Surface Mobility in Solids

For the purpose of placing events in the proper frame of reference, it is useful to do a simple calculation to estimate tihe mobility of atoms and molecules in a sohd surface. To obtain such an approximation, one can view the surface as being in a dynamic state where there is a constant interchange of molecules between the surface, the bulk, and the vapor phase. 

The number of moles of vapor hitting one cm of surface per second, Z, is given by 

FIGURE 7.1. Schematic illustration of the responses of liquid and solid surfaces to stress (a) a stressed hquid surface with vacancies (fe) molecules below the surface rapidly flow into the stressed area to heal the imbalance of forces (c) a stressed solid surface with various defects (d) due to lack of mobility, molecules cannot readily move into the stressed areas and the defects remain to produce a higher surface energy. 

Such calculations serve to illustrate the relatively low mobility of atoms or molecules in solids and help dramatize the differences that must be considered when discussing and comparing solid and liquid surfaces. They do not, however, exactly describe events when an atom or molecule moves within the surface region. For example, in bulk diffusion, one considers the ease with 

In summary, when one considers the nature of solid surfaces, one must always take into consideration the history (especially thermal) of the material. Since atoms and molecules in solid surfaces at room temperature have a very 

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