parallel alignment. In the first case, the director also lies parallel to the cell surfaces. However, the director orientation in the plane parallel to the cell walls is not uniform rather, it changes randomly over dimensions on the order of micrometers. This orientation is known as random parallel alignment (see Fig. 4B). 

In his work on optical textures in mesomorphic materials, Friedel presented an interesting discussion of both the uniform parallel and perpendicular states. However, the techniques he describes for achieving these two states are rather cumbersome. Since that time, a number of investigators have described different methods for obtaining perpendicular alignment. These have included chemical etch-ing,9,io coating with lecithin, and physical adsorption of organic surfactant additives such as the polyamide resin Versamid or impuri-... 

Zocher stated that uniform parallel alignment could be obtained if the cell surfaces were first unidirectionally rubbed. Various materials such as paper, tissue, and cotton wool were used to achieve uniform parallel alignment. Chatelain hypothesized that the parallel orientation resulted from the forces generated by the presence of an adsorbed layer of fatty contaminants on the cell surface, and the directionality was achieved by the unidirectional rubbing However, he could not eliminate the possibility that mechanical deformation of surface might have induced the observed alignment. 

For a nematic material with positive dielectric anisotropy, induced birefringence can also be observed. However, the liquid crystal must be in the uniform parallel orientation at zero volts.Above the threshold voltage, the director aligns itself parallel to the applied field. With crossed polarizer and analyzer, the voltage dependence of the light intensity is reversed from that described previously for a fluid of negative dielectric anisotropy. ... 

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