Alignment layer thickness

Figure 1.25 shows the boundary layer that develops over a flat plate placed in, and aligned parallel to, the fluid having a uniform velocity upstream of the plate. Flow over the wall of a pipe or tube is similar but eventually the boundary layer reaches the centre-line. Although most of the change in the velocity component vx parallel to the wall takes place over a short distance from the wall, it does continue to rise and tends gradually to the value vx in the fluid distant from the wall (the free stream). Consequently, if a boundary layer thickness is to be defined it has to be done in some arbitrary but useful way. The normal definition of the boundary layer thickness is that it is the distance from the solid boundary to the location where vx has risen to 99 per cent of the free stream velocity v . The locus of such points is shown in Figure 1.25. It should be appreciated that this is a time averaged distance the thickness of the boundary layer fluctuates owing to the velocity fluctuations.

Smectic A and smectic C mesophases characterised by anti-parallel (SmA2, SmAj, and SmC2, SmCj) and random (SmAi and SmCi) alignments of the molecular dipoles within the layer thickness in Fig. 10. 

In the in-channel detector, the working electrode is situated within the separation channel and analytes always migrate over the electrode while they are still confined to the channel. Therefore, the in-channel electrode does not need alignment. In this case, the layer thickness, the width and the distance to the outlet of the channel will be the parameters that influence the good performance of the detector. 

Furthermore, as the layer thickness will become smaller, exchange interactions between the particles will favour parallel alignment of the moments throughout the whole material. In a non-textured material, this will occur at the expense of coercivity. It may be expected that large nucleation fields can only be obtained with textured materials. The development of the corresponding preparation procedures constitutes a challenge for material scientists. 

The presentation of samples within the test chamber can have a significant effect on the outcome of the photostability study. Factors of importance are alignment of the samples relative to the irradiation source sample form and layer thickness and selection of protective material (Table 7.2). 

The pol5mier of an alignment layer may be a poly(imide), which is spin-coated from solution. After drying, a film with a thickness of 0.1 jtt is formed at 200°C. This film is subsequently rubbed in one direction so that it functions as the alignment layer. 

Fig. 4.8. Relaxation time r for the fundamental splay fluctuation mode as a function of sample thickness (circles) and best fit of the theoretically predicted relation for weak anchoring (solid line). The aligning layer was UV illuminated photoactive poly-(vinyl-cinnamate), the liquid crystal was 5CB in the nematic phase (T = 32°C) [59].

---