Grain motion

A second major problem involves the consequences of the multiscale nature of superplastidty, notably the correlation between the individual behavior of one grain under shear and normal stresses, and the average collective behavior of these grains. This is a very difficult task, as plastidty is a nonequilibrium phenomenon, and grain motion cannot be completely treated as being thermal in nature. Rather, it implies that the usual methods of thermodynamics and statistical physics cannot be applied -or perhaps they can, but under certain restrictions. 

Y ang X, Huan C, Candela D, Mair RW, Walsworth RL Measurements of grain motion in a dense, three-dimensional granular fluid, Phfs Rev Lett 88 044301, 2002. 

An important problem of this protection system is the durability. Frequent wave or current attack can lead to a failure of the wire mesh because of the continuously moving grains along the wires, finally cutting through. Another problem is the corrosion of the mesh. Therefore, meshes with plastic coating or corrosion resistant steel are used. On the other hand, the system is less suitable where waves and currents frequently lead to grain motion. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

D. Fan, L.-Q. Chen. Diffuse-interface description of grain boundary motion. Phil Mag Lett 75 187, 1997. 

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