Alignment, mechanical

In most experiments and observations we are confronted with a collection of very many particles unless special pains are taken to align the particles, or in the absence of a known alignment mechanism, we may reasonably assume that they are randomly oriented. Under these conditions the quantities of interest are the average cross sections (Cabs) and (Csca), which are independent of the polarization of the incident light provided that the particles are not intrinsically optically active. Let p(Cl) dti be the probability that one of the axes fixed relative to a particle, the x axis, say, lies within a solid angle dSl around the... 

Koch N, Kahn A, Ghijsen J, Pireaux J-J, Schwartz J, Johnson RL, Elschner A (2003) Conjugated organic molecules on metal versus polymer electrodes demonstration of a key energy level alignment mechanism. Appl Phys Lett 82 70 Cahen D, Kahn A (2003) Electron energetics at surfaces and interfaces concepts and... 

Clore et a/.81 calculated a structure of the B1 domain of the streptococcal protein G (56 residues) by RDCs (for 15N-Hn, l 5N l3C. and HN 13 C ) and 32 backbone hydrogen bond constraints (Section 8.3) without NOE-derived distances. They compared structural convergences for their calculations using 152 RDCs measured in tobacco mosaic virus, 150 RDCs measured in DMPC/DHPC (3 1) bicelles, and without RDCs. The rmsd values with RDCs were both improved significantly (four-fold). A further improvement was observed for calculations using the two sets of the constraints simultaneously by 20-30%. This indicated that the alignment mechanisms are different and complementary to the two medias. 

Because of the uncertainty about the alignment mechanisms, it is not possible to relate the observed polarization fraction to the strength of the magnetic field. Indirect estimates can be made, however, by measuring the dispersion of the polarization vectors which is related to the magnetic field strength and the gas density and turbulent velocity [13,14], but the field strength can be overestimated unless the polarization stmcture is resolved spatially. 

The usual approach is to use a tensile" or universal testing machine that is equipped with a compression load cell. The compression takes place between parallel steel plates, which may incorporate a self-alignment mechanism. Rather than a universal machine, an apparatus working in compression only is equally satisfactory if the volume of work justifies a dedicated apparatus. An alternative is to use a compression cage in a tensile machine effectively to reverse the motion of the machine. However, these are now seldom seen, as they can introduce considerable friction errors, and there is always difficulty in alignment. As will be appreciated, for stiff materials the forces and hence machine capacity required can be large. 

With the aim of quantitatively predicting the orientational order of rigid solutes of small dimensions dissolved in the nematic liquid crystal solvent, 4-n-pentyl-4 cyanobiphenyl (5CB), an atomistic molecular dynamics (MD) computer simulation has been applied. It is found that for the cases examined the alignment mechanism is dominated by steric and van der Waals dispersive forces. A computer simulation of the deuterium NMR spectra of molecules in a thin nematic cell has been carried out and the director distribution in the cell has been studied. An experiment for the direct estimation of an element of the order matrix from H NMR spectra of strongly dipolar coupled spins that is based on the multiple quantum spin state selected detection of single quantum transitions has been proposed. The experiment also enables obtaining nearly accurate starting dipolar... 

Choi, J. Lim, J. Song, K. Photo-alignment mechanism study of poly(siloxane cinna-mate). Polym. Korea 2006, 30, 417-421. 

Cempini, M., De Rossi, S.M.M., Lenzi, T., Vitiello, N., and Carrozza, M.C., Self-alignment mechanisms for assistive wearable robots A kinetostatic compatibility method. IEEE Trans. Robot., 29, pp. 236-250, 2013. 

A very plausible alignment mechanism involves movement of defects. Here, forces on defect structures in an electric field are described. The actual material contains a large number of defects in many configurations, so a full analysis would be prohibitively complex. However, it is instructive to consider instead some simple ca.ses a general wall defect, a +1/2, -1/2 pair of parallel disclination lines, and two compound edge dislocations with opposing Burgers vector. 

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