Screw motion

Usually, in AFM the position of the tip is fixed and the sample is raster-scanned. After manual course approach with fine-thread screws, motion of the sample is performed with a piezo translator made of piezo ceramics like e. g. lead zirconate tita-nate (PZT), which can be either a piezo tripod or a single tube scanner. Single tube scanners are more difficult to calibrate, but they can be built more rigid and are thus less sensitive towards vibrational perturbations. 

It is convenient to distinguish between particle or fluid rotation about axes normal and parallel to the direction of relative motion. These two types of motion may be termed respectively top spin and screw motion (Til). Top spin is of more general importance since this corresponds to particle rotation caused by fluid shear or by collision with rigid surfaces. Workers concerned with suspension rheology and allied topics have concentrated on motion at low Re, while very high Reynolds numbers have concerned aerodynamicists. The gap between these two ranges is wide and uncharted, and we make no attempt to close it here. 

Gas bubbles in screw motion show flattening as the angular velocity is increased (R5). Coriolis forces must be considered in predicting trajectories of fluid particles and a method of doing this is given by Catton and Schwartz (Cl). Slugs in rotating tubes are treated in Chapter 9. 

The effect of rotation on transfer to a translating sphere has been studied for both screw motion (El, F6, T2) and top spin (N3, T2) with Re > 1500. The effect of rotation on the transfer rate is less than 10% for vJU < 0.5. The ratio of the Sherwood number in screw motion to that in pure translation at the same Ur is correlated within 10% by... 

In the transition from Pair B to Pair C, the overall symmetry of the site is restored by an analogous 180° rotational translation of the second radical. In Pair C there is rapid internal rotation about the Ca—Cf bond, and the anisotropy of hyperfine splitting by the alpha hydrogens establishes the direction of the first C—C bonds in the radicals. This direction confirms the supposed screw motion. The breaking of symmetry in the first step, its subsequent restoration in the third, and the overall similarity in the motions of the two radicals give support to the hypothesis that the radicals move one by one. Infrared evidence discussed in Sections IX and VII-B.4.a confirms this inference. 

The laser vaporization approach allows the use of even the most refractory target materials. The source configuration used in Fig. 1 involves a target rod that is rotated and translated in a continuous screw motion to expose fresh metal to the laser beam. This has been found necessary to provide acceptable pulse-to-pulse reproducibility. Target rods of refractory metals, semiconductors, carbon, polyethylene, alumina, and alloys have all been vaporized successfully to make clusters in many laboratories. For some materials a disk target is preferred due to the ease in sample preparation. Molecular solids, liquids, and solutions could also be used, though care must be taken to consider the additional complex plasma chemistry one is likely to encounter. 

Loosening Screw Motion Wrong choice of screw type Osteoporotic bone... 

Ball screws vs. lead screws. Motion Syst Des, Jan. 2008. http //motionsystem... 

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