Alignment wedge

A small spot size for electron diffraction is used for three reasons i) to have a relatively small variation of thickness since most crystals are wedge shaped, ii) to reduce the amount of unwanted information like that of the matrix around a small precipitate and iii) to have a little variation in the crystal orientation. The latter reason is quite important which one can appreciate by moving the electron beam in nanodiffiaction mode over the specimen although the crystal is well aligned according to the selected area diffraction, fluctuations in orientation over 1 to 2° in all directions occur, even for areas which are very close to each other (10-50 nm). Such orientation variations should be considered as normal rather than an exception. 

A second general rule specifies that soaps of monovalent metal cations tend to produce O/W emulsions, while those of polyvalent metal cations will tend to produce W/O emulsions. Figure 7.3 illustrates the concept. In the example shown, the calcium ions each coordinate to two surfactant molecules that are aligned with their polar groups near the metal ion. This forces the hydrocarbon tails into a wedge-like orientation. The hydrocarbon tails in a close-packed interfacial layer are most easily accommodated if the oil phase is the continuous phase. Thus, the oriented-wedge... 

Finally, wedges and dashes are used for groups that are really aligned one behind another. It... 

Fig. 1.25 Tilt and alignment of seats (a) hemispherical design, (b) semicylindrical rockers, (c) stand-off screws, and (d) rotating wedges. (From reference 23 and the references therein. (Reprinted with permission from J. Rhys. E., 22, 913, (1989).)...

The Cano wedge can be used to measure the cholesteric pitch. Two substrates are wedged by a small angle 2y. The cholesteric liquid crystal between the substrates is homogeneously aligned. Under polarized microscopy, the equally spaced disclinations appear at the middle of the wedge. The separation of the lines 2d is associated with the pitch P... 

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